27. ROCK MAGNETIC PROPERTIES, MAGNETIC MINERALOGY, AND MAGNETIC FABRIC OF ROCKS … · 2006. 11....

Greene, H.G., Collot, J.-Y, Stokking, L.B., et al., 1994Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 134

27. ROCK MAGNETIC PROPERTIES, MAGNETIC MINERALOGY, AND MAGNETIC FABRICOF ROCKS IN THE D'ENTRECASTEAUX COLLISION ZONE1

L.B. Stokking,2,3 D. Merrill,3 X. Zhao,4 and P. Roperch5

ABSTRACT

During Leg 134, the influence of ridge collision and subduction on the structural evolution of island arcs was investigated bydrilling at a series of sites in the collision zone between the d'Entrecasteaux Zone (DEZ) and the central New Hebrides IslandArc. The DEZ is an arcuate Eocene-Oligocene submarine volcanic chain that extends from the northern New Caledonia Ridgeto the New Hebrides Trench. High magnetic susceptibilities and intensities of magnetic remanence were measured in volcanicsilts, sands, siltstones, and sandstones from collision zone sites. This chapter presents the preliminary results of studies of magneticmineralogy, magnetic properties, and magnetic fabric of sediments and rocks from Sites 827 through 830 in the collision zone.The dominant carrier of remanence in the highly magnetic sediments and sedimentary rocks in the DEZ is low-titaniumtitanomagnetite of variable particle size. Changes in rock magnetic properties reflect variations in the abundance and size oftitanomagnetite particles, which result from differences in volcanogenic contribution and the presence or absence of graded beds.Although the anisotropy of magnetic susceptibility results are difficult to interpret in terms of regional stresses because the coreswere azimuthally unoriented, the shapes of the susceptibility ellipsoids provide information about deformation style. The magneticfabric of most samples is oblate, dominated by foliation, as is the structural fabric. The variability of degree of anisotropy (P) anda factor that measures the shape of the ellipsoid (q) reflect the patchy nature of deformation, at a micrometer scale, that is elucidatedby scanning electron microscope analysis. The nature of this patchiness implies that deformation in the shear zones is accomplishedprimarily by motion along bedding planes, whereas the material within the beds themselves remains relatively undeformed.

INTRODUCTION

During Leg 134, the influence of ridge collision and subductionon the structural evolution of island arcs was investigated by drillingat a series of sites in the collision zone between the d'EntrecasteauxZone (DEZ) and the central New Hebrides Island Arc (Figs. 1 and2). The DEZ is an arcuate Eocene-Oligocene submarine volcanicchain extending from the northern New Caledonia Ridge to the NewHebrides Trench. Near the New Hebrides Trench, the DEZ comprisestwo parallel morphologic features that trend east-west: the fairlycontinuous North d'Entrecasteaux Ridge (NDR) and the South d'En-trecasteaux Chain (SDC), which are composed of seamounts andguyots. The impingement of the DEZ upon the central New HebridesIsland Arc has greatly disrupted and tectonically modified arc mor-phology and structure. Holes at Sites 827 and 829 were drilled topenetrate the lowermost accretionary wedge and the interplate thrustfault (décollement), where the NDR collides with the New HebridesIsland Arc. The primary objective of drilling at Site 828 was to obtaina reference section of rocks from the north ridge. Holes at Site 830were drilled to penetrate imbricated arc rocks in the collision zonebetween the Bougainville Guyot (eastern member of the SDC) andthe arc.

Preliminary interpretations of the results of Leg 134 suggest thateach ridge of the twin-ridge DEZ causes different forearc deformation.The sedimentary and surficial basement rocks of the NDR, whosebasement rocks (mid-ocean ridge basalts, or MORBs) are denser thanthose of the Bougainville Guyot, appear to have been scraped off andaccreted to the forearc during subduction (Collot et al., this volume;Fisher et al., 1986; Greene et al., this volume). This accretion has

1 Greene, H.G., Collot, J.-Y., Stokking, L.B., et al., 1994. Proc. ODP, Sci. Results,134: College Station, TX (Ocean Drilling Program).

2 ODP, College Station, TX 77845-9547, U.S.A.3 Department of Geophysics, Texas A&M University, College Station, TX 77843,

U.S.A.4 Earth Sciences Board, University of California, Santa Cruz, CA 95064, U.S.A.5 Laboratoire de Géodynamique Sous-Marine, Centre d'Etudes et de Recherches

Océanographiques de Villefranche-sur-Mer, B.P. 48,06230, Villefranche-sur-Mer, France.

formed the Wousi Bank, which consists of uplifted forearc rocksand stacked thrust sheets. The SDC impacts the forearc in a differentmanner: little deformational uplift, but considerable indenting, hasoccurred compared to the NDR collision zone, although the SDC isconverging at the same rate and at the same angle as the NDR (Collot,Greene, Stokking, et al., 1992).

This chapter presents the preliminary results of studies of mag-netic mineralogy, magnetic properties, and magnetic fabric of sedi-ments and rocks from Sites 827 through 830 in the collision zone.Additional work on these units is presented in two chapters byRoperch et al. (this volume).

LITHOSTRATIGRAPHY

Drilling results are presented in Collot, Greene, Stokking, et al.(1992), Reid et al. (this volume), and are summarized in Table 1.Typical sediments and rocks are illustrated in Plate 1. Drilling at Site827 (Figs. 1 and 2) recovered primarily arc-derived turbiditic volcanicsilts and silty sands. At Site 828 on the NDR drilling recovered high-susceptibility volcanic silts in the upper part of Hole 828A and pri-marily low-susceptibility oozes and chalks in Hole 828B.

At Site 829 collision has formed an accretionary complex that isat least 590 m thick at a distance of 2 km from the trench (Figs. 1 and2). Site 829 contains 21 lithostratigraphic units, several of whichrepresent repeated lithologies. Because of these repetitions, duringLeg 134 the lithostratigraphic units were divided into four compositeunits (Bigwan Wan, Bigwan Tu, Bigwan Tri, and Bigwan Fo, namedin Bislama, the official language of the Republic of Vanuatu; Collot,Greene, Stokking, et al., 1992). These composite units have beendefined on the basis of age and lithologies. Bigwan Wan comprisesPleistocene volcanic silt, sandstone, and gray silty chalk. Bigwan Tucontains upper Oligocene to lower Miocene foraminiferal and cal-careous chalk. Bigwan Tri is an upper Pliocene or Pleistocene chalkbreccia with upper Oligocene to lower Miocene clasts. Bigwan Fo isa breccia of unknown age with clasts of basaltic rock fragments,microgabbros, and gabbros. Lithostratigraphic units are numberedconsecutively starting with Hole 829A and ending with Hole 829C.

475

L.B. STOKKING, D. MERRILL, X. ZHAO, P. ROPERCH

Figure 1. The d'Entrecasteaux Zone-central New Hebrides Island Arc collisionzone. Bathymetry (in kilometers) after Kroenke et al. (1983). NDR = Northd'Entrecasteaux Ridge, NAB = North Aoba Basin, SAB = South Aoba Basin.

Initial interpretations of cores suggest that many of the units iden-tified in Hole 829A are similar to those observed in Hole 828A. Forexample, the calcareous chalk and pale brown chalk of Bigwan Tuare similar in age and lithology to the nannofossil chalk of litho-stratigraphic Unit II from Hole 828A. Igneous rocks collected at bothSites 828 and 829 are similar. Therefore the Leg 134 shipboard party(Collot, Greene, Stokking, et al., 1992) and post-cruise researchers(Collot et al., this volume; Greene et al., this volume; and Reid, et al.,this volume) concluded that an accretionary prism, composed in partof offscraped rocks and sediments from the downgoing NDR, waspenetrated at Site 829.

Site 830 was drilled on the forearc slope in a gently eastward-dipping thrust sheet. Rocks recovered include very coarse, Cataclas-tic volcaniclastic sandstones that are similar in composition to rocksexposed on nearby Espiritu Santo and Malakula islands (Mitchell andWarden, 1971; Mallick and Greenbaum, 1977).

METHODS

Two-hundred-nine samples were studied (Table 2). The set in-cluded 130 samples of volcanic silts and siltstones, 40 volcanic sandsand sandstones, 12 breccias and conglomerates, 19 oozes and chalks,and eight samples of mixed sediment and sedimentary rocks.

Magnetic hysteresis is measured by applying an increasing mag-netic field to a sample, then reversing the applied field, while continu-ously measuring the magnetization of the sample as it responds to thechanging applied field. Aplotof applied field vs. sample magnetizationresults in a loop, the shape of which is determined by the chemicalcomposition, microstructure, and particle orientation of the magneticmaterial within the sample (Stacey and Banerjee, 1974; Day et al.,

Site 831 Site 828 SH* 829 Site 827 Site 830

Figure 2. East-west cross sections of the collision zone with projected simpli-fied lithologic columns. Location of cross sections is shown in Figure 1.Legend is as follows: 1. Oceanic crust, 2. Western Belt volcanic rocks, 3.Eastern Belt volcanic rocks, 4. Central Chain volcanic rocks, 5. Basin fill, 6.Guyot volcanic rocks, 7. Volcanic sand/sandstone, 8. Volcanic silt/siltstone, 9.Volcanic sandstone/siltstone/claystone, 10. Sed-lithic breccia, 11. Volcanicbreccia, 12. Basalt, 13. Multiple slivers of siltstone and chalk, 14. Foraminiferalooze, 15. Nannofossil ooze, 16. Foraminiferal chalk, 17. Nannofossil chalk,18. Calcareous chalk, 19. Pelagic limestone, 20. Lagoonal limestone, 21.Unconformity, 22. Ash, 23. Thrust fault; 1. Pli./e. Pie is late Pliocene or earlyPleistocene, 1. O.-e. M. is late Oligocene to early Miocene, E. is Eocene, m.M.is middle Miocene, NDR is North d'Entrecasteaux Ridge, BG is BougainvilleGuyot, NAB is North Aoba Basin, NFB is North Fiji Basin.

1977; Cisowski, 1980; O'Reilly, 1984). Measurement of magnetichysteresis can provide information about several rock magnetic prop-erties: coercivity (Hc), saturation remanence (MRS), and saturationmagnetization (Ms). These parameters, in conjunction with coercivityof remanence (HCR), then provide constraints on the domain state ofthe magnetic minerals in the sample (Dunlop, 1969). Domain state isin turn related to particle size (see "Discussion" section, this chapter).Hysteresis behavior of nine samples was measured using a PrincetonApplied Research vibrating sample magnetometer housed in the Phys-ics Department at the University of California, Davis.

Magnetic susceptibility provides information about the type andconcentration of remanence-carrying minerals (such as titanomag-netite) in a sample. Magnetic susceptibilities of the archive halves ofcores were measured on board ship using a Bartington Instrumentsmagnetic susceptibility meter (model MSI) with an 80-mm sensorloop at a frequency of 0.47 kHz. Magnetic susceptibilities of 209discrete samples were measured at TAMU using a Bartington Instru-

476

ROCK MAGNETIC PROPERTIES, MINERALOGY, AND FABRIC

Table 1. Lithologic summary of Sites 827, 828, 829, and 830.

Lithologicunit

Site 827I

IIIII

IV

Site 828IIIIIIIIIIVIV

Hole 829AIIIIIIIVVVIVIIviπIXX

XIXIIxπiXIVXVXVI

Hole 829BXVII

Hole 829C

xvmXIXXXXXI

Site 830I

If11

Lithologicsubunit

IAIBIC

IIIAI1IBinc

IAIBIC

IC

Core, section,interval (cm)

827A-lH-l,0to-13H-l,60827A-lH-l,0to-3H-6, 150827A-3H-6, 150 to-8H-4, 50827A-8H-4, 50to-13H-6, 60827A-13H-6, 60 to -827B-4R-3, 90827B-4R-3, 90 to -15R-CC, 15

827B-4R-3, 90 to -10R-4, 120827B-10R-4, 120 to -12R-3, 90827B-12R-3, 90 to -15R-CC, 15827B-15R-CC, 15 to -31R-CC, 7

828A-1H-I,0to-8H-1,48828A-8H-l,48to-8H-6,42828A-8H-6,42to-llH-l,92828B-lR-l,0to-2R-l,0828A-llH-l,92to-15N-2,185828B-2R-l,0to-3R-l, 12

829A-lR-l,0to-8R-l,0829A-8R-l,0to-12R-l,40829A-12R-l,40to-19R-4, 88829A-19R-4, 88 to -23R-1, 0829A-23R-l,0to-41R-l,0829A-41R-1, 0 to -43R-1, 120829A-43R-1, 120 to -43R-CC, 20829A-43R-CC, 20 to -44R-1, 40829A-44R-1, 40 to -45R-CC, 5829A-45R-CC, 5 to -47R-1, 65

829A-47R-1, 65 to -48R-CC, 12829A-48R-CC, 12 to -51R-3, 10829A-51R-3, 10 to -54R-1, 0829A-54R-1, 0 to -55R-CC, 15829A-55R-CC, 15 to -57R-3, 72829A-57R-3, 72 to -64R-CC, 27

829B-lH-l,0to-3H-CC, 50

829C-1H-I,0to-9H-4,0829C-9H-4, 0 to-1 OH-1,0829C-10H-l,0to-llH-l,0829C-llH-l,0to-llH-CC, 15

830A-lH-l,0to-llX-l,34830A-lH-l,0to-3H-3, 150830A-3H-4, 0 to -6H-5, 110830A-6H-5, 110 to -11X-1, 34

830B-lR-l,0to-14R-l,0

830B-14R-l,0to-24R-CC, 15830C-1R-1, 0 to -12R-CC, 14

Depth(mbsf)

0.0-88.40.0-27.0

27.0-66.066.0-88.488.4-141.0

141.0-252.6

141.0-200.0200.0-218.0218.0-252.6252.6-400.4

0.0-61.961.9-69.369.3-90.890.0-100.090.8-111.4

100.0-119.4

0-60.560.5-99.499.4-171.9

171.9-205.2205.2-378.4378.4-398.9398.9-106.0406.0-413.0413.0-419.6419.6-435.0

435.0-442.0442.0-463.2463.2-484.5484.5-495.6495.6-517.2517.2-590.3

0.0-19.5

0.0-54.64.6-57.3

57.3-58.358.3-58.4

0.0-96.90.0-21.0

21.0-47.047.0-96.9

48.5-174.9

174.9-281.7235.0-350.6

Thickness(m)

88.427.039.022.452.6

111.0

59.018.034.6

147.8

61.97.4

21.510.020.619.4

60.538.972.533.3

173.220.5

7.17.06.6

15.4

7.02.2

21.311.121.673.1

19.5

54.62.71.00.1

96.921.026.049.9

126.4

106.8115.6

Dominantlithology

Volcanic silt, volcanic silty sandFew graded interbedsSeveral graded interbedsFew graded interbedsVolcanic silt and siltstoneCalcareous volcanic siltstone,

sed-lithic conglomerateVolcanic siltstoneSed-lithic conglomerate and brecciaVolcanic siltstoneVolcanic siltstone and sandstone,

breccia

Volcanic silt, volcanic sandy siltForaminiferal oozeNannofossil chalkNannofossil chalkVolcanic brecciaVolcanic breccia

Clayey volcanic siltForaminiferal chalkClayey volcanic siltSiltstone-chalk brecciaChalk brecciaCalcareous chalkVolcanic breccia, clayey chalkForaminiferal chalkSilty chalkCalcareous chalk and foraminiferal

chalkSilty chalkCalcareous chalkVolcanic sandstoneChalk and mixed sedimentSandy volcanic siltstoneIgneous sed-lithic breccia

Clayey sandy volcanic silt

Clayey sandy volcanic siltForaminiferal oozeForaminiferal silty mixed sedimentForaminiferal chalk

Volcanic silt and volcanic siltstoneVolcanic siltVolcanic sand interbedsInterbedded sandy silts and silty

sandsInterbedded sandy silts and silty

sandsVolcanic sed-lithic sandstoneVolcanic sed-lithic sandstone

Age

PleistocenePleistocenePleistocenePleistocenePliocene to PleistocenePliocene to Pleistocene

PleistocenePliocene or PleistocenePlioceneBarren

PleistocenePleistocene, PlioceneOligoceneOligocene/EoceneEocene(?)Barren

Pleistocenelate Oligocene to early MiocenePleistocenePleistocenelate Pliocene or PleistoceneOligocene and EoceneIndeterminate, Eoceneearly PliocenePleistoceneOligocene or Miocene

Pliocene (?) and PleistoceneOligocene to MiocenePliocene or PleistoceneOligocene or MiocenePliocene to PleistoceneEocene

Pleistocene

Pleistoceneearly MiocenePleistocenePleistocene

PleistocenePleistocenePleistocenePleistocene

Pleistocene

BarrenBarren

ments susceptibility meter, model MS2, at a frequency of 0.47 kHz.Data are reported as volume susceptibility (k).

Whole-rock samples from shear zones were observed using a JEOLJSM 6400 scanning electron microscope (SEM) with Tracor Northern,Series 2, energy-dispersive X-ray analysis. Magnetic separates ofvolcanic silt (Samples 134-827A-9H-1,118 cm, and -829A-6R-4,110cm) prepared by attracting particles in a slurry of ultrasonically sus-pended sediment and deionized water to a magnet, were also analyzed.

Alternating field (AF) demagnetization of the natural remanentmagnetization (NRM) of a sample provides information about itscoercivity spectrum, a property that, assuming the sample contains asingle type of magnetic carrier, is itself dependent on the range of sizesof magnetic particles in the rock. Aboard ship, both a 2-G Enterprises(Model 760R) pass-through cryogenic rock magnetometer and a Mol-spin spinner magnetometer were used to measure NRM. At TAMU,

NRM intensities were measured using a three-axis CTF cryogenicmagnetometer housed in a shielded room in the Paleomagnetics Labo-ratory at the Department of Geophysics. An AF demagnetizer (Model2G600) capable of producing a peak field up to 20 Mt was used on-linewith the pass-through cryogenic magnetometer on board ship. Ship-board AF demagnetization at higher fields and AF demagnetization atTAMU were performed using a single-axis Schonstedt geophysicalspecimen demagnetizer (Model GSD-1) capable of producing peakfields up to 100 mT. Exposure to strong magnetic fields in the corebarrel, in the steel drill pipe, and on the rig floor impart a steep negativeinclination to cores, particularly those containing low-coercivity mate-rial. The intensity of the overprint decreases radially from the edge ofthe core to its center; thus samples taken from the center of the coreare less affected than those from the edge (Collot, Greene, Stokking,et al., 1992). This spurious magnetization was removed from most Leg

477


Table 2. List of samples, depths, lithologic information, rock magnetic data. Acronyms and abbreviations are defined in the text.

Sample

Hole 827A9H-1, 1189H-2, 8311H-4, 75

Hole 827B5R-1,45R-3, 867R-5, 518R-4, 349R-1,959R-5, 849R-6, 210R-2, 2211R-1,11413R-2, 6213R-3, 1414R-2.11814R-3, 6215R-3, 50

Hole 828A4H-1.835H-4, 578H-1.448H-6, 698H-7, 59H-6, 899H-7, 2910H-l,116HH-1,20HH-3,126

Hole 828B1R-2, 451R-2, 981R-4, 60

Hole 829A5R-1.95R-3, 857R-1.3012R-1, 3517R-1, 13917R-2, 7017R-3, 8817R-4, 2317R-6, 1917R-6, 12317R-7, 2818R-U0718R-3, 7618R-4, 2818R-5.10718R-6, 3019R-1,7319R-4, 3719R-5, 7320R-U1620R-l,13520R-3, 224R-1.4443R-1.7743R-1, 14444R-2, 5751R-1.2351R-1.1O851R-2, 851R-2, 3852R-1,14456R-1.5056R-2.14156R-3, 1157R-2, 6064R-1.102

Hole 829BlH-1,312H-1,702H-2, 512H-3,1002H-4, 273H-1.413H-2, 213H-3, 25

Depth(mbsf)

68.269.382.9

146.5150.4172.3180.2186.1191.9192.6196.4205.5225.8226.8236236.9246.5

24.23861.869.670.579.380.281.690.194.2

9292.595.1

31.134.951.199.4

148.6149.4151.1151.9154.9155.9156.5157.9160.6161.6163.9164.6167.2171.4173.2177.3177.5179.1215.3398.5399.2409.5460.3461.2461.7462466.6504.3506.7506.9515.6582.2

0.31.22.54.55.3

10.411.713.3

Lithology

Vole, siltVole, siltVole, silt

Sltst.Sndst.Sndst.Sltst.Sltst.Mxd. sed.Mxd. sed.Mxd. sed.Cgt.Mxd. sed.Mxd. sed.Sltst.Sltst.Cgt.

SiltSiltOozeOozeOozeOozeOozeOozeOozeBreccia

ChalkChalkChalk

SiltSiltSiltChalkSiltSiltst.Siltst.Siltst.Siltst.Siltst.Siltst.Siltst.Siltst.Siltst.Siltst.Siltst.Siltst.Siltst.BrecciaBrecciaBrecciaSiltst.BrecciaChalkBrecciaChalkChalkChalkChalkChalkSandst.Siltst.Siltst.Siltst.Siltst.Breccia

SiltSiltSiltSiltSiltSiltSiltSilt

NRM(A/m)

0.600.620.19

0.060.120.170.170.070.020.030.040.160.230.130.220.030.09

0.380.330.090.010.030.000.020.010.010.00

0.030.040.04

0.890.680.290.050.370.360.190.230.420.650.400.210.100.350.260.150.481.260.930.020.010.130.050.010.020.160.080.080.070.060.340.290.060.120.480.02

0.550.470.490.450.910.610.59

RM10(A/m)

0.160.120.12

0.040.040.110.080.030.030.010.030.110.060.07

• 0.100.020.04

0.160.200.010.010.010.000.010.010.010.00

0.010.010.02

0.140.180.060.030.120.130.110.040.130.050.260.080.030.240.150.090.140.160.110.010.010.070.030.010.000.080.040.030.030.030.040.190.050.080.170.01

0.130.110.150.150.150.170.140.03

MDF(mT)

53

15

207.5151010557.5

35157.5

12.5101010

7.5203

202.5

405

27.5155

401

15

3052

1055

12555

125

15I0155

104

10101015105

1597873

155558

55555555

IRMIOOO(A/m)

120.66137.43109.81

79.65108.2567.3892.7071.1075.6267.7358.56

104.21138.54125.12129.0799.6795.73

88.10123.74

17.0411.1312.516.867.65

10.1913.650.50

5.686.38

13.85

149.80122.5093.3935.62

128.80144.9986.4182.71

122.95108.2090.64

117.02105.42130.39115.8387.4673.89

117.4981.7012.616.05

97.6124.8728.5540.0070.1949.6947.2540.6635.3752.30

209.93193.90192.19135.60

10.61

120.28125.84135.98120.14138.06131.22131.29105.31

ARM100(A/m)

2.153.682.52

1.842.481.462.461.801.531.531.563.003.153.034.273.902.69

2.323.210.560.560.570.540.470.740.650.03

0.340.340.44

3.994.262.260.923.394.022.822.842.963.323.783.053.593.232.872.313.532.552.210.710.302.300.940.994.021.331.981.911.491.030.953.913.804.325.140.18

2.942.983.102.892.843.043.173.04

K(S1)

0.00830.00410.0051

0.00280.00320.00290.00350.00250.00290.00250.00240.00230.00360.00310.00320.00220.0026

0.00350.00440.00120.00050.00060.00020.00030.00040.00060.0001

0.00030.00030.0006

0.00590.00390.0050.00140.00460.00410.00290.00240.00390.00510.00230.00480.00330.00580.00510.00370.00340.0080.00430.00030.00010.00520.00380.00150.00140.00640.00190.00180.00150.00280.00350.01990.01870.02490.00320.0003

0.00540.00650.0060.0060.00740.00680.00650.0047

Sample

3H-3,493H-4, 583H-5,68

Hnlp S9QP1H-2, 321H-3, 431H-4, 371H-5, 361H-6, 242H-1,792H-2, 722H-3, 792H-4, 792H-5, 792H-6, 793H-2, 803H-2, 1173H-2,1343H-2, 1443H-3,783H-4, 533H-5, 663H-6, 603H-7, 224H-1,354H-1.1194H-2, 294H-2, 1055H-1,295H-1, 1205H-2, 315H-2, 905H-3, 665H-4, 325H-4, 1245H-5, 205H-5, 627H-1,687H-1.1107H-2, 328H-1,289H-1,249H-3, 999H-4, 59

Hole 83OA1H-1, 81H-1,27

IH-l' 1001H-2,' 1031H-3, 821H-4, 711H-5, 402H-l! 862H-2, 632H-2^ 1002H-3, 832H-5, 362H-6, 812H-7, 513H-1, 933H-2, 473H-3, 603H-3, 813H-4, 844H-1, 1084H-2, 924H-3, 495H-1, 1235H-2, 385H-2, 1255H-3, 286H-1,746H-2, 147X-1.32

Hole 83OB1R-1,471R-2, 122R-1, 872R-2, 332R-2, 60

Depth(mbsf)

13.515.116.7

1.83.44.96.37.79.1

10.512.113.615.116.620.120.520.620.721.622.824.525.92727.728.529.129.930.631.532.132.73435.13636.536.948.248.649.350.352.25.6

57.1

0.10.30.412.53.85.26.47.99.19.5

10.813.415.316.517.418.520.120.321.827.128.429.531.632.333.233.740.641.548.8

4950.15.959.960.2

Lithology

SiltSiltSilt

SiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSillSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltMxd. sed.Mxd. sed.Mxd. sed.SiltSiltOoze/siltOoze

SiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSandSandSiltSandSiltSiltSilt

SiltSiltSiltSiltSill

NRM(A/m)

0.290.611.24

0.360.250.260.330.990.630.630.510.450.590.990.520.410.370.420.460.431.110.890.970.990.680.861.300.670.590.610.500.480.520.520.600.430.300.330.501.120.350.240.11

0.340.340.330.380.280.230.330.270.390.600.440.520.490.350.570.590.330.020.140.531.331.071.421.461.791.741.051.961.440.98

1.150.380.310.480.37

RM10(A/m)

0.030.160.17

0.120.110.130.110.110.240.220.200.190.200.200.210.170.130.130.190.200.190.190.160.230.050.100.190.090.110.140.130.100.110.110.130.090.060.070.100.270.090.070.03

0.230.250.220.210.140.090.180.120.110.080.150.230.220.190.240.190.110.010.000.110.160.150.260.200.370.330.280.390.260.28

0.340.180.130.390.29

MDF(mT)

55

30

57

105

1055555

1777555

1055

15255555555555555

55555

20251712107

12733373975

533

101520

33

303020

320

3010102020

IRM1000(A/m)

94.1799.66

106.03

141.21111.41127.87131.38130.76132.21126.45120.02120.18120.54115.72123.84125.38162.29141.78131.51147.53154.41144.96151.98137.48127.44155.05123.08137.47136.70127.51116.95121.78123.48116.55119.4595.5166.1869.7396.4152.1558.2146.6518.13

162.03114.87113.38122.46169.12116.9198.0499.6113.69

111.12154.18115.68115.88124.20115.35136.48121.85

5.626.43

105.74161.73125.31185.31172.81154.83205.46170.35163.20154.66148.92

213.15160.75194.54205.52

98.57

ARM100(A/m)

3.142.793.57

2.852.983.252.732.763.082.902.902.692.652.853.303.343.253.203.073.463.963.894.093.303.153.943.173.073.053.212.923.073.233.163.293.431.581.682.211.281.471.470.53

3.132.712.873.032.372.743.652.222.511.762.672.902.653.052.543.042.770.160.172.222.943.173.472.532.833.653.083.393.043.18

3.513.423.233.853.03

K(S1)

0.0040.00460.0039

0.00710.00380.00510.00650.0060.00620.00610.00570.006

0.00550.00520.0050.00520.00550.00630.00560.00610.00640.00580.00620.00580.00560.00680.00510.00580.00560.00530.00480.00450.00470.00440.00460.00280.00290.00260.00310.00250.00220.00170.0007

0.00330.00320.00330.00380.00380.00410.00220.00380.00420.0060.00470.00490.0050.00360.00430.00510.0050.00030.00020.00530.0090.00580.010.01730.01050.01230.01090.00730.00760.0069

0.01210.00770.01050.00930.0032

478


Table 2 (continued).

Sample

3R-1, 183R-1,693R-1.983R-1.1314R-1, 154R-1, 834R-2, 315R-2.515R-2, 935R-2, 1016R-l,206R-1,817R-1.878R-1, 1198R-2, 369R-2, 4410R-2, 812R-2.4913R-1,13915R-1.4115R-1,6315R-1.10717R-1,2718R-1.3918R-1.11219R-1.2019R-1.10019R-1,13119R-2, 1520R-1.4320R-l,9322R-1, 8323R-2, 4423R-2, 7724R-1.6024R-1, 69

Hole 830C1R-1,712R-1.712R-1.1162R-U454R-1.1234R-2.1116R-1.1137R-1.207R-1, 668R-1, 1258R-2, 59R-1, 139R-1.269R-1.8310R-1, 1510R-2, 2712R-2,6412R-2, 7412R-2, 111

Depth(mbsf)

67.968.468.76977.778.378.989.489.889.997.397.9

107.7117.7118.4128.1137.4157.3166.6185185.2185.7204.1213.8214.5223.3224.1224.4224.8233.3238.3253.2264.1264.5272.6272.7

235.7245.1245.6245.9264.9266.3284292.8293.3303.6303.9312312.2312.7321.7323.3343343.2343.5

Lithology

SiltSillSillSiltSiltSiltSiltSiltSiltSiltSiltst.Siltst.Siltst.Siltst.Siltst.Sandst.Sandst.Siltst.Siltst.Sandst.Sandst.Sandst.SiltSandst.BrecciaSandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.

Sandst.Sandst.Sandst.Sandst.Sandst.BrecciaBrecciaSandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.

NRM(A/m)

0.570.040.011.030.490.440.590.520.750.360.430.380.870.520.400.780.340.150.120.180.170.630.110.120.040.150.210.010.010.000.000.050.010.070.050.07

0.110.080.890.020.080.150.500.040.060.210.070.040.040.101.160.220.490.210.19

RM,0(A/m)

0.260.040.010.470.350.340.390.330.310.260.320.220.250.340.280.210.260.050.110.050.050.040.010.020.030.070.020.010.000.000.000.030.010.030.030.06

0.030.030.090.010.040.020.140.010.010.010.020.010.020.070.080.020.130.020.11

MDF(mT)

7352030202015155

2020125

15175

207

3055555

20105

1057

108

40103020

5453

1155355655

20155

57

IRM1000(A/m)

146.7160.8626.41

169.09143.22126.54166.96147.22157.26138.63170.32150.56153.50186.87243.43144.60128.77152.98127.5186.93

100.8065.9434.54

225.84136.57165.4393.2719.756.212.663.86

197.4168.3851.59

207.38220.21

154.95149.12452.45

39.3384.0339.00

267.4392.38

118.7387.5971.4787.6674.14

205.22212.68

93.72160.3357.86

209.43

ARM100(A/m)

3.712.831.223.233.382.993.393.192.882.594.333.102.896.374.252.682.962.912.972.152.172.682.268.014.087.135.301.380.330.330.467.791.652.415.685.09

3.604.989.031.454.051.837.184.446.71

10.323.776.104.474.785.134.832.242.119.15

K(S1)

0.00560.00140.00120.00790.005

0.00470.00770.00640.0080.00630.0070.00710.00870.00960.01480.00660.00470.00640.00440.00490.00540.00190.00090.00540.00470.00480.00230.00070.00030.00010.00020.00530.00210.00210.00490.0102

0.01090.00380.01440.00170.00360.00190.00510.00310.00570.00230.00280.00260.00220.00920.01180.00340.01030.00190.0065

134 samples by AF demagnetization at a peak field of 10 mT. Hence,both NRM intensity and intensity remaining after 10-mT (RM10) de-magnetization are presented.

Ten well lithified mini-core samples of siltstone and sandstone fromSites 829 and 830 were thermally demagnetized using a Schonstedtthermal demagnetizer (Model TSD-1) aboard ship. Samples wereheated at temperature intervals between 20°C and 50°C up to 500°C,and the remanence was measured between steps.

The manner in which isothermal remanent magnetization (IRM)and anhysteretic remanent magnetization (ARM) are acquired anddemagnetized also provides information on the type of magnetic car-riers and the coercivity spectrum of the sample. After AF demagneti-zation, the acquisition behavior of ARM was studied for a suite of 10pilot samples. The Schonstedt AF demagnetizer and a DTECH dou-ble-coil anhysteretic magnetizer were used to produce the ARM. Thealternating field was progressively increased from 0 to 100 mT in aDC-bias field of 0.1 mT. The ARM was then AF demagnetized so thedemagnetization behavior could be compared to that of NRM andIRM. Subsequently, an ARM (ARM100) was imparted to the 209samples in the study using an alternating field of 100 mT and a DC-

bias field of 0.1 mT. The samples were then AF demagnetized at peakfields of 95 or 100 mT prior to giving the samples an IRM.

The IRM acquisition behavior of the pilot samples was studied atTAMU. Impulse fields increasing from 0 to approximately LOT wereapplied along the -Z-axis of the sample using an ASC impulse mag-netizer (Model IM-10). The remanence was measured between stepsusing the spinner magnetometer. The coercivity of saturation rema-nence (HCR) of the samples was determined by applying an increasingreverse-field IRM (along the +Z-axis) to the samples. A second IRMwas then applied (at a saturating field of about 1.0 T) and demagnet-ized using the Schonstedt AF demagnetizer so that the demagnetiza-tion behavior of IRM could be compared to that of NRM and ARM.An IRM was then applied to all samples in the study at a field of about

LOT(IRMloOo).Anisotropy of magnetic susceptibility of several samples from

Hole 829A was measured by B. Ellwood at the University of Texas,Arlington, using a low-field torque magnetometer. All samples weretaken near or within shear zones. Magnetic fabric is the three-dimen-sional variation of magnetic properties within a sample. This variationis determined by measuring the magnetic property in several differentsample orientations, a procedure that reveals the anisotropy of mag-netic properties in the sample. The magnitudes and directions of theproperty being measured define an ellipse, the shape of which pro-vides information about the orientation of the magnetic minerals inthe rock (Hrouda, 1982). This type of analysis is useful for determin-ing orientation of foliation planes and degree of deformation (Hrouda,1982). The samples were not corrected for orientation: most samplescame from cores for which recovery was poor, so that structural andpaleomagnetic corrections could not be made. Nevertheless, fabricinformation can be obtained from the shape of the ellipsoid.

Either ratios or differences among the principal axes (maximum,intermediate, and minimum) of the susceptibility ellipsoid may beused as a measure of ellipsoid shape. In the following equations, KMAXis the maximum axis, KINT is the intermediate axis, KMIN is the min-imum axis of the anisotropy ellipsoid, and KAVG is {KMAX + KINT +KM1N)ß. The total anisotropy is defined by (KMAX - KMIN)/KAVG. Thedegree of anisotropy, P, is defined as the ratio KMAX/KMIN and de-scribes the amount of preferred orientation of magnetic minerals inthe sample. The q factor, defined as (KMAX - K[NT)/[(KMAX + KINT)/2 -(KMIN)], is used to measure the shape of the ellipsoid in sediments.For most sedimentary rocks, q ranges from about 0.06 to 0.67 (Tar-ling, 1983, and references therein). Increases in deformation of sedi-ment in rocks whose degree of anisotropy (P) is less than about 1.05have been correlated with increases in q (Hrouda, 1982). Magneticlineation (L) is (KMAX - KINT)/KAVG × 100%, and measures the degreeto which magnetic minerals parallel each other in a linear fashion,defining a prolate anisotropy ellipsoid. Magnetic foliation (F) is (KINT- KMIN)/KAVG × 100%, and determines the degree of planar parallelismof magnetic minerals, which describe an oblate anisotropy ellipsoid.

RESULTS

Hysteresis

Hysteresis data from samples from Sites 827 through 829 arepresented in Table 3 and Figure 3. The MRS/MS values all fall near theboundary between pseudo-single-domain and multidomain behavior.The shapes of Figures 3A, 3C, and 3D are typical of samples domi-nated by magnetite, whereas the shape of Figure 3B may result froma mixture of magnetite and hematite.

Susceptibility

The variation of susceptibility determined using the shipboardmultisensor track with depth and lithologic type in Holes 827B, 828 A,829A, 830A, and 830B is illustrated and described in the appropriatesite chapters of the Initial Reports volume (Collot, Greene, Stokking,et al, 1992) and in two chapters by Roperch et al. (this volume).

479


I 4Φ

Sample 828A-1H-1, 58-βOcm B field vs. moment

-2

C

i 1 • 1 r

Moment (emu)

15

-J 0

£ 5

Sample 829A-57R-1, 76-78cm B field vs moment

-5-2000

• Moment (emu)

2000 4000 6000B field (Oβ)

8000

B

10

8

6

4

2

0

-2

D

5

4

3

2

1

0

-1

Sample 829A-18R-2, 71 73cm B field vs moment

Moment (emu)

i

Sample 829A-43R-3, 78-80cm B field vs moment

• Moment (emu)

J i I u10000 -2000 2000 4000 6000 8000 10000

B field (Oe)

Figure 3. Hysteresis loops of representative samples. A. Sample 134-828A-1H-1,58-60 cm. B. Sample 134-829A-18R-2, 71-73 cm. C. Sample 134-829A-57R-1,76-78 cm. D. Sample 134-829A-43R-3, 78-80 cm.

Table 3. Hysteresis parameters.

Sample

134-827A-9H-1, 24-27 cm827A-12H-3, 73-75 cm828A-1H-1, 58-60 cm828A-3H-1, 62-64 cm829A-4R-3, 98-101 cm829A-18R-2, 71-73 cm829A-43R-3, 78-80 cm829A-51R-3, 24-26 cm829A-57R-1, 76-86 cm1. Volcanic silt2. Volcanic siltstone3. Ig-lithic breccia4. Volcanic sandstone

Lith.

111122344

MRSIMS

0.180.140.180.220.160.120.130.190.10

BCR

(mT)

372727273027152121

IRM I O O O(A/m)

79.8102.6112.7110.0103.2111.891.283.083.3

Note: Lith. = lithology, MRS/MS = saturation remanent magnetiza-tion/saturation magnetization, BCR = coercivity of remanence,I R M 1 0 0 0 = isothermal remanent magnetization at 1.0 T appliedfield.

Volume magnetic susceptibility of discrete samples of silt and silt-stone ranges from 0.0002 to 0.0249 S1, with an average of 0.0056S1. The susceptibility of sand and sandstone averages 0.0052 S1 andvaries from 0.0001 to 0.0173 S1, whereas that of breccias and con-glomerates averaging 0.0022 S1 ranges from 0.0001 to 0.0051 S1.Susceptibility of discrete samples of calcareous oozes and chalksaverages 0.0013 S1 and ranges from 0.0007 to 0.0064 S1, and that ofmixed sediment samples varies from 0.0024 to 0.0036 S1 with an

average of 0.0029 S1 (Table 2). The downhole variation of magneticsusceptibility in Holes 827B, 829A, 829C, 830A, and 830B is illus-trated in Figure 4.

NRM

Intensity of NRM ranges from 0.010 to 1.964 A/m (average 0.52A/m) in the silts and siltstones, from 0.12 to 1.794 A/m (average 0.16A/m) in the sands and sandstones, from 0.0005 to 0.93 A/m in thebreccias and conglomerates, from 0.003 to 0.24 A/m (average 0.06A/m) in the calcareous oozes and chalks, and from 0.02 to 0.50 A/m(0.20 A/m) in mixed sediment (Table 2). The NRM of all samples inthis study most probably includes a depositional remanence (DRM),possibly a post-depositional remanence (pDRM), and a drilling-induced remanence.

Demagnetization Behavior

Remanent intensity after AF demagnetization at 10 mT (RM !0)varies from 0.003 to 0.471 A/m (average 0.16 A/m) in the silts andsiltstones, from 0.001 to 0.366 A/m (average 0.04 A/m) in the sandsand sandstones, from 0.0005 to 0.14 A/m (average 0.06 A/m) in thebreccias and conglomerates, from 0.005 to 0.083 A/m in the calcare-ous oozes and chalks, and from 0.014 to 0.102 A/m (average 0.06A/m) in mixed sediment (Table 2). Vector endpoint diagrams illus-trating the AF demagnetization behavior of samples from the pilotstudy are presented in Figure 5. The steep, negative, drilling-inducedcomponent is evident in all samples and is significantly reduced by

480


demagnetization at 10 mT. The downhole variation of RM j 0 in Holes827B, 829A, 829C, 830A, and 83OB is illustrated in Figure 6.

The median destructive field (MDF) is the peak demagnetizingfield at which half the NRM has been removed. MDF values forsamples in this study are provided in Table 2. Although these dataprovide information about the relative coercivities of the samples, thevalues should be interpreted with caution because of the drilling-induced component of the NRM. Nevertheless, the remanence ofmost samples is demagnetized at relatively low peak fields, consistentwith the dominant carrier of remanence being titanomagnetite, witha relatively large multidomain component.

The results of thermal demagnetization of samples of well lithi-fied siltstone from Core 134-829A-56R are illustrated in Figure 7.The remanence of all samples measured was unblocked by heatingat temperatures between 450°C and 500°C, consistent with eithertitanomagnetite, magnetite, or hematite as the carrier of remanence.

Laboratory-induced Remanences

Figures 8 and 9 illustrate IRM and ARM acquisition behavior andremanence decay after AF demagnetization of typical samples fromthe pilot study. The shapes of the acquisition curves are typical oftitanomagnetite. The IRM acquisition curves rise sharply, then flatten,indicating that little, if any, hematite is present. In most samples,saturation is achieved at an impulse field of approximately 0.1 T.

IRM1 0 0 0 ranges from 5.62 to 243.4 A/m (average 125. 58 A/m) insilts and siltstones, from 2.66 to 452.4 A/m (average 121.65 A/m) inthe sands and sandstones, from 0.50 to 267.4 A/m (average 68.27A/m) in the breccias and conglomerates, from 5.68 to 70.19 A/m(average 25.11 A/m) in the calcareous oozes and chalks, and from58.56 to 138.5 A/m (average 87.24 A/m) in mixed sediment (Table2). Especially in the silts, siltstones, sands, and sandstones, IRM1 0 0 0intensities are quite high, reflecting the abundance of titanomagnetitein the samples. ARM100 ranges from 0.16 to 6.37 A/m (average 3.05A/m) in silts and siltstones, from 0.33 tolθ.32 A/m (average 3.97 A/m)in the sands and sandstones, from 0.03 to 7.18 A/m (average 2.26A/m) in the breccias and conglomerates, from 0.34 to 1.98 A/m(average 0.89 A/m) in the calcareous oozes and chalks, and from 1.53to 3.15 A/m (average 2.03 A/m) in mixed sediment (Table 2). TheARM100 and IRM1 0 0 0 of the pilot samples were demagnetized so thatthe stability of ARM and IRM could be compared to that of NRM.

Magnetic Fabric

Results of measurements of anisotropy of magnetic susceptibilityfrom deformed intervals in Hole 829A are listed in Table 4, and acomparison of lineation and foliation is presented in Figure 10. Plate2 contains photomicrographs of samples of breccia (Samplel34-829A-43R-1, 144-146 cm) and brown clay (Sample 134-829A-47R-1, 57-59 cm) from shear zones.

DISCUSSION

Magnetic Mineralogy

Preliminary scanning electron and energy-dispersive x-ray micro-analyses of magnetic mineral separates from typical samples of vol-canic silt from Holes 827A and 829A are illustrated in Plate 3. SEMand qualitative X-ray analysis indicate that titanomagnetite, relativelylow in titanium, is the dominant carrier of remanence (Pis. 2 and 3).The shapes of the IRM and ARM acquisition curves and the behaviorduring AF and thermal demagnetization are consistent with titano-magnetite as the predominant magnetic mineral.

Particle Size

Rock magnetic data can be used to constrain the size of magneticparticles in the samples. For equidimensional grains, pure magnetite

Hole 827B

K(SI)0.002 0.003 0.004 0.005

140

160

180

£ 200

Q.

Q 220

240

260

MIA

1MB

•MIC

Hole 829C

K(SI)

0 0.005 0.01 0.015

0

100

200

300

400

500

600

Hole 829A

K(SI)

0.01 0.02 0.031 1 '

-

- #

• . i

1

l l _

III -

I V -

v -VVI

VIII-XI

XIII

xvr

Holes 830A and 830B

K(SI)) 0.01 0.02 0.03

300

Figure 4. Downhole variation of magnetic susceptibility (K) in Holes 827B,

829A, 829C, 830A, and 830B.

is superparamagnetic when less than about 0.03 µm in diameter,single domain from 0.03 to 0.08 µm in diameter, and typically exhibitsmultidomain at diameters greater than 0.2 µm (Butler and Banerjee,1975). Particles larger than typical single-domain grains that showsingle-domain behavior are considered pseudo-single domain. Thediameters at which transitions between stable domain states occurdepend upon the composition of the particle, however, and vary withthe concentration of cations in solid solution with Fe, particularlyTi, but also Al, Mg, and Mn. Considering only the influence of Ti,titanomagnetite containing 60% ulvospinel (Fe2Ti04) does not showsingle domain behavior until its diameter has increased to 0.08 µm(Dunlop, 1981); the transition from single domain to multidomainbehavior theoretically should occur at 0.2 µm (Butler and Banerjee,1975) and has been observed to occur at about 0.6 µm (Soffel, 1971).In addition, domain behavior is affected by the elongation of theparticle (Butler and Banerjee, 1975), the structure of the grain,oxidation, or microcracks (Haggerty, 1970; Johnson and Hall, 1978;Henshaw and Merrill, 1980) that may produce regions within a largegrain that act as single domains.

The ratio of hysteresis parameters MRSIMS has been used to dis-tinguish between single-domain and multidomain behaviors: valuesgreater than 0.5 indicate single-domain particles, ratios between about0.1 and 0.5 represent pseudo-single domain particles, and values lessthan about 0.1 indicate multidomain or superparamagnetic particles(StonerandWohlfarth, 1948; Day et al., 1977; Thompson and Oldfield,

481


NRM

40

Scale: i.β IA/m

40

9 N R M NRM

Scale: 1.β-1 A/m

E40

Scale: 1e-1 A/m Scale: 1.e-1 A/m

oNRM

10

V 20

Scale: 1.e-1 A/m

Figure 5. Vector endpoint diagrams illustrating the results of AF demagnetization of NRM of representative samples. Solid circles represent the horizontal

component of remanence and open circles represent the vertical component. NRM, RMi0, final, and some additional peak demagnetizing fields (mT) are indicated.

A. Volcanic silt (Sample 134-829C-5H-2, 31 cm). B. Volcanic siltstone (Sample 134-829A-17R-2, 70 cm). C. Breccia (Sample 134-830C-6R-1, 113 cm). D.

Chalk (Sample 134-829A-44R-2, 57 cm). E. Mixed sediment (Sample 134-827B-9R-6, 2 cm).

Hole 827B

RM 10 (A/m)0 0.05 0.1 0.15

140

Hole 829A

RM 10

0.1 0.2 0.3

Table 4. Anisotropy data. Acronyms and abbreviationsare defined in the text.

160

c 180< Λ

E200

220

240

260

MIA

1MB'

inc.1

•IV-

VI

Vlll-X

• ,xv

0

100

200

300

400

500

600

o (

50

100

150

200

250

300

Figure 6. Downhole variation of RM 1 0 in Holes 827B, 829A, 829C, and 830A

and 830B.

Hole 829C

RM 100.1 0.2 0.3

Hole 830A and 830B

RM10 (A/m)0.2 0.4 0.6

0

10

ST 20

17

•1

i • •

• * ••t•

i

IAIB

-

IC

, l l ~

Sample

134-829A-12R-1.2412R-1.5543R-1.2043R-1, 13943R-2, 2743R-2, 6947R-1, 5147R-1.5751R-1,851R-1.5951R-2, 13058R-1, 132

Depth(mbsf)

99.2499.55

397.27399.09399.47399.89427.11427.17460.18460.69462.90524.52

P

1.011.061.111.041.061.051.041.081.061.061.051.22

0.760.610.850.610.871.260.590.630.440.190.570.38

F(%)

0.553.324.362.072.241.042.194.123.344.612.7313.2

L(%)

0.682.936.431.793.423.521.823.751.910.982.26.11

Aniso.(%)

1.236.24

10.793.865.664.564.017.875.35.594.92

19.27

Note: Aniso. = total anisotropy.

1986). Although domain state is not a direct measure of particle size,the ratios may provide some indication of relative changes in domainbehavior that reflect changes in particle size. Thus, on the basis ofhysteresis parameters, the samples analyzed in this study fall near theboundary between pseudo-single domain and multidomain behavior.

Differences in the stabilities to AF demagnetization of natural andartificial remanences carried by magnetite are governed by particlesize, the relative proportions of low- and high-coercivity particles, thedegree of particle alignment, and the interplay between internal stresseswithin a particle and magnetostatic interactions between pinned andunpinned domain walls (Heider et al., 1992; Moon and Merrill, 1986;Stacey and Banerjee, 1974; Xu and Merrill, 1990; Xu and Dunlop,1993). Lowrie and Fuller (1971) demonstrated that the relative stabili-ties to AF demagnetization of IRM and thermoremanent magnetization(TRM) may be used to evaluate the domain state that predominatesin the primary carrier of remanence in a sample, assuming that theprimary magnetization is carried by magnetite. For single-domain par-ticles, TRM is more stable than IRM, whereas for multidomain parti-cles, IRM is more stable than TRM. The stability of ARM to AFdemagnetization is similar to that of TRM (Johnson et al., 1975; Leviand Merrill, 1976), which enabled the Lowrie-Fuller test to be modifiedto compare stabilities of ARM and IRM, thereby avoiding problemsassociated with mineralogical changes caused by heating the samples.

4S2


f - Y

Sample 829C-5H-2, 31 cm

50°C

2 - 100

Sample 134-829A-56R-5, 10-12 cm

- 0.1 A/m

Sample 134-829A-56R-4, 122-124 cm

Figure 7. Thermal demagnetization behavior of volcanic siltstone samples

from Hole 829A. Symbols as in Figure 5, temperature steps in °C.

The validity of ARM as an analog to TRM may, however, depend onthe concentration of the magnetic material (Sugiura, 1979).

Recent studies question interpretations of the Lowrie-Fuller test:the stability of multidomain particles depends on an interplay betweenthe internal stresses that act to pin domain walls and a screening effectin which magnetostatic interactions between unpinned and pinneddomain walls increases the particle's resistance to AF demagnetiza-tion (Moon and Merrill, 1986; Xu and Merrill, 1990; Heider et al.,1992; Xu and Dunlop, 1993). In grains with high internal stress, suchas the crushed magnetite particles used by Lowrie and Fuller (1971),the effect of screening is small, and stability to AF demagnetizationdepends mostly on microcoercivity, and hence, particle size. Stabilityto AF demagnetization of particles whose internal stress is low,however, is controlled by screening, and thus shows less size depend-ence (Xu and Dunlop, 1993).

Experiments in which the Lowrie-Fuller test was modified by sub-stituting ARM for TRM were not designed to simulate DRM, and didnot take into account the effects of concentration or particle align-ment on stability against AF demagnetization. Nonetheless, the modi-fied Lowrie-Fuller test has been applied to marine sediments. Lowrie-Fuller test results were consistent with particle sizes determinedusing transmission electron microscopy when the test was applied todeep-sea sediments that contained single-domain biogenic and multi-

-0.2-200

B

-20

1.2

1.0 "

1 0.6

£ 0.4

200 400 600 800 1000 1200Impulse field (mT)

1.2

inte

nsity

OB

O

5 0.6

11 0.4o

2 0.2

0.0

1 1 '

-

-

1 ' 1

•

t i l l

1 ' 1 '

-

1 1 1 1

20 40 60ARM field (mT)

80 100

1 1 •

•D

D

•

i

O

O

-

, 1 .

1

]

U

»

•

I

1

D

D

#Oi

1

α

s1

1 1 •

D

i ,

i

α0

•

i

• i

IRMARMNRM

-

-

•

i I i

o.o -

-0.2-20 0 20 40 60 80 100 120

AF demagnetizing field (mT)Figure 8. Rock magnetic behavior of volcanic silt (Sample 134-829C-5H-2,31 cm). A. IRM acquisition. B. ARM acquisition. C. Intensity decay, AFdemagnetization of NRM, IRM 1 0 0 0, and ARM1 0 0.

domain lithogenic magnetite (Petersen et al., 1986) and to carbonatesthat contained single-domain biogenic magnetite (McNeill, 1990).Apparent particle sizes indicated by the hysteresis parameters differedfrom those implied by the Lowrie-Fuller test, however, for suboxicsediments that probably contained magnetite from a variety of sources(Karlin, 1990). The test is probably most useful in sediments that

483


.ε

1.2

1.0

0.8

0.6

0.4

0.2

0.0

0 •>

Sample 829A-44R-2, 57 cm• i > i - i i

•---

i i i i i i i i

i i i i i

• ----i i i i i

B

-200 0 200 400 600 800 1000 1200Impulse field (mT)

1.2

1.0

ëo.β&_ç2 0.6<"8 0.413

3§ 0.2oz

0.0

-0.2

'

---

•

••

•

i 1 i 1 i 1

i I i | i

Φ

*

-

-

-

-

-

-

i 1 i 1 i

-20

-20

20 40 60ARM field (mT)

80 100

1.2

1.0

£0.8

§.1 0.6"SΦ

iθ.2

0.0

-0.2

1 l

•-

-

-

-i i

1 l

π

D

• D0 D

i t i

1

OV

1

1 1

D

i i

1

•

D0

1 •

IRM -ARM -NRM -

-

-

-

U D-

i

1000 20 40 60 80AF demagnetizing field (mT)

Figure 9. Rock magnetic behavior of chalk (Sample 134-829A-44R-2,57 cm).

A. IRM acquisition. B. ARM acquisition. C. Intensity decay, AF demagneti-

zation of NRM, IRM 1 0 0 0, and ARM1 0 0.

contain uniformly stressed magnetic particles whose stability is gov-erned by microcoercivity rather than screening, and in which magne-tostatic interactions between aligned particles are not significant.

In most Leg 134 samples, the stabilities of IRM and ARM to AFdemagnetization are similar: ARM is slightly more stable to AF demag-netization than IRM, and both are much more stable than the NRM(Figs. 8C and 9C). A simple application of the Lowrie-Fuller test to

43 R#

-

#43R /

// 47R^3R // 2 R

/ 51R 1R/ 43R

051R

#58R

Figure 10. Hole 829A. Lineation vs. foliation.

these results would suggest that the samples contain single-domainmaterial. Alternatively, the samples may contain multidomain particleswhose stability against AF demagnetization is increased by magneticscreening or particles with a wide range of internal stresses. Consideringthat hysteresis data implied the presence of multidomain material, thesamples may well contain mixtures of particle sizes and domain states.

Ratios of IRMi000 (or ARM100) to susceptibility reflect changes ingrain size of magnetic particles (Banerjee et al., 1981; King et al., 1982;Bloemendal et al, 1989; Sager and Hall, 1990). Figure 11 shows thedownhole variation of IRM1000/ÅT and Figure 12 is a plot of log Kvs. log IRM1000. Susceptibility reflects the concentration of magneticminerals, whereas IRM1000 and ARM100 are controlled both by theconcentration and particle size of magnetic minerals. If grain size isthe same, susceptibility can be used to normalize out concentrationvariations. In Figures 11 and 12, a large proportion of fine-grainedparticles results in high ratios, whereas a preponderance of coarse-grained particles produces low ratios. The presence of superparamag-netic material will increase susceptibility, resulting in low ratios thatdo not reflect the presence of coarse-grained material (Thompson andOldfield, 1986). This tests elucidates relative changes in particle sizethat correspond to sedimentological differences discussed below.

Correlation between Magnetic Propertiesand Lithostratigraphy

Figure 4 shows the downhole variation of magnetic susceptibilitymeasured in discrete samples from Holes 827B, 829A, 829C, and830A and 830B. Susceptibility peaks in all plots correspond to inter-vals of volcanic silt, siltstone, sand, and sandstone, and thus indicateincreases in the contribution of volcanogenic material, either derivedby erosion of arc rocks or from a volcanic eruption. Oozes, chalks,and calcareous breccias are marked by low susceptibilities. Intensitiesof NRM and RM10 (Collot, Greene, Stokking, et al., 1992; and Figure6) also reflect the abundance of titanomagnetite. Both display trendssimilar to the susceptibility.

In Hole 827B, Unit III contains highly bioturbated, partially lith-ified calcareous volcanic siltstone with intervals of conglomerate.Increases in susceptibility and intensity of remanence (Figs. 4 and 6)correspond to decreases in carbonate percentage (Collot, Greene,Stokking, et al., 1992). The IRM1000/£ratio increases in Subunits IIIAand UIC, suggesting a fining upward of the relatively dense titano-magnetite grains which was not observed in sedimentological de-scriptions of the cores.

Units in Hole 829A that contain abundant volcanogenic mineralsare characterized by high magnetic susceptibilities and intensities, as

484


|

20140

160

180

200

220

240

260

10

Hole 827B

SIRM/k30 40 50

Hole 829A

S1 RM/k20 40 60

•

-•

, 1

•

••, 1

-

-

1IIA

IIIB"

-

MIC-

Hole 829C

S1 RM/k20 30

0

100

200

300

400

500

600Hole 830A and Hole 830B

SIRM/k0 10 20 30 40 50

V-VL

VIN—XL

XV

ß xvi

60

Figure 11. Downhole variation of IRM1000/tf in Holes 827B, 829A, 829C, and830Aand830B.

expected. In Unit III, interbeds of volcanic sand are common, and anincrease in grain size in the lower 15 m of the unit is reflected by adecrease in IRM1000/ÅT. Differences in magnetic susceptibility andintensity in Unit XIII, a volcanic sandstone, reflect differences in theamount of carbonate: intervals low in carbonate correspond to highmagnetic susceptibility and intensity. Unit XV, a partially lithifiedsandy volcanic siltstone with chalk clasts, displays a wide range ofsusceptibility and intensity values as well as varying IRM QOQ/Kratios.

Cores from Hole 829C are described as structureless clayey sandyvolcanic silt with some poorly defined ash layers (Collot, Greene,Stokking, et al., 1992). Trends in magnetic susceptibility and intensityare similar: both suggest a gradational variation in the contribution ofvolcanogenic material. Peaks correspond to intervals described ascontaining ash layers. IRM1000/X" ratios tend to increase downhole,which corresponds to the observed decrease in grain size from clayeysandy volcanic silt to clayey volcanic silt (Collot, Greene, Stokking,et al., 1992). Lower IRMIOOO/K ratios near the top of Unit XVIII inHole 829C correspond to clay-rich intervals; higher ratios near thebase of the unit reflect increases in the amount of sand. Fluctuationsin the IRM1000/K ratio reflect the variable amounts of clay and sandreported by shipboard sedimentologists.

Two major lithostratigraphic units were defined and describedat Site 830 (Collot, Greene, Stokking, et al, 1992; Table 1). Litho-stratigraphic Unit I is a dark-gray volcanic silt and siltstone that issubdivided into three subunits based primarily on the presence and

-2.40

-2.45 -

-1.5

-3..2 1.4 1.6 1.8 2.0 2.2 2.4

log SIRM0.0 0.5 1.0 1.5 2.0 2.5

log SIRMFigure 12. Log K vs. log IRM1Ooo

character of sandy interbeds. Subunit IA consists of nearly structure-less unlithified silt and contains ash layers 1 to 5 cm thick. Carbonateis more abundant in the upper part of the subunit. The decrease incarbonate downhole is reflected by an increase in magnetic suscepti-bility and intensity. A decrease in IRM1000/^ near the base of Sub-unit IA corresponds to an increase in the frequency of sandy layers.Subunit IB is clayey sandy silt with several normally graded interbedsof black sand and an increased proportion of volcanogenic minerals,notably Plagioclase, clinopyroxene and opaque minerals. Subunit ICis a sequence of interbedded sandy silt and sand that contains lessclinopyroxene and opaque minerals than Subunit IB. Variations inIRM1000//C also reflect grading observed in Subunits IB and IC. Anincrease in the contribution of volcanogenic sediment is reflected byan increase in magnetic susceptibility and intensity in Subunit IB. UnitII comprises partially lithified, very poorly sorted, very coarse, vol-caniclastic, silty sandstones. The sand grains in the sandstone areparticles of well-lithified volcanic siltstone. The substantial scatter inall rock magnetic properties from Unit II samples results from thewide range of particle sizes observed in the unit.

Samples from Hole 829A define a fairly linear trend in a plot oflog K vs. log IRM1000, although some scatter is apparent. Samplesfrom Holes 829C and 830A and B lie along linear trends in log K vs.log IRM1000 plots.

Fabric: AMS and SEM Observations

Anisotropy parameters listed in Table 4 vary widely, even betweenvery closely spaced samples. Figure 10 is a plot of lineation (L) vs.foliation (F). The straight line drawn on the plot indicates a slope of

485


unity and forms the boundary between ellipsoids that are predomi-nantly prolate and those that are oblate (Hrouda, 1982). The magneticfabric of Samples 134-829A-12R-1, 24 cm, -43R-2, 27 cm, and-43R-2, 69 cm, is thus prolate and dominated by lineation, whereasthat of the remaining samples is oblate, and dominated by foliation,as is the structural fabric (Meschede and Pelletier, this volume).Scatter of anisotropy parameters was also observed in sediments fromabove the décollement in the Nankai Trough, and was attributed tostrain inhomogeneity; below the décollement at Nankai, magneticfabric was predominantly oblate (Owens, 1993).

Even in intensely deformed sediments, SEM observations revealregions within individual beds in which deformation and parallelorientation of clay particles is not as apparent at high magnifications(e.g., 2000X) as it is at low magnifications (


Levi, S., and Merrill, R.T., 1976. Acomparison of ARM and TRM in magnetite.Earth Planet. Sci. Lett., 32:171-184.

Lowrie, W., and Fuller, M., 1971. On the alternating field demagnetizationcharacteristics of multidomain thermoremanent magnetization in magnet-ite. J. Geophys. Res., 76:6339-6349.

Mallick, D.I.J., and Greenbaum, D., 1977. Geology of Southern Santo. Reg.Rep.—New Hebrides Condominium Geol. Surv.

McNeill, D.F., 1990. Biogenic magnetite from surface Holocene carbonatesediments, Great Bahama Bank. /. Geophys. Res., 95:4363^371.

Mitchell, A.H.G., and Warden, A J., 1971. Geological evolution of the NewHebrides island arc. J. Geol. Soc. London, 127:501-529.

Moon, T, and Merrill, R.T., 1986. Magnetic screening in multidomain mate-rial. J. Geomagn. Geoelectr., 38:883-894.

O'Reilly, W., 1984. Rock and Mineral Magnetism: New York (Chapman andHall).

Owens, W.H., 1993. Magnetic fabric studies of samples from Hole 808C,Nankai Trough. In Hill, LA., Taira, A., Firth, J.V., et al., Proc. ODP, Sci.Results, 131: College Station, TX (Ocean Drilling Program), 301-310.

Petersen, N., von Dobeneck, T, and Vali, H., 1986. Fossil bacterial magnetitein deep-sea sediments from the South Atlantic Ocean. Nature, 320:611-615.

Sager, W.W., and Hall, S.A., 1990. Magnetic properties of black mud turbiditesfrom ODP Leg 116, Distal Bengal Fan, Indian Ocean. In Cochran, J.R.,Stow, D.A.V., et al, Proc. ODP, Sci. Results, 116: College Station, TX(Ocean Drilling Program), 317-336.

Soffel, H.C., 1971. The single-domain/multidomain transition in intermediatetitanomagnetites. J. Geophys., 37:451^-70.

Stacey, F.D., and Banerjee, S.K., 1974. Developments in Solid Earth Geophys-ics (Vol. 5): The Physical Principles of Rock Magnetism: Elsevier (NewYork).

Stoner, E.C., and Wohlfarth, E.P., 1948. A mechanism of magnetic hysteresisin heterogeneous alloys. Philos. Trans. R. Soc. London A, 240:599.

Sugiura, N., 1979. ARM, TRM and magnetic interactions: concentrationdependence. Earth Planet. Sci. Lett., 42:451^455.

Tarling, D.H., 1983. Paleomagnetism, Principles and Applications in Geol-ogy, Geophysics and Archaeology: London (Chapman and Hall).

Thompson, R., and Oldfield, F, 1986. Environmental Magnetism: London(Allen and Unwin).

Xu, S., and Dunlop, D.J., 1993. Theory of alternating field demagnetization ofmultidomain grains implications for the origin of pseudo-single domainremanence. J. Geophys. Res., 98:4183-4190.

Xu, S., and Merrill, R.T., 1990. Toward a better understanding of magneticscreening in multidomain grains. J. Geomagn. Geoelectr., 42:637-652.

Date of initial receipt: 14 May 1992Date of acceptance: 2 September 1993Ms 134SR-026

487


Acm

7 0 -

7 5 -

8 0 -

Ccm

65 -

Bcm

25 -

3 0 -

35 -

70 -

7 5 -

80 -J

Dcm

2 0 -

2 5 -

3 0 -

Plate 1. Core photographs illustrating typical sediments and rocks from Leg 134. A. Volcanicsiltstone with ash layer (Interval 134-830A-3H-3, 70-81 cm). B. Deformed breccia (Interval134-829A-43R-1,22-39 cm). C. Volcanic sandstone (Interval 134-830C-7R-1,63-80 cm). D. Chalk(Interval 134-829A-51R-1, 18-33 cm).


Plate 2. SEM photomicrographs illustrating the fabric of shear-zone sam-ples. A. Sample 134-829A-43R-1,144-146cm, describedinCollot, Greene,Stokking, et al. (1992) as a highly deformed breccia. The scale bar is 1 mm.B. Region in center of right flank of fold in upper-right corner of A. Scale baris 10 µm. C. Sample 134-829A-47R-1, 55-57 cm, a brown volcanic clay inwhich wavy lamina were observed (Collot, Greene, Stokking, et al., 1992).The scale bar is 100 µm.


Sample 829A-6R-4,110 cm Sample 829A-6R-4,110 cm

Plate 3. SEM photomicrographs illustrating titanomagnetites separated from volcanic silts. A, B. Sample 134-827A-9H-1, 118 cm. C, D. Sample 134-829 A-6R-4, 110 cm.

490

27. ROCK MAGNETIC PROPERTIES, MAGNETIC MINERALOGY, AND MAGNETIC FABRIC OF ROCKS … · 2006. 11....

Documents

Transcript of 27. ROCK MAGNETIC PROPERTIES, MAGNETIC MINERALOGY, AND MAGNETIC FABRIC OF ROCKS … · 2006. 11....