1. PALEOMAGNETISM AND MAGNETIC PROPERTIESOF IGNEOUS ROCK SAMPLES—LEG 38

Dennis V. Kent and Neil D. Opdyke, Lamont-Doherty Geological Observatoryof Columbia University, Palisades, New York

INTRODUCTION

Basaltic rock was recovered from nine sites on Leg38. Initially, one to three partially oriented specimens inthe form of right-circular cylinders, 2.5 cm in diameterand 2.5 cm high, were obtained from each of these sitesfor preliminary magnetic studies. The studies includedmeasurement of intensity and direction of remanentmagnetism, susceptibility, stability of remanenceagainst alternating fields, and thermomagneticcharacteristics. Additional samples were taken subse-quently from cores some of the sites in order to resolvesome discrepancies apparent in our initial observations,particularly at Site 336, as well as between these andmeasurements on the same rocks made by Russianworkers. Results from the additional samples are incor-porated in this report.

MAGNETIC PROPERTIESNatural Remanent Magnetization and Susceptibility

The direction and intensity of natural remanentmagnetization (NRM) were measured in each specimenwith a spinner magnetometer; the specimens initialsusceptibilities were measured with an AC bridge(Figure 1). The site mean values of NRM intensity(Table 1) were within the range 1 × lO~3 to 2 × 10~2G,and similar to the range of NRM intensities found inother DSDP basalts (Lowrie, 1974), except at Site 344which gave an unusual low site mean. The site meansusceptibilities were also within the range of otherDSDP basalts (10~4 to 10"2G/oe), but at seven of thenine sites, the mean susceptibility was a factor of two ormore greater than the mean (0.63 × 10~4G/oe) forother DSDP basalts (Lowrie, 1974).

The modified Königsberger, Qri (=NRM/k) variesfrom 0.1 to 45, with most values falling between 1 and10 (Figure 1). The site mean values of the Königsbergerratio, calculated with the 1965.0 InternationalGeomagnetic Reference Field at each site, were greaterthan 1, except at Site 344 which had a low value of 0.46(Table 1). Due to the high susceptibilities, the site meanKönigsberger ratios generally were less than theaverage Königsberger ratio (7.92) reported for otherDSDP basalts (Lowrie, 1974). Although the remanencewas the dominant magnetization component in mostcases, induced magnetization components cannot beconsidered insignificant at several sites, particularly atSite 344 in which the induced component dominatedthe magnetization.

Stability and Direction of MagnetizationAlternating field (AF) demagnetization in pro-

gressively higher fields to 500-oe peak was used to in-

vestigate the stability of the NRM of each specimen.Vector diagrams illustrating the effects of this treat-ment on two representative specimens are shown inFigure 2. The direction of stable remanence, thedemagnetizing field at which it was obtained, and thedirection of NRM for comparison are shown for eachspecimen in Table 2. The median destructive field(MDF) and the fraction of the NRM remaining afterAF treatment in 100 oe were determined for eachspecimen and are also shown in Table 2.

Stable magnetic properties were found at Sites 336and 337. The basalts at these sites possessed high MDFand experienced little change in magnetic directionswith AF treatment (Figure 2). Appreciable soft com-ponents of magnetization often were present in thebasalts from the other sites, reflected either by lowMDF (e.g., Sites 338, 343, and 345), or large directionalchanges in remanence after AF demagnetization (Site344; Figure 2). The least stable magnetizations werepresent at Sites 348 and 350. The NRM at these siteshad low MDF, and the magnetic inclination directionsoften changed from positive (directed down) to steeplyinclined upward after partial AF demagnetization.Nevertheless, it was possible to isolate a stablemagnetization at these and the other sites after theremoval of soft magnetic components by AFdemagnetization.

The magnetic inclinations of DSDP samples aremeaningful because the vertical direction usually isknown, provided that care is taken in preserving theoriginal orientation of the sample. In addition, the highpaleolatitudes of these sites result in steep inclinationsfrom whose sense the magnetic polarity can be deter-mined readily. The polarity of the geomagnetic fieldwas most likely normal at the time of basement forma-tion at Sites 338, 343, and 345, and reversed during theformation of Sites 336, 342, 344, 348, and 350: bothnormally and reversely magnetized samples occurred atSite 337 (Table 2). The remanence of some specimensfrom three reversely magnetized sites (342, 344, and350) showed an initial increase in magnetization afterAF treatment (7100/NRM > 100%; Table 2). Thisbehavior can be attributed to the removal of a soft com-ponent directed along the present in situ magnetic field,antiparallel to the stable reversed remanence, and sup-ports the interpretation of reversed magnetization in atleast three of the five reversed sites.

The cleaned remanent inclinations, regardless ofsign, were, on the average, steeply dipping at most sites(Table 3) as would be expected from the highpaleolatitudes of Greenland and western Europe sincethe lower Tertiary. Two exceptions were Sites 336 and337, which had mean inclinations substantiallyshallower than at the other Leg 38 sites. Moreover,

D. V. KENT, N. D. OPDYKE

•

o•D

A

336

337

338

342

343

KEY

*

•

O

344

345

348

350

10" 2 10"

SUSCEPTIBILITY (GAUSS/OE)

Figure 1. Intensity of natural remanent magnetization (NRM) and the susceptibility (k) of Leg 38 basalts. Qn' is the ratioNRM/k

TABLE 1Site Mean Values of Natural Remanent

Magnetization (NRM), Susceptibility (k),and Königsberger Ratio

Site

336337338342343344345348350

Numberof

Samples

967315225

NRM(KHG)

33.684.930.914.4

164.41.45

82.747.046.2

k(lO^G/oe)

23.92.41

13.112.057.1

3.8627.233.219.9

KönigsbergerRatio

2.9269.55.012.335.590.465.852.75

13.0

both normally and reversely magnetized samples oc-curred at Site 337, although the reversed samplesgenerally had steeper inclinations than the normallymagnetized samples. It is possible that the polarity ofthe geomagnetic field was reversed during the forma-tion of this site, reflected by the steep negative in-clinations. The more shallow inclinations, both positiveand negative, are probably due to sampling of basaltbreccia that has been identified at this site, in which

fragments of basalt may have been physically rotatedafter they had acquired a magnetization.

Magnetic MineralogyCurie point analysis was carried out on represen-

tative specimens from each site to determine the domi-nant magnetic mineralogy. Small chips of basalt wereground to a coarse powder and heated in air in a ver-tical motion type, automatically recording Curiebalance. It was not possible to obtain thermomagneticcurves for samples from Site 344; the concentration ofmagnetic minerals at this site was evidently insufficientto produce a usable signal under the experimental con-ditions used. The Curie temperatures observed in thethermomagnetic heating curves of the other samples arelisted in Table 4.

Samples from a majority of the sites (337, 343, 345,348, and 350) gave irreversible thermomagnetic curves(Figure 3a) typical of those found in most rapidlyquenched, submarine basalts (Ozima and Ozima, 1971;Lowrie, 1974). The Curie points observed duringheating ranged from 190°C to 405°C, and during cool-ing they ranged from 505°C to 530°C, corresponding toa low-titanium magnetite. The Curie point observedduring heating can be attributed to the presence of

PROPERTIES OF IGNEOUS ROCK SAMPLES

344-37-2, 135 cm

MAGNETIZATION UNITS = 10 GAUSS

Figure 2. Vector diagram showing the AF demagnetizationstability of the NRM of two basalt specimens from Leg38. The northward (N) and eastward (E) componentsdefine the declination variation (solid circles) relative toan arbitrary zero; the horizontal (H) and vertical (V)components define the inclination variation (open cir-cles). Numbers next to selected points refer to the peakdemagnetizing field intensity in oersteds.

either titanomagnetite, or the cation-deficient form,titanomaghemite as the dominant magnetic mineral.

Samples from Site 336 each gave a nearly reversiblethermomagnetic curve (Figure 3b). A single hightemperature Curie point (range from 545 °C to 565°C)was observed during both heating and cooling and canbe attributed to a magnetic mineral close incomposition to magnetite.

The thermomagnetic behavior of samples from Sites338 and 342 was more complex, and as for Site 336samples, uncharacteristic of normal oceanic basalt.Two Curie points were observed during heating:between 230°C and 315°C and between 485°C and565°C. Only a single Curie point, in the range from510°C to 535°C, was observed during cooling (Figure3c). The double Curie points indicate the presence oftwo magnetic mineral phases, most probablytitanomagnetite (or titanomaghemite) and a lowtitanium magnetite.

DISCUSSIONIf we make the gross assumption that the limited

number of basalt samples studied here are represen-tative of the magnetized layer at each site, it is apparentthat at many of the sites necessary, but insufficient,magnetic properties occur for straightforward inter-pretation of any marine magnetic anomalies accordingto the Vine and Matthews (1963) hypothesis. Althoughthe Königsberger ratio was greater than 1 at most of thesites, the generally low values indicate that inducedmagnetization components may have a perturbing in-fluence on the anomaly profiles over some areas. In thevicinity of Site 344, the low NRM intensities andKönigsberger ratios of the sampled igneous rock

suggest that variations in susceptibility, as opposed toalternations in the magnetization polarity, may be moreimportant in producing any local magnetic anomalies.

The low median destructive field of basalts from Sites343, 345, 348, and 350 indicate the presence of a largesoft component of magnetization. At two of the sites(348 and 350), the soft magnetization was of sufficientmagnitude to mask the stable, reversed remanence ofthe basalt. DSDP basalts with these unstable remanentcharacteristics generally have been found in regionswhere linear magnetic anomaly patterns are absent orpoorly developed (Lowrie, 1974) and are known to bevery susceptible to acquisition of viscous remanence(Lowrie et al., 1973; Peirce et al., 1974). Remagnetiza-tion of the oceanic crust by viscous remanence acquisi-tion has been offered as an explanation of somemagnetic quiet zones (Lowrie, 1973). However, well-developed linear magnetic anomalies are reported tooccur in the vicinity of at least two of the sites (343 and348) which had unstable magnetic properties. Thissuggests that the recovered basaltic rocks may not berepresentative of the magnetized layer giving rise to themagnetic anomalies.

Curie point analysis revealed that the basalts at threesites (336, 338, and 342) had thermomagnetic char-acteristics not normally associated with submarinebasalts. Curie temperatures of greater than 500°C wereobserved at these sites and may represent low-titaniumtitanomagnetite produced by the subsolidus exsolutionof ilmenite from an originally homogeneoustitanomagnetite, following high-temperature oxidationduring initial cooling. Such features of deuteric, hightemperature oxidation commonly have been observedin subaerial basalts (e.g., Ade-Hall et al., 1964). In con-trast, most submarine basalts including those at theother Leg 38 sites have single, low Curie temperatures(100°C to 400°C) attributed to the presence of ahomogeneous titanomagnetite (or titanomaghemite).

At each of the sites with atypical thermomagneticcharacteristics, evidence from overlying sedimentsuggests shallow water or even subaerial emplacementof the basalt. Site 336 was taken on the northern flankof the Iceland-Faeroe Ridge; the basaltic basement wascovered by rubble probably derived from the basalt bysubaerial erosion (Talwani et al., 1975). Basaltic rockunderlying Sites 338 and 342 on the inner V^ringPlateau may represent oceanic basement formedimmediately after the opening of the Norwegian Sea(Talwani and Eldholm, 1972). The character of theoverlying sediment at these two sites would imply thatthe basaltic basement was initially at or above sea level.In addition, the basalts recovered at all three of thesesites are more similar petrologically to plateau basaltthan to typical oceanic tholeiites.

Atypical thermomagnetic characteristics, similar tothose observed at Sites 336, 338, and 342, weremeasured also on basalts recovered on Leg 26, at Sites253 and 254 on the southern end of Ninetyeast Ridge(Ade-Hall, 1974). The overlying sediment at these sitessuggested that this portion of Ninetyeast Ridge wasclose to sea level during its formation (Davies et al.,1973), although in contrast to the Leg 38 samples, the

D. V. KENT, N. D. OPDYKE

TABLE 2Stability of Each Specimen Against AF Demagnetization

Sample(Interval in cm)

Site 336

41-1, IB41-1, 14B42-1, IB42-1,14442-2, 8243-1, 12344-1,9444-2, 7144-2, 105

Site 337

13-1,8013-2, 3713-2, 14013-3, 3814-2,9115-2, 137

Site 338

43-2,11543-2,4143-4,5444-3,4044-3,9145-1, 12745-2, 56

Site 342

7-2, 1377-5, 1268-2, 65

Site 343

13-2, 20

Site 344

34-2, 27354,8736-2, 7537-1, 8537-2, 135

Site 345

33-2,5635-1, 145

Site 348

32-4,9334-2, 107

Site 350

14-2, 10814-3, 8416-2, 3016-2, 11516-3, 73

D

2126622928413320410110837

9070272303132235

32227984433336354

34117242

133

19442197170357

27254

3326

14516627017438

NRMI

° -51°-51-A6-53-20-35-84-32-49

12-79-2722-65-77

71703152706368

-7-86-79

75

-69-34-3-27-27

7970

4338

-54-6484776

After AFDemagnetizationD

24°25023528610220412510155

8470271302138242

300301564730312358

35417590

134

21137191267358

25262

279259

94161224227258

I

-54°-46^9-57-46-34-77-36-58

15-79-3220-65-75

71727957687373

-75-86-82

78

-77-29-72-71-70

8174

-76-70

-67-65-87^9-66

Hd(oe)

100100100100200100100100100

200200150200150150

100100100100100100100

100100100

100

100300200100100

150100

400200

100100100200100

MDF(oe)

23318719526125720981132135

405368226205239235

23418159691277486

94265175

75

36314943189396

9161

3231

2112939628437

^lOO/NRM(%)

877984937688286260

949885939391

89824322613241

4692126

35

941073468113

4116

74

10899289679

Note: Declination (D) and inclination (I) of the natural remanent magnetism (NRM)compared to that of the stable component obtained after partial demagnetizationin a peak alternating field (AF) intensity (Hd). The declination direction is foran arbitrary fiducial in each specimen. MDF is the median destructive field, andJ100/NRM i s t h e fraction of the NRM remaining after AF treatment in 100 oe.

PROPERTIES OF IGNEOUS ROCK SAMPLES

TABLE 3Magnetization Polarity and Mean Remanent

Inclinations of Igneous Rocksat Leg 38 Sites

Site

Numberof

Samples Polarity

MeanInclination(± S. D.)

336337338342343344345348350

967315225

Reversed(Mixed)NormalReversedNormalReversedNormalReversedReversed

TABLE 4

-50.8

70.4°-81.0°78.0°

-63.8°77.5°

-73.0°-66.8°

Curie Temperatures of IgneousRock Sample;5

±12.9"

±6.8°±5.6°

±19.6°

±13.5°

Sample(Interval in cm)

Curie Point DuringHeating Cycle (°C)

Site 33641-1, IB42-1, 14444-2,71

Site 33713-2, 14014-2,9115-2, 137

Site 338

43-2, 11543-4,5445-2,56

Site 342

7-2, 1377-5, 1268-2, 65

Site 34313-2, 20

Site 344

34-2, 2735-4, 8737-2, 135

Site 345

33-2,5635-1, 145

Site 348

32^,9334-2, 107

Site 350

16-2, 30

560565545

285315305

405,570315,565310,485

230,480270,560245,520

310

-

315405

340315

190

recovered basalts were described as oceanic tholeiites(Kempe, 1974).

It thus appears that the presence of high Curietemperature magnetic minerals in oceanic basement

(b)

Site 33845-2, 56 cm

(c)

600TEMPERATURE (°C)

Figure 3. Thermo magnetic curves representative of thebasalts from Leg 38. Heating and cooling were performedin air in afield of 3000 oe.

volcanics may be associated with shallow water or sub-aerial emplacements of the basalt, although theirabsence would not necessarily preclude such an origin.The mechanism by which this can occur may be relatedto a greater availability of oxygen to enhance the hightemperature oxidation of the original titanomagnetitein the cooling basalt. Alternatively, high regionalhydrothermal alteration can also produce high Curiepoints (Ade-Hall et al., 1971). However, petrologicdescriptions suggest that the rocks at the high Curietemperature sites range from little altered (Sites 336 and342) to highly altered (Site 338), not substantiallydifferent from the alteration states at the other Leg 38sites with low Curie temperatures. In addition, equal orgreater thicknesses of sediment overlie basaltic base-ment at most of the other Leg 38 sites with low Curietemperature magnetic mineralogies (Talwani et al.,1975). Unless some appreciable thickness of sedimentor basalt has been eroded at the high Curie point sites,it seems unlikely that hydrothermal alteration (e.g.,burial metamorphism) is the mechanism responsible forthe high Curie temperatures, at least in these cases.

ACKNOWLEDGMENTSThis research was supported by Grant DES 74-17965 of the

National Science Foundation.

D. V. KENT, N. D. OPDYKE

REFERENCES

Ade-Hall, J.M., 1974. Strong field magnetic properties ofbasalts from DSDP Leg 26. In Davies, T.A., Luyendyk,B.P., et al., Initial Reports of the Deep Sea Drilling Pro-ject, Volume 26: Washington (U.S. Government PrintingOffice), p. 529-532.

Ade-Hall, J.M., Palmer, H.C., and Hubbard, T.P., 1971. Themagnetic and opaque petrological response of basalts toregional hydrothermal alteration: Geophys. J., v. 24,p. 137-174.

Ade-Hall, J.M., Wilson, R.L., and Smith, P.J., 1964. Thepetrology, Curie points and natural magnetizations ofbasic lavas: Geophys. J., v. 9, p. 323-336.

Kempe, D.R.C., 1974. The petrology of the basalts; Leg 26.In Davies, T.A., Luyendyk, B.P., et al., Initial Reports ofthe Deep Sea Drilling Project, Volume 26: Washington(U.S. Government Printing Office), p. 465-504.

Lowrie, W., 1973. Viscous remanent magnetization in oceanicbasalts: Nature, v 243, p. 27-30.

, 1974. Oceanic basalt magnetic properties and theVine and Matthews hypothesis: J. Geophys., v. 40,p. 513-536.

Lowrie, W., Lovlie, R., and Opdyke, N.D., 1973. Magneticproperties of Deep Sea Drilling Project basalts from the

North Pacific Ocean: J. Geophys. Res., v. 78, p. 7647-7660.

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

Peirce, J.W., Denham, C.R., and Luyendyk, B.P., 1974.Paleomagnetic results of basalt samples from DSDP Leg26, Southern Indian Ocea'n. In Davies, T.A., Luyendyk,B.P., et al., Initial Reports of the Deep Sea Drilling Pro-ject, Volume 26: Washington (U.S. Government PrintingOffice), p. 517-527.

Davies, T.A., Luyendyk, B.P., Rodolfo, K.S., Kempe,D.R.C., McKelvey, B.C., Leidy, R.D., Horvath, G.J.,Hyndman, R.D., Theirstein, H.R., Boltovskoy, E., andDoyle, P., 1973. Deep Sea Drilling Project Leg 26:Geotimes, v. 18, p. 16-19.

Talwani, M. and Eldholm, O., 1972. Continental margin offNorway: A geophysical study: Geol. Soc. Am. Bull., v. 83,p. 3575-3606.

Talwani, M., Udintsev, G., Bj^rklund, K., Caston, V.N.D.,Faas, R.W., Kharin, G.N., Morris, D.A., Müller, C ,Nilsen, T.H., van Hinte, J.E., Warnke, D.A., and White,S.M., 1975. Deep Sea Drilling Project Leg 38: Geotimes,v. 20, p. 24-26.

Vine, F.J. and Matthews, D.H., 1963. Magnetic anomaliesover oceanic ridges: Nature, v. 199, p. 947-949.