28. VRM STUDIES IN LEG 37 IGNEOUS ROCKS1

D. V. Kent, Lamont-Doherty Geological Observatory, Columbia University, Palisades, New Yorkand

W. Lowrie, Institüt für Geophysik, ETH—Hönggerberg, CH-8049 Zurich, Switzerland

ABSTRACT

A representative set of igneous rock samples from Hole 332B andSites 334 and 335 were studied to determine their ability to acquireviscous remanence (VRM). The results for samples from Sites 334and 335 indicate that VRM cannot be considered to be a serioussecondary component in the remanence of these rocks; thesesamples have stable magnetizations characterized by high mediandestructive fields (MDF). The ability to develop VRM is quitevariable in samples from Hole 332B. In high MDF samples, thedeveloped VRM is of low intensity, but in samples with low MDFand VRM can account for a large portion of the measured NRM in-tensity. The quantities VRM/NRM and MDF were approximatelyinversely proportional for samples from the three holes studied here.

INTRODUCTION

Some magnetic properties, including the acquisitionof viscous remanent magnetization (VRM), werestudied in a limited number (16) of Leg 37 igneous rocksamples. Ten samples were available from Hole 332B,two samples from Site 334, and four samples from Site335. Most samples are of basaltic rock although at leastone sample (from Site 334) is of ultramafic composi-tion.

REMANENT MAGNETIC PROPERTIESThe NRM intensity and direction of each sample

were measured with a Digico complete results spinnermagnetometer, and the weak-field susceptibilities weremeasured with an a.c. bridge (Figure 1). The geometricmean NRM intensity is 2.7 × lO~3 Gauss and the meansusceptibility is 6.9 × 10~4 G/oe, similar to the meanvalues of 2.1 X lO~3 Gauss and 6.3 × 10~4 G/oe,respectively, reported for 26 DSDP sites by Lowrie(1974). The Q'n ratios are (with one exception at Site334) greater than unity (Chapters 2, 4, and 5, thisvolume).

The remanence of each sample was progressivelydemagnetized in alternating fields to at least 400 oe. Ingeneral, the remanent directions were very stable dur-ing AF demagnetization (Figure 2), although in somesamples the data quality deteriorated in fields greaterthan 800 oe, due probably to the acquisition ofanhysteretic remanence (ARM) components. Fromeach demagnetization curve the median destructivefield (MDF) was determined, as well as the stable direc-tion. In the site report chapters the declinations and in-clinations of NRM and the stable direction are com-pared, although the declinations themselves have no

'Institüt für Geophysik Contribution No. 118.

absolute meaning because of the absence of azimuthorientation.

The NRM intensities and inclinations are verysimilar to those measured in nearby samples onboardship (Table 1 and Ade-Hall, 1974; personal com-munication). Both sets of paleomagnetic data indicatethe occurrence of both normal and reversed polarityzones within basement rocks penetrated by Hole 332Band Site 334; the four samples from Site 335 have uni-form negative inclinations, slightly steeper than the ex-pected axial dipole field inclination for the site lati-tude.

THERMOMAGNETIC ANALYSISThin slices from each sample were ground to a fine

powder for Curie temperature studies. These ex-periments were carried out in air with a vertical motionCurie balance, the field (4 koe) being switched off fre-quently to monitor possible weight loss in thespecimens. The thermomagnetic curves were mostlyirreversible (Figure 3a) and have the general charactertypical of oceanic basalts in which titanomagnetites arefrozen in a metastable condition by rapid chilling(Ozima and Ozima, 1971). The Curie temperature (θt)of the initial mineral (titanomagnetite) is lower thanthat of the exsolved magnetite (0/), and the initial roomtemperature magnetization (Jt) is less than the finalroom temperatures magnetization (//), so that (Ji/Jf) istypically less than unity (Table 2).

In two samples (332B-45-1, 104 cm and 334-26-1, 140cm) the curves were almost reversible, apart from somehigh temperature oxidation, and show none of theirreversibility associated with titanomagnetite phaseseparation. The Curie temperatures of these samplesare similar to the values of 0/ for the other samples, cor-responding to a magnetite containing a slight amountof titanium. Sample 332B-45-1, 104 cm is from a com-

525

D. V. KENT, W. LOWRIE

10"°:

. <é//

-

i '

A ©

AA

// ©

/

Θ

Θ

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o Θ

y

y

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A

o

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332B334335

<é//

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V/ .

-310 ' 10SUSCEPTIBILITY

(GAUSS/oe)

Figure 1. NRM intensities and susceptibilities of the basaltsamples.

plex brecciated unit of basalt near the base of Hole332B whereas Sample 334-26-1, 140 cm is from an ul-tramafic unit (near the base of Site 334) containing inpart serpentinized peridotite. We infer that the oc-currence of magnetite in these samples was due to eitherexsolution of titanomagnetite carried to completion insitu or to secondary formation of magnetite, particular-ly in the serpentinized peridotite (Saad, 1969). Thelatter explanation can help to account for the mixedremanent polarities encountered at Site 334 if thesecondary magnetite formed and acquired a chemicalremanent magnetization in a magnetic field differentfrom that recorded by the overlying basaltic rocks.

In 10 of the 16 thermomagnetic curves, an initial in-crease in magnetization was observed (e.g., Figure 3a,

Table 2), as also reported by Ade-Hall (1974) in someLeg 26 basalts and attributed to a possible lowtemperature chemical change. In two specimens in-complete exsolution was observed; in 332B-9-2, 92 cm a

N,-V

4

3r>Z)c

2

1

SITE 332BSAMPLE 6 - 1 , 100

_

NRM ^ \ ^

^ \

i 1 i

<pNRM100©

\

200

i , 1

©\

300©

400 © y

500©

W,H

( 10~ 3 GAUSS)

Figure 2. Vector diagram showing stability against AF de-magnetization of the declination (D), defined by N andW (open dots), and the inclination (I), defined by V andH (solid dots) in DSDP Leg 37 oceanic basalt. The num-bers indicate the AF intensity in peak oersted.

TABLE 1Comparison of NRM Intensities and Inclinations Measured in

the Laboratory With Shipboard Measurements in Nearby Samples

Laboratory Shipboard

Sample(Interval in cm)

Intensity(10 Gauss) Inclination

Approx.Subbasement

Depth (m)Sample

(Interval in cm)Intensity

(10~4 Gauss) Inclination

Hole 332B

2-5, 1136-1, 1009-2, 9211-1, 11014-2, 8417-1,622-4, 4827-2, 11233-1, 12745-1, 104

Site 334

9-2, 7026-1, 140

Site 3356-5, 339-4,5810-3, 7613-2, 13

9.0550.912.519.27.02

36.838.615.311.463.4

26.217.8

41.324.331.127.1

-11-26

204048

-31-818

-43-4

65-23

-63-61-65-70

18144192220249276328373429543

40106

10374572

Hole 332B2-5, 1066-1,779-2, 10411-1, 3914-2, 8118-1,4322-4, 4027-2, 433-1, 10045-1, 104

Site 334

17-1, 1726-1, 20

Site 3356-3, 1089-5, 9410-3, 7613-2, 13

7.6243.923.319.813.153.038.635.0

8.274.3

18.521.4

64.127.443.215.1

13-38

392642

-29-5

-11-30

-2

41-77

-69-71-60-81

526

VRM STUDIES OF IGNEOUS ROCKS

SITE 332B

SAMPLE 6 - 1 , 100

TABLE 2Curie Temperature Analysis

300 400

TEMPERATURE ( °C)

Figure 3. (a) Typical irreversible thermomagnetic curvemeasured in air in 4000 oe, showing the slight initial risein magnetization (A), the initial and final magnetizations(Jj and Jf, respectively), and the initial (öj) and final (of)Curie temperatures, (b) Thermomagnetic curve of samplein which exsolution after thermal cycling was incomplete.

Curie temperature in the cooling curve was visible nearthe Curie temperature of the initial mineral which wasalso comparatively low (210°C). In Sample 332B-11-1,110 cm, the exsolution was so incomplete that amagnetite Curie point could not reasonably be deter-mined (Figure 3b). The initial Curie point (160°C) islower than any of the others, which range from 212°Cto 570°C (Table 2).

Thermomagnetic curves in vacuum permit a descrip-tion of the state of oxidation of titanomagnetites(Ozima and Ozima, 1971). It is possible to use thermo-magnetic curves in air for this same purpose althoughthis method is not unambiguous (Lowrie et al., 1973).Assuming at first stoichiometric titanomagnetites, thecomposition of the initial and final minerals may bedetermined from their Curie points, 0, and θf. Ap-propriate initial and final magnetizations may becalculated and the expected ratio (J,/Jf) determined.The stoichiometric titanomagnetites should lie on theline ST in Figure 4. Points lying above this line may ormay not be stoichiometric, but points lying below theline are nonstoichiometric. Thus, most of the pointsfrom Hole 332B lie above the stoichiometric line, andmay be stoichiometric although this is not an unam-biguous conclusion. However, the samples from Sites

Sample(Interval in cm)

Hole 332B

2-5, 1136-1, 100*9-2, 9211-1, 11014-2, 84*17-1,6*22-4, 48*27-2, 112*33-1, 127*45-1, 104

Site 334

9-2, 70*26-1, 140

Site 335

6-5,33*9-4, 58*10-3, 76*13-2, 13

255245212160290300270240280545

300570

280320310345

530565505

_550555555555545545

540555

550550545535

0.6970.5130.439

—0.2260.5840.4970.4200.3081.235

0.3581.590

0.3300.4280.3880.425

Note: Asterisks identify samples with thermo-magnetic curves showing initial increase inmagnetization.aCurie temperature before heating.

Curie temperature of final mineral.cRatio of initial to final magnetizations.

334 and 335 as well as two of the Hole 332B samples, liebelow the line. The only way this can occur is as a resultof maghematization of the original mineralogy.

0.8

© SITE 332B

• SITE 334

A SITE 335

IOC 200 300

INITIAL CURIE TEMPERATURE (°C)

400

Figure 4. Plot of magnetization ratio (Jj/Jf) against Oi for13 samples. Line ST represents the stoichiometric curve;points below ST indicate the initial titanomagnetite isnonstoichiometric.

527

D. V. KENT, W. LOWRIE

VISCOUS REMANENT MAGNETIZATIONEXPERIMENTS

The specimen magnetizations were measured afterdemagnetization and the specimens were placed in auniform field of 1 oe. After 1 hr, and again atlogarithmically spaced intervals for a total of about 580hr, the remanences were remeasured, and the VRMcomponent computed. In this way the growth of VRMin each sample was monitored. A representative VRMgrowth curve is shown in Figure 5, in which the acquisi-tion appears to proceed in three distinct segments. Thisinterpretation is consistent with observations of VRMacquisition in many other DSDP basalts (Lowrie,1974). The growth of VRM proceeds logarithmicallywith time, but at different rates in each segment.Instead of attempting to determine a representativeaverage viscosity coefficient for each curve, the VRMintensity acquired in the 1-oe field over 500 hr was usedto calculate the ratio VRM/NRM for each sample; theNRM intensities, VRM intensities, and the ratioVRM/NRM are listed in Table 3. The ratio

DSDP SITE 332B

SAMPLE 9-2, 92

10 100ACQUISITION TIME (hrs.)

1000

Figure 5. Acquisition of VRM in three distinct stages in asample from Hole 332B.

VRM/NRM is an expression of the potential serious-ness of the effect of VRM on the sample. It varies fromabout 1.5% in Sample 335-13-2, 13 cm to 79% in Sample332B-11-1, 110 cm, and averages (geometric mean) only8.5%. It is quite variable from sample to sample in Hole332B without any consistent correlation with depth andis uniformly low at Site 335 where the median destruc-tive fields are relatively high.

The VRM/NRM ratio is inversely related to themedian destructive field (MDF) as shown in Figure 6.Only Samples 332B-45-1, 104 cm and 334-26-1, 140 cmappear to differ from the trend defined by the otherspecimens. The thermomagnetic analysis showed thatthese two samples have different magnetic min-eralogies than the others, probably dominated bymagnetite rather than titanomagnetite or titano-maghemite. In Sample 332B-45-1, 104 cm the NRM in-tensity and susceptibility are much higher than in theother samples, and although the actual VRM is fairlystrong, it is small in proportion to the NRM. Sample334-26-1, 140 cm is unusually coarse grained and has a

TABLE 3Comparison of VRM, Acquired in a Field of 1 oefor 500 hr, With NRM Intensity in Each Sample

Sample(Interval in cm)

Intensity(lO^G)

NRM VRM VRM/NRM (%)

Hole 332B2-5, 1136-1, 1009-2, 9211-1, 11014-2, 8417-1,622-4, 4527-2, 11233-1, 12745-1, 104

Site 33419-2, 7026-1, 140

Site 335

6-5, 339-4,5810-3, 7613-2, 13

9.0550.912.519.27.02

36.838.615.311.463.4

26.217.8

41.324.331.127.1

4.353.194.64

15.10.491.452.641.571.423.86

1.482.12

1.041.331.280.41

48.06.3

37.079.0

7.03.96.8

10.012.06.1

5.612.0

2.55.54.11.5

100 200 300 400

MEDIAN DESTRUCTIVE FIELD (OE)

Figure 6. The ratio of the VRM acquired over 500 hr in 1oe to the NRM intensity (VRM/NRM) plotted againstmedian destructive field (MDF). Inset: the slope of thedouble logarithmic plot suggests that the best-fit relation-ship is approximately (VRM/NRM) = 56.5 (MDF)T6I5.Solid circles are for Samples 332B-45-1, 104 cm and334-26-1, 140 cm (see text).

high susceptibility and correspondingly low Q'n value.In this sample the VRM developed is of moderate in-tensity, but low in proportion to the susceptibility ofthe sample.

528

VRM STUDIES OF IGNEOUS ROCKS

CONCLUSIONSIt has been observed (Lowrie and Kent, 1975) that

VRM acquired from the NRM state can be developedalmost twice as rapidly as from the demagnetized state.Nevertheless, the present results indicate such lowamounts of VRM from the demagnetized state insamples from Sites 334 and 335 that VRM cannot beconsidered to be a serious component in the remanenceof the basaltic layer at these sites. These sites havestable magnetizations characterized by high mediandestructive fields.

In Hole 332B, however, the ability to develop VRMis quite variable. In high MDF samples the developedVRM is generally of low intensity, but in samples withlow MDF the VRM (especially in the NRM state)ought to be able to account for a large portion of themeasured NRM intensity. The inverse relationshipbetween NRM/VRM and MDF follows an ap-proximately -6/5 power law (Figure 6), or, to a rougherapproximation, the two quantities are inversely propor-tional.

No attempt has been made to analyze the shapes ofthe VRM acquisition curves, which for experimentalreasons were in many cases not of sufficiently highquality to permit separation of each curve into morethan one distinct stage as shown in Figure 5. The sam-ple illustrated indicates, however, that, as reported inmany other DSDP basalts (Lowrie, 1974), VRM is notusually carried in these rocks by a single phase. It is ap-parent from thermomagnetic analysis that manysamples may be at least partially oxidized, and thatboth titanomagnetite and titanomaghemite may bepresent. This would allow VRM to be acquired by twodistinct minerals in such samples at the two differentobserved rates.

Where only a single mineralogical phase is present(titanomagnetite or titanomaghemite), it would bepossible for two grain-size fractions to contributedifferently to the VRM. For example, the center of thethree acquisition stages in Figure 4 evidently cor-responds to a discrete range of activation times which,

after initiation, are fairly quickly exhausted. It is notpossible on the basis of present data to satisfactorily ex-plain these phases, or to say whether other discreteranges exist beyond the limits of the period of ex-perimental observation. If this is not the case, and therate of VRM acquisition observed in the latest stage istaken to be representative for longer periods of time,then it appears that a large portion of VRM is acquiredby the time the second stage has passed, in comparisonto the VRM that might be acquired during very longtimes, of the order of the duration of a geomagneticpolarity epoch.

ACKNOWLEDGMENTSWe are grateful to N.D. Opdyke and R.L. Larson for

critically reading the manuscript and to M.N. Israfil and M.Sundvik for technical assistance in the laboratory. The workwas supported by National Science Foundation Grant DES74-17965.

REFERENCESAde-Hall, J.M., 1974. Strong field magnetic properties of

basalts from DSDP Leg 26. In Davies, T.A., Luyendyk,B.P., et al., Initial Reports of the Deep Sea Drilling Proj-ect, Volume 26: Washington (U.S. Government PrintingOffice), p. 529-532.

Lowrie, W., 1974. Oceanic basalt magnetic properties and theVine and Matthews hypothesis: Zeit. für Geophysik/J.Geophys., v. 40, p. 513-536.

Lowrie, W. and Kent, D.V., 1975. Viscous remanentmagnetization in basalt samples. In Hart, S.R., Yeats,R.S., et al., Initial Reports of the Deep Sea Drilling Proj-ect, Volume 34: Washington (U.S. Government PrintingOffice), p. 479-484.

Lowrie, W., L^vlie, R., and Opdyke, N.D., 1973. Magneticproperties of Deep Sea Drilling Project basalts from theNorth Pacific Ocean: J. Geophys. Res., v. 78, p. 7647-7660.

Ozima, M. and Ozima, M., 1971. Characteristic thermo-magnetic curve in submarine basalts: J. Geophys. Res.,v. 76, p. 2051-2056.

Saad, A.H., 1969. Magnetic properties of ultramafic rocksfrom Red Mountain, California: Geophysics, v. 34,p. 974-987.

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