45. ROCK AND PALEOMAGNETISM OF LEG 42A, HOLE 373A BASALTS

N. Petersen, Institut für Allgemeine und Angewandte Geophysik, Ludwig Maximilians Universitat,Munich, West Germany

U. Bleil, Institut für Geophysik, Ruhr Universitat, Bochum, West Germanyand

P. Eisenach, Institut für Allgemeine und Angewandte Geologie, Ludwig Maximilians Universitat,Munich, West Germany

ABSTRACT

Thirteen basalt samples have been investigated, eleven of whichwere oriented with respect to vertical. The natural remanentmagnetization, low field susceptibility, a.c. and d.c. field stability,saturation remanence, saturation magnetization, and Curie temper-ature have been measured. These measurements have been comple-mented by ore microscopic analysis. The remanence directions ofthe samples closely group around the magnetic dipole fielddirection to be expected at the latitude of Hole 373A. In allsamples apart from one, cation-deficient titanomagnetite is themain carrier of magnetization. It is concluded that the sampleshave undergone low temperature oxidation in a submarine environ-ment.

INTRODUCTION

Thirteen basalt samples from Hole 373A have beenanalyzed for paleomagnetic and rock magnetic proper-ties. The measurements have been complemented byore microscopic observation. Eleven of the rocks wereoriented with respect to vertical, two were non-ori-ented; only the inclination of magnetization directioncan be given in absolute values.

MAGNETIC MEASUREMENTS

MethodsRemanent magnetization of the rocks was measured

with a Digico spinner magnetometer. Stepwise alternat-ing field demagnetization in 25, 50, 75, 100, 150, 200,300, 400, 500, and 1000 Oe was carried out in order todetermine the "stable direction" of magnetization.

The natural remanent magnetization (NRM) ofbasalts normally is of a composite nature; it consists ofthe original thermoremanent magnetization acquiredduring cooling in the earth magnetic field after erup-tion, and also of other magnetization componentsacquired subsequently, like viscous magnetization andchemical magnetization. The stepwise alternating fielddemagnetization is suitable for removing these second-ary, normally less stable components of magnetization;the more stable original thermoremanent magnetiza-tion can thus be determined.

The volume susceptibility was measured with aBison magnetic susceptibility bridge. Isothermal satura-tion remanent magnetization (Jsr) was produced in a104 Oe field. The bulk coercivity (Hc) and the coerciv-ity of remanence (Hcr) were determined by a progres-sive reduction of this remanence with d.c. magneticfields applied in opposite direction. Curie temperature

of the rocks has been determined by measuring thetemperature dependence of strong field magnetization(measured in 1800 Oe) with a magnetic balance.

ResultsThe results of the magnetic measurements are sum-

marized in Table 1. Some of the samples that weresufficiently large have been subdivided into subspeci-mens (1-cm cubes) to test the within-sample scatter ofthe results. For comparison, data for other ocean-floorbasalts have been included in Table 1. (It should benoted that the data of Lowrie (1974) were obtainedfrom dredged samples and from very shallow DSDPdrillholes only).

Figure 1 is a downhole plot of NRM-intensity,stable inclination, Q-factor and Curie temperature. Inthis figure the theoretical axial dipole field value forthe inclination of the earth's magnetic field at thelatitude of Hole 373A (56°) has been included forcomparison as has the mean initial Curie temperatureof unaltered Leg 37 basalts (Bleil and Petersen, inpress). Figure 2 gives the thermomagnetic curves usedin the determination of Curie temperatures.

ORE MICROSCOPIC INVESTIGATION

MethodPolished sections of all samples have been examined

under the ore microscope using a Leitz Ortholux Polmicroscope. To aid in the identification of the magneticminerals magnetic colloids have also been used.

ResultsWith the exception of Sample 7-4, 132-135 cm,

which is aphyric, all samples are slightly altered phyric

881

N. PETERSEN, U. BLEIL, P. EISENACH

TABLE 1Different Magnetic Parameters of Hole 373A Basalts

Sample(Interval in cm)

3-3,40-45 12

Sample mean6,CC7-1,108-110 1

2Sample mean

7-2, 51-547-2, 100-106 1

23

Sample mean7-2, 143-1467-3, 55-577-3,105-110 1

23

Sample mean7-4, 3-57-4, 132-13511-1,83-8511-1,137-13912, outer barrel 1

23

Sample meanMean Values

All 373AsamplesDSDP-basalts(Lowrie,1974)

Leg 34 basalts(Ade-Halletal., 1976)b

Leg 37 basalts(Bleil andPetersen,1976)

N

Intensity(10-3Gauss)

2.440.901.671.688.486.257.373.282.431.261.011.573.543.841.611.261.531.471.761.315.957.113.531.532.422.49

3.31±2.18

5.08±4.38

6.11±6.94

3.16±3.13

RM

Inclination(°)

55.113.134.1-

62.362.362.364.441.050.958.750.261.373.469.061.459.663.355.875.939.051.9_——-

57.4±13.0

_

_

_

StableInclinationa

o3.9

-8.3-2.2—58.158.158.161.351.752.546.450.265.456.662.457.053.557.658.862.331.443.6———-

49.4±19.7

_

_

—

Suscepti-bility13

(10-3

Gauss/Oe)

0.370.240.310.481.601.741.670.350.570.400.310.430.350.290.370.340.400.370.330.501.301.431.992.382.172.18

0.77±0.64

2.61±2.48

3.07±3.38

0.34±0.29

Qc

158

118

128

102110

778

222910899

126

10114133

12±7_

15±15

26±29

MedianDestruc-

tiveFieldd(Oe)

6316111212691

10699

1247883

12094

12816912714212213013594996059382440

109±33

74±38

177±194

328±157

H c(Oe)

_—--879089

—7896

12198-_

147145145146—62554838424241

77±37

_

_

Her(Oe)

_—--

146152149-

124135177145-_

208216220215

—120989180828382

129±46

_

—

_

Jsr(Gauss)

_---

0.400.400.40-

0.131.191.220.85-_0.250.250.270.26—

0.140.240.220.220.230.190.22

0.33±0.24

_

_

0.32±0.11

T c(°C)

287--

270510—-

295---

-—

285----

270—295215---

274±28

_

253±94

287±71

,1800 Oehorc

(Gauss cm-Vgr)

0.15--0.280.96--0.34--—--—0.28----0.43-0.660.73---

0.48±0.27

_

1.38±1.20

0.28±0.20

aAfter removal of unstable magnetization components by alternating field demagnetization.^Volume susceptibility - low field susceptibility.cKoenigsberger Q Factor."Alternating field necessary to erase half the original intensity of magnetization — a remanence stability measure.

basalt containing feldspar phenocrysts and brokenplagioclase laths. Large variations in the pyroxene and,particularly, olivine content exist. The most abundantore phase in the rocks is skeletal titanium-rich titano-magnetite showing varying degree of maghemitization.Sample 7-1, 108-110 cm, is the only specimen wherethe grains of titanomagnetite consist of intergrowths oflamellae of secondary ilmenite with titanium-poortitanomagnetite, which indicate that high temperaturedeuteric oxidation has taken place (Figure 3[B]). In allthe other samples many titanomagnetite grains containpatterns of curved and branching cracks (Figure 3[A]),similar to those described by Ade-Hall et al. (1976a).These cracks are probably due to low-temperature oxi-dation (maghemitization) and associated volume

change. The cracks are often subsequently filled withsulfide.

A large number of discrete grains of primary ilmen-ite, sometimes mantled by titanomagnetite, is presentin all samples. The sulfides, also present in all samples,though to a much smaller amount than the Fe-Tioxides, predominantly consist of grains of pyrrhotiteand chalcopyrite (the latter intergrown with bornite)and to a minor degree of intergrowths of pyrite andmarcasite. In general the petrography of the opaqueminerals strongly resembles that of the Leg 34 oceanfloor basalts of the Nazca Plate as described by Ade-Hall et al. (1976a). Tables 2 and 3 give a briefdescription of the opaque mineralogy of the individualsamples.

882

ROCK AND PALEOMAGNETISM OF BASALTS

250-

300-

£350-

1

400-

450-

Re

co

very

••i

•i

-

Co

re

2

3

4

5

6

z

10

11

11

Petro-graphy

marl. ||vole,ashes

bas.breccia

IV

basaltandbas.breccia

V

basalt

-80

Stable

-40

nclination (

0 40

• •

80

1

i

i

NRM

•

< -

-iπt

•

•

ens

i

. . .

ty ( 1 0 3 Gauss)

\ 6 8

• •

Q factor

5 10 50

•

1

•

Curie temp.

100 300

•

CO

500

axial dipole field initial Curie temp.Leg 37 basalts

Figure 1. Down hole plot of different magnetic parameters for Hole 373A basalts. Theinclination of the earth's magnetic axial dipole field at the latitude of Hole 373A hasbeen given as a reference value. The mean value of the initial Curie temperatures forDSDP Leg 37 basalts (Bleil and Petersen, 1976) prior to any subsequent alteration ofthe magnetic mineral phase is also indicated.

DISCUSSION AND SUMMARY

Titanomagnetite is the dominant opaque phase inall investigated samples, with ilmenite less and sulfidesmuch less abundant. The skeletal shape of the titano-magnetites (Figure 3[A]) is indication of rapid coolingof the basaltmagma. The carrier of the remanent mag-netization are the grains of titanomagnetite. There mayalso be a negligible contribution from the sulfides (pyr-rhotite). With one exception all samples show featuresof maghemitization of the titanomagnetites which isdue to low temperature oxidation (below 250°C) andtypical of deep ocean weathering (halmyrolysis) as ithas been observed in ocean-floor basalts from, for ex-ample, Leg 34 (Ade-Hall et al., 1976a) and Leg 37(Bleil and Petersen, in press). The frequent occurrenceof irregular patterns of cracks in the titanomagnetitegrains is probably an indication of volume change as-sociated with low temperature oxidation.

The exception is sample 7-1, 108-110 cm, where thetitanomagnetite grains consist of intergrowths of sec-ondary ilmenite lamellae with titanium-poor titano-magnetite indicating deuteric high temperature oxida-tion. The high Curie temperature of 510°C of thesample is in good agreement with the ore microscopicobservation of high-temperature oxidation. In constrastto subaerial basalts, high-temperature oxidation oftitanomagnetites seems to be rare in ocean-floor basaltsand may occur only in the center of massive flows(Watkins and Haggerty, 1967; Grommé et al., 1969).

All other samples, showing the features of low-temperature oxidation, have lower Curie temperatureswith a mean of 274°C. This value is distinctly higherthan the average Curie temperature of unaltered ba-salts with stoichiometric titanomagnetites. The mean"unaltered" Curie temperature of the Leg 34 basalts,for example, is 140°C (Ade-Hall et al., 1976); of theLeg 37 basalts it is 119°C (Bleil and Petersen, inpress). Another value of mean "unaltered" Curietemperature obtained from 269 analyses of a greatvariety of continental basalts is 168°C (Petersen,1976). The difference between these low Curie temper-atures and the elevated mean Curie temperature of274°C of the samples investigated here can mostreasonably be explained by the microscopically ob-served maghemitization of the titanomagnetites, as theCurie temperature of titanomagnetite increases withthe degree of maghemitization (Readman andO'Reilly, 1972). In other words the elevated Curietemperatures support the microscopic observation ofmaghemitization of the titanomagnetites.

The remanence directions of the samples groupclosely around the magnetic dipole field direction to beexpected at the latitude of Site 373A. The exception issample 3-3, 40-45 cm, which has a very shallowinclination and is from a different lithological unit thanthe deeper samples. Sample 7-1, 108-110 cm, beingunusual because of the high temperature oxidationfeatures, falls well into this group (see Figure 1) whichvery likely means that it also had been formed under

883

N. PETERSEN, U. BLEIL, P. EISENACH

6cc

200 400 600 200 400 600 200 400 600Temperature (°C)

1.2

0.8

0.4

7-1 (108-110)

\ \

0.8

0.4

1 1 1

(51-54)

\

200 400 600

0.8

0.4

\

: \ \

(137-139)

\

\\

1.2

0.8

0.4

200 400 600 200 400 600 200

Temperature (°C)

400 600 200 400 600

Figure 2. Strong field magnetization (measured in a magnetic field of 1800 Oe) versus temperature for Hole 373A basalts.The distinct irreversibility of heating and cooling curves is indicative of titanomaghemite as carrier of magnetization.

the same conditions as the other samples, namely thesubmarine environment.

ACKNOWLEDGMENTSThis study is a combined contribution of the Institut für

Allgemeine und Angewandte Geophysik, of the Institut fürAllgemeine und Angewandte Geologie, both Ludwig-Maxi-milians Universitàt, Munich, and of the Institut fúr Geo-physik, Ruhr-Universitàt, Bochum. We thank Prof. Dr. G.Angenheister, Prof. Dr. D. Klemm, and Prof. Dr. H. Baulefor generously making available the facilities of the abovenamed institutions and for their support throughout thestudy. We are grateful to the members of the shipboard partyProf. Dr. K. Hsü and Dr. F. Fabricius who provided thesamples for this investigation. The paper was also reviewedby Dr. F. Fabricius and by Prof. G. Angenheister, Munich.

Financial support of the Deutsche Forschungsgemein-schaft is gratefully acknowledged.

REFERENCESAde-Hall, J. M. and Johnson, H. P., 1976. Paleomagnetism of

basalts, Leg 34. Initial Reports of the Deep Sea DrillingProject, Volume 34: Washington (U.S. Government Print-ing Office), p. 513-532.

884

Ade-Hall, J. M., Fink, L. K., and Johnson, H. P., 1976a.Petrography of opaque minerals, Leg 34. Initial Reportsof the Deep Sea Drilling Project, Volume 34: Washington(U.S. Government Printing Office), p. 349-362.

Ade-Hall, J. M., Johnson, H. P., and Ryall, P. J. C, 1976b.Rockmagnetism of basalt, Leg 34. Initial Reports of theDeep Sea Drilling Project, Volume 34: Washington (U.S.Government Printing Office), p. 459-468.

Bleil, U. and Petersen, N., in press. Magnetic properties ofbasement rocks, Leg 37, Site 332. Initial Reports of theDeep Sea Drilling Project, Volume 37: Washington (U.S.Government Printing Office).

Grommé, C. S., Wright, T. L., and Peck, D. L., 1969.Magnetic properties and oxidation of iron-titanium oxideminerals in Alae and Makaopuhi lava lakes, Hawaii: J.Geophys. Res., v. 74, p. 5277-5293.

Lowrie, W., 1974. Oceanic basalt magnetic properties andthe Vine and Matthews hypothesis: J. Geophys., v. 40, p.513-536.

Petersen, N., 1976. Notes on the variation of magnetizationwithin basalt lava flows and dikes: Pageoph, v. 114, p.177-193.

Readman, P. W. and O'Reilly, W., 1972. Magnetic propertiesof oxidized (cation-deficient) titanomagnetites (Fe,Ti,D)3O4: J. Geomag. GeoeL, v. 24, p. 69-90.

Watkins, N. D. and Haggerty, S. E., 1967. Primary oxidationvariation and petrogenesis in a single lava: Contr. Min.Petr., v. 15, p. 251-271.

ROCK AND PALEOMAGNETISM OF BASALTS

30 µm

Figure 3. (A) Skeletal titanomagnetite (white) and laths ofilmenite (white, center of the picture). Sample373A-6, CC; (B) Skeletal titanomagnetite (light grey) with volume change cracks, partly filled by a redbrown mineral (ilmenite?). Sample 373A-7-2,100-106 cm; (C) Subhedral to anhedral titanomagnetite(grey) with exsolution lamellae of ilmenite (light grey and dark grey). Partly crossed nicols. Sample373A-7-1, 108-110 cm; (D) Euhedral, porous grains of chrome-aluminum spinel (dark grey). Titano-magnetite with ilmenite exsolution lamellae at the rim (light grey). Sample 373A-7-1, 108-110 cm.

885

N. PETERSEN, U. BLEIL, P. EISENACH

TABLE 2Ore Microscopic Description of the Hole 373A Basalts

Sample(Interval in cm)

Ore PhasesGeneral Petrography Titanomagnetite Ilmenite Sulfides Other Phases

3-3, 40-45

6,CC

7-1, 108-110

7-2,51-54

7-2, 100-106

7-3, 55-57

7-3, 105-110

7-4, 3-5

14, 132-135

11-1,83-85

Phyric basalt with tra-chytic texture. Largeeuhedral glomerocrysts ofalkali feldspar, in agroundmass of smallerplag.-laths. Pyroxene andchlorite between thelaths. Few olivine grains.Phyric, amygdaloidal ba-salt with ophitic texture.Compared to Section 3-3less Pl-phenocrysts, butmore Cl.Phyric, coarser grainedbasalt. Compared to Sec-tion 3-3 more and largereuhedral to subhedral 01.

Similar to Section 7-1,but more and largerfeldspar glomerocrysts.Phyric basalt, less coarsegrained than Section 7-1.

7-2, 143-146 Phyric basalt

Phyric basalt

Phyric basalt, increasingcontent of large feldsparglomerocrysts.Phyric basalt, very littleolivine only.Aphyric, amygdaloidalbasalt, vesicles filled withwhite calcite.Aphyiic basalt

11-1,137-139 Phyric basalt

12, outer barrel Phyric, coarser grainedbasalt with ophitictexture.

Skeletal, mostly lyingbetween the laths.Most of the grains have aTi-rich core and a broadrim of maghemite.

Skeletal to anhedralgrains. Maghemitizationsimilar as in Section 3-3.

Large skeletal grains withilmenite exsolution paral-lel (100) and (010).Deuteric high tempera-ture oxidation.Like6,CC.

Skeletal to anhedral withvolume change cracks,partly filled by a red-brown mineral(ilmenite?).Skeletal, with volumechange cracks, oftenlined by tiny hematitecrystals.Like Sample 7-2, 143-146.

Like Sample 7-2, 143-146.

Like Sample 7-2, 143-146.Very fine, skeletal, mag-hemitization similar as inSample 7-2, 143-146.Skeletal with volumechange cracks, which arefilled by tiny hematiteand spinel crystals.Skeletal, partly inter-grown with sulfide, Vol-ume change cracks filledby sulfide.Like Sample 11-1,137-139.

Small, long laths,scattered over thegroundmass. Of-ten mantled by Ti.mag.

Similar to Section3-3.

Similar to Section3-3.

Few small roundishgrains, consisting ofan intergrowth ofmarcasite withlimonite.

Similar to Section3-3.

Few small pyrrhotiteinclusions in Ti. mag.

Like Section 3-3. Like Section 3-3.

Similar to Section3-3.

Similar to Section3-3.

Similar to Section3-3.

Similar to Section3-3.

Similar to Section3-3.Like Section 3-3.

Small grains of chal-copyrite with inter-growths of bornite.

Only traces.

Only traces.

Tiny grains lined upto garlands.

Tiny grains lined upto garlands.Like Section 3-3.

Like Section 3-3. Like Section 3-3.

Similar to Section3-3.

Like Sample 11-1,137-139.

Mostly intergrownwith ti.-mag.

Like Sample 11-1,137-139.

Limonite often forms theseam of decomposed oli-vine, often replaces ti.mag.Chrome-aluminum spinel,sometimes zoned with rimof magnetite, sometimesporous with tiny spots ofmag.Vesicles and veinlets in andaround the 01 lined bylimonite.

Limonite and chrome-aluminum spinel similar toSection 3-3.

Limonite similar to Section3-3.

Limonite similar to Section3-3.

Limonite similar to Section3-3. Chrome-aluminumspinel with vaguely devel-oped rims of magnetite.Limonite and chrome-aluminum spinel similar toSection 3-3.Similar to Section 3-3.

Similar to Section 3-3.

No record

No record

No record

No record

TABLE 3Mean Grain Diameter in Microns of the Different Ore Phases

in the Hole 373A Basalts

Sample(Interval in cm)

3-3, 40-456,CC7-1,108-1107-2,51-547-2, 100-1067-2, 143-1467-3, 55-577-3, 105-1107-4, 3-57-4, 132-13511-1,83-8511-1,137-13912, outer barrel

Titano-magnetite

181954433322362627

9121426

Ilmenite

15161830262020181811151529

Sulfide

6756

20trtr

216

10101310

Limonite

86353647322612—

77———-

Chromite

__10——

144_3186_

_-

886