37. MAGNETIC STABILITY OF ELEVEN BASALT SPECIMENS FROM DSDP LEG 341

Charles R. Denham and J.C. Guertler, Department of Geology and Geophysics,Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

ABSTRACTSix basalt specimens from Deep Sea Drilling Project Hole 319A

and five from Site 321 exhibited mixed stabilities during progressivealternating-field demagnetization and in a 500-hr test of viscousremanent magnetization. The samples from Hole 319A were general-ly less stable than those at Site 321. The results of 19 measurementson each specimen are given here in tabular form.

INTRODUCTION

Laboratory tests were conducted on 11 basalt speci-mens from DSDP Hole 319A and Site 321 to study theirbehavior under progressive alternating-field demag-netization and to discover their susceptibility to con-tamination by viscous remanence acquired in a weakfield. Figure 1 shows examples of the results.

PROGRESSIVE ALTERNATING-FIELDDEMAGNETIZATION

Each specimen was measured after partial demag-netization at steps of 25, 50, 100, 200, 400, and 800 oepeak alternating field, using a two-axis tumbler exposedto the earth's field. The results are given in Tables 1 and2.

Magnetic intensities at Site 321 decreased smoothlywith increasing alternating fields, while those at Hole319A generally were erratic, characteristic of these lessstable specimens. In all but one case in Hole 319A, themagnetization initially increased, suggesting previousaccumulation of secondary magnetization in directionsopposite to the original remanence. Unfortunately, the319A specimens sometimes changed their magnetizationnoticeably during the course of a measurement, so thattheir apparent demagnetization behavior may be arti-ficial in part. At Site 321, two specimens displayed slightbut perhaps insignificant increases of intensity afterapplication of 25-oe peak alternating field.

In most of the specimens, a stable direction of originalremanence was never isolated. Frequently, the direc-tions progressed along a smooth path up to 200-oe peakfield, then diverged at 400 and 800 oe. Because theprogressive demagnetizations were performed in theearth's field, the acquisition of a net anhysteriticremanence (ARM) not only was possible, but actuallyappears to have occurred, often at 400 oe and usually at800 oe, where the results bear little resemblance to thoseof weaker fields. Several years experience with our de-

'WHOI Contribution No. 3451.

magnetization apparatus has shown that ARM does notbecome significant in stable rocks until alternating fieldsexceeding 400 oe are applied. In the case of these rocks,however, we regret not having performed replicatedemagnetizations at each level in order to identify theonset of net ARM more closely. A special test involvingsix-fold repetition of the 800-oe demagnetization andmeasurement using Samples 319A-3-4, 96-99 cm and321-14-1, 99-102 cm showed that the spurious ARM wasnot systematic. The results of 400 and 800 oe generallyhave no value here, except that the latter served as thestarting point for the test of viscous remanent mag-netization (VRM).

VISCOUS REMANENT MAGNETIZATIONImmediately following the alternating-field demag-

netization at 800 oe, a 500-hr test of viscous remanence(Lowrie, 1973; Peirce et al., 1974) was conducted, withmeasurements performed at 1, 2, 5, 10, 20, 50, 100, 200,and 500 hr into the experiment. The samples remainedin a constant position during the test, except for briefperiods during which they were removed for measure-ment. The earth's field in our laboratory was used toproduce VRM nominally in the westward direction at ashallow inclination, relative to the coordinate system ofthe specimens. Results are given in Tables 1 and 2. Twospecimens from Hole 319A were especially unstable,whereas all five from Site 321 displayed only weak VRMduring the test.

DISCUSSIONStatistically significant conclusions are not possible

with only five or six specimens from each site. Hopefullythese data will prove useful when combined with theHole 319A and Site 321 results of other workers, how-ever. By themselves, the 11 specimens examined heremerely confirm the common observation of magnetic in-stability in near-surface basalts recovered by DSDP.

ACKNOWLEDGMENTSFunds for this brief investigation were provided by National

Science Foundation Grant GA-41183 to D.A. Johnson,Woods Hole Oceanographic Institution.

469

C. R. DENHAM, J. C. GUERTLER

200 r

(NJ

er—>

319A-3-3-119-122

100

10 r

NRM2 25 50 100 200 400

PEAK FIELD (Oersted)

800

1 1 1 1 1 1 1

5 10 20 50 100 200 500HOURS

Figure 1. Examples of results. The lower plots show 19 magnetic directions for each of two specimens, where the numbersbeside each point correspond to line numbers in Table 1. All points lie on the lower hemisphere of an equal-area projec-tion, except for five small dots at the lower right, which are upper hemisphere directions too closely spaced for larger sym-bols. The upper two graphs show the effect of partial demagnetization in alternating-fields up to 800 oe peak (left) and theaccumulation of viscous remanent magnetization in the westward direction (relative to the sample coordinates) over aperiod of 500 hr. The volume in each case is 8.6 cc.

470

MAGNETIC STABILITY OF ELEVEN BASALT SAMPLES

TABLE 1Measurements of Magnetic Direction, Hole 319A and Site 321

Hole 319A

1 NRMl2NRM23 25 oe4 50oe5 100 oe6 200 oe7 4U0 oe8 800 oe9 lhr

10 2 ro-ll 5hr12 10hr13 20 hr14 50hr15 lOOhr16 200 hr17 500 hr18 25 oe19 50 oe

Site 321

1 NRMl2NRM23 25oe4 50 oe5 100 oe6 200 oe7 400 oe8 800 oe9 lhr10 2hr11 5hr12 10 hr13 20 hr14 50 hr15 100 hr16 200 hr17 500 hr18 25 oe19 50 oe

3-1

I

-50-34-42-44-144871423535343434333231313131

14-1

-10-9-11-8-8-6178791013141515151715

, 50-53 cm

D

56494745167

338291290289289288287288287287287286287

J

4.3E"4

2.9E"4

3.2E-4

3.3E"4

2. IE" 4

2.0E"4

1.9E"4

5.7E"5

6.6E"5

6.7E"5

6.9E"5

7.1E~5

7.2E"5

7.3E"5

7.6E-5

7.7E-5

8.0E"5

7.9E~5

7.8E"5

, 99-102 cm

134134132131132132131127130128131134138139141141144141138

1.2E"2

1.2E-2

1.2E"2

LIE" 2

8.8E"3

3.9E"3

2.2E"3

1.0E"3

8.4E"4

9.8E"4

8.6E"4

8.IE-4

7.4E"4

7.1E-4

6.7E"4

6.7E"4

6.5E"4

6.9E"4

7. IE" 4

3-2,

I

34474646495362536062616366676768696868

14-1

-7-7-8-9-8115367182767961585551444958

104-107 cm

D

71666571767678107108105106107106107111107112110108

J

7.5E-4

6.8E"4

6.8E"4

7.2E"4

7.2E"4

5.0E"4

3. IE" 4

2.4E"4

2.2E"4

2.2E~4

2.2E"4

2.1E-4

2.1E-4

2.1E-4

2.1E-4

2.1E-4

2.1E-4

2.1E-4

2.1E-4

, 142-145 cm

121123124124128127130110139180217211260261267266268266261

1.4E-2

1.2E"2

1.3E"2

LIE" 2

5. IE"3

2.IE"3

8.0E"4

4.6E-4

2.9E"4

2.9E"4

2.9E"4

3.0E-4

3.4E"4

3.5E"4

3.6E-4

3.9E"4

4.4E"4

4. IE"4

3.7E"4

3-3

I

13141437546362785548474540393938364043

14-;

-17-13-15-13-13-466605445404232323029686672

, 14-17 cm

D

108117117121133134125322300298295293290290288289288288290

J

2.5E":i

1.9E"3

2.OE"3

2.3E"3

2.IE"3

1.2E"3

4.2E"4

2.1E"4

2.5E"4

4.0E-3

2.8E"4

2.9E"4

3.2E"4

3.4E"4

3.5E"4

3.5E"4

3.7E-4

3.4E"4

3.2E"4

!, 94-97 cm

150147153159161163189172253259264262272270271273191222225

9. IE"3

1.0E"2

9.3E"3

7.7E"3

4.8E"3

1.4E"3

3.4E"4

3.9E"4

3.7E-4

4.4E"4

4.8E"4

4.6E-4

5.9E"4

5.9E"4

6.2E"4

6.7E"4

3.2E"4

3.3E"4

3.5E"4

3-3, 119-122 cm

I

16676568777959406579555849385454397169

D

1142341701751733051401008329322298289288297293292284241

J

1.7E-J

8.OE-4

1.4E"3

1.3E"3

8.6E"4

4.2E"4

3.2E"4

5.3E"4

3.6E-4

3.1E-4

2.8E-4

3.0E-4

3.8E"4

4.4E-4

3.6E-4

3.2E-4

3.9E"4

2.3E-4

2.7E"4

14-3, 97-100 cm

-16-12-9-11- 9943542119211913121515136666

1411551511501481429127628428628728128128127999282244333

LIE" 2

LIE" 2

LIE" 2

7.5E"3

3.8E~3

1.3E-3

4.1E-4

3.3E"4

5.IE"4

6.3E-4

4.3E"4

6.OE-4

9.4E"4

9.3E"4

8.7E"4

9.2E"4

9.0E"4

3.4E"4

3.7E"4

3-4, 96-99 cm

I

230845656625-5-6-112233351034

D

754366626980317345317314310314304303300301297296300

J

1.3E--*7.7E"4

1.1E"3

1.0E"3

6.9E"4

3.5E-4

3.7E"4

4.2E"4

5.3E"4

5.8E-4

5.6E"4

4.8E-4

6.7E"4

6.9E"4

7.3E-4

6.IE-4

8. IE-*5.2E"4

2.9E"4

14-4, 35-38 cm

-10-10-14-14-14-936474138313226263230283966

279281281281280279352264276277275274277279278277279261295

1.4E"2

1.4E"2

1.2E"2

9.6E"3

5.5E"3

1.6E"3

3.2E"4

6.9E"4

8. IE"4

8.2E"4

8.5E"4

8.3E"4

9.5E"4

LIE'3

8.7E"4

8.4E"4

9.8E"4

5.5E"4

4.3E"4

I

128935767065838382807978777676747475

3-5, 44-47 cm

D

5572694730527528653324314307304299300298299295297296

J

4.9E"4

4.6E-4

4.2E"4

2.4E"4

3.4E"4

3.7E"4

2.9E"4

1.5E"4

1.5E"4

1.5E"4

1.5E"4

1.5E"4

1.5E-4

1.5E"4

1.5E-4

1.5E-*1.6E-*1.6E-4

1.6E"4

Note: I = inclination; D - relative declination, in degrees; and J = intensity in Gauss or emu/cc for 11 basalt specimens. In the numbers expressing J, E refersto the order of magnitude. NRMl, NRM2 Natural remanent magnetization of specimens when first received from DSDP (NRMl) and several monthslater when laboratory testing began (NRM2). 25-800 oe = Results of stepwise demagnetization in 25-800 oe peak AF. 1-500 hr = Results during the VRMtest, with measurements conducted at logarithmic intervals up to 500 hr from the time of AF demagnetization at 800 oe. 25-50 oe= Results of AF de-magnetization following the VRM test.

471

C. R. DENHAM, J. C. GUERTLER

TABLE 2Summary of Progressive Alternating-Field Demagnetization and Viscous

Remanent Magnetization Results Given in Table 1

Sample(Interval in cm)

Hole 319A

3-1,50-533-2,104-1073-3,14-173-3,119-12234, 96-993-5, 44-47

Site 321

14-1,99-10214-1,142-14514-2, 94-9714-3, 97-10014-4, 35-38

MDF(oe)

48035023014014050?

15090958075

DirectionalStability

Very poorModerateSmoothly varyingVery poorVery poorMod. above 100 oe

GoodGood to 200 oeMod. to 200 oeGood to 200 oeGood to 200 oe

§W(10~á Gauss)

0.00560.0140.0630.250.150.0098

0.180.140.150.290.19

SW/JNRM2(%)

223

3120

2

21231

Note: MDF is the median destructive field, chosen as the point where themagnetization fell below half of the previous highest value. S\y is thebase-10 viscosity coefficient estimated from the westward (relative) com-ponent of VRM acquired in the earth's field of our laboratory. Values ofS\v/JNRM2 show that the Site 321 specimens were less vulnerable to VRMthan those of Hole 319A.

472