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Title: The Nature of the Lower Crust and Moho at Slower Spreading Ridges (SloMo)

Proponent(s): Henry J.B. Dick, Woods Hole Oceanographic Inst., Shoji Arai, Kanazawa University James H. Natland, University of Miami, Christopher J. MacLeod, Cardiff University, Paul T. Robinson, Dalhousie University, Maurice Tivey, Woods Hole Oceanographic Institution and SloMo Proponent group (attached)

Keywords: (5 or less)

Ocean crust, Gabbro, Peridotite, Moho Area:

Indian Ocean, SW Indian Ridge

Contact Information: Contact Person: Henry J.B. Dick

Department: Geology & Geophysics Organization: Woods Hole Oceanographic Institution

Address McLean Laboratory, MS#8, Woods Hole, MA 02543-1539 USA Tel.: 508-289-2590 Fax: 508-457-2183

E-mail:

Permission to post abstract on iSAS Web site: X Yes No Abstract: (400 words or less)

This proposal is to drill through the Atlantis Bank gabbroic massif into mantle 2.2 km NE of 1.5-km deep Hole 735B to 500-m below Moho. There are 2 major objectives. First to recover the lowermost gabbros and crust-mantle transition in order to understand the processes creating Mid-Ocean Ridge Basalt – the most abundant magma type on Earth, and second, resolve the controversy as to whether the Moho at slow spreading ridges can be a serpentinization front. Based on geologic mapping, geochemistry, and seismic refraction, it is believed that the igneous crust-mantle boundary below Atlantis Bank is ~2.5 km above Moho. This is an ideal location, then, to test the serpentinization front hypothesis. The drill site is also positioned at the center of the 700-km2-gabbro massif to recover the crust-mantle transition where it is most fully developed at the likely point of focused melt flow from the mantle. This will test competing hypotheses for MORB petrogenesis: one supported by experimental petrology that it segregates at depths of 10 to 30 km where primary MORB melts were last in equilibrium with the olivine plus two pyroxene mantle assemblage, and then transported to the crust with little additional mantle interaction. The alternative hypothesis is that MORB aggregates and pools in the mantle at the base of the crust, where melt-rock reaction with the mantle and lower crust, significantly modifies the melt composition prior to intrusion to higher levels and eruption to the seafloor. The latter process has two major implications: 1) the assumed composition of primary magmas, based on compositions calculated assuming that MORB is produced by simple fractional crystallization of a parental melt is incorrect, and that 30 years of experimental petrology has used the wrong composition in predicting mantle-melt equilibrium, and 2) that hybridized mantle produced by melt-rock reaction at the base of the crust is a

IODP Proposal Cover Sheet New Revised Addendum

Above For Official Use Only

significant contributor to the bulk composition of the crust. The results will profoundly affect our understanding of magma generation and the fundamental linkage between the mantle, melt, and crust. In addition, the new hole, combined with the existing holes will produce a transit spaced at ~ 1-km intervals to look at the lateral heterogeneity of the crust, while at the same time testing the nature of magnetic reversals in plutonic rock, as well as document the stress-strain evolution of a plate boundary undergoing asymmetric seafloor spreading.

Scientific Objectives: (250 words or less)

v Test competing models for the nature of the lower ocean crust beneath a typical magmatic accretionary ridge segment.

v Constrain the nature of the Moho at a location where it is has been suggested that it is a serpentinization front.

v Determine the lateral heterogeneity of the lower ocean crust and the scale and manner of melt intrusion.

v Determine the Nature of magnetic anomaly transitions in the lower crust.

Please describe below any non-standard measurements technology needed to achieve the proposed scientific objectives.

None

Proposed Sites:

Site Name Position Water Depth

(m)

Penetration (m) Brief Site-specific Objectives Sed Bsm Total

AtBk-1A

AtBk-2A

AtBk-3A

32°42.75’S,

57°17.11’E

32°41.00’S,

57°20.35’E

32°40.3’S,

57°17.5’E

700

1700

700

0

1+

0

6000

500+

500+

6000

501+

500+

Crust-mantle boundary &

Moho

Core the dike-gabbro

transition

Determine the lateral

heterogeneity of the lower

ocean crust

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Nature of the Lower Crust and Moho at Slower-spreading Ridges

List of Proponents with Expertise Senior & Mid-Career Proponents: 1) Henry J.B. Dick, Senior Scientist

Dept. of Geology & Geophysics Woods Hole Oceanographic Inst. Igneous petrology, ridge tectonics

2) James H. Natland, Professor Rosenstiel School of Marine & Atmospheric Sciences University of Miami Igneous petrology

3) Dr. Shoji Arai, Professor Dept. of Earth Sciences Kanazawa University Mantle Petrology

4) Paul T. Robinson, Professor Dalhousie University Igneous & metamorphic petrology

5) Christopher J. MacLeod, Professor School of Earth & Ocean Sciences Cardiff University Igneous petrology, ridge tectonics

6) Maurice Tivey, Senior Scientist Dept. of Geology & Geophysics Woods Hole Oceanographic Inst. Paleomagnetics

7) Benoit Ildefonse, Directeur de Recherche Geosciences Montpellier, CNRS Université Montpellier Structural Geologist

8) Georges Ceuleneer, Professor Observatoire Midi-Pyrénées, CNRS Igneous Petrology

9) Damon Teagle, Senior Researcher National Oceanography Centre University of Southampton & NERC Ocean Floor Hydrothermal Systems

10) Kazuhito Ozawa, Professor University of Tokyo Igneous Petrologist

11) Marguerite Godard, Professor Geosciences Montpellier, CNRS Université Montpellier Geochemist

12) Jay Miller, Manager Ocean Drilling Program

Texas A & M University Petrology, Physical Properties

13) Ricardo Tribuzio, Professor Dipartimento di Scienze della Terra e dell'Ambiente, University of Pavia Igneous Petrologist

14) Hidenori Kumagai, Senior Scientist Institute for Research on Earth Evolution, JAMSTEC Isotope Geochemist

15) Mark Kurz, Senior Scientist Woods Hole Oceanographic Inst. Isotope Geochemist

16) Juergen Koepke, Professor Institut fur Mineralogie Leibniz Universitaet Hannover Metamorphic Petrologist

17) Sumio Miyashita, Professor Dept. of Geology Faculty of Science, Nigata University Igneous Petrologist

19) Jinichiro Maeda, Professor Hokudai University Igneous Petrologist

20) Rolf-Birger Pedersen, Professor Centre for Geobiology Dept. of Earth Science University of Bergen Geobiologist & Petrologist

21) Juan Pablo Canales, Associate Scientist Dept. of Geology & Geophysics Woods Hole Oceanographic Inst. Seismologist

22) Greg Hirth, Professor Dept. of Geological Sciences Brown University Structural Geology and Rheology

Early Career Proponents: 23) C. Johan Lissenberg, Lecturer

School of Earth & Ocean Sciences Cardiff University Igneous Petrologist

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24) Frieder Klein, Assistant Scientist Dept. of Geology & Geophysics Woods Hole Oceanographic Inst. Alteration/Serpentinization

25) Alessio Sanfilippo, Post-doc Dipartimento di Scienze della Terra e dell'Ambiente, University of Pavia Igneous Petrologist

26) Lydéric France, Maître de Conférences Centre de Recherches Pétrographiques et Géochimiques CNRS UPR 2300, Nancy-Université Igneous Petrologist

27) Aaron Yoshinobu, Assoc. Professor Department of Geosciences Texas Tech University Structural Geologist

28) Masako Tominaga, Assistant Prof. Dept. of Geological Sciences

Michigan State University Down Hole Physical Properties

29) Tim Schroeder, Faculty Dept. of Natural Sciences Bennington College Structural Geologist

30) Natsue Abe, Research Scientist Institute for Research on Earth Evolution, JAMSTEC Igneous Petrologist

31) Betchaida Payot, Post-doc Dept. of Earth Sciences Kanazawa University Mantle Petrology

32) Marie Python, Assistant Professor Department of Natural History Science Hokaido University Igneous Petrologists

Potential Reviewers 1) Laurence Coogan

University of Victoria lacoogan@uvic.ca

2) Sherman Bloomer, Oregon State University Bloomer Sherman sherman.bloomer@oregonstate.edu

3) Jeffrey Alt University of Minnesota jalt@umich.edu

4) Richard Naslund State University of New York. Albany

5) Thierry Juteau University de Bretagne Occidental

6) Jeff Gee Scripps Institution of Oceanography jsgee@ucsd.edu

7) Toshiya Fujiwara JAMSTEC toshi@jamstec.go.jp

8) Eric Hellebrand University of Hawaii ericwgh@hawaii.edu

9) Anthony Morris amorris@plymouth.ac.uk

10) Susumu Umino Kanazawa University sesumin@staff.kanazawa-u.ac.jp

11) Wolfgang Bach University of Bremen wbach@uni-bremen.de

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1. Scientific Objectives

This proposal is to drill to the crust-mantle boundary in a tectonic window at Atlantis Bank on the ultraslow-spreading SW Indian Ridge. There are two principle objectives:

I. Test the hypothesis that the Moho beneath Atlantis Bank is a serpentinization front.

II. Recover the igneous lower crust and the crust-mantle transition at an average melt flux for slow and ultraslow-spreading ridges.

From this we seek to understand:

• The igneous stratigraphy of the lower crust

• How much mantle material is incorporated into the lower crust.

• How melt is transported through and emplaced into the lower crust

• How the lower crust shapes the composition of mid-ocean ridge basalt the most abundant magma on Earth?

• The primary modes of accretion of the lower crust.

• Lateral heterogeneity of the lower crust at magmatic time scales.

• The distribution of strain in the lower crust and shallow mantle in the shallow lithosphere during asymmetric seafloor spreading.

• The nature of magnetic anomaly transitions in the lower crust.

• The role of the lower crust and shallow mantle in the global carbon cycle.

• Life in the lower crust and hydrated mantle.

This drilling will:

Provide an important step towards a full penetration in the Pacific by providing critical needed experience in deep drilling in lower crustal rocks

Create a laboratory for hole-to-hole and ship-to-hole experiments for in-situ determination of the seismic character of lower crust and mantle rock at a seismically appropriate scale.

2. Relationship to the IODP Science Plan 2013-2023

“How are seafloor spreading and mantle melting linked to ocean crustal architecture” “- - - In contrast, crust formed at slower-spreading centers, which comprise the majority of the global mid-ocean ridge system, differs substantially in composition and architecture from that of faster spreading crust. At slow-spreading ridges, mantle melting produces insufficient magma to form the full layered sequence observed in crust produced at faster spreading rates.” Science Plan Challenge 9 3.

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At slow and ultraslow-spreading ridges the lower crust uniquely preserves the critical link described by Challenge 9, where magmatic and tectonic processes directly reflect plate dynamics, melt input, and the pattern of mantle flow. Thus, a substantial portion of plate spreading is accommodated in the lower crust by ‘tectonic extension’ due to faulting and ductile deformation. These faults have a strong control on melt distribution and transport in the lower crust and delivery to the seafloor. At fast-spreading ridges, it is accepted that the crust principally undergoes magmatic accretion by the injection of melt into the lower crust, diking, and eruption of magmas on the seafloor. Thus, rollover and corner flow by ductile flow accompanying mantle upwelling and plate spreading is believed largely limited to the mantle. With only ephemeral magma chambers, however, at slower-spreading ridges, the lower crust can support a shear stress. As a consequence, the lower rates of magma supply, and colder stronger lithosphere, slower-spreading ridges have very different morphology and crustal architecture. Thus, a full picture of crustal architecture and accretion can only be drawn if both fast- and slow-spreading environments are addressed, as stated in Challenge 9.

Unlike the Pacific mantle tectonites and gabbros are widely exposed at slower-spreading ridges, largely intact sections of lower crust and shallow mantle can be drilled without going through a carapace of lavas and sheeted dikes. The latter is challenging due to extensive brittle deformation, and the very tough nature of fine-grained often-recrystallized diabase. As a result, despite numerous attempts over 40 years, the dike-gabbro transition has only been barely reached through unusually thin volcanics and dikes in Hole 1256D in the Pacific. By contrast, drilling in lower crust and mantle at slower-spreading ridges has been very successful with average recoveries reaching 87% and penetrations of 1415 m at Hole U1309D and 1508 m at Hole 735B in the Atlantic and Indian Ocean.

Emplacement of lower crust and mantle to the seafloor occurs in several modes. The most prominent are extensional block faulting, and long-lived detachment faulting that exposes lower crust and mantle for up to several million years on a single fault plane. The latter produces large domal highs, known as oceanic core complexes (or megamullions), where intact lower crust and shallow mantle can be explored in three dimensions by geologic mapping and drilling. Numerical modeling shows this ‘asymmetric accretion’ occurs when 30-50% of extension is accommodated by magmatic accretion 4. At lower magma supply, extension occurs largely block faulting, with widely spaced magmatic centers and intervening long amagmatic ridge segments 5,6.

During detachment faulting the lower crust and mantle spreads in one direction, while the sheeted dike and lava carapace spread in the other (Fig 1). This ‘asymmetric accretion’, is distinct from ‘symmetric’ where the volcanic carapace spreads in both directions away from the ridge with the plutonic rocks. Although the latter mode of spreading has long been regarded as producing ‘normal crust’, asymmetric spreading is

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now recognized as a distinct and fundamental mode of crustal accretion at slower-spreading mid-ocean ridges 7. In well-mapped areas of slow and ultraslow-spreading ridges close to 50% or more of the ocean crust can be identified as formed in this manner (e.g.: 5,8-10).

The new picture of crust formed at slower-spreading ridges based on core complex studies departs radically from the Penrose model (Fig. 2). While the overall sequence of pillow lavas, dikes, gabbro and mantle tectonite generally persists; sometimes the gabbros are missing or intruded into the mantle beneath dikes and pillow lavas. Elsewhere, only scattered basalt occurs overlying mantle rock directly (Fig. 2b-e). Of the 5 models shown, only that for fast-spreading ridges (Fig. 2a) is not well documented geologically. Seafloor mapping and drilling has demonstrated that each of the others occurs in various tectonic settings. This extreme variability presents something of a dilemma as how to address Challenge 9. Notably, none of the sections in Figure 2 represents the architecture of the lower crust formed during symmetric spreading, which presumably ranges from similar to Pacific crust at high magma fluxes to something similar to Atlantis Bank at moderate flux. Atlantis Bank (Fig. 3), then, represents what might be termed “fully magmatic crust”, in that the gabbro massif, even if relatively thin (~2-km) extends nearly the full length of the paleo-ridge segment. By contrast, seismic tomography and mapping at the three best exposed and studied MAR core complexes indicate that the gabbros occur in bodies 10’s to ≥ 100 km2 with a heterogeneous distribution in the shallow lithosphere e.g.: 11. Since symmetric extension at higher magma fluxes represents the ~50% of slow and ultraslow-spreading crust, Atlantis Bank is likely close to the average magma flux for slower-spreading ridges. Since amagmatic and weakly magmatic spreading segments, with huge mantle exposures can be fairly well explored by geologic mapping on the seafloor, we suggest there is the most to gain by focusing on the fully magmatic case, as this is as close to a “representative piece of the lower ocean crust formed at slower-spreading ridges as we are likely to get.

“To gain a better understanding of processes, composition, and architecture at slower-spreading ridges, drilling of a single deep hole should be complemented by drilling through “tectonic windows” in slow- and ultraslow-spreading crust where faulting exposes the deep crust and mantle. This drilling strategy would test three-dimensional models of the oceanic crust developed through seismic surveys and ocean-bottom mapping of rock outcrops.” Drilling & Research Strategy, Challenge 9 3

Atlantis Bank, with two existing ODP Holes, provides a unique, opportunity to examine lower crustal stratigraphy in three dimensions (Fig. 4). It is also the best-exposed plutonic massif yet found in the oceans. This is due to its deep dissection by late transform-parallel block faults and numerous large slumps produced by tectonic uplift over a 12 m.y. period initiated by a spreading direction change at ~19.5 Ma 12. Uplift to sea level created a ~15-km2 wave cut platform where sediment cover and fault gouge

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were eroded off, revealing the bedrock below (Fig. 4). Unusually high current activity and scouring by carbonate bioclastic sand then prevented accumulation of manganese so geologic structures and layering can be seen as in glacially polished outcrops (Fig. 5). This permitted an extraordinary level of seafloor geologic mapping.

The mapping at Atlantis Bank shows continuous gabbroic accretion and emplacement to the seafloor on a single detachment fault for ~4 Ma on a segment scale. The detachment footwall is only broken by ridge-parallel faults at the northern and southern ends of the massif so observations made by drilling can be extrapolated to a broad region. This cannot be done at the best-known Atlantic core complexes, where far more heterogeneous basement is exposed - reflecting lower magma supply. Seismic tomography and mapping at these complexes indicate that the gabbros occur in bodies tens to >hundred kilometers with a heterogeneous distribution within the shallow mantle 11 as opposed to the 700 km2 Atlantis Bank Massif.

A single deep hole into an oceanic core complex is of little use if it is not in a representative section suitable to test three-dimensional models for accretion. This requires the geologic context provided by geologic mapping. Otherwise, it is a 1-D view of a 3-D problem with the other dimensions blank. Continuity of lithology and structure insures the hole is representative of the igneous and tectonic processes operative during accretion, and can provide a laboratory for testing the seismic character of the ocean crust at seismically appropriate scales. Estimations of seismic velocity of Earth materials are generally made using small cylinders of rock a centimeter or two across, whereas the characteristics of a massive outcrop, with its internal structures may be very different. With a hole that penetrates >2 km, and passes into different lithologies, the seismic character of the rocks and structures can be measured directly at variable lithostatic loads using down-hole seismometers, provided that there is continuity of outcrop at the appropriate scale.

3. Scientific Justification:

3.1 Is the Moho a serpentinization front?

Deeper crustal drilling in these settings, where the shallow crust is either removed by faulting or never developed initially, will resolve whether the Moho is a lithologic, alteration, or tectonic boundary, and how the nature of Moho varies in space and time. Drilling & Research Strategy, Challenge 9 3

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The Mohorovičić seismic discontinuity, or Moho, is one of the fundamental seismic boundaries within the Earth. Its exact nature has been a subject of speculation and theory for over a hundred years since first recognized by Andrija Mohorovičić. At present this is limited to inference by remote geophysical sensing and studies of the crust-mantle boundary in ophiolites. While it lies 20 to 90 km beneath continents, it is only 5 to 8 km below the ocean floor – and thus, unlike the continents, potentially drillable. Seismic P-wave velocities above the Moho are those associated with basalt and gabbro, generally lying between 6-7 to 7.2 km/s, while those below the Moho (7.6-8.6 km/s) are similar to mantle peridotite and dunite. Thus the 1971 Penrose Ophiolite Conference agreed that the oceanic Moho represented the boundary between a homogenous layered crust of pillow lavas, sheeted dikes, and gabbros overlying mantle peridotite tectonite 13 (Fig. 2a). The basis for this model was the supposedly uniform seismic character of the ocean crust, seafloor gravity, magnetics, and laboratory measurements on abyssal rocks, and ophiolite studies c.f.: 14,15,16. This ‘Penrose model’ is generally accepted for the East Pacific Rise, where the upper crust, at least, is demonstrably layered in seismic units that reflect lithologic and/or porosity changes with depth e.g.: 17,18.

Although the Moho is defined in terms of seismic velocity alone, the traditional working model based on early limited resolution seismic studies is a 1st order zero thickness discontinuity in rock composition across which homogeneous bodies of mafic and ultramafic rocks are in contact. More recent high-resolution seismic refraction and reflection studies of the ocean crust and field studies of exposed ‘Moho’ remnants in ophiolites and continental suture zones call this into question, showing that Moho varies considerably in character, and likely has a finite thickness. The preponderance of the evidence points to an oceanic Moho that is a complex, interlayered 0- to 3-km thick transition zone of crystal cumulates composed predominantly of mafic compositions at the top and ultramafic compositions at the base overlying residual upper mantle (19 and reference therein).

It is generally, though not universally, believed that: “The oceanic Moho is formed remarkably quickly, is laterally continuous over great distances, and is not subjected to pervasive serpentinization 19”. This, however, is the elephant in the closet of marine seismology, and it is this we propose to test by drilling at Atlantis Bank. Hess 20 first suggested that Moho was a serpentinization front. This was virtually abandoned because seismic observations in fast-spread crust did not conform the predictions of Hess’ hypothesis, while studies in “intact” (but not necessarily fully representative) ophiolites found a consistent simple layered structure corresponding to the 1971 Penrose Conference model. Considerable evidence, however, has been building based on observations at slow and ultraslow-spreading ridges that the Moho in some tectonic settings cannot be simply equated such a simple sequence (igneous crust-mantle boundary. In these cases, it is now believed that the Moho may be an alteration front in mantle peridotite 21-26.

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Jokat et al. 24,26,27 found that crustal thicknesses defined by depth to Moho in amagmatic segments along the ultraslow Gakkel Ridge varied from 1.4 – 2.8 km and in some locations seismic stations indicate that mantle rocks are exposed on the seafloor. “The intercept times of the fast travel time branches are so close to those of the seafloor reflection that either no basalts are present or they are so thin that they cannot be detected by our experimental setup. These stations lie also in areas where mainly peridotites were sampled. Here, it is very likely that no crust exists and the mantle crops out at the seafloor as the sampling suggests. In this case, the modeled crust–mantle boundary is not the base of the crust, but indicates the lowermost boundary of altered peridotites 27.”

At the slow-spreading Mid-Atlantic Ridge between 34°N-35° 30’N, Canales et al. 23 found that crustal and mantle seismic structure within a non-transform offset indicated that serpentinization may be as much as 40% ~3.8 km below the seafloor and 10% at ~6.2 km, while at the center of spreading segments the seismic Moho likely represents the lower boundary of an interlayered gabbro-dunite transition zone. Thus, it is entirely likely that in this setting the nature of Moho varies spatially from mafic/ultramafic at the segment center to a serpentinization front that develops during asymmetric spreading at segment ends with unroofing, intense fracturing, and cooling into the serpentine stability field (~300-400°C) accompanying uplift into the inside-corner rift-mountains,.

Muller and coworkers 22,28 found Moho at 5±1 km beneath Atlantis bank but concluded that it was likely a serpentinization front. This was based on geochemical arguments that the original igneous crustal thickness there was originally ~4-km thick, and with basaltic carapace removed by detachment faulting, the remainder was likely to be ~2 to 2.5-km thick. However this interpretation is non-unique, primarily because of the overlap in P-wave seismic velocity between gabbros and ~20-40% partially serpentinized peridotites e.g.: 29. Seismic velocities beneath Atlantis Bank in the 2-5 km depth range are consistent with a gabbroic composition 28, and the observed very-low vertical velocity gradient above the seismic Moho argues against a gradual decrease in extent of serpentinization.

The proposition that Moho beneath Atlantis Bank is a serpentinization front, however, is strongly supported by geologic mapping that shows the igneous crust-mantle boundary outcrops along the transform wall there for 40 km (Fig. 3). This boundary was traversed at two locations by Shinkai 6500 Dives 466 and 458 at 4500 and 4650-m respectively, while Dives 467 and 459 topped the detachment surface at 3000 and 1755-m; intervals of 1550 and 2895-m immediately above them. Projecting the detachment surface to the locations of the traversed crust-mantle boundary indicates that the crustal thickness at these points prior to mass wasting on the transform wall was significantly less than 2000-m, while the depth to Moho below the transform wall remains ~5-km (Fig. 7). Given that the high core complex dome is produced by a flexure during uplift into the

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rift-mountains do to a spreading direction change, not present on crust of the same age north of the plate boundary, it seems reasonable that the mapped boundary would likely project sub-horizontally to beneath the center of Atlantis Bank consistent with a crustal thickness of 2-2.5 km as suggested by Minshull et al. 22.

Similarly, the seismic determination of SWIR crustal thickness at 61°E seems at odds with the observations of Cannat et al. 30 based on mapping and dredging that the section east of the Melville FZ consists of local magmatic centers linked by oblique segments with “smooth” seafloor and nearly amagmatic spreading.

It is not clear then, based on observations at slow and ultraslow-spreading ridges, that the Moho can be simply equated to an interlayered sequence of primitive gabbros and dunite overlying mantle peridotite tectonites. Serpentine is stable up to 400°-500°C 31,32 while numerous normal faults and cracks associated with rifting at ridges could allow penetration of fluids up to 6-km below the seafloor e.g.: 33, which is coincident with Moho depths. If this is correct, then the total flux of melt and volatiles from the Earth’s interior to the crust, oceans and atmosphere could be in question as well as many models for the igneous ocean crust.

3.1.1 Site Requirements

Given the lateral heterogeneity of the ocean crust at slow and ultraslow-spreading ridges, where to test the concept that the Moho can be an alteration front is worth some consideration. It depends on what one wishes to learn. It seems very likely that the Moho, as determined by Jokat et al. (2003) at amagmatic segments on the Gakkel Ridge is an alteration front. However, the depth to Moho there is significantly shallower than the accepted value for most of the ocean crust (~6-7 km). One could drill adjacent to the transform wall at Atlantis Bank, where the crust – mantle boundary appears exposed, such as at the site of dives 643 and 649: there the crust-mantle boundary consists of a thin body of highly evolved oxide gabbro ~100-m thick intruded over granular peridotite tectonites 34. This is not the crust-mantle transition that concerns igneous petrologists and geochemists, but an apophysis of the larger gabbro body to the east intruding the mantle. As Canales et al. 23 suggest, the alteration of the mantle here is likely strongly influenced by the hydrothermal circulation associated with faulting along the transform and uplift into the rift-mountains. What is of interest generally is whether the Moho can be an alteration front beneath a fully magmatic crust, which may therefore be substantially thinner than generally believed. That is the elephant in the marine seismology closet.

Wherever we choose to test the serpentinization hypothesis a measured Moho discontinuity beneath the drill site is a requirement. The crust also needs to be old and cold enough to drill to sufficient depth to resolve this issue. For example, although Moho has been identified at the Atlantis Massif beneath 1415-m Hole U1309D at >4.5 km 35, it is already 120°C at the bottom of the hole. Thus, increasing temperature down-hole may

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make it technically impossible to drill deep enough to resolve this issue there. Another requirement, that should be considered, is lateral continuity of structure and lithology. It is little good, leaving aside the technical challenges, to drill at sites where the lateral continuity of the rocks is unknown or tectonically disrupted and heterogeneous. A major drilling objective is to create a laboratory for seismic characterization of known sections of the lower crust and shallow mantle at seismic scales. This requires mapping out the exposures to insure that there is an appropriately sized body of gabbro emplaced to the seafloor without major tectonic disruptions or large variations in magmatic activity. This requires a good regional geologic map: requirements uniquely met at Atlantis Bank.

3.2.1 Recovering the igneous lower crust

Crust formed at mid-ocean ridges extends across three fifths of the Earth’s surface, comprising ~60% by area and ~30% by volume. The ocean crust on average is about 6-7 km thick, with the lower portion, or seismic layer 3, thought to consist of gabbroic rocks. Yet to date there are only two penetrations of the lower crust further than two hundred meters and those no deeper than 1500-m. Until ocean drilling, the stratigraphy of the gabbroic layer at ocean ridges was unknown and widely misinterpreted. The results of drilling deep in the Indian Ocean and the Atlantic, with the relatively shallow East Pacific Rise Hess Deep sections 36, show that the lower crust at fast and slow-spreading ridge is profoundly different (see 37,38 for a complete review). Slow and ultraslow ridges represent ~60% of the global ridge system, and it seems astounding then that ocean drilling has not penetrated further into layer 3 there when this could be easily done in tectonic windows. Our objective here, then, is to go deeper, as deep as possible, and as soon as possible, to recover historic and more fully representative section of this vast portion of our planet, from which erupts the most abundant magma type on earth.

The composition, diversity and architecture of the lower crust and shallow mantle are critical to understanding the global geochemical cycle, particularly the exchange of heat, mass and volatiles between the Earth’s interior, oceans and atmosphere. Until now, the composition and structure of the ocean crust has largely been inferred from ophiolites - fossil ocean crust tectonically exposed on land, seismology, and the composition of mid-ocean ridge basalts (MORB). Ophiolites, however, largely represent atypical sections of ocean crust, generally formed above subduction zones and small ocean basins, and are no longer seen as perfect paradigms for crustal formation in the large ocean basins. Seismology, though extremely useful, is an imperfect tool in that it gives little direct information on the composition of the ocean crust, little detail on its internal structure, and has difficulty distinguishing serpentinized mantle rock from the gabbro, troctolite and dunite thought to form the lower crust (e.g.: 23,39). MORB, the most abundant magma type on earth, is also now believed to integrate a host of complex mantle and crustal processes, and is of limited use in assessing the bulk composition, thickness, or internal stratigraphy of the lower crust e.g.: 40). For example, melt

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inclusions hosted in early crystallizing phases, such as olivine and plagioclase, are physically isolated from many crustal-level processes and preserve a far greater compositional range than that found in MORB e.g.: 41,42-44. This is compounded by the discovery of olivine-rich troctolites in the middle of the 1415-km long gabbro section drilled at IODP Hole U1309D, believed to have formed by reaction of parental liquids with mantle peridotite at the crust-mantle boundary and then rafted up into the crust e.g.: 45,46.

Thus it is clear that understanding the accretion process, melt-rock interaction within and beneath the lower-crust, the diversity of MORB parental magmas, and the bulk composition of the ocean crust cannot be achieved simply by studying erupted basalts. Rather, several key pieces of information are needed: (1) what is the diversity of parental liquids delivered to the base of the crust from the mantle, (2) what processes occur in the crust-mantle transition zone, where melts likely accumulate prior to transport through the overlying crust, and (3) how do interactions between the ocean crust and the melts rising through it influence the compositions of erupted MORB, and (4) how much mantle rock is incorporated into the crust as it accretes?

As the compositional diversity of melts entering the lower crust is not faithfully preserved and the processes that shape it are obscured in MORB, independent sources of information are needed. The gabbros and peridotites that constitute the lower crust and mantle are the key source of this information. With the complex stratigraphy, and many processes that affect the composition of MORB, however, the disconnected loose spot samples taken from the ocean floor are hard to interpret at best, and likely not fully representative of what is occurring at depth. They form a poor and inadequate basis on which to reconstruct the geology and processes occurring in the lower crust, the crust mantle transition and the shallow mantle. In order to address the fundamental questions of MORB genesis and the constitution of the lower crust and mantle the critical stratigraphic relationships must be recovered, and an intact section that is fully representative of the lower crust and shallow mantle must be obtained from both slow and fast-spreading tectonic environments.

The two deep sections drilled to date (1508-m Hole 735B and 1415-m U1309D, Fig. 8) are both from slower-spreading ridges. They share basic similarities and represent numerous small intrusions, melt flow channels, and evidence for late stage upward percolative melt flow and melt-rock reaction. Both represent sections from immediately beneath the dike-gabbro transition. While only one dike was drilled near the top of Hole 735B, inliers of the dike-gabbro transition in the detachment footwall occur all over the bank. At U1309D, the top of the section penetrated numerous dikes (Fig. 8), so the detachment fault that exposing both sections must have passed through the zone of diking beneath the rift valley. However, the two sections in other respects are critically different.

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Hole 735B consists of roughly 3/4 olivine gabbro, and 1/4 oxide gabbros (Fig. 8), with the latter concentrated toward the top of the hole by late-stage penetrative deformation and percolative melt flow 2. Alteration is largely in the upper amphibolite to granulite facies, and is clearly related to shear zones and the localization of ductile deformation beneath the sheeted dikes in often still partially molten gabbro during the initial stages of detachment faulting. The most primitive gabbros in Hole 735B are the two olivine-gabbro units in the upper 500-m, below which are three olivine gabbro units that each unit show an upward decrease in (Mg# (100 x Mg / [Mg + Fe]). These sequences gradually become more olivine rich down hole. The section in the upper two thirds is riddled with highly evolved oxide gabbros, gabbronorites, and felsic veins far more evolved than anything in Hole U1309D. The overwhelming majority of Hole 735B gabbros represents crystallization of melt that is too evolved to represent or be in equilibrium with the residues of primitive MORB, but rather later fractionates that solidified within the lower crust 2,47 (Fig. 9). Thus, the essential stratigraphy representing MORB evolution has not been recovered, and must lie beneath the hole or out of the section.

By contrast, Hole U1309D cores are significantly less deformed than from Hole 735B, with no downward progression from gabbro mylonite and gneiss to relatively undeformed gabbros (Fig. 8). Alteration is dominantly in the lower-amphibolite to greenschist facies, and is controlled by brittle faulting – occurring largely in the late stages of uplift and emplacement. Unlike Hole 735B, the igneous section contains numerous intervals of olivine-rich troctolite. These are believed to represent mantle peridotite that reacted with MORB melt at the base of the crust, and then were rafted, with continued intrusion of melt from below, up into the section. These hybridized rocks may be directly related to the processes deeper in the section that shaped the composition of MORB, and occur with ‘cumulus’ troctolites that appear to have directly crystallized from it during and prior to its eruption to the seafloor.

The olivine-rich troctolites in Hole U1309D presents a major finding – that the ocean crust, previously presumed to have the bulk composition of primitive parental basalt, actually may incorporate a considerable volume of the mantle itself, and is therefore more magnesian and less-enriched in incompatible elements than previously thought. It also supports the idea that rather than melts segregating from the mantle at great depth, MORB actually aggregates and pools in the mantle at the base of the crust, from where it erupts to the seafloor after reacting with a significant volume of the shallow mantle. Evidence for this has been seen beneath the East Pacific Rise where it is reported that melt accumulates at the Moho and mid-crustal levels in addition to the melt lens beneath the sheeted dikes 48,49. This is supported by findings at the Kane Megamullion, where seafloor mapping and sampling recovered samples from what is believed to be the crust-mantle transition: harzburgite tectonites, dunites, olivine rich troctolites, cumulate troctolites and primitive olivine gabbros (Fig. 10, 50). Analysis of

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the Kane olivine rich troctolites show that they too appear to represent hybridized mantle rock that reacted with primitive MORB, consistent with the hypothesis proposed for the U1309D gabbros.

Thus, one explanation for the differences between the Hole 735B and U1309D sections is that the former represents the top of the lower crustal section, while the latter represents a deeper section, emplaced by late-stage faulting within an older core complex to the seafloor. The critical crust-mantle transition zone, however, was clearly not drilled at Hole U1309D, however, so this would place the section at mid-depths in the lower crust – similar to what we might find by drilling deeper than Hole 735B at Atlantis Bank.

There is, however, a problem with this. The lower crustal section at the Kane Megamullion appears to be significantly thinner than either 735B or U1309D. In fact, primitive troctolites are found intercalated with oxide and olivine gabbros with sheeted dikes in the upper portion of a single fault face, while the crust-mantle transition rocks likely come from its base. Thus the entire gabbro section may be little more than a kilometer thick (Figure 9). The gabbro body drilled at Atlantis Massif also appears much smaller than the 700-km2 Atlantis Bank gabbro massif, perhaps around 100-150 km2 (see Fig. 6 in Blackman and Collins 11), and while a gabbro net-vein complex that likely represents apophyses of the main gabbro body riddle the massive peridotite tectonites on the flanking transform wall, the Atlantis Bank gabbro is exposed in massive outcrop from the base to the crest of the transform wall in the northern half of the complex. Finally a major difference between the two sections is that one is entirely missing the interspersed peridotite and mantle-derived troctolites that so classically characterizes the other. Thus an equally likely hypothesis is that the stratigraphy at Atlantis Massif recovered in Hole U1309D, is that for crust at a magma-poor rather than a fully magmatic ridge segment, which consists of interspersed gabbro and hybridized peridotite. By contrast Atlantis Bank represents the more typical case of fully magmatic accretion, and thus more representative of the 50-60% of the crust that is spreading symmetrically at higher melt fluxes. The only way to tell is to drill deeper at Atlantis Bank while progressing to the crust-mantle transition. In this case, rather than seeing a complex intercalation of primitive and differentiated gabbros throughout the section, as at Kane Megamullion and Atlantis Massif, we would likely find a sharper distinction with the primitive cumulates, largely missing from Hole 735B, concentrated at the base of the section.

A third real possibility is that the differences between the two deep holes are due to a lateral changes in lithology from the center to the flanks of large axial intrusion zones, or to their flanking margins. In this case, the present stratigraphy in Hole 735B might undergo little change deeper in the section, and even become less primitive as the crust-mantle transition is reached. This would be explored by siting the new hole several kilometers from Hole 735B as recommended by previous SEP reviews.

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3.2.2 Recovering the Crust-Mantle Transition

For 40 years it has been accepted that mid-ocean ridge basalt segregates from the mantle at great depth, and then emplaced into the crust with only minor interaction with the overlying mantle (e.g.: 51,52). The latter process creating the dunite channels that riddle many outcrops in mantle massifs on land (e.g.: 53,Quick, 1981 #3446), and in abyssal peridotites suites {e.g.: 54,55). The principle line of argument used to justify this is that the range of MORB compositions require crystallization of high-magnesian clinopyroxene to explain their diversity. Experimental petrology showed that the required mineral assemblage does not exist in equilibrium with MORB except at high pressures (e.g.: 56,57,58). Thus, the prevailing model assumes that MORB evolved by simple fractional crystallization – and you could determine the composition of parental MORB by simply backing out olivine and plagioclase crystallization to equilibrium with mantle olivine.

There are several problems with this hypothesis, not the least of which is the required large mass of clinopyroxene-olivine cumulates has never been found in the many ophiolite mantle massifs around the world, and are conspicuously absent in abyssal peridotite suites 50. Rather, there has been an accumulating evidence to suggest that MORB accumulates and aggregates in the mantle at the base of the crust, from whence it intrudes into the crust and erupts to the seafloor. This evidence, reviewed earlier, consists of the complex crust-mantle transitions zones of interlayered primitive gabbro, troctolite, dunite and harzburgite found in many ophiolite sections e.g.: 59 and at the Kane Megamullion where the transition zone is believed to be exposed in a fault face 50 as well as studies of olivine-rich troctolites from Hole U1309D 45,46,60,61. This coupled with abundant petrologic evidence for impregnation and pooling of MORB melt in the shallow lithosphere beneath the crust 53,62-67, and seismic evidence for accumulation of melt in the mantle at the base of the crust in the Pacific 48,49 all argue against the prevailing hypothesis for high pressure melt segregation followed by simple fractional crystallization in the crust for MORB. This is compounded by recent work that shows that high-magnesian pyroxene is produced by melt-rock reaction with shallow mantle peridotite and troctolite in the crust-mantle transition zone at ocean ridges 65,68. Thus, a new hypothesis has arisen that the crust-mantle transition represents the process zone that shapes the composition of MORB prior to its intrusion higher in the crust and eruption to the seafloor. Thus, recovering the full in-situ crust-mantle transition zone at fast and slow-spreading ridges is critical to test the competing hypotheses for the evolution and origin of the most abundant magma type on Earth.

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3.3 Site Requirements

The requirements for drilling the crust-mantle transition zone are much the same as those for determining the nature of Moho discussed earlier: Lateral continuity of a large section in fully magmatic crust. The added caveat is that it is absolutely critical to position the drill site as close to the mid-point of the gabbro body to be drilled as possible in order to capture the full crust-mantle transition zone. Observations in large mantle massifs, such as Oman show that dunite bodies marking the location at which melts pool and then are emplaced into the overlying crust are not uniformly developed along the crust-mantle contact, but rather occur in specific regions related to the pattern of focused melt flow out of the mantle 59,69,70. This is supported by the observation of local magmatic centers at the Kane Megamullion, away from which both gabbros and dunites become scarce 50. Drilling into the transition zone as it pinches out away from a magmatic center, or where it is missing entirely, as at the location of the mantle-crust contact located by Shinkai 6500 Dives 643 and 645 (Fig. 3b), would be pointless.

4. Drill Hole Location

The exact position in which the deep hole at Atlantis Bank should be sited was a subject of considerable discussion among the various review panels, and in fact, with disagreements with the site survey panel over the extent and nature of site documentation, held up the previous proposal long enough that when it came to SPC for consideration, it was far too late in the program for this multi-leg expedition to be seriously considered. The principle disagreement between the proponents and the review panel was over whether the drill site should be located next to the existing Hole 735B, or moved in order to capture the lateral variability of the lower crust and to drill where it was expected the hole would pass through the magnetic reversal believed to lie beneath Atlantis Bank (Fig. 11). In the end, the proponents moved the hole to satisfy the latters requirements to its present position, which also determined at what scale of lateral variability we would look at. Before that, the proponents followed the recommendation of the TAMU IODP drilling engineers that the best approach would be to drill to the depth of Hole 735B without coring to obtain as clean a hole as possible, and then, after logging the section corresponding to the unlogged, but blocked lower kilometer of Hole 735B, drill ahead to our objective.

AtBk 1 is sited so there will be three holes spaced ~1.1 km apart (1508-m 735B, 180-m 1105A, and AtBk1) representing ~220,000 yr.’s of spreading and will allow direct correlation of structures – critical for petrologic models for accretion and measuring seismic characteristics. The panels also requested backup sites, which we have situated at the far end of the wave cut platform, to examine lateral variability at a larger scale (Fig. 4), and on the bench to the east below the platform, where Shinkai Dive 648 documented an intact section of the dike-gabbro transition (Fig. 3).

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5. Site Survey Requirements

As noted above, there was disagreement over the necessary site survey information between the proponents and the Site Survey Panel. This was not a problem for Leg 176, which had similar objectives, where the Panel was satisfied with the existing information and seismic data. This was also the case for the previous proposals for AtBk1 up until a rotation of Site Survey panelists and a new chairman who requested extensive new seismic reflection and refraction surveys, including a tomography experiment and OBS program. This effectively killed the proposal. Moving goalposts are a frustration, particularly when they are moved out of reach. The real question is whether additional site survey would change in any way the desire to drill here by the proponents – which is easily answered in the negative and would be very unlikely to change the site location.

It is also questionable whether the crust-mantle boundary would be visible seismically if it does not coincide with Moho. There are many potential scenarios as shown in Figure 12, including some combination of those models or variants yet to be imagined. Based on the excellent exposures in the arc-related Karmoy Ophiolite, the transition from lower crust to mantle could be a gradient with a gradual appearance of enclaves of dunite and other ultramafics increasing gradually in abundance downward to ultramafic tectonite – all overprinted by an alteration gradient. Carlson and Miller 71 did find that while there is overlap in the velocities of gabbro and peridotite from ophiolites and the ocean crust, they were distinguishable on the basis measured P and S wave velocities (but not by one of these alone). However, they also found that seismic data might fail to distinguish between gabbro and serpentinite where seismic P-wave velocities are greater than 7-km/sec – that is in the deepest levels of the ocean crust: precisely the region we are concerned with here. In any case, as a practical matter, determining whether the lower crust is serpentine or gabbro seismically (altered or otherwise) proves very difficult, and can only be resolved by drilling 23,39.

There is another consideration here. The available geologic information for Atlantis Bank (Fig. 3) exceeds that which can be obtained in any other manner than by drilling. Seismic experiments of the scale required by the “latter day” site survey panel are extraordinary expensive and difficult to mount. Thus, they have occurred largely only in the those regions of the ocean basins of the most immediate access to the community – that is the East Pacific Rise and the Central North Atlantic. This parochial interest is one reason why it has only been in the last ten years, as geologic mapping and survey have slowly expanded to other more remote regions that the community has realized the ocean crust is far more heterogeneous and diverse than previously supposed. In fact, it can be reasonably argued that the existing seismic database for refraction and reflection is not particularly representative of the ocean crust at all. Given, the remoteness of Atlantis Bank, then, there is likely more to gain here by having additional seismic surveys follow the drilling rather than preceding it. A major point of this work is to create a seismic

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laboratory, where seismic measurements can be made using down-hole seismometers to directly measure the in-situ velocities and characteristics of the formations drilled. So it makes sense to plan, propose and schedule such surveys after, not before, a deep hole is drilled at the site.

As a final word, only once has a group with relevant expertise been convened to examine the question of what site survey information is needed for drilling in tectonic windows: the Offset Drilling Working Group of the JOIDES Planning Committee chaired in 1992 by Fred Vine. The results of their deliberations were reported in the JOIDES Journal volume 18, no. 3. Their principle recommendation with respect to site survey requirements was: “Each proposed drill site should be located on the geologic map or maps and cross-section(s). Using all available information and a hypothesis-testing rational, specific predictions should be made, to the extent possible of approximate depths, attitudes, and nature of important seismic and lithologic transitions, faults and other features. The consequences of particular predicted observations to the hypothesis or hypotheses being tested should be clearly stated as a justification for drilling.” In their list of possible target areas, Atlantis Bank on the SW Indian Ridge was ranked #1 and short listed as first choice for drilling in a tectonic window adjacent to a fracture zone on a transverse ridge (AKA core complex) at a slow-spreading ridge. “This transverse ridge provides ideal sites for further long gabbro sections, ultramafic sections and, hopefully, the gabbro/ultramafic transition.”

6. Drilling Strategy

There are several possible scenarios with which to address the proposal objectives. It should be possible for the Chikyu to go to Atlantis Bank and drill a full penetration to 6-km. This would have several advantages, among which are gaining experience in deep drilling in lower crust and mantle rock to the depth of Moho in the Pacific, avoiding the challenging pillow lavas, dikes, and dike-gabbro transition. The results would be unambiguous and complete. Politically, such drilling as a prelude to the full penetration in the Pacific would provide a major success that would help that program go forward. Moreover, if there are nasty surprises waiting in further deep drilling, as encountered by Leg 335 at the dike-gabbro transition, this will permit for them to be anticipated and engineered for in advance for the Pacific hole, greatly increasing the chances of success.

Alternatively, it is entirely possible that the crust-mantle transition can be drilled in two legs by the JOIDES resolution, as a penetration to 3 km is a reasonable expectation based on the actual drilling records from Legs 118 and 176 (Fig. 12). If this drilling cores 500-m of partially serpentinized mantle below the hole, this can be characterized seismically in-situ to determine if the remaining rock down to Moho is similar. It took exactly six weeks to drill to 1508-m, including bit changes (Fig. 12). This is one of the great advantages of drilling in 700-m of water, and having excellent hole conditions and drillability. Perhaps most important, the penetration rates and recovery were unchanged

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once 150 meters was reached and full weight put on bit. Thus, there is the reasonable expectation that even with a generous weather window, two legs devoted to drilling and coring could achieve the major objectives of our proposal.

In any case, the proponents feel that the exact strategy for drilling at Atlantis Bank should be determined by the planning structure, particularly at the PEP level in conjunction with the drilling engineers at JAMSTEC and IODP. The program at Atlantis Bank should also involve an extensive down-hole measurements program – but once again, this might be best done on a follow-up dedicated logging leg that coincides with the seismic experiments. In this case only a few basic logs would be run at the conclusion of each drilling leg. The logging community could then plan and staff a full logging program based on what has been drilled, rather than what is anticipated, and coordinate it with the seismic program for a much more valuable two-ship experiment.

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Figure Captions

Figure 1 Modes of emplacement for plutonic rocks at slow and ultraslow-spreading ridges representing moderate, low and very low melt fluxes at slower-spreading ridges. A. Fully magmatic crust, asymmetric spreading as at Atlantis Bank. B. Heterogeneous crust, asymmetric, as at Kane Megamullion representing low magma fluxes. C. Block faulting at very low magma fluxes as at 9°-16°E on the SW Indian Ridge.

Figure 2 Models for crustal accretion at ocean ridges. A. Classic interpretation of the Penrose model for the EPR. B. Modified Penrose model modified for slow-spreading ridges at moderate melt flux. C. Heterogeneous crust formed at low melt fluxes. D. Heterogeneous crust formed at low melt supply in a region believed to represent cooler lithosphere. E. Amagmatic crust formed at ultraslow-spreading rates.

Figure 3 A. Location of Atlantis Bank oceanic core complex on ~8.8 – 13.7 Ma crust in the southern rift mountains of the SW Indian Ridge showing the position of the geologic map of the core complex (Figure 4) on the paleo-spreading segment. Proposed drill site, AtBk 1, shown as red dot. Box on right side of the figure shows the conjugate crust of the same age as Atlantis Bank. The dimensions are different as the SWIR here spreads faster parallel to the transform than away from it. B. Geologic map of Atlantis Bank, SW Indian Ridge showing locations of ROV and submersible dives, dredge hauls, and existing and proposed ODP and IODP drill sites. Note that additional ROV tracks and diamond coring locations are shown in the higher resolution map of the platform at the top of the bank in Figure 4. Regions consisting of slump deposits and talus are shaded, while unshaded regions are based on outcrop sampling and observations. Location labeled ‘peridotite pebbles’ represents carbonate cemented beach gravels that contained a few small chips of serpentinized peridotite, indicating that serpentinite schists once overlay this region prior to erosion.

Figure 4 High-resolution contoured single narrow-beam map of the Atlantis Bank wave-cut platform with ROV and submersible dive locations. Diamond drill core locations and lithologies are shown by black-rimmed symbols, dive samples by symbols with rims colored the same as the tracks. Locations of primary site AtBk1, and backup site AtBk3 are also shown.

Figure 5 ROV ROPOS vertical images of wave-cut platform outcrops. A. Dive 436 amphibolite mylonites with eroded ledges typical of much of the detachment fault footwall over Atlantis Bank. B. Dive 429 coarse-grained gabbro cut by two generations of amphibolite veins, and a late gabbroic pegmatite. C. Dive 433 diabase dikes cutting coarse gabbro. D. Dive 430 amphibole vein set cutting gabbro gneiss.

Figure 6 Velocity models for N-S seismic line CAM101 over Atlantis Bank [3, 4]. 0.4 km/s velocity contours. Upper panel shows an undifferentiated layer 3, while the lower panel shows a lower serpentinized mantle layer 3 and an upper gabbroic layer 3. The models fit the data equally well. Triangles mark OBH’s. AtBk-1A lies on the line, and Hole 735B projected from a km to the west. Moho gap due to OBS placement & is not real (White pers. Com.).

Figure 7 Cross section of Atlantis Bank through Hole 735B. Projected onto this line, ROV & submersible dives, over-the-side rock drills, Hole 735B and dredges show

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continuous gabbro outcrop, locally covered by a thin detachment fault zone assemblage of talc-serpentine schist, diabase cataclasite and weathered pillow basalt and cataclasized gabbro mylonites where the original fault surface is uneroded. Yellow is olivine gabbro, purple is ferrogabbro, red is ferrogabbro mylonite. Best guess model for Moho and crust-mantle boundary are shown - but only drilling will tell. Seafloor geology is real - subsurface geology including dunite zone, location of crust-mantle boundary are all inference. Gap in MOHO reflector is again due to the geometry of OBS emplacement and ray tracing. Insets above the figure show dive profiles on the west and east sides of the bank, and the lithostratigraphy of Hole 735B (cold colors are progressively olivine-rich, hot colors progressively more Fe-Ti rich).

Figure 8 Lithostratigraphy of Holes 735B and U1309D modified from the Initial Reports Volumes of the Proceedings of the Ocean Drilling Program. Lithologies are shown as running 20-m averages of the proportions of different rock types down hole, with those for 735B normalized to 100% recovery, while those for U1309D shown as a proportion of the intervals drilled. Different symbols for the points in the whole rock Mg# diagram are for different lithologies based on modal mineralogy, and can be found in the Initial Reports volumes. We note that these sections are based on the descriptions of two different scientific parties, and although both followed the AGI conventions on nomenclature, the exact definitions for each rock type described may vary between them. The red line at Mg# 80 is to emphasize the overall chemical differences in the two sections. Primary melts emerging from the mantle would be in equilibrium with gabbroic cumulates and dunites with an Mg# ≥ 90, and the absence of these in both sections indicates that what has been recovered from these holes does not record the evolution of primitive MORB liquids.

Figure 9 Plot of percent crystallization versus Mg# of experimentally crystallized MORB melt (blue symbols). The curve is the best-fit polynomial expression to the data. The general range of compositions for corresponding cumulate gabbros from Hole 735B, based on olivine compositions and using Roeder and Emslie 72 with Kd = 0.30, are shown for comparison (J. Natland, pers. comm., 2012). Series ranges are from Natland and Dick 73. The plot shows that cumulates representing close to 40% of Indian ocean basalts are nearly entirely missing from Hole 735B.

Figure 10 Intercalated dikes and olivine-rich troctolites from the west scarp of Adam Dome, Kane Megamullion, Mid-Atlantic Ridge and the bathymetric map of the dome in the upper two panels. Inferred cross-section of Adam Dome based on analysis of the dredge samples and bathymetry from Dick et al. 50.

Figure 11 A. Location and magnetization of Atlantis Bank platform drill cores. B. Contoured deep-towed magnetization. C. Magnetic anomaly models [1] with position of Hole 735B and proposed Hole AtBk1.

Figure 12. Feasibility of a two-leg Resolution drilling program. A) Models for the crust below Hole 735B. Muller model based on seismic data for intact SWIR crust & basalts REE inversions. Cannat model preferred by Coogan et al. 1 to explain missing primitive cumulates at Atlantis Bank, while Dick et al. 2 prefer a combination of the Penrose and Muller models, believing the base of the crust will be shallower than MOHO. B)

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Penetration rates and drilling time for Hole 735B compared to drilling through the pillow lavas and dikes at Hole 504B.

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Figure 1 Modes of emplacement for plutonic rocks at slow and ultraslow-spreading ridges representing moderate, low and very low melt fluxes at slower-spreading ridges. A. Fully magmatic crust, asymmetric spreading as at Atlantis Bank. B. Heterogeneous crust, asymmetric, as at Kane Megamullion representing low magma fluxes. C. Block faulting at very low magma fluxes as at 9°-16°E on the SW Indian Ridge.

Figure 2 Models for crustal accretion at ocean ridges. A. Classic interpretation of the Penrose model for the EPR. B. Modified Penrose model modified for slow-spreading ridges at moderate melt flux. C. Heterogenous crust formed at low melt fluxes. D. Heterogeneous crust formed at low melt supply in a region believed to represent cooler lithosphere. E. Amagmatic crust formed at ultraslow-spreading rates.

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Figure 3 A. Location of Atlantis Bank oceanic core complex on ~8.8 – 13.7 Ma crust in the southern rift mountains of the SW Indian Ridge showing the position of the geologic map of the core complex (Figure 4) on the paleo-spreading segment. Proposed drill site, AtBk 1, shown as red dot. Box on right side of the figure shows the conjugate crust of the same age as Atlantis Bank. The dimensions are different as the SWIR here spreads faster parallel to the transform than away from it. B. Geologic map of Atlantis Bank, SW Indian Ridge showing locations of ROV and submersible dives, dredge hauls, and existing and proposed ODP and IODP drill sites. Note that additional ROV tracks and diamond coring locations are shown in the higher resolution map of the platform at the top of the bank in Figure __. Regions consisting of slump deposits and talus are shaded, while unshaded regions are based on outcrop sampling and observations. Location labeled ‘peridotite pebbles’ represents carbonate cemented beach gravels that contained a few small chips of serpentinized peridotite, indicating that serpentinite schists once overlay this region prior to erosion.

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Figure 4 High-resolution contoured single narrow-beam map of the Atlantis Bank wave-cut platform with ROV and submersible dive locations. Diamond drill core locations and lithologies are shown by black-rimmed symbols, dive samples by symbols with rims colored the same as the tracks. Locations of primary site AtBk1, and backup site AtBk3 are also shown.

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Figure 5 ROV ROPOS vertical images of wave-cut platform outcrops. A. Dive 436 amphibolite mylonites with eroded ledges typical of much of the detachment fault footwall over Atlantis Bank. B. Dive 429 Coarse-grained gabbro cut by two generations of amphibolite veins, and a late gabbroic pegmatite. C. Dive 433 diabase dikes cutting coarse gabbro. D. Dive 430 amphibole vein set cutting gabbro gneiss.

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Figure 6 Velocity models for N-S seismic line CAM101 over Atlantis Bank[3, 4]. 0.4 km/s velocity contours. Upper panel shows an undifferentiated layer 3, while the lower panel shows a lower serpentinized mantle layer 3 and an upper gabbroic layer 3. The models fit the data equally well. Triangles mark OBH’s. AtBk-1A lies on the line, and Hole 735B projected from a km to the west. Moho gap due to OBS placement & is not real (White pers. Com.).

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Figure 7 Cross section of Atlantis Bank through Hole 735B. Projected onto this line, ROV &

submersible dives, over-the-side rock drills, Hole 735B and dredges show continuous gabbro outcrop, locally covered by a thin detachment fault zone assemblage of talc-serpentine schist, diabase cataclasite and weathered pillow basalt and cataclasized gabbro mylonites where the original fault surface is uneroded. Yellow is olivine gabbro, purple is ferrogabbro, red is ferrogabbro mylonite. Best guess model for Moho and crust-mantle boundary are shown - but only drilling will tell. Seafloor geology is real - subsurface geology including dunite zone, location of crust-mantle boundary are all inference. Gap in MOHO reflector is again due to the geometry of OBS emplacement and ray tracing. Insets above the figure show dive profiles on the west and east sides of the bank, and the lithostratigraphy of Hole 735B (cold colors are progressively olivine-rich, hot colors progressively more Fe-Ti rich).

Lithologic units in the gabbro massif are identified by color in the inset for two dives

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Figure 8 Lithostratigraphy of Holes 735B and U1309D modified from the Initial Reports Volumes of the Proceedings of the Ocean Drilling Program. Lithologies are shown as running 20-m averages of the proportions of different rock types down hole, with those for 735B normalized to 100% recovery, while those for U1309D shown as a proportion of the intervals drilled. Different symbols for the points in the whole rock Mg# diagram are for different lithologies based on modal mineralogy, and can be found in the Initial Reports volumes. We note that these sections are based on the descriptions of two different scientific parties, and although both followed the AGI conventions on nomenclature, the exact definitions for each rock type described may vary between them. The red line at Mg# 80 is to emphasize the overall chemical differences in the two sections. Primary melts emerging from the mantle would be in equilibrium with gabbroic cumulates and dunites with an Mg# ≥ 90, and the absence of these in both sections indicates that what has been recovered from these holes does not record the evolution of primitive MORB liquids.

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Figure 9 Plot of percent crystallization versus Mg# of experimentally crystallized MORB melt (blue symbols). The curve is the best-fit polynomial expression to the data. The general range of compositions for corresponding cumulate gabbros from Hole 735B, based on olivine compositions and using Roeder and Emslie (1970) with Kd = 0.30, are shown for comparison (J. Natland, pers. comm., 2012). Series ranges are from Natland and Dick (2002). The plot shows that cumulates representing close to 40% of Indian ocean basalts are nearly entirely missing from Hole 735B.

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Figure 10 Intercalated dikes and olivine-rich troctolites from the west scarp of Adam Dome, Kane Megamullion, Mid-Atlantic Ridge and the bathymetric map of the dome in the upper two panels. Inferred cross-section of Adam Dome based on analysis of the dredge samples and bathymetry from Dick et al. (2008).

31

Figure 11 A. Location and magnetization of Atlantis Bank platform drill cores. B. Contoured deep-towed magnetization. C. Magnetic anomaly models [1] with position of Hole 735B and proposed hole AtBk1.

32

Figure 12. Feasibility of a two-leg Resolution drilling program. A) Models for the crust below Hole 735B. Muller model based on seismic data for intact SWIR crust & basalts REE inversions. Cannat model preferred by Coogan et al. 1 to explain missing primitive cumulates at Atlantis Bank, while Dick et al. 2 prefer a combination of the Penrose and Muller models, believing the base of the crust will be shallower than MOHO. B) Penetration rates and drilling time for Hole 735B compared to drilling through the pillow lavas and dikes at Hole 504B.

33

1 Coogan, L. et al. Whole rock geochemistry of gabbros from the Southwest Indian Ridge: constraints on geochemical fractionations between the upper and lower crust and magma chamber processes at (very) slow spreading ridges. Chemical Geology 178, 1-22 (2001).

2 Dick, H. J. B. et al. A Long In-Situ Section of the Lower Ocean Crust: Results of ODP Leg 176 Drilling at the Southwest Indian Ridge. Earth and Planetary Sciences 179, 31-51 (2000).

3 IODP. Illuminating Earth's Past, Present, and Future, International Ocean Discovery Program Science Plan 2013-2023. (Integrated Ocean Drilling Program Management International, Washington DC, 2012).

4 Tucholke, B. E., Behn, M. D., Buck, R. & Lin, J. The role of melt supply in oceanic detachment faulting and formation of megamullions. Geology 36, 455-458, doi:10.1130/G24639A.1 (2008).

5 Dick, H. J. B., Lin, J. & Schouten, H. An ultraslow-spreading class of ocean ridge. Nature 426, 405-412 (2003).

6 Standish, J. J., Dick, H. J. B., Michael, P. J., Melson, W. G. & O'Hearn, T. MORB generation beneath the ultraslow-spreading Southwest Indian Ridge (9°-25°E): Major element chemistry and the importance of process versus source. Geochemistry, Geophysics, Geosystems 9, 39 p., doi:Q05004, doi:10.1029/2008GC001959 (2008).

7 Cheadle, M. & Grimes, C. To fault or not fault. Nature Geoscience 3, 454-456 (2010).

8 Cannat, M. et al. Thin crust, ultramafic exposures, and rugged faulting patterns at the Mid-Atlantic Ridge (22°-24°N). Geology 23, 49-52 (1995).

9 Cannat, M. et al. Modes of seafloor generation at a melt-poor ultraslow-spreading ridge. Geology 34, 605-608 (2006).

10 Escartin, J. et al. Central role of detachment faults in accretion of slow-spreading oceanic lithosphere. Nature 455, 790-794, doi:10.1038/nature07333 (2008).

11 Canales, J. P., Tucholke, B. E., Xu, M., Collins, J. A. & DuBois, D. L. Seismic evidence for large-scale compositional heterogeneity of oceanic core complexes. Geochemistry, Geophysics, Geosystems 9, 22 p., doi:10.1029/2008GC002009 (2008).

12 Baines, G. et al. Mechanism for generating the anomalous uplift of oceanic core complexes: Atlantis Bank, southwest Indian Ridge. Geology 31, 1105-1108 (2003).

13 Conference Participants. Penrose Field Conference: ophiolites. Geotimes, 24-26 (1972).

14 Salisbury, M. H. & Christensen, N. I. The Seismic Velocity Structure of a Traverse Through the Bay of Islands Ophiolite Complex, Newfoundland, an Exposure of Oceanic Crust and Upper Mantle. Journal of Geophysical Research 83, 805-817 (1978).

34

15 Christensen, N. I. Ophiolites, seismic velocities and oceanic crustal structure. Tectonophysics 47, 131-157 (1978).

16 Kempner, W. C. & Gettrust, J. F. Ophiolites, synthetic seismograms, and ocean crustal structure 1. Comparison of oceanbottom seismometer data and synthetic seismograms for the Bay of Islands Ophiolite. Journal of Geophysical Research 87, 8447-8462 (1982).

17 Detrick, R., Collins, J., Stephen, R. & Swift, S. In situ evidence for the nature of the seismic layer 2/3 boundary in oceanic crust. Nature 370, 288-290 (1994).

18 White, R. S., McKenzie, D. & O'Nions, R. K. Oceanic crustal thickness from seismic measurements and rare earth element inversions. Journal of Geophysical Research 97, 19683-19715 (1992).

19 Jarchow, C. M. & thompson, G. A. The nature of the Mohorovicic discontinuity. Annual Reviews of Earth and Planetary Sciences 17, 475-506 (1989).

20 Hess, H. H. in Petrologic Studies: A volume in honor of A.F. Buddington (eds A.E.J. Engel, H.L. James, & B.F. Leonard) 599-620 (Geological Society of America, 1962).

21 Clague, D. A. & Straley, P. F. Petrologic nature of the oceanic Moho. Geology 5, 133-136 (1977).

22 Minshull, T. A., Muller, M. R., Robinson, C. J., White, R. S. & Bickle, M. J. Is the oceanic Moho a serpentinization front? Geological Society Special Publications 148, 71-80 (1998).

23 Canales, J. P., Detrick, R. S., Lin, J., Collins, J. A. & Toomey, D. R. Crustal and upper mantle seismic structure beneath the rift mountains and across a non-transform offset at the Mid-Atlantic Ridge. Journal of Geophysical Research 105, 2699-2720 (2000).

24 Jokat, W., Ritzmann, O., Schmidt-Aursch, M. & Schmidt, T. Geophysical structure along and off Gakkel Ridge, Arctic Ocean. EOS, Transactions of the American Geophysical Union 82, F1097 (2001).

25 Jokat, W. Gakkel Ridge: Geophysical constraints on its crustal structure. EOS, Transactions of the American Geophysical Union 83 (2002).

26 Jokat, W. et al. Geophysical evidence for reduced melt production on the super-slow Gakkel Ridge (Arctic Ocean). Nature 423, 962-965 (2003).

27 Jokat, W. & Schmidt-Aursch, M. C. Geophysical characteristics of the ultraslow spreading Gakkel Ridge, Arctic Ocean. Geophysical Journal International 168, 983-998 (2007).

28 Muller, M. R., Robinson, C. J., Minshull, T. A., White, R. S. & Bickle, M. J. Thin crust beneath ocean drilling program borehole 735B at the Southwest Indian Ridge? Earth and Planetary Science Letters 148, 93-107 (1997).

29 Miller, D. J. & Christensen, N. I. Seismic velocities of lower crustal and upper mantle rocks from the slow spreading Mid-Atlantic Ridge, south of the Kane

35

transform zone (MARK). Proceedings of the Ocean Drilling Program, Scientific Results 153, 437-454 (1997).

30 Cannat, M. et al. Spreading rate, spreading obliquity, and melt supply at the ultraslow spreading Southwest Indian Ridge. Geochemistry, Geophysics, Geosystems 9, 26 (2008).

31 O'Hanley, D. S. Serpentinites: Records of Tectonic and Petrological History. Vol. 34 (Oxford University Press, 1996).

32 Coleman, R. G. & Keith, T. E. A chemical study of serpentinization - Burro Mountain, California. Journal of Petrology 12, 311-328 (1971).

33 Toomey, D. R., Solomon, S. C. & Purdy, G. M. Microearthquakes beneath median valley of Mid-Atlantic Ridge near 23°N: Tomography and tectonics. Journal of Geophysical Research 93, 9093-9112 (1988).

34 Matsumoto, T. et al. Magmatism and "Crust-mantle Boundary" on the ultra-slow Spreading Ridge as Observed in Atlantis Bank, Southwest Indian Ridge. Journal of Geography 112, 705-719 (2003).

35 Blackman, D. K. & Collins, J. A. Lower crustal variability and the crust/mantle transition at the Atlantis Massif oceanic core complex. Geophysical Research Letters 37, 5 pp, doi::10.1029/2010GL045165 (2010).

36 Gillis, K., Mével, C., Allan, J. & al., e. Vol. 147 352 (Ocean Drilling Program, College Station, TX, 1993).

37 Coogan, L. A. in Treatise on Geochemistry Vol. 3 1-45 (Elsevier, 2007). 38 Sinton, J. M. & Detrick, R. S. Mid-Ocean Ridge magma chambers. Journal of

Geophysical Research 97, 197-216 (1992). 39 Canales, J. P., Collins, J. A., Escartin, J. & Detrick, R. S. Seismic structure across

the rift valley of the Mid-Atlantic Ridge at 23°20' (MARK area): Implications for crustal accretion processes at slow spreading ridges. Journal of Geophysical Research 105, 28411-28425 (2000).

40 Niu, Y. & O'Hara, M. J. Global correlations of ocean ridge basalt chemistry with axial depth: a new perspective. Journal of Petrology 49, 633-664, doi:10.1093/petrology/egm051 (2008).

41 Dungan, M. A., Long, P.E., and Rhodes, J.M. in Initial Reports of the Deep Sea Drilling Project Vol. 45 (ed W.G. Melson and P.D. Rabinowitz) 461-478 (US Government Printing Office, 1978).

42 Danyushevsky, L. V. et al. The H2O content of basalt glasses from Southwest Pacific back-arc basins. Earth and Planetary Science Letters 117, 347-362 (1993).

43 Shimizu, N. The geochemistry of olivine-hosted melt inclusions in a FAMOUS basalt ALV519-4-1. Physics of the Earth and Planetary Interiors 107, 183-201 (1998).

36

44 Nielsen, R. L. et al. Melt inclusions in high-An plagioclase from the Gorda Ridge: an example of the local diversity of MORB parent magmas. Contrib Mineral Petrol 122, 34-50 (1995).

45 Drouin, M., Godard, M., Ildefonse, B., Bruguier, O. & Garrido, C. J. Geochemical and petrographic evidence for magmatic impregnation in the oceanic lithosphere at Atlantis Massif, Mid-Atlantic Ridge (IODP Hole U1309D, 30°N). Chemical Geology 264, 71-88 (2009).

46 Suhr, G., Hellebrand, E., Johnson, K. & Brunelli, D. Stacked gabbro units and intervening mantle: A detailed look at a section of IODP Leg 305, Hole U1309D. Geochemistry, Geophysics, Geosystems 9, 31 pp, doi:10.1029/2008GC002012 (2008).

47 Bloomer, S. H., Natland, J. H., Meyer, P. S. & Dick, H. J. B. in Proceedings of the Ocean Drilling Program Vol. 118 (eds R.P. Von Herzen, P.T. Robinson, & et al.) 21-40 (Ocean Drilling Program, 1991).

48 Crawford, W. C., Webb, S. C. & Hildebrand, J. A. Constraints on melt in the lower crust and Moho at the East Pacific Rise, 9°48'N, using seafloor compliance measurements. Journal of Geophysical Research 104, 2923-2939 (1999).

49 Dunn, R. A. & Toomey, D. R. Three-dimensional seismic structure and physical properties of the crust and shallow mantle beneath the East Pacific Rise at 9°30'N. Journal of Geophysical Research 105, 23537-23555 (2000).

50 Dick, H. J. B., Tivey, M. A. & Tucholke, B. E. Plutonic foundation of a slow-spreading ridge segment: Oceanic core complex at Kane Megamullion, 23°30'N, 45°20'W. Geochemistry, Geophysics, Geosystems 9, 44, doi:doi:10.1029/2007GC001645 (2008).

51 Herzberg, C. Partial crystallization of mid-ocean ridge basalts in the crust and mantle. Journal of Petrology 45, 2389-2405 (2004).

52 Kelemen, P. B., Kikawa, E., Miller, D. J. & al., e. Proceedings of the Ocean Drilling Program, Initial Reports. Vol. 209 (Ocean Drilling Program, Texas A&M University, 2004).

53 Dick, H. J. B. & Bullen, T. Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas. Contributions to Mineralogy and Petrology 86, 54-76 (1984).

54 Arai, S. & Matsukage, K. in Proceedings of the Ocean Drilling Program Vol. 147 (eds C. Mevel, K.M. Gillis, J.F. Allan, & P.S. Meyer) 157-172 (Ocean Drilling Program, 1996).

55 Dick, H. J. B. & Natland, J. H. in Scientific Results Vol. 147 (eds K. Gillis, C. Mevel, & J. Allan) 103-134 (Ocean Drilling Program, Texas A&M University, 1996).

56 Elthon, D. High magnesia liquids as the parental magma for ocean floor basalts. Nature 278, 514-518 (1979).

37

57 Elthon, D. Mineral chemistry of gabbroic rocks from the Mid-Cayman Rise. Eos, Transactions, American Geophysical Union 63, 1135 (1982).

58 Elthon, D. in Magmatism is the Ocean Basins (eds A.D. Saunders & M.J. Norry) 125-136 (Geological Society Special Publication No. 42, 1989).

59 Boudier, F. & Nicolas, A. The nature of the Moho transition zone in the Oman ophiolite. Journal of Petrology 36, 777-796 (1995).

60 Drouin, M., Godard, M. & Ildefonse, B. Origin of olivine-rich gabbroic rocks from the Atlantis Massif (MAR 30°N, IODP Hole U1309D): petrostructural and geochemical study. Geophysical Research Abstracts 9, 06550 (2007).

61 Drouin, M., Godard, M. & Ildefonse, B. Origin of olivine-rich troctolites from IODP Hole U1209D in the Atlantis Massif (Mid-Atlantic Ridge): Petrostructural and geochemical study. Eos Trans. AGU 88 (52), Abstract T53B-1300 (2007).

62 Girardeau, J. & Mercier, J. C. C. in Ophiolites and Their Modern Analogues Vol. 60 Geological Society Special Publication (eds L. M. Parsons, B. J. Murton, & P. Browning) 241-250 (1992).

63 Girardeau, J. & Francheteau, J. Plagioclase-wehrlites and peridotites on the East Pacific Rise (Hess Deep) and the Mid-Atlantic Ridge (DSDP Site 339) evidence for magma percolation in the oceanic upper mantle. Earth and Planetary Science Letters 115, 137-149 (1993).

64 Dijkstra, A. H., Drury, M. R. & Vissers, R. L. M. Structural petrology ofr plagioclase peeridotites in the West Othris Mountains (Greece): Melt impregntion in mantle lithosphere. Journal of Petrology 42, 5-24 (2001).

65 Dick, H. J. B., Lissenberg, C. J. & Warren, J. Mantle melting, melt transport and delivery beneath a slow-spreading ridge: The Paleo-MAR from 23°15'N to 23°45'N. Journal of Petrology 51, 425-467 (2010).

66 Muntener, O., Piccardo, G. B., Pettke, T. & Zanetti, A. Plagioclase peridotite and gabbroic rocks: thermochemical erosion followed by igneous crystallization in a thick thermal lithosphere. Ofioliti 30 (2005).

67 Rampone, E. & Borghini, G. Melt migration and intrusion in the Erro-Tobbio peridotites (Ligurian Alps, Italy): Insights on magmatic processes in extending lithospheric mantle. European Journal of Mineralogy 20, 573-585 (2008).

68 Lissenberg, C. J. & Dick, H. J. B. Melt-rock reaction in the lower ocean crust and its implications for the genesis of mid-ocean ridge basalt. Earth and Planetary Science Letters 271, 311-325 (2008).

69 Boudier, F. & Coleman, R. G. Cross section through the peridotite in the Samail ophiolite, southeastern Oman Mountains. Journal of Geophysical Research 86, 2573-2592 (1981).

70 Godard, M., Jousselin, M. D. & Bodinier, J.-L. Relationships between geochemistry and structure beneath a paleo-spreading centre: A study of the mantle section in the Oman ophiolite. Earth and Planetary Science Letters 180, 133-148 (2000).

38

71 Carlson, R. L. On the Abundance of Serpentinites in the Oceanic Crust. EOS, Transactions, American Geophysical Union 77 (1996).

72 Roeder, P. L. & Emslie, R. F. Olivine-liquid equilibrium. Contributions to Mineralogy and Petrology 29, 277-302 (1970).

73 Natland, J. H. & Dick, H. J. B. in Proceedings of the Ocean Drilling Program, Scientific Results Vol. 176 (eds J.H. Natland, H.J.B. Dick, D.J. Miler, & R. Von Herzen) 1-69 (Ocean Drilling Program, 2002).

IODP Site Summary Forms: Form 1 - General Site Information

Please fill out information in all gray boxes

Section A: Proposal Information

Title of Proposal: Nature of the Lower Crust and Moho at Slower Spreading Ridges (SloMo)

Date Form Submitted: April 1, 2012

Site Specific Objectives with

Priority (Must include general

objectives in proposal)

I. Test the hypothesis that the Moho beneath Atlantis Bank is a serpentinization front. II. Recover the igneous lower crust and the crust-mantle transition at an average melt flux for slow and ultraslow-spreading ridges.

List Previous Drilling in Area:

Hole 735B, ODP Legs 118, 176 Hole 1105A, ODP Leg 179

Section B: General Site Information

Site Name: (e.g. SWPAC-01A)

AtBk-1A If site is a reoccupation

of an old DSDP/ODP Site, Please include former Site #

Area or Location:

Atlantis Bank Southwest Indian Ridge

Latitude:

Deg:32 Min:42.75 Jurisdiction: International waters

Longitude:

Deg:57 Min:17.11 Distance to Land: 1393 km to Mauritius

Coordinates System:

WGS 84, Other ( )

Priority of Site: Primary: Yes Alt: Water Depth: 700 m

X New

Revised Revised 7 March 2002

X

Section C: Operational Information Sediments Basement

Proposed Penetration:

(m)

0 6000 mWhat is the total sed. thickness?

cm Total Penetration: 6000 m

General Lithologies: Gabbro and partially serpentinized -peridotite. peridotite, fresh peridotite

Coring Plan: (Specify or check)

1-2-3-APC VPC* XCB MDCB* PCS RCB X Re-entry X HRGB X

* Systems Currently Under Development Wireline Logging

Plan: Standard Tools Special Tools LWD Neutron-Porosity X Borehole Televiewer X Formation Fluid Sampling Density-Neutron

Litho-Density X Nuclear Magnetic Resonance

Borehole Temperature & Pressure

Resistivity-Gamma Ray

Gamma Ray X Geochemical Borehole Seismic Acoustic

Resistivity X Side-Wall Core Sampling

Acoustic X

Formation Image X Others ( ) Others ( ) Max.Borehole

Temp. : Expected value (For Riser Drilling) <<115°C @ 3 km, <<216°C @ 6km

Estimates are maximums based on conductive geotherm and matching thermochronology constraints from John et al. 2004, assumes 1D cooling & ignores sidewall cooling.

Mud Logging: (Riser Holes Only)

Cuttings Sampling Intervals from m to m, m intervals from m to m, m intervals Basic Sampling Intervals: 5m

Estimated days: Drilling/Coring: Phase I – 80 Phase II - ~ 90

Logging: Phase I – 10 Phase II – 10

Total On-Site: Phase I - 100 (10 days weather. Phase II – 120 (10 days weath.)

Future Plan: Longterm Borehole Observation Plan/Re-entry Plan Longterm borehole re-entry site for seismic and borehole experiments

Hazards/ Weather:

Please check following List of Potential Hazards What is your Weather window? (Preferable

period with the reasons) Shallow Gas Complicated Seabed Condition Hydrothermal Activity

Hydrocarbon Soft Seabed Landslide and Turbidity Current Nov. – March Austral summer

Shallow Water Flow Currents Methane Hydrate

Abnormal Pressure Fractured Zone Diapir and Mud Volcano

Man-made Objects Fault High Temperature

H2S High Dip Angle Ice Conditions

CO2

New Revised

Please fill out information in all gray boxes

Proposal # 535 FULL6: Site #: AtBk-1A Date Form Submitted: 10/1/07

Data Type

SSP Requir-ements

Exists In DB

Details of available data and data that are still to be collected 1

High resolution seismic reflection

no Primary Line(s) :Location of Site on line (SP or Time only) Crossing Lines(s):

2 Deep Penetration seismic reflection

no Primary Line(s): Location of Site on line (SP or Time only) Crossing Lines(s):

3 Seismic Velocity† no

4 Seismic Grid no

5a Refraction (surface)

no

5b Refraction (near bottom)

Yes OBS Survey Muller Line 101 crosses over site

6 3.5 kHz No Location of Site on line (Time)

7 Swath bathymetry

Yes Rc 27 cruise 9, Kaire & Yokosuka cruises

8a Side-looking sonar (surface)

no

8b Side-looking sonar (bottom)

no

9 Photography or Video

Yes Representative images sent to data bank & included here. ROPOS ROV dive surveyed just west of site, and BGS diamond rock drill cores nearby

10 Heat Flow Yes Temperature measurements downhole from Leg 118 (11°C @ 500-m) 11a Magnetics Yes RC 27 Leg 9, Kaire & Yokosuka, James Clark Ross 31 cruises 11b Gravity Yes RC 27 Leg 9, Kaire & Yokosuka cruises, including deep tow

12 Sediment cores Yes 50 Diamond drill rock & limestone cores @ Cardiff University 13 Rock sampling Yes RC27-9, JCR31, ODP Legs 118, 176, 179

14a Water current data Yes Drilling operations above 14b Ice Conditions none

15 OBS microseismicity

no

16 Navigation

17 Other Yes 2-meter resolution contoured narrow beam map from JCR31

SSP Classification of Site: SSP Watchdog: Date of Last Review: SSP Comments:

X=required; X*=may be required for specific sites; Y=recommended; Y*=may be recommended for specific sites; R=required for re-entry sites; T=required for high temperature environments; † Accurate velocity information is required for holes deeper than 400m.

Yes

IODP Site Summary Forms: Form 2 - Site Survey Detail

New Revised

Proposal #: 535Full6 Site #: AtBk-1A Date Form Submitted: 10/01/07 Water Depth (m): 700 Sed. Penetration (m): 10 cm Basement Penetration (m): 3 – 5.5 km

Do you need to use the conical side-entry sub (CSES) at this site? Yes No X

Are high temperatures expected at this site? Yes X No

Are there any other special requirements for logging at this site? Yes No X If “Yes” Please describe requirements:

What do you estimate the total logging time for this site to be:

Measurement Type

Scientific Objective Relevance

(1=high, 3=Low) Neutron-Porosity Formation characteristics

2

Litho-Density Formation characteristics

2

Natural Gamma Ray Formation characteristics

2

Resistivity-Induction Formation characteristics – particularly occurrence of oxide rich layers in

gabbroic crust

1

Acoustic Formation characteristics

2

FMS Determine strikes and dips of layers, shear zones, dikes

1

BHTV Determine strikes and dips of layers, shear zones, dikes

1

Resistivity-Laterolog Formation characteistics

2

Magnetic/Susceptibility

Determine magnetization of anomaly transitions 1

Density-Neutron (LWD)

Resitivity-Gamma Ray

(LWD)

Other: Special tools (CORK,

PACKER, VSP, PCS, FWS,

WSP

For help in determining logging times, please contact the ODP-LDEO Wireline Logging Services group at: borehole@ldeo.columbia.edu http://www.ldeo.columbia.edu/BRG/brg_home.html Phone/Fax: (914) 365-8674 / (914) 365-3182

Note: Sites with greater than 400 m of

penetration or significant basement penetration require deployment of standard toolstrings.

IODP Site Summary Forms: Form 3 - Detailed Logging Plan

New Revised

Please fill out information in all gray boxes

Proposal #: Site #: Date Form Submitted:

1 Summary of Operations at site:

(Example: Triple-APC to refusal, XCB 10 m into basement, log as shown on page 3.)

Deploy HRGB or re-entry cone, establish hole and RCB core to 500-m, set casing to 200+ m, then drill and core to 500-m below the crust-mantle boundary, or below Moho, whichever comes first.

2 Based on Previous DSDP/ODP drilling, list all hydrocarbon occurrences of greater than background levels. Give nature of show, age and depth of rock:

None

3 From Available information, list all commercial drilling in this area that produced or yielded significant hydrocarbon shows. Give depths and ages of hydrocarbon-bearing deposits.

None

4 Are there any indications of gas hydrates at this location?

None

5 Are there reasons to expect hydrocarbon accumulations at this site? Please give details.

None

6 What “special” precautions will be taken during drilling?

None

7 What abandonment procedures do you plan to follow:

None

8 Please list other natural or manmade hazards which may effect ship’s operations: (e.g. ice, currents, cables)

Storms estimated to reduce drilling time by ~ 4 days per leg

9 Summary: What do you consider the major risks in drilling at this site?

This is about as safe as it gets

IODP Site Summary Forms: Form 4 – Pollution & Safety Hazard Summary

New Revised New Revised X

Proposal #: Site #: AtBk-1A Date Form Submitted: 1 April 2012

Sub-

bottom depth (m)

Key reflectors, Unconformities,

faults, etc

Age

Assumed velocity (km/sec)

Lithology

Paleo-

environment

Avg. rate of sed. accum. (m/My)

Comments

0 Moho @ 5.3 km 11 Ma 6.8 Gabbro & Peridotite

Ocean Island 0

IODP Site Summary Forms: Form 5 – Lithologic Summary

IODP Site Summary Forms: Form 1 - General Site Information Please fill out information in all gray boxes

Section A: Proposal Information

Title of Proposal: Nature of the Lower Crust and Moho at Slower-spreading Ridges

Date Form Submitted: April 1, 2012

Site Specific Objectives with

Priority (Must include general

objectives in proposal)

Drill the dike-gabbro transition in ultraslow spread crust to examine the history of alteration, deformation and intrusion at this key transition zone in an asymmetric seafloor spreading environment.

List Previous Drilling in Area:

Hole 735B, ODP Legs 118, 176 Hole 1105A, ODP Leg 179

Section B: General Site Information

Site Name: (e.g. SWPAC-01A)

AtBk-2 If site is a reoccupation of an old DSDP/ODP Site, Please include former Site #

Area or Location:

SW Indian Ridge

Latitude:

Deg:32 Min:41.0 Jurisdiction: International waters

Longitude:

Deg57: Min:20.35 Distance to Land: 1393 km to Mauritius

Coordinates System:

WGS 84, Other ( )

Priority of Site: Primary: Alt: first alternate Water Depth: 1700 m

New X

Revised Revised 7 March 2002

Section C: Operational Information

Sediments Basement Proposed

Penetration: (m)

1+ m 500+ m

What is the total sed. thickness? m Total Penetration: 501+ m

General Lithologies: Cemented gabbro beach cobbles Dikes and gabbro

Coring Plan: (Specify or check)

1-2-3-APC VPC* XCB MDCB* PCS RCB Re-entry HRGB

* Systems Currently Under Development Wireline Logging

Plan: Standard Tools Special Tools LWD Neutron-Porosity X Borehole TeleviewerX Formation Fluid Sampling Density-Neutron

Litho-Density X Nuclear Magnetic Resonance

Borehole Temperature & Pressure

Resistivity-Gamma Ray

Gamma Ray X Geochemical Borehole Seismic Acoustic

Resistivity X Side-Wall Core Sampling

Acoustic X

Formation Image X Others ( ) Others ( ) Max.Borehole

Temp. : Expected value (For Riser Drilling) 10°C

Mud Logging: (Riser Holes Only)

Cuttings Sampling Intervals from m to m, m intervals from m to m, m intervals

Basic Sampling Intervals: 5m Estimated days: Drilling/Coring: 20+ Logging: 3 Total On-Site: 23+

Future Plan: Longterm Borehole Observation Plan/Re-entry Plan

Hazards/ Weather:

Please check following List of Potential Hazards NONE What is your Weather window? (Preferable

period with the reasons) Shallow Gas Complicated Seabed Condition Hydrothermal Activity

Hydrocarbon Soft Seabed Landslide and Turbidity Current Oct – March – southern summer

Shallow Water Flow Currents Methane Hydrate

Abnormal Pressure Fractured Zone Diapir and Mud Volcano

Man-made Objects Fault High Temperature

H2S High Dip Angle Ice Conditions

CO2

°C

New: YES Revised

Please fill out information in all gray boxes

Proposal #: 535 FULL5 Site #: AtBk-2 Date Form Submitted: 2/15/07

Data Type

SSP Requir-ements

Exists In DB

Details of available data and data that are still to be collected

1 High resolution seismic reflection

no Primary Line(s) :Location of Site on line (SP or Time only) Crossing Lines(s):

2 Deep Penetration seismic reflection

no Primary Line(s): Location of Site on line (SP or Time only) Crossing Lines(s):

3 Seismic Velocity† 4 Seismic Grid no

5a Refraction (surface)

no

5b Refraction (near bottom)

Yes OBS Survey Muller line 101 4 km from site.

6 3.5 kHz no Location of Site on line (Time) 7 Swath

bathymetry Yes RC 27 cruise 9, Kaire and Yokosuka cruises

8a Side-looking

sonar (surface) no

8b Side-looking

sonar (bottom) no

9 Photography

or Video yes Representative images sent to data bank & included here.

Dive traverse up 1 km fault scarp adjacent to site exposing full section 10 Heat Flow no

11a Magnetics Yes RC 27 cruise 9, Kaire and Yokosuka cruises, James Clark Ross Cruise 31

11b Gravity Yes RC 27 cruise 9, Kaire and Yokosuka cruises

12 Sediment cores Yes Diamond over-the-side rock and limestone cores @ Cardiff University 13 Rock sampling Yes RC 27-9, James Clark Ross 31, ODP Legs 118, 176, 179

14a Water current data Yes Drilling operations above 14b Ice Conditions none

15 OBS microseismicity

no

16 Navigation ??? 17 Other Yes Analogue map of contoured high res survey by James Clark Ross

SSP Classification of Site: SSP Watchdog: Date of Last Review: SSP Comments:

X=required; X*=may be required for specific sites; Y=recommended; Y*=may be recommended for specific sites; R=required for re-entry sites; T=required for high temperature environments; † Accurate velocity information is required for holes deeper than 400m.

IODP Site Summary Forms: Form 2 - Site Survey Detail

New X Revised

Proposal #: 535 Full5 Site #: AtBk 2 Date Form Submitted: 2/14/07 Water Depth (m): 1700 Sed. Penetration (m): 0 Basement Penetration (m): 500+

Do you need to use the conical side-entry sub (CSES) at this site? Yes No X Are high temperatures expected at this site? Yes No X Are there any other special requirements for logging at this site? Yes No X

If “Yes” Please describe requirements:

What do you estimate the total logging time for this site to be: 3 days

Measurement Type

Scientific Objective Relevance

(1=high, 3=Low) Neutron-Porosity Formation characteristics

2

Litho-Density Formation characteristics

2

Natural Gamma Ray Formation characteristics

2

Resistivity-Induction Formation characteristics

2

Acoustic Formation characteristics

2

FMS Determine strikes and dips of layers, shear zones, dikes 1

BHTV Same as above 1

Resistivity-Laterolog Formation characteristics 2

Magnetic/Susceptibility Determine magnetization of dike-gabbro transition 1

Density-Neutron (LWD)

2

Resitivity-Gamma Ray

(LWD)

2

Other: Special tools (CORK,

PACKER, VSP, PCS, FWS,

WSP

For help in determining logging times, please contact the ODP-LDEO Wireline Logging Services group at: borehole@ldeo.columbia.edu http://www.ldeo.columbia.edu/BRG/brg_home.html Phone/Fax: (914) 365-8674 / (914) 365-3182

Note: Sites with greater than 400 m of

penetration or significant basement penetration require deployment of standard toolstrings.

IODP Site Summary Forms: Form 3 - Detailed Logging Plan

New Revised

Please fill out information in all gray boxes X

Proposal #: Site #: Date Form Submitted:

1 Summary of Operations at site: (Example: Triple-APC to refusal, XCB 10 m into basement, log as shown on page 3.)

Using either a HRGB or a ReEntry cone, establish a hole to drill to 500+ meters in massive gabbro.

2 Based on Previous DSDP/ODP drilling, list all hydrocarbon occurrences of greater than background levels. Give nature of show, age and depth of rock:

None

3 From Available information, list all commercial drilling in this area that produced or yielded significant hydrocarbon shows. Give depths and ages of hydrocarbon-bearing deposits.

None

4 Are there any indications of gas hydrates at this location?

None

5 Are there reasons to expect hydrocarbon accumulations at this site? Please give details.

None

6 What “special” precautions will be taken during drilling?

None

7 What abandonment procedures do you plan to follow:

Sail away at the end of drilling

8 Please list other natural or manmade hazards which may effect ship’s operations: (e.g. ice, currents, cables)

Storms

9 Summary: What do you consider the major risks in drilling at this site?

This is about as safe as it gets

IODP Site Summary Forms: Form 4 – Pollution & Safety Hazard Summary

New Revised New X X

Revised

Proposal #: 535-full5 Site #: AtBk-2 Date Form Submitted: Feb. 14, 2007

Sub- bottom

depth (m)

Key reflectors, Unconformities,

faults, etc

Age

Assumed velocity (km/sec)

Lithology

Paleo-

environment

Avg. rate of sed. accum. (m/My)

Comments

0 Moho @ 5.3 km 11 Ma 6.8 Gabbro Ocean island 0

IODP Site Summary Forms: Form 5 – Lithologic Summary

IODP Site Summary Forms: Form 1 - General Site Information Please fill out information in all gray boxes

Section A: Proposal Information

Title of Proposal: Nature of the Lower Crust and Moho at Slower-spreading Ridges

Date Form Submitted: April 1, 2012

Site Specific Objectives with

Priority (Must include general

objectives in proposal)

AtBk-3 lies on the northernmost lip of the Atlantis Bank Platform and has the objective of examining the shallow igneous and high-temperature detachment deformation history at a significantly later point in its history (~500,000 yrs) than at either AtBk-1 or 1105A or 735B. We would occupy this location in the event that we were unsuccessful in spudding in at AtBk-2.

List Previous Drilling in Area:

Hole 735B, ODP Legs 118, 176 Hole 1105A, ODP Leg 179

Section B: General Site Information

Site Name: (e.g. SWPAC-01A)

AtBk-3 If site is a reoccupation of an old DSDP/ODP Site, Please include former Site #

Area or Location:

SW Indian Ridge

Latitude:

Deg:32 Min:40.3 Jurisdiction: International waters

Longitude:

Deg57: Min:17.5 Distance to Land: 1393 km to Mauritius

Coordinates System:

WGS 84, Other ( )

Priority of Site: Primary: Alt: second alternate Water Depth: 700 m

New X

Revised Revised 7 March 2002

Section C: Operational Information

Sediments Basement Proposed

Penetration: (m)

0+ m 500+ m

What is the total sed. thickness? m Total Penetration: 500+ m

General Lithologies: B Gabbro mylonites, some dikes

Coring Plan: (Specify or check)

1-2-3-APC VPC* XCB MDCB* PCS RCB Re-entry HRGB X

* Systems Currently Under Development Wireline Logging

Plan: Standard Tools Special Tools LWD Neutron-Porosity X Borehole TeleviewerX Formation Fluid Sampling Density-Neutron

Litho-Density X Nuclear Magnetic Resonance

Borehole Temperature & Pressure

Resistivity-Gamma Ray

Gamma Ray X Geochemical Borehole Seismic Acoustic

Resistivity X Side-Wall Core Sampling

Acoustic X

Formation Image X Others ( ) Others ( ) Max.Borehole

Temp. : Expected value (For Riser Drilling) 10°C

Mud Logging: (Riser Holes Only)

Cuttings Sampling Intervals from m to m, m intervals from m to m, m intervals

Basic Sampling Intervals: 5m Estimated days: Drilling/Coring: 20+ Logging: 3 Total On-Site: 23+

Future Plan: Longterm Borehole Observation Plan/Re-entry Plan

Hazards/ Weather:

Please check following List of Potential Hazards NONE What is your Weather window? (Preferable

period with the reasons) Shallow Gas Complicated Seabed Condition Hydrothermal Activity

Hydrocarbon Soft Seabed Landslide and Turbidity Current Oct – March – southern summer

Shallow Water Flow Currents Methane Hydrate

Abnormal Pressure Fractured Zone Diapir and Mud Volcano

Man-made Objects Fault High Temperature

H2S High Dip Angle Ice Conditions

CO2

°C

New: YES Revised

Please fill out information in all gray boxes

Proposal #: 535 FULL5 Site #: AtBk-3 Date Form Submitted: 2/15/07

Data Type

SSP Requir-ements

Exists In DB

Details of available data and data that are still to be collected

1 High resolution seismic reflection

no Primary Line(s) :Location of Site on line (SP or Time only) Crossing Lines(s):

2 Deep Penetration seismic reflection

no Primary Line(s): Location of Site on line (SP or Time only) Crossing Lines(s):

3 Seismic Velocity† 4 Seismic Grid no

5a Refraction (surface)

no

5b Refraction (near bottom)

Yes OBS Survey Muller line 101 crosses over site.

6 3.5 kHz no Location of Site on line (Time) 7 Swath

bathymetry Yes RC 27 cruise 9, Kaire and Yokosuka cruises

8a Side-looking

sonar (surface) no

8b Side-looking

sonar (bottom) no

9 Photography

or Video yes Representative images sent to data bank & included here.

ROPOS ROV dive surveyed around site and BGS drill cores nearby 10 Heat Flow no

11a Magnetics Yes RC 27 cruise 9, Kaire and Yokosuka, James Clark Ross 31 cruises 11b Gravity Yes RC 27 cruise 9, Kaire and Yokosuka cruises

12 Sediment cores Yes Diamond over-the-side rock and limestone cores @ Cardiff University 13 Rock sampling Yes RC 27-9, James Clark Ross 31, ODP Legs 118, 176, 179

14a Water current data Yes Drilling operations above 14b Ice Conditions none

15 OBS microseismicity

no

16 Navigation ??? 17 Other Yes Analogue map of contoured high res survey by James Clark Ross

SSP Classification of Site: SSP Watchdog: Date of Last Review: SSP Comments:

X=required; X*=may be required for specific sites; Y=recommended; Y*=may be recommended for specific sites; R=required for re-entry sites; T=required for high temperature environments; † Accurate velocity information is required for holes deeper than 400m.

IODP Site Summary Forms: Form 2 - Site Survey Detail

New X Revised

Proposal #: 535Full5 Site #: AtBk-3 Date Form Submitted: 2/14/07 Water Depth (m): 700 Sed. Penetration (m): 0 Basement Penetration (m): 500+

Do you need to use the conical side-entry sub (CSES) at this site? Yes No X Are high temperatures expected at this site? Yes No X Are there any other special requirements for logging at this site? Yes No X

If “Yes” Please describe requirements:

What do you estimate the total logging time for this site to be: 3 days

Measurement Type

Scientific Objective Relevance

(1=high, 3=Low) Neutron-Porosity Formation characteristics

2

Litho-Density Formation characteristics

2

Natural Gamma Ray Formation characteristics

2

Resistivity-Induction Formation characteristics

2

Acoustic Formation characteristics

2

FMS Determine strikes and dips of layers, shear zones, dikes 1

BHTV Same as above 1

Resistivity-Laterolog Formation characteristics

2

Magnetic/Susceptibility Determine magnetization of dike-gabbro transition 1

Density-Neutron (LWD) 2

Resitivity-Gamma Ray

(LWD)

2

Other: Special tools (CORK,

PACKER, VSP, PCS, FWS,

WSP

For help in determining logging times, please contact the ODP-LDEO Wireline Logging Services group at: borehole@ldeo.columbia.edu http://www.ldeo.columbia.edu/BRG/brg_home.html Phone/Fax: (914) 365-8674 / (914) 365-3182

Note: Sites with greater than 400 m of

penetration or significant basement penetration require deployment of standard toolstrings.

IODP Site Summary Forms: Form 3 - Detailed Logging Plan

New X Revised

Please fill out information in all gray boxes

Proposal #:535Full5 Site #: AtBk-3 Date Form Submitted: 2/14/07

1 Summary of Operations at site: (Example: Triple-APC to refusal, XCB 10 m into basement, log as shown on page 3.)

Using either a HRGB or a ReEntry cone, establish a hole to drill to 500+ meters in massive gabbro.

2 Based on Previous DSDP/ODP drilling, list all hydrocarbon occurrences of greater than background levels. Give nature of show, age and depth of rock:

None

3 From Available information, list all commercial drilling in this area that produced or yielded significant hydrocarbon shows. Give depths and ages of hydrocarbon-bearing deposits.

None

4 Are there any indications of gas hydrates at this location?

None

5 Are there reasons to expect hydrocarbon accumulations at this site? Please give details.

None

6 What “special” precautions will be taken during drilling?

None

7 What abandonment procedures do you plan to follow:

Sail away at the end of drilling

8 Please list other natural or manmade hazards which may effect ship’s operations: (e.g. ice, currents, cables)

Storms

9 Summary: What do you consider the major risks in drilling at this site?

This is about as safe as it gets

IODP Site Summary Forms: Form 4 – Pollution & Safety Hazard Summary

New Revised New X X

Revised

Proposal #: 535-full5 Site #: AtBk-2 Date Form Submitted: Feb. 14, 2007

Sub- bottom

depth (m)

Key reflectors, Unconformities,

faults, etc

Age

Assumed velocity (km/sec)

Lithology

Paleo-

environment

Avg. rate of sed. accum. (m/My)

Comments

0 Moho @ 5.3 km 11 Ma 6.8 Gabbro Ocean island 0

IODP Site Summary Forms: Form 5 – Lithologic Summary