Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1990Proceedings of the Ocean Drilling Program, Initial Reports, Vol. 125

11. SITE 7831

Shipboard Scientific Party2

HOLE 783A

Date occupied: 24 March 1989

Date departed: 26 March 1989

Time on hole: 1 day, 5 hr, 45 min

Position: 30°57.86'N, 141°47.27'E

Bottom felt (rig floor; m; drill-pipe measurement): 4660.0

Distance between rig floor and sea level (m): 11.20

Water depth (drill-pipe measurement from sea level; m): 4648.8

Total depth (rig floor; m): 4828.20

Penetration (m): 168.20

Number of cores: 18

Total length of cored section (m): 168.20

Total core recovered (m): 46.95

Core recovery (%): 27.9

Oldest sediment cored:Depth (mbsf): 150.27Nature: glass-rich silty clay, sheared phacoidal serpentineEarliest age: early Pliocene(?)Measured velocity (km/s): 1.6

Hard rock:Depth (mbsf): 158.60Nature: serpentinized ultramafic rock

Principal results: Site 783 is located on the northern, midflank part ofa seamount that forms part of a 500-km-long ridge running alongthe lowermost, inner wall of the Izu-Bonin Trench. The singlehole, 783A, attempted at this site was drilled to 168.2 m belowseafloor (mbsf) with 27'.9% recovery before being abandonedbecause of high torque. Two lithologic units were defined. Unit I,from 0 to 120.0 mbsf, is middle or lower Pleistocene to lowerPliocene or older, glass-rich, silty clay to claystone. Unit II(120.0-158.6 mbsf) is phacoidal sheared serpentine with clasts ofserpentinized harzburgite and metabasalt. The harzburgite has aprimary mineralogy of olivine (80-90 modal%) and orthopyroxene(10—15 modal%) with minor clinopyroxene and spinel. Thus, theseamount is confirmed as serpentinite in composition, although theearly Pliocene or older age demonstrates that it predates theMariana serpentinite seamount drilled at Sites 778 through 780.

Shear fabrics and convolute plastic folding indicative of post-emplacement tectonic activity can be seen within the claystones ofUnit I and the phacoidal serpentine of Unit II. Studies of physicalproperties show that the bulk densities increase from 1.47 to 1.59g/cm3 in the sediments of Unit I and cluster at about 2.1 g/cm3 inthe serpentine of Unit II. The average compressional-wave veloc-ity in the sediments is 1.9 km/s. Thermal conductivities averageabout 1 W/mK in sediments and 1.91 W/mK in the serpentine.Heat flow in the sediments is 23 mW/m2. Interstitial fluids show asharp chemical discontinuity at the sediment/serpentine boundarywith rapid chemical changes thereafter, which include increases in

1 Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1990. Proc. ODP, Init.Repts., 125: College Station, TX (Ocean Drilling Program).

2 Shipboard Scientific Party is as given in the list of participants precedingthe contents.

pH, calcium, and sodium, depletions in silica, magnesium, sulfate,and potassium, and little change in chlorinity. The fluid chemistry,therefore, shows that serpentinization is still taking place, butwithout the low-chlorinity component identified at the Marianaseamount (Sites 778 through 780).

BACKGROUND AND SCIENTIFIC OBJECTIVES

The eastern 50 km of the Izu-Bonin forearc is morpholog-ically, compositionally, and structurally different from theremainder of the forearc. Within 50 km of the trench axis, aridge runs for more than 500 km along the lowermost, innerwall of the Izu-Bonin Trench, at a latitude of about 30°N (Fig.1). This ridge is separated from the outer-arc high by a narrow(—10 km wide) sediment trough, which has a series of sea-mounts along its eastern and western boundaries. Dredgesfrom two of these seamounts, one at 32°N (K. Saboda, pers.comm., 1987) and the other at about 31°N (Kobayashi, 1989),recovered serpentinized ultramafic rocks. The seamount at31°N, chosen for Ocean Drilling Program (ODP) Site 783, wasdredged in 1988 at three locations (Fig. 2), yielding a widevariety of sedimentary, igneous, and metamorphic rocks(Kobayashi, 1989). Most of the samples dredged from the twosummit locations are serpentinized ultramafic and maficrocks, showing slickensides and fractures. Some pumicefragments and sedimentary rocks were also retrieved from thesummit and these comprise the bulk of the rocks recoveredfrom the lower west flank of the seamount. A very smallamount of serpentine mud adhering to the surfaces of a few ofthe metamorphosed rocks was recovered.

Several multichannel seismic lines were run across the innertrench wall region, and the structure of the ridge and theseamounts within it are presented in Horine et al. (this volume).The presence of fault planes associated with the seamounts anddetected within the ridge suggests that the ridge is still tectoni-cally active. By comparison with Conical Seamount, the sea-mount at Site 783 is far more complex. The summit region of thisseamount is broken by numerous small fault blocks and lackscoherent reflectors ("Underway Geophysics" chapter, this vol-ume). Thus, the seamount is most likely not currently producingserpentine flows, but may be, however, a formerly active ser-pentinite diapir. The study of the interstitial waters within thesediments (both pelagic and serpentine) at this site is critical tothe understanding of the history of this seamount, the flux ofsubduction-derived fluids through the outer forearc wedge, andthe interaction of serpentinite with the associated country rockand overlying sediments.

The history of sedimentation in this part of the forearc hasnot been studied in detail. The rate of recent sedimentation inthis part of the forearc, though expected to be lower than therate closer to the active arc, is constrained only by seismicprofiling. The sequence, if condensed, should provide a com-prehensive history of outer-forearc sedimentation as well as arecord of the interaction between pelagic sedimentation andinflux from seamount serpentinite. In addition, an influx ofarc-derived sediment is likely from cross-arc transport via the

253

SITE 783

33°N

140°E 142°

Figure 1. Location of sites in the Izu-Bonin forearc from Legs 125 and 126. Note the north-trending ridge on the inner trench wall. Site783 was located at proposed Site BON-7. Crosses represent dredge sites on two seamounts of the outer forearc ridge, one at 32°N andthe other at Site 783 at 31°N. Bathymetry in kilometers.

large canyon systems of the Izu-Bonin forearc. North of theseamount, a reentrant in the outer forearc ridge forms part ofthe distal portion of a prominent canyon system that stretchesmore than halfway across the forearc. Some of the sedimentfilling this part of the canyon laps onto the seamount. Thus,the nature of sediment transport along this channel can beinvestigated by drilling into the material on the north flank ofthe seamount.

The objectives of drilling at this site were as follows: (1) todetermine the nature of sediments in the outermost forearcregion; (2) to study the sedimentary sequences on the flank ofthe seamount, and hence to constrain the regional tectonichistory of the ridge upon which the seamount is located, theage of the seamount, and the mode of emplacement of theseamount; (3) to study the composition and structure ofbasement underlying the sedimentary column in order todetermine whether the seamount is a block of upfaulted lowercrustal material or an older, tectonized analog of ConicalSeamount; and (4) to determine the composition of interstitialwaters present in the sediments. To achieve these objectives,Site 783 was located at 30°57.86'N, 140°47.27'E, in a waterdepth of 4648.8 m, on the northern, midflank part of the

seamount (Fig. 1). The sediment cover over acoustic base-ment is approximately 200 m (see "Underway Geophysics"chapter, this volume).

OPERATIONS

Transit from Sites 782 to 783 (Proposed Site BON-7)After a 26-nmi transit from Site 782, Site 783 was established

at 2030UTC, 23 March 1989, by the deployment of a beacon.

Hole 783A

Because the nature of the material on the surface at Site783 was unclear from the seismic records, a standard rotarycore barrel (RCB) bottom-hole assembly with a hydraulic bitrelease was deployed. The pipe trip began at 2200UTC, 23March, and Hole 783A was spudded at 0500UTC, 24 March.The first RCB retrieval recovered 9.71 m of core, establishingthe mud line at 4648.8 m below sea level (mbsl). Recovery wassporadic in the lithologic units rich in ash, silt, and serpentine.No notable problems occurred until Core 125-783A-18R wasbeing cut, when drill-string torque and circulation pressure

254

SITE 783

31°10'N

31°00'

30°50'

141°45'E 141°55'

Figure 2. Bathymetry of the inner trench wall seamounts at 31°N. Site783 is located on the north flank of the southern seamount. Arrowsmark the tracks of three dredge hauls made on the seamount in 1988(Kobayashi, 1989).

increased considerably. A mud pill, circulation of high-vis-cosity mud, and a wiper trip did not improve hole conditions,so the hole was abandoned.

The RCB was deployed 18 times to a total depth of 168.2 mbsfin Hole 783A, recovering 46.95 m of core for a recovery rate of27.9% (Table 1). Heat-flow measurements were taken at 42.7 and90.0 mbsf. Site 783 officially ended at 0215UTC, 26 March, whenthe drill string was pulled to 4088.8 mbsl and the vessel wasunder way for Site 784 in dynamic positioning mode.

Table 1. Coring summary for Site 783.

Core no.

125-783A-1R2R3R4R5R6R7R8R9R10R11R12R13R14R15R16R17R18R

Coring totals

Date(March

1989)

242424242424242424242425252525252525

Time(UTC)

063008000920105012101500161017201815191522300000023005000800114015001750

Depth(mbsf)

0.0-9.79.7-16.4

16.4-26.026.0-35.735.7-42.742.7-52.452.4-62.162.1-71.871.8-81.381.3-90.990.9-100.6

100.6-110.2110.2-119.9119.9-129.6129.6-139.2139.2-148.9148.9-158.6158.6-168.2

Lengthcored(m)

9.76.79.69.77.09.79.79.79.59.69.79.69.79.79.69.79.79.6

168.2

Lengthrecovered

(m)

9.711.890.026.725.250.004.704.330.021.870.220.400.110.424.493.871.671.26

46.95

Recovery(%)

100.028.20.2

69.375.00.0

48.444.60.2

19.52.34.21.14.3

46.839.917.213.1

27.9

LITHOSTRATIGRAPHY

The stratigraphic section recovered at Site 783 is dividedinto two lithologic units: lithologic Unit I, composed primarilyof olive brown (2.5Y 4/4) to greenish-gray (5G 4/1) vitricclayey silt and glass-rich silty clay or clay stone, and lithologicUnit II, composed of greenish-gray (5GY 6/1 to 5GY 4/1) andbluish-gray (5B 6/1 to 5B 4/1) phacoidal, sheared serpentinewith vertical convolute bedding (Table 2).

Lithologic Unit ISections 125-783A-1R-1, 0 cm, to 125-783A-14R-1, 12 cm; depth,

120.0 mbsf.Age: middle or early Pleistocene to early Pliocene(?).

Lithologic Unit I has varying amounts of silt-sized detri-tus, most of which represents a volcanic influx: glass (7%-58%), feldspar (trace-15%), pyroxene (0%-2%), chlorite(0%-2%), and epidote (0%-7%). A trace to 5% zeolitepresent in a few cores (Cores 125-783A-1R, 125-783A-3Rthrough 125-783A-5R, 125-783A-7R, 125-783A-11R, and125-783A-12R) is interpreted as an alteration product ofvolcanic glass.

The ash layers in Cores 125-783A-1R, 125-783A-4R,125-783A-5R, and 125-783A-10R of Unit I all have sharpbasal contacts, are 1 to 2 cm thick, and are composed ofvolcanic glass (60%-90%) and feldspar (5%-10%), variableamounts of pyroxene (as much as 15%), opaque minerals (asmuch as 5%), chlorite (trace amounts), and clays (as much as15%). In addition to the ash layers are a few layers of vitricsands and silts with large admixtures of clay (30%-60%) thatare interpreted as reworked ash.

Pumice, in millimeter- to centimeter-sized clasts, is scat-tered throughout the unit. The pumice is usually locatedseveral centimeters above an ash layer and may be related tothe same eruptive event as the underlying ash layer, with thepumice requiring a longer time for gravitational settlingthrough the water column than the smaller vitric fragments.

Lithic fragments, such as marlstone and clay stone, wereobserved in Cores 125-783A-1R, 125-783A-4R, and 125-783A-5R. Thulite, which was commonly observed in asso-ciation with serpentine at Site 780 (see "Lithostratigraphy"section, "Site 780" chapter, this volume), occurs in Cores125-783A-4R, 125-783A-5R, and 125-783A-8R. Serpentinewas also observed (in Cores 125-783A-5R, 125-783A-7R,125-783A-8R, 125-783A-10R, 125-783A-12R, and 125-783A-14R) and increases downhole from trace amounts to 10%.

Lithologic Unit I contains a small biogenic contribution:spicules (as much as 10%), radiolarians (as much as 5%),diatoms (as much as 10%), silicoflagellates (rarely present, atrace amount to 1%), nannofossils (rarely present, as much as15%), and foraminifers (rarely present, 3%-4%). Parts of theunit have sufficient biogenic constituents to be nannofossil-bearing or -rich, or biogenic silica-rich. Calcareous biogenicmaterial is lacking in the lower section of this unit, whereassiliceous biogenic material is present throughout the section.Diatoms are present from the top of the unit down to at leastSection 125-783A-10R-CC, and radiolarians occur throughoutthe section, including the base of Unit I in Core 125-783A-14R.

Nannofossils present in Sample 125-783A-1R-1, 1 cm, fromthe top of the first core, provide a stratigraphic age of middle orearly Pleistocene for the uppermost sediment in Unit I. Section125-783A-10R-CC, near the base of the unit, has a diatombiostratigraphic age of early Pliocene; the age of the oldeststratum in Unit I is unknown but is at least early Pliocene.

255

SITE 783

Table 2. Lithologic units recovered at Site 783, Leg 125.

Lithologicunit Cores

Depth(mbsf) Dominant lithology Stratigraphic age

I

II

783A-lR-l,0cm, to783A-14R-1, 12 cm

783A-14R-1, 12 cm, to783A-18R-1, 147 cm

0.0-120.0

120.0-158.6

Vitπc clayey silt, glass-nchsilty clay/clay stone

Phacoidal shearedserpentine with verticalconvolute bedding

middle or early Pleistoceneto early Pliocene(?)

?

The change from clay to claystone is typical of burialdiagenesis. The clay gradually is compacted, becomesfirmer, and eventually is sufficiently lithified in parts of thecore so that it becomes necessary to cut the core with a sawto minimize disruption of the sediments. The transition fromclay to claystone occurs in Section 125-783A-8R-1. Inaddition to the burial diagenesis, there is an apparentoverprint of tectonic diagenesis beginning in Section 125-783A-8R-2, 130 cm, and continuing into Section 125-783 A-8R-3. This is seen as a scaly fabric with slickensidedsurfaces of claystone and the development of a spongytexture with higher apparent permeability than the overlyingclaystone (for further discussion see "Structural Studies"section, this chapter). The sediment in this region may haveprovided a pathway for fluids to escape from the underlyingserpentine in lithologic Unit II.

Lithologic Unit IISections 125-783A-14R-1, 12 cm, to 125-783A-18R-1, 147 cm;

depth, 120.0-158.6 mbsf.Age:?

The contact between lithologic Units I and II was notrecovered. The immediate interval is represented by serpen-tine-bearing, feldspar- and glass-rich silty clay underlain by a1-cm interval of volcanic ash, which is disseminated along thecore liner for the next 4 cm (a 4-cm void in the core), followedby a 2-cm-diameter clast of serpentinized ultramafic rock.Lithologic Unit II is composed of phacoidal sheared serpen-tine that displays convolute bedding and blocks of serpenti-nized harzburgite; in Core 125-783A-18R the composition isprimarily blocks of serpentinized harzburgite with some ma-trix of phacoidal sheared serpentine.

No fossils or obvious detrital components were found inthis unit. The age of these strata is unknown. In the absence ofevidence for faulting, it is likely that this unit is in stratigraphiccontinuity with Unit I, in which case Unit II is probably atleast early Pliocene, the oldest diatom biostratigraphic age inthe overlying unit.

BIOSTRATIGRAPHY

Calcareous nannofossil and diatom evidence from Site 783indicates that the sedimentary interval from Samples 125-783A-1R-1, 28 cm, to 125-783A-10R-CC ranges in age frommiddle to early Pleistocene to the early Pliocene. No agescould be assigned to the remaining sedimentary cores (125-783A-llRto 125-783A-15R).

The sedimentary interval in Hole 783A may have beendeposited near the carbonate compensation depth (CCD), asshown by strong dissolution of the calcareous plankton assem-blages. Nannofossil evidence suggests that Sample 125-783 A-13R-CC was probably reworked.

Calcareous Nannofossils

Samples from Cores 125-783A-1R, 125-783A-3R, 125-783A-4R, and 125-783A-13R contain rare to common, poor tomoderately preserved nannofossil assemblages. The poor

preservation probably resulted from deposition at or near theCCD. Samples 125-783A-1R-1, 28 cm, 125-783A-3R-CC, and125-783A-4R-CC can be given a generic middle/early Pleis-tocene age based upon the presence of rare Pseudoemilianialacunosa and unidentified species of Gephyrocapsa sp. Acommon, moderately preserved assemblage in Sample 125-783A-4R-1, 69 cm, is recognized to have an early Pleistoceneage corresponding to the "Small Gephyrocapsa Zone" (Gart-ner, 1977) within Zone CN14a.

A rare, well-preserved late Eocene nannofossil assem-blage (Zones CP14b/15) containing Dictyococcites bisectus,Sphenolithus predistentus, Discoaster barbadiensis, andCribrocentrum reticulatum, together with dissolved frag-ments of coccoliths that appear to be younger in age, ispresent in Sample 125-783A-13R-CC. The good preservationand rarity of the Eocene nannofossils (usually well-pre-served Eocene assemblages are abundant) suggest rework-ing.

Foraminifers

No samples examined from Site 783 (Samples 125-783A-1R-CC to 125-783A-15R-CC) contained planktonic or benthicforaminifers. This is probably because deposition took placeat or below the CCD.

Diatoms

Rare to common, poorly preserved to well-preserved dia-tom assemblages are present in the sedimentary interval atHole 783A. In the majority of the samples, however, frag-ments (of Thalassionema nitzschioides and some Ethmodiscusrex) dominate. Therefore, it was only possible to date threesamples.

Sample 125-783A-1R-CC is characterized by Pseudoe-unotia doliolus and Nitzschia reinholdii. Sample 125-783A-1R-1, 1 cm, also contains N. reinholdii, assigning the wholeof this core to the lower Pleistocene (N. reinholdii Zone).

Sample 125-783A-5R-CC was dated as late Pliocene by thepresence of Rhizosolenia praebergonii and the absence of P.doliolus (R. praebergonii Zone).

Nitzschia jouseae is present in Sample 125-783A-10R-CC;therefore, this sample is early Pliocene in age.

IGNEOUS AND METAMORPHIC PETROLOGY

The rocks recovered from Site 783 are serpentinizedharzburgite (90%-95%), dunite (5%-10%), and subordinatemetabasalt and meta-volcaniclastic rocks. These form clastsof isolated fragments and occur as inclusions in a matrix ofhighly sheared and pulverized serpentinite. The clasts are 1to 10 cm in diameter and subangular to subrounded in shape.

Ultramafic Rocks

Serpentinized Harzburgite

The clasts of serpentinized harzburgite are massive anddark bluish gray (5B 4/1) to black (5G 3/2). The primarymineralogy of the harzburgite is olivine (85%-90%), orthopy-

256

SITE 783

roxene (10%—15%), clinopyroxene (l%-2%), and chromium-spinel (trace-2%). Olivine is anhedral and 0.5 to 3 mm in size.Orthopyroxene is subhedral to anhedral and 1 to 5 mm indiameter. Clinopyroxene is present as blebs and anhedralcrystals (less than 1 mm), closely related to orthopyroxene,and as exsolution lamellae (on [100)) in orthopyroxene. Chro-mium-spinel is dark red in color, 0.2 to 1 mm in diameter, andexhibits a subhedral to anhedral shape. Wavy extinction andrare kink-banding are recognizable in olivine, orthopyroxene,and clinopyroxene crystals.

One distinctive feature of Sections 125-783A-17R-CC and125-783A-18R-1 is the presence of 0.5- to 1.5-cm-thick pyrox-ene-rich lherzolitic bands within the harzburgite (Fig. 3).These contain olivine (60%-75%), orthopyroxene (15%-20%),clinopyroxene (5%-20%), and spinel (trace-2%). Orthopyrox-ene and olivine are anhedral, 1 to 4 and 1 to 3 mm in diameter,respectively. Clinopyroxene forms subhedral to anhedralcrystals (1-2.5 mm) and exsolution lamellae (on [100)) inorthopyroxene. Spinel is subhedral to anhedral and 0.2 to 1mm in diameter. Spinel is dark brown and probably low inchromium and high in aluminum. The contact between theband and the host rock is obscure.

Olivine and orthopyroxene are variably replaced by ser-pentine minerals, which appear to be mostly chrysotileand/or lizardite. A mesh texture after olivine is predominant,and undeformed bastite pseudomorphs of orthopyroxene arecommon. Clinopyroxene is less serpentinized. Anastomos-ing veins of serpentine can be seen in some clasts. Thedegree of serpentinization is variable, from 60% to 95%.

cmi—

75

80

Serpentinized Dunite

Highly serpentinized (greater than 95% serpentine) dunitesare rare. They are massive and black (5G 3/2) in color. Therocks consist of olivine (greater than 95%), orthopyroxene(less than 5%), and chromium-spinel (less than 1%). Serpen-tinized olivine exhibits a massive mesh-textured fabric, andthere are rare, undeformed bastite pseudomorphs of orthopy-roxene. Dark reddish chromium-spinel is well preserved.

Metavolcanic Rocks

Metabasalt

851— **

Figure 3. A pyroxene-rich (lherzolitic) band in the upper harzburgiteclast in Section 125-783A-18R-1, 72-85 cm.

Several clasts of metabasalt, 1 to 1.5 cm in diameter, wererecovered. Most of the clasts are grayish green (5G 4/2-5G5/2), and one clast containing abundant limonite is darkreddish brown (5YR 2.5/2). The metabasalts are composed ofPlagioclase, clinopyroxene, olivine(?), and glass as primaryphases and chlorite, albite, sphene, dusty brownish clay,zeolite, and limonite as secondary phases. The primary igne-ous textures, such as intersertal and aphyric textures, are wellpreserved, but almost all the primary minerals except clinopy-roxene are fully chloritized. All of the clasts are fractured tovarying degrees and contain 0.2- to 0.8-mm chlorite veinsrunning throughout. Zeolite fills in vesicles greater than 0.2mm.

Meta-Volcaniclastic Rock

Only one example of a clast of meta-volcaniclastic rockwas recovered. The rock is 5 cm in diameter, light greenish-gray (5G 7/1), and medium- to fine-grained and consists ofPlagioclase and clinopyroxene as relic minerals and prehnite,aluminum-rich epidote, actinolite, chlorite, sphene, and prob-ably hydrogrossular as secondary minerals. The clast containsabundant subangular to subrounded fragments of Plagioclaseand clinopyroxene within a fine-grained dusty matrix. The sizeof the fragments is variable, ranging from less than 0.1 to 1mm. Most of the clinopyroxene fragments are altered tocolorless actinolite, and those of Plagioclase to aluminum-richepidote and prehnite. The textural relationships suggest thatprehnite, epidote, and actinolite are an equilibrium assem-blage. Later-stage Cataclastic deformation has developedalong the boundaries of fragments.

IGNEOUS AND METAMORPHIC GEOCHEMISTRYFive of the ultramafic clasts (harzburgites) from Site 783

were selected for shipboard geochemical analysis. Preparationand analytical techniques are given in the "ExplanatoryNotes" chapter of this volume. Rock and thin-section descrip-tions are given in the preceding "Igneous and MetamorphicPetrology" section. The geochemical data are listed in Tables3 and 4.

Ultramafic SamplesModal estimates of the original (pre-serpentinization) com-

position of ultramafic rocks range from 74% to 86% olivine,12% to 20% orthopyroxene, 1% to 5% clinopyroxene, and lessthan 2% spinel. The degree of serpentinization of the samplesvaries from 55% to 92%.

Major-Element Chemistry

Loss on ignition (LOI) of these samples ranges from 12.62to 15.75 wt% and like that of the samples from Site 779, showsa positive correlation with the degree of serpentinization (Fig.4). (See "Igneous and Metamorphic Geochemistry" sectionsof "Site 778" and "Site 779" chapters, this volume, and"Explanatory Notes" for an explanation of LOI.) Concentra-

257

SITE 783

Table 3. Major-element data for serpentinized harzburgites from Hole783A.

Core, section:Interval (cm):

SiO2

TiO2A12O3

Fe2O3

MnOMgOCaONa2OK2OP2O5

NiOCr2O3aLOITotalMg#%Serpentinization

16R-CC,14-17

36.410.000.417.280.09

40.180.430.000.000.000.280.16

14.83100.0791.6270.00

16R-CC,19-22

39.210.000.597.370.11

38.191.660.000.000.000.250.21

12.62100.2191.1255.00

17R-1,9-12

35.900.000.357.420.11

39.390.310.000.000.000.290.18

15.7599.7091.3292.00

18R-1,49-51

36.010.000.477.640.12

39.350.660.000.000.000.300.21

14.9999.7591.0760.00

18R-1,96-97

36.840.000.457.870.11

39.910.630.000.000.000.290.25

13.5599.9090.9460.00

Note: Data (in wt% oxides) from whole-rock analyses by XRF.a LOI = loss on ignition between 105° and 1030°C.b Estimated from thin sections.

tions of major-element oxides in these rocks are similar toconcentrations in the harzburgites from Sites 778, 779, and780. SiO2 ranges from 35.90 to 39.21 wt%, MgO from 38.19 to40.18 wt%, A12O3 from 0.35 to 0.59 wt%, and Fe2O3 from 7.28to 7.87 wt%. Magnesium number is also similar to that in theharzburgites from Sites 778, 779, and 780 and varies from90.94 to 91.62. The concentrations of Na2O, K2O, and P2O5are all below detection limits. The general absence of clinopy-roxene crystals and very low concentrations of A12O3 and CaOindicate that the rocks are residues from high degrees ofpartial melting.

Trace-Element Chemistry

In the trace-element data presented in Table 4, compatibleelements are high in all samples, as would be expected. Valuesof chromium range from 1074 to 1716 ppm, and those ofnickel, from 2210 to 2357 ppm, with the exception of low-nickel Sample 125-783A-16-CC (Piece 2, 19-22 cm), whichcontains 1980 ppm. Average concentrations of nickel inharzburgites from Sites 783 (2215 ppm) and 780 (2261 ppm) areslightly lower than those of the harzburgites from Sites 778(2644 ppm) and 779 (2320 ppm). Chromium and nickel do notcorrelate with one another or with the degree of serpentiniza-tion.

Table 4. Trace-element data for serpentinized harzburgitesfrom Hole 783A.

Core, section:Interval (cm):

NbZrYSrRbZnCuNiCrVCeBa

16R-CC,14-17

tr1

nd10

nd390

22101074

9trtr

16R-CC,19-22

tr01

12nd

3129

19801469

23trtr

17R-1,9-12

tr109

nd342

22671255

21tr13

18R-1,49-51

tr11

130

412

23571451

3912

tr

18R-1,96-97

tr10

13nd

381

22601716

36tr15

16

15 —

ó 1 4

13 —

12

1

—

-

o

1

1 '

o

o

1 1

1 '

o

1 .

1 ' 1

1 1 1

o

—

-

1

50 60 70 80Serpentinization (%)

90 100

Note: Data (in ppm) from whole-rock analyses by XRF; nd = nonedetected; tr = below detection limit (<5 ppm for Nb, <IO ppm forBa and Ce).

Figure 4. Modal percent of serpentinization vs. weight percent loss onignition (LOI) of between 105° and 1030°C.

Niobium, zircconium, yttrium, rubidium, and titanium areall present in trace amounts or are below detection limits.Cerium and barium values range from below their detectionlimits (less than 10 ppm) to 12 and 15 ppm, respectively,although the cerium values at least may result from analyticalerror. Strontium varies from 9 to 13 ppm, similar to itsconcentrations in harzburgites from Sites 778, 779, and 780.Zinc ranges from 31 to 41 ppm, with an average concentrationof 37 ppm, which is similar to that of the harzburgites fromSite 780 (36 ppm) and slightly lower than those of harzburgitesfrom Sites 778 (42 ppm) and 779 (39 ppm). There is nosignificant correlation between the geochemical parametersand depth.

Sample 125-783A-16R-CC (Piece 2, 19-22 cm) is anoma-lously rich in copper (29 ppm). It shows the highest concen-trations of SiO2 (39.21 wt%) and A12O3 (0.59 wt%) of all thesamples analyzed from this site. It also shows the highestconcentration of CaO (1.66 wt%) and lowest nickel (1980 ppm)of all harzburgite samples analyzed from Sites 778 through 780and 783. Note that this sample contains 5 modal% clinopyrox-ene (see "Igneous and Metamorphic Petrology" section) andrepresents the highest concentration of clinopyroxene in anyanalyzed sample. The petrogenetic implications are not clear,however, as the distribution of clinopyroxene indicates thatthis mineral was introduced by melt impregnation and was nota genuine residual phase. The hand sample appears to containa 10-mm anhedral patch of a bronze-colored mineral that maybe native copper or a copper sulfide.

In general, the samples analyzed from Site 783 are similarto those from Sites 778, 779, and 780 ("Igneous and Metamor-phic Geochemistry" sections of the respective site chapters).

SEDIMENT/FLUID GEOCHEMISTRY

Sediment GeochemistryCores from Site 783 were analyzed aboard ship for inor-

ganic carbon and for total carbon, nitrogen, and sulfur usingthe techniques described in the "Explanatory Notes" (thisvolume). The organic carbon content was then calculated bydifference. These results are presented in Table 5 and Figure5. The CaCO3 content of the sediments and the phacoidal,sheared serpentine is extremely low, generally less than 1%,except for two intervals of sediment at 0 to 6 and 26 to 30

258

SITE 783

Table 5. Total nitrogen, carbon, inorganic carbon, organic carbon,and carbonate carbon in sediments and serpentine at Site 783.

Core, section,interval (cm)

119-783 A-1R-1,77-791R-2, 23-251R-3, 76-781R-4, 97-99IR-5, 76-771R-6, 74-761R-7, 7-92R-1,34-362R-2, 15-174R-1,74-764R-2, 84-864R-3, 67-694R-4, 76-784R-5, 24-264R-CC, 8-105R-1,56-585R-2, 62-645R-3, 40-425R-4, 25-275R-CC, 11-137R-1, 30-327R-2, 50-527R-3, 139-1417R-4, 6-88R-1, 37-398R-2, 71-738R-3, 76-7810R-1, 33-3510R-2, 5-711R-1, 16-1812R-1, 7-915R-1, 125-12715R-2, 76-7815R-3, 76-7816R-1, 83-8516R-2, 82-8416R-CC, 5-7I7R-1, 127-12917R-CC, 10-1218R-1,82-84

Depth(mbsf)

0.771.733.765.476.768.249.07

10.0411.3526.7428.3429.6731.2632.2432.6136.2637.8239.1040.4540.8352.7054.4056.7956.9662.4764.3165.8681.6382.8591.06

100.67130.85131.75133.25140.03141.52142.89150.17150.50159.42

Totalnitrogen(wt%)

0.19

0.15

0.13

0.18

0.20

0.14

0.190.070.11

0.06

0.09

nd

Totalcarbon(wt%)

2.88

0.41

2.93

0.54

0.61

0.35

0.480.250.28

0.57

0.22

0.31

0.18

Inorganiccarbon(wt%)

7.250.152.470.490.030.110.140.020.040.430.802.590.090.470.060.090.040.020.030.120.040.080.070.030.110.030.050.010.040.030.020.020.440.050.100.080.070.090.100.05

Organiccarbon(wt%)

0.41

0,37

0.34

0.50

0.54

0.32

0.440.220.26

0.13

0.14

0.22

0.13

CaCO3

(wt%)

60.41.2

20.64.10.20.91.20.20.33.66.7

21.60.73.90.50.70.30.20.21.00.30.70.60.20.90.20.40.10.30.20.20.23.70.40.80.70.60.70.80.4

Note: nd = not detected. Total sulfur was below the detection limit of 0.06 wt% in allsamples analyzed for total carbon, nitrogen, and sulfur.

mbsf. The near-surface sediment interval contains up to 60wt% CaCO3, whereas the deeper interval contains up to 22wt%. The organic carbon content decreases with depth fromabout 0.5 to 0.1 wt%. The nitrogen content varies with that oforganic carbon, decreasing from 0.2 wt% to less than 0.01wt% with depth. Sulfur is below the detection limit of 0.06wt% in all samples.

Fluid Geochemistry

The concentration of methane measured for 5-cm3 head-space samples is uniformly low, less than 20 mL/L (Table 6).A single sample from 31 mbsf may have contained a trace ofethane. Other than this, methane was the only hydrocarbondetected.

The interstitial waters at Site 783 (Table 7 and Fig. 6)contrast markedly with those at Site 782 in response to thepresence of serpentine at Site 783 at 120 to 158.6 mbsf. Theserpentine consists of a matrix of phacoidal, sheared serpen-tine containing clasts of serpentinized harzburgite and meta-basalt. It underlies 120 m of glass-rich silty clay to claystone.Between the lowest interstitial-water sample from the clay-stone at 83 mbsf and the highest from the serpentine at 131mbsf, there are abrupt changes in pH, alkalinity, silica,sulfate, magnesium, calcium, sodium, potassium, and salinity.Only ammonia, chlorinity, and bromide are unaffected by thepresence of the serpentine. Between the claystone and theserpentine, pH rises from 8 to almost 10, alkalinity decreasesfrom 2.9 to 1.3 meq/kg, silica decreases from 643 to 2

16020 40 60

CaCO3 (%) ^organic (%) Total nitrogen (wt %)

Figure 5. Weight percent of calcium carbonate, total organic carbon,and total nitrogen in sediments and serpentine at Site 783.

mmol/kg, and sulfate decreases from 28 to 6 mmol/kg. Mag-nesium decreases sharply from 50 to less than 1 mmol/kg,potassium decreases from 11 to 9 mmol/kg, calcium increasesfrom 11 to 39 mmol/kg, and sodium increases from 480 to 499mmol/kg, before decreasing again to 482 mmol/kg. Salinitydecreases from 3.58 to 3.32 wt%, and chlorinity increases withdepth within the claystone, probably as a result of the in-creased chlorinity of ocean bottom water during the Pleis-tocene glaciations (McDuff, 1985). It shows no further changewithin the serpentine. The decrease in salinity therein resultsfrom the decrease in sulfate.

The changes in pore-water composition between the clay-stone and serpentine almost certainly result from ongoingreactions between seawater-derived solutions and the par-tially serpentinized peridotite. Some of the changes are similar

Table 6. Results of headspace-gas analysesof cores at Site 783.

Core, section,interval (cm)

125-783A-1R-6, 0-32R-2, 0-34R-4, 0-3c7R-2, 0-38R-2, 0-310R-2, 137-14016R-3, 0-317R-1, 149-150

Depth(mbsf)

7.5211.2230.5253.9263.6282.69

142.21150.40

Methane

(µL/L)a

109

111315131315

(µM)b

0.90.81.01.21.31.21.21.3

a Microliters of methane per liter of wet sedi-ment or wet unconsolidated serpentine, assum-ing a sample volume of 4.2 cm of sediment orserpentine.b Micromoles of methane per liter of interstitialwater, assuming a porosity of 50%.c Trace of ethane present.

259

SITE 783

Table 7. Composition of interstitial waters from cores at Site 783.

Samplenumber

Core, section,interval (cm)

Surface seawater (17 MarchIW-1IW-2IW-3IW-4IW-5IW-6IW-7IW-8IW-9

1R-1, 145-1501R-2, 145-1501R-5, 145-1504R-3, 140-1507R-1, 140-15010R-1, 140-15015R-2, 140-15016R-1, 135-15017R-1, 140-150

Depth(mbsf)

1989)1.482.987.48

30.4553.8582.75

131.05140.63150.35

Vol-ume(mL)

35413673583510109

Squeezetemperature

(°C)

212222677

pH

8.257.727.857.957.958.188.018.539.969.49

(R.I., %)

35.635.435.435.435.635.936.133.833.033.9

Salinity

(calcium, %)

34.6535.0035.0335.3535.6135.5135.7933.9933.2733.35

Chlorinity(mmol/kg)

539.2543.6544.5547.5553.3552.3557.2554.3549.4555.2

Alkalinity(meq/kg)

2.6893.7813.2123.3343.2943.7682.8901.2741.3771.449

Sulfate(mmol/

kg)

27.7827.3127.4128.2027.7727.1127.6111.547.856.06

Sodium(mmol/

kg)

462.0468.2468.3473.1476.1474.4480.5498.1489.1481.8

Potassium(mmol/

kg)

10.1110.6710.8711.6111.4911.5611.119.128.468.63

to those found in the serpentine sediments and flows at Sites778,779, and 780 on Conical Seamount in the Mariana forearc.Interstitial waters from the Conical Seamount sites wereinterpreted as mixtures of solutions from three sources: (1)seawater that had reacted with ultramafic rocks, (2) seawaterthat had reacted with mafic rocks and sediments, and (3)low-chlorinity water from a deep source probably associatedwith de watering of the subducted lithospheric slab (see "Sed-iment/Fluid Geochemistry" section, "Site 780" chapter). Thedeep source of low-chlorinity water predominated at Site 780on the summit, whereas reacted seawater predominated onthe seamount flank at Site 779 and especially Site 778.

Pore waters at Site 783 on the Izu-Bonin seamount resem-ble the pore waters at the Mariana seamount (Sites 778-780) intheir downhole increase in pH and ammonia and their strongdownhole depletions in magnesium and silica. Although theconcentrations of ammonia, magnesium, and silica are com-parable between the two seamounts, the pH is not as high atSite 783 as it is at the Conical Seamount sites, with theexception of Site 778. For other dissolved species, Site 783resembles Site 779 and especially Site 778 on the flank muchmore than Site 780 on the summit. Whereas alkalinity in-creases from the seawater value at Sites 779 and 780, itdecreases at Sites 778 and 783. Sulfate likewise increases atSite 780 and decreases slightly and then increases at Site 779,but decreases greatly at Sites 778 and 783. Calcium decreasesat Sites 779 and 780, but increases at Sites 778 and 783.Sodium increases at Site 780, but increases at Sites 778, 779,and 783. Potassium increases at Site 780, but decreases atSites 778, 779, and 783. Chlorinity falls at all three ConicalSeamount sites, especially at the summit (Site 780), whereas itincreases within the claystone at Site 783 and shows no furtherchange within the serpentine. Methane is abundant at Sites779 and 780, moderately abundant at Site 778, and nearlyabsent at Site 783. Ethane and propane are present at Sites 779and 780, but are absent at Sites 778 and 783.

Comparison of the interstitial waters from serpentinite atSites 778, 779, 780, and 783 supports interpretation of theConical Seamount waters as mixtures from three sources, aspreviously summarized. Because the waters at Site 783 mostclosely resemble those at Site 778, it is likely that these twosites represent the clearest examples of seawater-harzburgiteinteraction at low temperature. The associated reactions canbe inferred to consume H+, alkalinity, silica, sulfate, Mg2+,and K+ and to produce ammonia, Ca2+, and Na+ whileconserving chlorinity. Although a small fraction of low-chlo-rinity, hydrocarbon-rich water from a deep source is presentat Site 778 on Conical Seamount, there is no evidence for suchwater at Site 783 on the Izu-Bonin seamount.

STRUCTURAL STUDIES

Deformation observed at Site 783 includes (1) shear fabricsdeveloped in poorly lithified muds and in sheared phacoidalserpentine; (2) a possible small fault in poorly lithified mate-rial; (3) high-temperature, pre-serpentinization fabrics in peri-dotites; and (4) lower temperature, post-serpentinization fab-rics in serpentinite cobbles. We will discuss these in turn, andthen briefly discuss the structural and tectonic significance ofthe rheological measurements made at this site.

Structures in Pelagic/Volcaniclastic MudsWe observed no tectonic structures within the unlithified

sediment that makes up the upper 62 m of material recoveredfrom Hole 783A. The first partially lithified sediment occurs inSection 125-783A-8R-1 (62.1 mbsf). Section 125-783A-8R-3consists of claystone with several intervals containing weaklydeveloped shear and fracture foliations. From 95 to 107 cm inthis section, the foliation consists of at least three sets of shearplanes spaced from less than 1 mm to about 3 mm apart. It waspossible to measure the orientations of two of these sets; oneset is approximately vertical, whereas the other dips approx-imately 60°. The three sets of shear planes divide the siltstoneinto angular phacoids, or lozenges, the long axes of whichdefine the macroscopically obvious foliation (Fig. 7). Between113 and 120 cm is a "columnar" cleavage that consists ofvertical fracture planes spaced from less than 1 to perhaps 3mm apart. This foliation may be related to the presence of thesmall normal(?) fault described in the following text. Between120 and 129 cm, the bottom of the section, the siltstonecontains a lozenge foliation similar to that described for theinterval between 95 and 107 cm, but this lower interval isdifficult to study because it is shattered by drilling.

Two small faults cut the partially lithified claystone in thelower part of Section 125-783A-8R-3, one in the upper part ofthe interval containing the vertical ("columnar") fracturecleavage and one below it (Fig. 7). The upper fault sharplytruncates a silty clay or claystone clast, dips about 40°, andflattens downward. Although the nature of the fault is unclear,its sharpness and the lack of anastomosing fault strands,associated gouge or breccia, or drag features suggest that itsdisplacement is quite small, perhaps only a few centimeters.

The abrupt appearance of these brittle strain features(fracture and microfault foliations and small faults) shortlybelow the first visible lithification suggests that the greatercohesion of the lithified sediment facilitates brittle failure.Thus, diagenetic effects, including overburden pressure, prob-ably determine the interval in which the structures are formed.However, overburden stress is probably not responsible for

260

SITE 783

Table 7

Calcium(mmol/

kg)

10.1810.5110.5310.6910.8011.1311.3929.4734.0039.21

(continued).

Magnesium(mmol/kg)

52.4851.0451.1850.5451.4651.0450.476.240.450.00

Bromide(mmol/

kg)

0.810.890.750.870.850.790.770.840.840.89

Silica(µ.mol/

kg)

0471503526643654643

1724

Ammonia(µmol/kg)

107

101037094

120188151125

the foliations or the small faults. The former indicate a greatestprincipal stress oriented at 15° or more from vertical; the latterindicate horizontal extension that might accompany larger-scalefaulting or slumping of adjoining sediment, but would probablynot take place at such a small scale during normal compaction inthe absence of tectonism. We do not have sufficient data todemonstrate the origin of these structures, but suggest thefollowing: (1) all the structures may be related to large-scalebasement faulting (compressional or extensional) in the forearc;(2) the normal(?) faults may have formed during lateral extensionallowed by the slumping of material downslope from the hole;and (3) all the structures may have formed in response to stresses

exerted by the continuing downhill creep of the underlyingserpentine sediments and flows.

Structures in Serpentine Sediments and Flows(Flow Matrix)

The material recovered in Cores 125-783A-15R through125-783A-18R consists of a matrix of sheared phacoidal ser-pentine that bears fine-sand- to cobble-sized clasts, largely ofserpentinized harzburgite tectonite (Fig. 8). The matrix exhib-its a spectacular anastomosing shear foliation defined bydeformed color banding (light blue to light blue-green to lightgreen to pale yellow-green) and by the long axes of phacoidal(lens- or pie-shaped) serpentinite fragments. The shear folia-tion is always parallel to the layering defined by color bandingand variations in clast size and concentration. Axial-planefoliations are not observed.

The phacoids are from 1 to 12 cm long and from a fewmillimeters to a few centimeters across (Fig. 8, 124-127 cm,and Fig. 9). Close inspection shows that much of the materialbetween large phacoids consists of smaller phacoids. Thefoliation is whorled, contorted, and convolute with foliationstreams swirling, branching, and rejoining around the largerclasts. Scattered through the material are smaller, angular tosubangular and, less commonly, subrounded, sand- to small-pebble-sized clasts of serpentinite. Although some of these arelight green, many are dark green to dark blue-gray; because

100 —

200 —

400

300 —

1.2 2.0 2.8 3.6Alkalinity (meq/kg) pH

10 0 0.5 1.0 0 100 200 6 18 30 10 20 30 40 0 30 60Silica (µmol/kg) Ammonia (µmol/kg) Sulfate (mmol/kg) Ca2+ (mmol/kg) Mg2+ (mmol/kg)

100

200

300

400 I I I7 9 11 13 460 480 500 33 34 35 36 33.2 34.0 34.8 35.6 0.8 0.9 1.0 539 543 547 551555

K+ (mmol/kg) Na+ (mmol/kg) Salinity by R.I. (‰) Salinity as sum of salts (‰) Bromide (mmol/kg) Chlorinity (mmol/kg)

Figure 6. Composition of interstitial waters from sediments at Site 783 (circles) compared with that from Site 782 (squares) and surface seawatercollected 22 February and 4 and 17 March 1989 (crosses).

261

SITE 783

W-

cm115

<r E

Figure 7. Stereonet showing the orientations of fractures that defineshear foliation in Section 125-783A-8R-3, 95-107 cm. 1, 2, and 3 =faults with displacement; 3 is a normal fault. 4, 5, and 6 = cleavageplanes. P = pole of plane.

their concentration varies greatly from one layer to the next,they help to define the foliation (Fig. 9).

Phacoids within the matrix typically exhibit pinch-and-swell texture (boudinage) along their long axes and commonlyare cut by serpentine veins and/or microfaults at high tomoderate angles with respect to their long axes. These micro-faults almost universally show a normal sense of movementwith respect to the long axes of the phacoids (Fig. 8, 124-127cm, and Fig. 10, especially between 75 and 77 cm and between83 and 85 cm). This texture, together with the boudinage andveining, clearly indicates that the phacoidal matrix has under-gone major extension (pure shear) parallel to the layering,foliation, and the direction of alignment of the phacoids. Thisextension was both brittle and ductile, the difference presum-ably depending on confining stress, strain rate, and the degreeof lithification and water content of phacoids and interphacoidmatrix.

The phacoids are commonly bounded, and their taperingends terminated, by shear bands (zones in which the bulkshear is concentrated) within the surrounding finer-grainedmatrix. Phacoids that lie parallel to the local direction of thematrix foliation are typically lensoid (i.e., their long axes arestraight). Phacoids that are oblique to the surrounding folia-tion are typically sigmoid, with their long axes bent and theirends sheared out parallel to the foliation. Pure and simpleshear have thus both played an important part in forming thefabric. Shear bands may extend across the entire core or dieout within the matrix. Therefore, the overall geometry ofmatrix foliation and phacoid alignment outlines the pattern ofmatrix streamlines, along which extensional flow has oc-curred.

Although we have not yet measured the orientation of theshear foliation in detail, we observe no strong preferredorientation. Whorled foliation, in which the whorls appear tohave long vertical limbs, appears to dominate some parts of

120

125

Figure 8. Sheared, phacoidal serpentine matrix enclosing serpenti•nized harzburgite tectonite clast (Section 125-783A-18R-1, 115-126cm).

the cores. This texture, described elsewhere in this chapter as"vertical convolute lamination," is probably an artifact of theaspect ratios of our core sections, which are 150 cm long and6.5 cm wide; fabrics parallel to the core will thus appear todominate the geometry.

We interpret these textures as flow fabrics developed insheared serpentinite as it continued to hydrate, soften, andflow downhill under the influence of gravity. Although webelieve that the original shearing of the serpentinite wastectonic in the sense that it was caused by deep-earth pro-cesses, we suggest that these near-surface processes formedmuch of the present fabric of the rock.

High-Temperature Peridotite Deformation TexturesClasts larger than 1 cm, mostly of serpentinized harzburg-

ite tectonite, were encountered from Section 125-783A-15R-1,15 cm, through the bottom of Core 125-783A-18R, but aredominant only in Core 125-783-18R, where they make upabout 80% of the material recovered. These clasts preservesome relict mineralogy, but the specimens from which our thinsections were made have been so highly serpentinized thatrelict textures developed at depth under elevated pressure andtemperature could not be observed. Deformation texturesobserved in similar clasts from Sites 778, 779, and 780 includesubgrain bands and deformation lamellae in olivine, kink-bands in orthopyroxene, and undulatory extinction in both.

Serpentinization TexturesThe serpentinization textures in clasts are similar to those

observed in clasts from Sites 778, 779, and 780 and includeserpentinization veins, cross-fiber serpentine veins, and"Frankenstein" veins (see "Structural Studies" sections in"Site 779" and "Site 780" chapters, this volume). A spectac-

262

SITE 783

cm

40

45

50

cm

55

Figure 9. Layering, shear foliation, and phacoids in serpentine sedi-ments and flows (Section 125-783A-15R-2, 36-56 cm).

ular dark serpentinization vein with an unusual light center isshown in the large clast in Figure 8. The upper edge of thelarger clast from 116 to 124 cm in Section 125-783A-18R-1 isbounded by a small, poorly developed "Frankenstein" vein.Textures seen in thin section include cross-fiber veins andfiber haloes surrounding relict and pseudomorphed olivinesubgrains.

Many fiber haloes in serpentinized harzburgites from Site783 are asymmetric, in striking contrast to those from Site 780,which are almost all highly symmetric (see Fig. 7). Whiledetailed study will be necessary to confirm and quantify thisobservation, it probably implies that many of the Site 783harzburgites were serpentinized in anisotropic stress fields(i.e., under deviatoric stresses).

75

80

85 >—

Figure 10. Normal microfaults, boudinage, and veining (evidence oflayer-parallel extension) in phacoidal serpentine. The interval shown(Section 125-783A-16R-1, 72-85 cm) probably represents a singleserpentinite clast that is being softened by continuing hydration,disaggregated, and sheared out into the matrix.

Important vein textures are developed in the shearedserpentine. Section 125-783A-14R-1, 10-20 cm, preservesmuch of the contact between more "normal" pelagic/volcani-clastic sediment, containing about 4 modal% serpentine, andthe underlying serpentinite. Section 125-783A-14R-1, 13-15cm, is a serpentinized harzburgite tectonite clast; the intervalfrom 15 to 20 cm is unfoliated serpentine sediment thatcontains a well-developed, branching network of cross-fiberserpentine veins up to 3 mm across that are clearly growing inthe material. This is important because it provides unambig-uous evidence that serpentinization is continuing in the disag-gregated, unlithified sediment.

Within the sheared phacoidal serpentines below thislevel, both phacoid and interphacoid material are highlyveined with serpentine. Although some veins in phacoidsend at the phacoid margin, others pass outward into thematrix. Many of these veins have become deformed by,drawn out into, and incorporated within the matrix. More-over, in at least two intervals (76-85 and 100-125 cm inSection 125-783A-16R-1) large serpentinite clasts are cut byintricate, anastomosing networks of veins, which pass outinto narrow channels of finely phacoidal, serpentine matrixthat also cut the clasts (Fig. 10). The clasts were quite softand water saturated when recovered. We postulate that

263

SITE 783

these intervals preserve in place a late stage of the fragmen-tation of large clasts by serpentine veins, their continuinghydration and softening, and their shearing and incorpo-ration into the phacoidal matrix.

Inspection under the binocular microscope shows that thematrix consists of slickensided lenses of serpentine that, inturn, consist of smaller scales and lenses; adding water to thismaterial and agitating it gently with a probe causes it todisaggregate completely into submillimeter-sized serpentinefragments, interspersed with which are subangular to sub-rounded sand-sized grains of serpentine. We suggest that thefragility of the matrix is a consequence of its hydration.

Rheologic Properties of the Materials and Structuraland Tectonic Implications

We used a Wykeham-Farrance torsion-vane apparatus toperform stress-strain tests on the unlithified materials recov-ered from Site 783. The sheared phacoidal, serpentine matrixis weak, with failure strengths that range between 7.3 and 51kPa and average 23 kPa. These materials are comparable instrength to similar materials recovered from other sites,although they are significantly weaker than the overlyingclaystones and significantly stronger than the average unfoli-ated serpentine at Site 780 (Table 8). Moreover, the mode offailure was quite different: at low strains, the stress-straincurves were much more linear, and the strain appeared to bemore elastic. Moreover, the material failed in a brittle manner,by cracking, rather than yielding plastically at an ultimatestrength (Fig. 11).

We interpret these results as the consequence of thegreater dewatering and compaction of these materials thanthose studied previously on Conical Seamount. This conclu-sion is compatible with the nature of the material, which isquite different from the pasty, weak, plastic, little-foliated

Table 8. Ultimate strength ofserpentine sediments and flowsrecovered at Site 783.

Core, section

119-783A-1R-11R-31R-52R-12R-24R-24R-45R-15R-37R-17R-38R-115R-115R-115R-215R-316R-116R-216R-317R-1Sediment

Average

Depth(mbsf)

0.733.706.68

10.9311.3228.1631.2236.5639.5653.4456.2862.97

130.41130.83131.87133.33139.92141.92142.39149.97

Standard deviationMaximumMinimum

SerpentineAverageStandard deviationMaximumMinimum

Stress(kPa)

13.137.129.117.544.434.941.532.852.429.869.234.916.051.016.735.719.725.526.941.5

36.3914.8869.2013.10

29.1312.5851.0016.00

serpentine sediment and flows recovered from the shallowlevels of Sites 778 and 779 on the flanks of Conical Seamount.We have interpreted this pasty material as representing youngserpentine debris flows that have only begun to dewater. AtSite 783, pasty, unfoliated serpentine is restricted to Core125-783A-14R, at the volcanogenic sediment/serpentine inter-face, which is likely to be a zone of extensive fluid transfer andis clearly a zone of active serpentinization. The sheared,phacoidal serpentine from Site 783 is also different from theunfoliated serpentine material from Site 780, which we postu-late to be a site of active upwelling of water-charged mud. Ourinterpretation is also compatible with the similarity betweenthe material from Site 783 and that from deep levels at Sites778 and 779, which we think represent older debris flows thathave undergone extensive dewatering. Finally, our interpre-tation is compatible with the location of the serpentine 60 mbelow the level at which the volcanogenic sediments may havefirst lithified.

The serpentine at Site 783 is blanketed by virtually serpen-tine-free sediment that is at least as old as early Pliocene.Thus, the stratigraphy, sedimentology, paleontology, andrheology of the materials recovered from Site 783 are allconsistent with a scenario in which serpentinite debris-flowprocesses were once active on Torishima Forearc Seamountbut have since ceased, allowing the serpentine and serpen-tinite to be blanketed with pelagic and volcanogenic sediment.Serpentinization and dewatering have, however, continueduntil the present.

PALEOMAGNETISM

Magnetic RemanenceThe natural remanent magnetization (NRM) of the sedi-

ments contained within the archive halves of Cores 125-783A-1R, 125-783A-2R, 125-783A-4R, 125-783A-5R, 125-783A-7R, 125-783A-8R, and 125-783A-10R was measuredusing the cryogenic magnetometer. Intensities vary typicallybetween 10 and 100 mA/m, but there is no trend in thesevalues downhole. Cores were then demagnetized at 10 mT,which reduced the intensity typically by 10% to 20%. Addi-

Sample 125-783A-15R-3, 84 cm -

COSample 125-780D-7X-1, 40 cm

43 D D-

10 20 30Strain angle (degrees)

40 50

Figure 11. A representative stress-strain test of serpentine sedimentsand flows from Site 783 (failure strength = 35.7 kPa), compared withmaterial from Site 780 on the summit of Conical Seamount (ultimatestrength =11.7 kPa).

264

SITE 783

Table 9. Remanence data for the discrete samples taken from Hole 783A.

Core, section,top of

interval (cm)

119-783 A-1R-1, 1031R-2, 331R-3, 61R-4, 11IR-5, 861R-6, 891R-7, 452R-1, 522R-2, 34R-1, 1144R-2, 1134R-3, 1134R-4, 1134R-5, 405R-1, 265R-2, 265R-3, 265R-4, 137R-1, 907R-2, 907R-3, 907R-4, 38R-1, 1208R-2, 1208R-3, 91

Natural remanent magnetization

Declination(degrees)

325.40.5

343.063.0

302.4344.5333.024.7

252.133.0

227.7220.2238.692.0

171.4190.987.1

262.2122.7110.5182.1339.0262.8

86.5278.8

Inclination(degrees)

30.015.560.274.762.347.874.0

-54.2-55.0-48.0

28.5-32.5-79.1-19.0-82.9-75.8-51.7-50.7-67.7-53.6-64.3-46.2

53.755.344.5

Intensity(mA/m)

77.371.463.6

101.090.773.37.6

32.744.225.714.623.134.562.247.360.599.267.168.982.584.6

118.0123.048.741.6

Declination(degrees)

348.012.3

351.084.4

301.2344.3270.3

5.5250.229.434.1

210.9236.793.987.9

190.689.7

261.3120.3108.8179.5338.2262.092.3

279.6

(10 mT)

Inclination(degrees)

52.023.062.169.663.348.053.9

-55.8-51.9-58.3

19.0-33.0-76.1-23.8-51.9-63.8-54.6-53.0-67.5-53.6-63.8-46.6

53.056.749.4

Remanent magnetization

Intensity(mA/m)

47.362.551.769.282.461.7

7.937.145.822.833.420.337.657.036.670.9

110.069.567.277.279.1

112.0111.041.537.1

Declination(degrees)

349.514.9

351.989.0

302.4345.7268.1

0.7251.036.841.9

210.2238.695.788.1

190.789.6

254.4119.9107.9179.2338.7261.992.3

274.1

(20 mT)

Inclination(degrees)

51.828.963.169.665.448.862.3

-56.6-51.4-52.4

6.8-31.2-74.2-25.3-53.1-65.6-51.2-49.9-67.0-52.9-64.4-46.7

54.456.450.0

Intensity(mA/m)

36.852.137.949.862.148.16.3

33.136.717.52.6

16.828.944.230.460.580.748.453.661.562.987.498.338.431.0

Polarity

NNNNNNNRRRRRRRRRRRRRRRNNN

Table 10. The polarity of each sediment core from Hole 783A andthe corresponding diatom and nannofossil zones.

Core

119-783 A-1R2R4R5R7R8R10R

Polarity

NRRRRN7

Diatom zone

N. reinholdii

R. praebergonii

N. jouseae

Nannofossil zone

CN14a

CN14a (lower/middle part)

tionally, one specimen was removed from the working half ofeach processed section for a more detailed analysis. The NRMof these specimens was also measured using the cryogenicmagnetometer. Intensities vary typically between 50 and 80mT. Each specimen was then demagnetized at 5, 10, 15, and20 mT and remeasured between each step. Demagnetizationreduced the intensity typically by 25% to 35% at the 20 mTstep. The demagnetization data for the discrete specimens aresummarized in Table 9. Only small changes in the remanencedirection are observed between the 10 and 20 mT steps. Thissuggests that the 10-mT whole-core data can be used to makereliable polarity evaluations. The polarity of each core issummarized in Table 10. (No polarity transitions were re-corded within any of the cores.)

Unfortunately, the biostratigraphic data, obtained almostentirely from core-catcher material (see "Biostratigraphy"section, this chapter), are poor. However, a biostratigraphiczonation, based primarily on diatoms, but also including twohorizons dated using nannoplankton bio markers, allows us tocorrelate tentatively the polarity sequence with the time scaleof Berggren et al. (1985). Barron (1985) correlated the inter-nationally recognized low-latitude diatom zones with thepolarity time scale of Ness et al. (1980). At least for the

interval from 0 to 4 Ma, the age of each magnetochronboundary in that time scale is not much different from thatpresented in the time scale of Berggren et al. (1985). Forpractical purposes, the relative positions of the diatom zonesof Barron (1985) can be transferred directly onto the time scaleof Berggren et al. (1985). This modified scheme was adoptedhere.

The magnetostratigraphy of Hole 783A is summarized inFigure 12. We have been able to identify partial records of theBrunhes Normal Polarity Chron (Core 125-783A-1R) and theMatuyama Reverse Polarity Chron (Cores 125-783A-2Rthrough 125-783A-7R). Core 125-783A-8R may record part ofthe Gauss Normal Polarity Chron.

The pelagic sediment/serpentine sediment contact wasidentified within Section 125-783A-14R-1. Cores 125-783A-15R through 125-783A-18R were processed in the cryogenicmagnetometer at 0 and 10 mT. The serpentine material has anNRM intensity typically between 50 and 200 mA/m. BothCores 125-783A-16R and 125-783A-17R carry normal polaritymagnetizations. However, as the serpentine material is notpelagic sediment, we are unable to fit the magnetic polaritydata from these cores into any geochronological framework.

Magnetic SusceptibilityMagnetic susceptibilities were measured on the multisen-

sor track (MST). A downhole plot of these values for Hole783A is presented in Figure 13. The sediments (between 0 andjust past 100 mbsf) have values typically between 3 × I0"4 and6 × 10~4 S1 units, whereas the serpentinite has values typi-cally between 1 × 10~3 to 3 × 10~3 S1 units. The high magneticsusceptibility (and NRM intensity) associated with the serpen-tinite almost certainly results because magnetite is producedduring the process of serpentinization. The serpentinite typi-cally contains on the order of 2% by volume of magnetite("Igneous and Metamorphic Petrology" section, this chap-ter).

265

SITE 783

Age

latePliocene

80 —

90 —

Nanno-fossilzone

CCN14a)

Diatom | Magneto-zone löl chron

Brunhes

? Gauss

Figure 12. The magnetostratigraphy of Hole 783A (based on whole-core and discrete-sample data).

PHYSICAL PROPERTIES

The MST was used to determine bulk density (using theGRAPE), sonic velocity, and magnetic susceptibility in eachcore section from Hole 783A. Representative samples wereselected from each section for measurement of bulk density,grain density, porosity, water content, and thermal conduc-tivity. Where possible, shear strength, resistivity, and sonicvelocity (using the Hamilton Frame device) were determined.Shear strength measured in material from this hole is reportedin the "Structural Studies" section. Physical-property mea-surements are listed in Tables 11 and 12.

A total of 120 m of volcaniclastic and pelagic sediment wascored in Hole 783A. At about 120 mbsf, serpentine sedimentsand flows with dispersed hard-rock clasts were encountered.The physical properties of the volcaniclastic/pelagic and ser-pentine material in Hole 783A are distinct (Fig. 14). Theseparation between the volcaniclastic/pelagic and serpentinematerial can easily be seen between the two groups on thedepth plot of the GRAPE-determined bulk-density data. Thediscontinuities in data points with depth are gaps in corerecovery. The average bulk densities of the volcaniclastic/pelagic and serpentine material are 1.54 ± 0.08 g/cm3 and 2.01± 0.19 g/cm3, respectively. The P-wave logger, which ismounted on the MST, gave reliable compressional-wave sonicvelocities for parts of the core from Hole 783A. Figure 15

1000 2000 3000 4000 5000

Susceptibility (4π x 10"6SI)

Figure 13. A downhole plot of magnetic susceptibility for Hole 783A.

illustrates how the velocities vary with depth; the averagecompressional-wave velocity is 1620 ± 106 m/s and is rela-tively constant over the depth range shown.

Table 11 lists the index properties of the discrete samplesfrom Hole 783A. The bulk density of the volcaniclastic andpelagic sediment is relatively constant or decreases slightly to100 mbsf (Fig. 16). At about 120 mbsf, where the serpentinematerial was encountered, the bulk density sharply increases.Values of bulk density of the serpentine material clusteraround 2.14 ± 0.03 g/cm3 and show no obvious increase withdepth. Although the values of grain density are scattered, theiraverage is relatively constant with depth. The average graindensities of the volcaniclastic and pelagic sediment and ser-pentine material are 2.59 ± 0.13 g/cm3 and 2.71 ± 0.13 g/cm3,respectively.

The average thermal conductivity of sediment is 0.937 ±0.070 W/mK (Table 11 and Fig. 16). At 120 mbsf, in theserpentine material, the thermal conductivity increases to 1.6W/mK and rises steadily to 2.1 W/mK at 150 mbsf, resulting ina gradient of 0.029.

Measurements of sonic velocity were obtained from dis-crete samples of the serpentine material. The A velocities aremeasured parallel to the surface of the split core and the Bvelocities are measured perpendicular to the surface of thesplit core. With the exception of the harzburgite clast at 159mbsf, the velocities are constant and isotropic with depth,with average A and B velocities of 194 and 196 m/s, respec-tively.

DOWNHOLE MEASUREMENTS

Two downhole temperature measurements were obtainedat Site 783 using the Barnes-Uyeda temperature tool.

266

SITE 783

Table 11. Index properties for Hole 783A.

Core, section,interval (cm)

125-783A-1R-1,77-791R-2, 23-251R-3, 76-781R-4, 97-99IR-5, 76-781R.6, 74-761R-7, 7-92R-1,33-352R-2, 15-174R-1,65-674R-2, 84-864R-4, 76-794R-5, 24-264R-CC, 8-105R-1,56-585R-2, 62-645R-3, 40-425R-4, 25-275R-CC, 11-137R-1, 30-327R-2, 50-527R-3, 139-1407R-4, 6-88R-1,37-398R-2, 71-738R-3, 76-7810R-1, 33-3510R-2, 5-71IR-I, 16-1812R-1,7-915R-1, 125-12715R-2, 76-7815R-3, 76-7816R-1, 83-8516R-2, 82-8416R-CC, 5-717R-1, 127-12917R-CC, 10-1218R-1, 82-84

Depth(mbsf)

0.771.733.765.476.768.249.07

10.0311.3526.7428.3431.2632.2432.6136.2637.8239.140.4540.8352.754.456.7956.9662.4764.3165.8681.6382.8591.06

100.67130.85131.75133.25140.03141.52142.89150.17150.5159.42

Bulkdensity(g/cm3)

.47

.48

.5

.46

.48

.49

.4

.42

.55

.5

.47

.49

.52

.55

.63

.58

.53

.58

.54

.54

.58

.58

.59

.49

.55

.52

.5

.48

.62

.442.12.152.162.072.112.122.232.172.56

Graindensity(g/cm3)

2.582.692.732.562.592.522.542.182.912.592.622.632.62.542.612.652.622.572.62.42.642.452.582.422.532.412.642.682.592.542.672.612.842.572.662.682.542.582.57

Porosity(%)

7273.272.972.471.369.27166.272.67072.471.769.16662.566.168.764.668.263.366.461.864.467.365.564.970.871.562.673.234.629.538.132.434.234.420.926.50.4

Watercontent

(%)

5252.551.752.85149.253.949.449.749.552.251.248.345.340.844.447.643.447.143.744.741.643.14844.945.45051.241.154.117.514.618.816.617.217.210130.2

Voidratio

0.720.730.730.720.710.690.710.660.730.70.720.720.690.660.620.660.690.650.680.630.660.620.640.670.660.650.710.720.630.730.350.290.380.320.340.340.210.260

The temperature record from Run 06R at 42.7 mbsf isshown in Figure 17A and at an expanded scale in Figure 17B.The probe was firmly seated in the sediment at 51 min into therun and remained relatively undisturbed until 66 min. Afterthe first few minutes of emplacement, the temperature decaycurve is proportional to I/time. A linear least-squares regres-sion on the curve, extrapolated to I/time = 0, is a goodestimate of the equilibrium sediment temperature, 3.29°C (Fig.18). The correlation coefficient (r2) has the somewhat lowvalue of 0.527, but the slope of the regression line is negligible,so there should be no significant error.

The temperature record from Run 11R at 90.9 mbsf isshown in Figure 19A and at an expanded scale in Figure 19B.The probe was seated in the sediment from 55 to 66 min intothe run, but the minor temperature increases at 59 and 63min indicate slight disturbances to the probe. The equilib-rium temperature of 3.93°C estimated from the decay curve(Fig. 20) has a low r2 of only 0.334 because of the distur-bances.

The temperature data must be combined with the thermal-conductivity data ("Physical Properties" section; Fig. 21) todetermine the heat flow at the site. Note that the large increasein thermal conductivity below 120 mbsf takes place below thedepth of the temperature measurements from the two runs.The most useful way to analyze the temperature data is to plottemperature vs. the vertically integrated thermal resistivity.Thermal resistivity is simply the inverse of thermal conduc-tivity. In the plot of integrated thermal resistivity vs. depth inFigure 22 the decreased slope of the line below 120 mbsf

results from the higher thermal conductivity of the deepersediment.

The equilibrium temperature and integrated thermal resis-tivity data are plotted in Figure 23. The slope of the linearleast-squares regression is the heat flow at this site, 23mW/m2. The quality of the fit is surprisingly poor.

Several factors might affect these data. The first is instru-ment performance, but that appears acceptable because theindicated temperature is similar in the bottom water beforeand after the penetration (Figs. 17 and 19). The second factoris a variation in bottom water temperature with time, but sucha change would have to be extremely large, probably over1°C. The bottom water temperature that we now observe isconsistent with temperatures at similar depths in this regionfrom the other temperature runs on this leg and from historicOceanographic data. The third factor is an exothermic chem-ical reaction in the sediment, but this is unlikely as thesediments were of an unexceptional nature. A fourth factor isslow advection of pore fluids upward through the sediments. Ifadvection is occurring, the total heat flow at this site is higherthan the least-squares estimate given here.

SUMMARY AND CONCLUSIONS

Site 783 (30°57.86'N, 141°47.27'E; water depth of 4648.8 m)is located on the northern, midflank part of a seamount thatforms part of a 500-km-long ridge running along the lower-most, inner wall of the Izu-Bonin Trench. The single hole,783A, attempted at this site was drilled to 168.2 mbsf with27.9% recovery (Fig. 24). Heat-flow measurements weretaken at 42.7 and 90 mbsf. Hole 783A was abandoned whenthe torque and pressure of the drill string suddenly increased.

Two lithologic units were defined as follows:Unit I (0-120.0 mbsf) is middle or early Pleistocene to early

Pliocene (or older) glass-rich silty clay to clay stone;Unit II (120.0-158.6 mbsf) is phacoidal, sheared serpentine

with clasts of serpentinized harzburgite.Dating is based on calcareous nannofossil and diatom

biostratigraphy. Microfossil studies also indicate that theupper 100 m was deposited at or near the CCD and thatreworking may have been important in places lower in thesequence. Lithologic Unit I as a whole has a small biogeniccomponent, the siliceous biogenic contribution being greaterthan the calcareous contribution; it also has varying volcanicinflux, primarily as glass with lesser amounts of feldspar,pyroxene, and amphibole.

The harzburgites of Unit II consist of olivine (80-90modal%), orthopyroxene (10-15 modal%) with minor clinopy-roxene (as exsolution lamellae in orthopyroxene), and spinel.Other rock types are very rare, although one piece ofharzburgite contains a 5- to 15-mm-wide clinopyroxene-richband and there are several small metabasaltic clasts. Thedegree of serpentinization is variable, but rarely below 50%.

Deformation observed in the sediments at Site 783 beginswith the first partially lithified sediment at 62.1 mbsf and includesshear fabrics and convolute plastic folding within the claystonesof Unit I and the phacoidal serpentine of Unit II. High-temper-ature pre-serpentinization and lower temperature, post-serpenti-nization fabrics similar to those described from Conical Sea-mount are found in the ultramafic clasts of Unit IL

Studies of physical properties show that the bulk densitiesincrease from 1.47 to 1.590 g/cm3 in the sediments of Unit Iand cluster about 2.1 g/cm3 in the serpentine of Unit IL Theaverage compressional-wave velocity in the sediments is 1.9km/s. Thermal conductivities average about 1 W/mK in sedi-ments and 1.91 W/mK in the serpentine. Heat flow in thesediments is 23 mW/m2.

267

SITE 783

Table 12. Physical properties for Hole 783A.

Core, section,interval (cm)

Depth(mbsß

Thermalconductivity

(W/mK)

Ultimatestrength

(kPa)

Compressional-wavevelocity

Resistivity Formation A direction B direction(ohms) factor (km/s) (km/s)

125-783A-1R-1, 76-801R-2, 751R-3, 76-801R-4, 751R-5, 76-801R-6, 751R-7, 6-101R-7, 302R-1, 30-342R-1, 752R-1, 123-1252R-2, 16-202R-CC, 84R-1, 60-644R-1, 754R-2, 754R-3, 754R-3, 86-904R-4, 754R-5, 20-244R-CC, 75R-1, 10-145R-1, 755R-2, 755R-3, 10-145R-3, 755R-4, 247R-1, 10-147R-1, 607R-2, 607R-3, 10-147R-3, 607R-3, 88-908R-1, 10-148R-1, 678R-1, 87-898R-2, 678R-3, 10-148R-3, 6710R-1, 30-3410R-1, 5810R-2, 2111R-1, 1212R-1, 1914R-1, 1514R-CC, 815R-1, 64-6615R-1, 75-8115R-1,125-12715R-2, 54-5615R-2, 74-7615R-3, 40-4215R-3, 74-7615R-3, 87-9115R-CC, 416R-1, 16-2016R-1, 7516R-1, 83-8516R-2, 8016R-2,122-12416R-3, 19-2116R-3, 3016R-3, 48-5216R-CC, 5-716R-CC, 2117R-1, 30-3417R-1, 6917R-1,107-10918R-1, 80

0.752.253.755.256.758.259.089.3

10.0210.4510.9311.3211.5426.6226.7528.2529.7529.8831.2532.2632.635.8236.4537.9538.8239.4540.4452.525354.555.525656.2862.2262.7762.9764.2765.2265.7781.6281.8883.0191.02

100.79120.05120.22130.26130.35130.85

131.66131.74133.02133.24133.49134.03139.38139.95140.03141.5141.92

142.39142.5142.7142.89143.05149.22149.59149.97

159.4

0.8880.8630.9390.9210.98

0.988

1.039

0.991.0060.991

0.9190.9250.976

0.9140.9940.989

0.9550.931

0.8970.912

0.9980.998

0.959

1.004

0.906

0.833

0.90.8470.6820.9651.6240.638

1.123

1.839

1.961

1.606

1.94

1.827

1.348

1.93

2.092

2.322

13.1

37.1

29.1

17.544.4

34.9

41.5

32.8

52.4

29.8

69.2

34.9

10.8

11.2

11.4

11

12.5

13.9

16.3

15.4

15.4

23

18.5

35.1

20.1

13.5

18.1

16.5

1.26

1.31

1.33

1.28

1.46

1.62

1.9

1.8

1.8

2.68

2.16

4.1

2.35

1.58

2.11

1.93

1651

16.7

35.7

19.7

25.5

26.9

37

91

43

100

4.32

10.62

5.02

11.67

11.44

2.06

1.92

1.97

1.98

68.2

45.7

7.96

5.33

41.5

1.981.86

1.9

5.62 5.76

268

SITE 783

1.6 1.8 2.0Bulk density (g/cm3)

2.2 2.4

Figure 14. Bulk density (determined by the GRAPE) vs. depth forHole 783A.

Interstitial-fluid compositions show only small variations withdepth in the sediment, a sharp discontinuity at the clay stone/serpentine boundary, and rapid chemical changes thereafter.Important variations with depth in the serpentine include anincrease in pH from about 8 at the claystone/serpentine bound-ary to 9.5 to 10 at the bottom of the hole, a decrease in alkalinity(from about 3 to 1.3 meq/kg), depletions in silica (from 43 to 2

mmol/kg), magnesium (from 50 to mmol/kg), and sulfate (from28 to 6 mmol/kg), increases in calcium (from 11 to 39 mmol/kg)and sodium (from 480 to 499 mmol/kg), decreases in potassium(from 11 to 9 mmol/kg) and salinity (from 3.59 to 3.32 wt%), andlittle change in ammonia, chlorinity, and bromide. Significantdifferences from the values measured at Conical Seamount arethat calcium was depleted at Conical Seamount (by carbonateprecipitation) and chlorinity decreased (possibly by mixing withdeeper, fresher water).

The principal results of this site are (1) the confirmationthat this seamount is made up, at least in part, of serpentinite;(2) the biostratigraphic evidence that the serpentine sedimentis at least early Pliocene in age and hence older than ConicalSeamount in the Mariana forearc; (3) the structural evidencefor deformation within the overlying sediments as well aswithin the serpentinite; and (4) the evidence from fluid chem-istry that serpentinization is still taking place, but without thelow-chlorinity component identified at Conical Seamount(Sites 778-780).

REFERENCES

Barron, J. A. 1985. Miocene to Holocene planktic diatoms. In Bolli,H., Saunders, J. B., and Perch-Nielsen, K. (Eds.), PlanktonStratigraphy: Cambridge (Cambridge Univ. Press), 763-810).

Berggren, W. A., Kent, D. V., Flynn, J. J., and Van Couvering, J.,1985. Cenozoic geochronology. Geol. Soc. Am. Bull., 96:1407-1418.

Gartner, S., 1977. Calcareous nannofossil biostratigraphy and revisedzonation of the Pleistocene. Mar. Micropaleontol., 2:1-25.

Kobayashi, K. (Ed.), 1989. Preliminary Report of the Hakuho MaruCruise KH-87-3, July-August 13, 1988, Izu-Ogasawara (Bonin),East Mariana Basin and Yap Trench (WESTPAC, ODP SiteSurvey): Tokyo (Ocean Research Institute, Univ. of Tokyo).

McDuff, R. E., 1985. The chemistry of interstitial waters, Deep SeaDrilling Project Leg 86. In Heath, G. R., Burckle, L. H., et al.,Init. Repts. DSDP, 86: Washington (U.S. Govt. Printing Office),675-687.

Ness, G., Levi, S., and Crouch, R., 1980. Marine magnetic anomalytime scales for the Cenozoic and Late Cretaceous. A precis,critique and synthesis. Rev. Geophys. Space Phys., 18:753-770.

Ms 125A-111

NOTE: AH core description forms ("barrel sheets") and core photographs have beenprinted on coated paper and bound as Section 4, near the back of the book, begin-ning on page 383.

269

SITE 783

A 0

100

200

O®

oo

• SedimentO Serpentinite

B 1 1 1(

/ <

-

• SedimentO Serpentinite

I I I

*••o o o

° COoo

o

1 . I .

>

i

m—0

0

0

• Sediment0 Serpentinite

i 1

1 1 '

—

0

o °ooθ

0

0

1 1 1

1.2 1.6 2.0

Bulk density (g/cm3)

2.4 2.2 2.6

Grain density (g/cm3)

3.0 0 1 2 3

Thermal conductivity (W/mK)

Figure 15. Physical and index properties vs. depth for Hole 783A. A. Bulk density (from discrete samples). B. Grain density. C. Thermalconductivity.

A 0 B 130

140

150

|

mo

l 1 ^

—

• A Velocity

- o B Velocity

1550 1600 1650Sonic velocity (m/s)

1601700 1

1 i o2 3 4

Sonic velocity (km/s)

Figure 16. Sonic velocity vs. depth for Hole 783A. A. Velocity determined by the P-wave logger. B. Velocity determined fromdiscrete samples; "A" velocities are parallel to core; " B " velocities are perpendicular to core.

270

SITE 783

A 20

ü

B 7

40 60Time (min)

55 65Time (min)

75

Figure 17. A. Temperature vs. time for Run 06R of the Barnes-Uyedatemperature tool at 42.7 mbsf in Hole 783A. B. An expanded plotshowing the data when the tool was in the seafloor.

y = 3.2866 + 36.000 x r2 = 0.527

0.001 0.002 0.0031/Time (s)

0.004 0.005

Figure 18. The reduced temperature fit of the first Barnes-Uyedatemperature run. The intercept value at I/time = 0 is the equilibriumtemperature of the sediment.

A 30

120

Time (min)

Figure 19. A. Temperature vs. time for Run 11R of the Barnes-Uyedatemperature tool at 90.9 mbsf in Hole 783A. B. An expanded plotshowing the data when the tool was in the seafloor.

y = 3.9306 - 25.755 x r2 = 0.334

0.004 0.0050 0.001 0.002 0.0031/Time (s)

Figure 20. The corresponding reduced temperature fit for the secondBarnes-Uyeda run.

271

SITE 783

Q.α>Q

200

100 —

1 2

Conductivity (W/mK)

Figure 21. Plot of thermal-conductivity results vs. depth for Hole783A.

200Integrated thermal resistivity (m2K/W)

Figure 22. Plot of integrated thermal resistivity vs. depth for Hole783A.

( L y = 1 -8455 + 2.2718e - 2 x r2 = 0.894

20 40 60 80

Integrated thermal resistivity (m2K/W)

100

Figure 23. Temperature vs. integrated thermal resistivity for Hole783A. The slope of the best-fitting linear regression is a heat-flowvalue of 23 mW/m2. See text for discussion of possible explanationsfor the poor fit.

Q.α>Q

-

50 —

-

100 —

150 —

Age

middle orearly

Pleistocene

earlyPliocene

Co

re

Re

cove

ry

1R 1

2R 13R 1

4R 1

"δR~^•

6R

7R

8R

9R

10R

11R

12R

13R

••i

Lith.unit

I

14RF

1 5 R ^ ^

16RT II

17R|

18R|~

Lithology

Vitric clayey silt,glass-rich siltyclay/claystone.

Phacoidal, shearedserpentine with

verticalconvolute bedding.

Figure 24. Core recovery and lithologic summary, Hole 783A.

272