24. MAGNETIC VISCOSITY OF SUBMARINE BASALTS,DEEP SEA DRILLING PROJECT, LEG 37

C. Plessard and M. Prévot, Laboratoire de Géomagnétisme du Parc Saint-Maur 4,Avenue de Neptune, 94100 Saint-Maur (France)

INTRODUCTION

Magnetic viscosity or magnetic after effect is the in-crease or decrease of magnetization with time in a con-stant (eventually zero) applied field.

These studies on the magnetic viscosity of submarinebasalts have two main objectives:

1) to estimate the intensity of the induced magnetiza-tion carried in situ. Usually, it is assumed that the in-duced magnetization is given by \H where x is t n e

magnetic susceptibility measured in a short time (lessthan 1 min in most cases) and H the intensity of thegeomagnetic field at the site of drilling. This assumptionis wrong if the growth of the induced magnetization withtime is important. In this case, the induced mag-netization resulting from the influence of the geo-magnetic field since the beginning of the Brunhes epoch(0.7 m.y.) can be rather higher than the magnetizationmeasured in short times.

2) to determine the relative importance of the viscousremanent magnetization (VRM) with respect to theprimary remanent magnetization and the behavior ofthe VRM when affected by alternating magnetic fields.As VRM is probably the most important secondarymagnetization in submarine lavas (Lowrie et al., 1973),both these points are of particular interest for inter-preting the paleomagnetic data obtained from theserocks.

EXPERIMENTAL PROCEDURESFor the entire collection (40 samples), we have deter-

mined the viscosity index v (Thellier and Thellier, 1944)which is equal to the ratio of the VRM obtained during2 weeks in the geomagnetic field to the "stable" naturalremanent magnetization (NRM) measured after a longstorage (at least 2 weeks) of the sample in a free-fieldspace. The measurements have been carried out with a"bit sample spinner magnetometer" (Thellier, 1967).

The intensities of the viscous magnetizations of thesesamples being weak, further experiments have beenmade with an astatic magnetometer (Pozzi and Thellier,1963) especially built for studies of magnetic viscosity.This apparatus requires cylindrical samples with well-defined dimensions (length 7 cm, maximum diameter 1.7cm). Because the shape of the DSDP samples was notsuitable for this instrument, we crushed the samples andslightly pressed the powder (the grain size of which wasa few millimeters) into a sample holder with the correctdimensions. The grain size of most titanomagnetites inthese samples being less than 1 mm, we may supposethat the grain size of the magnetic minerals is not

modified. Twenty samples, which were intended for pur-poses other than magnetic studies, were not available forcrushing and could not be studied further. The 20powdered samples were exposed to a 450-oe alternatingmagnetic field (peak value) before further experiments.This treatment was performed to destroy any remanentmagnetization and to obtain an initial state:

1) reproducible (each experiment being carried outtwice for verification). Note that thermal demagnetiza-tion does not lead to a magnetically stable body, themagnetic viscosity decreasing sharply with increasingelapsed time following thermal demagnetization(Plessard, 1971).

2) comparable to the initial state considered fortheorectical purposes (Néel, 1951).

The change with time or "trainage" of the inducedand remanent magnetizations has been studied forviscous magnetizations given in a 10-oe constant fieldapplied for 20 min. Measurements are valid between 5sec and 20 min after the constant field is applied (growthof the induced magnetization) or removed (decrease ofthe remanent magnetization). Using the method of leastsquares, two functions have been fitted to each set ofdata (both for the increase of induced magnetizationand for the decrease of remanent magnetization): alinear function (σ = a log t + b) and a parabolic func-tion (σ = a log2 t •+ b log t + c). We define also aparameter k given by k = Δσi/ σ2 where σi andΔσ2 are, respectively, the increase (or decrease) ofmagnetization when time increases from 5 sec to t m andfrom tm to 20 min (Figure 1) with log tm = 1/2 (log 5 sec+ log 20 min). If k differs from 1, the "trainage" (in-crease or decrease) deviates from a logarithmic function.For volcanic rocks, all the parameters characterizing the"trainage" of the viscous magnetizations are propor-tional to the intensity of the applied field within therange 0 to 15-20 oe. This fact allowed us to calculate thevarious parameters for h 0.5 oe (Table 1).

The method used for the alternating field treatment ofVRMs has been previously described (Biquand andPrévot, 1970). The VRMs studied here were obtained ina 3-oe field applied for 1 month. The AF treatment takesplace half an hour after the sample is removed from thefield. It has been shown that the demagnetization curvesare not significantly different for h = 3 oe and h = 0.5 oe(Prévot, 1975).

VISCOUS INDUCED MAGNETIZATION (VIM)We see from Table 1 that:1) The various parameters can be quite different from

sample to sample;

503

C. PLESSARD, M. PREVOT

σ1 Cio'2 e.m.u./g) ( 7 r , ( 1 0 " 4 e.m.u./g)

1.10 -

1.05 -

1.00log t

(1200") ( t in s.) (5")

3 1 log t

(1200") ( t i n s.)

Figure 1. Sample 332B-37-2, 102-105 cm. Variation with time of the induced (left) and remanent magnetizations (right), t isthe time since the 10-oe field is applied and t 'the time after this field is removed.

2) For most of the samples, ratio ù^σjσi is quitelarge.

3) For all the samples the growth in magnetizationwith time is better expressed by a second-order function(Figure 1). The /:, parameter being lower than 1, exceptfor Sample 332B-44-4, 24-26 cm, the extrapolated valueof σ, for t = 0.7 m.y. is greater than indicated by theusual logarithmic extrapolation.

4) Because of the importance of the growth of the in-duced magnetization with time, the extrapolated valueof σ,•| for t = 0.7 m.y. is generally several times greaterthan σ, for t = 5 sec, the maximum difference corre-sponding to a factor of 20.

5) The geometric mean for the σ N R M /σ / ( e x t r >ratio(Table 1) is about 3.5. Assuming the direction of theNRM is parallel (or antiparallel) to the Brunhesgeomagnetic field, we deduce from this result that theglobal magnetization (equal to NRM + VMI 0.7 m.y.)carried in situ by these basalts is statistically twicelarger—or so—for normally magnetized blocks than forblocks with the opposite polarity. Clearly, calculationmodels for magnetic anomalies, which assume the sameintensity of magnetization for both the normally andreversely magnetized crust, are unrealistic.

6) For most of the samples, and in spite of the strongincrease of the induced magnetization with time, the in-

tensity of the NRM is higher than the intensity of the in-duced viscous magnetization calculated for t - 0.7 m.y.For such samples the magnetic anomalies reflect somechanges in the distribution of the remanent mag-netizations on the sea floor. However, for 6 of the 20samples studied, the VIM carried in situ is probablycomparable to the natural remanent magnetization.

VISCOUS REMANENT MAGNETIZATION (VRM)

1) Except for Sample 332B-35-1, 46-49 cm thedecrease with time of the remanent magnetizationfollows approximately a logarithmic function (k β 1,Table 1)

2) The viscosity index v varies from less than 1% to60% (Table 2). Extensive measurements on subaerialvolcanic rocks have shown that v is log normally dis-tributed (Prévot, 1975). The logarithmic mean value of vfor the samples studied here is 3.4% (95% confidence in-terval for the mean: 2.2% to 5.3%). Note that the timeduration in the geomagnetic field from the beginning ofthe Brunhes epoch results in a "natural" VRM which isprobably 3 to 4 times greater than the VRM acquiredwithin 2 weeks (Prévot, 1975; Prévot and Grommé,1975). The mean magnetic viscosity of these submarinesamples, probably 3.5 m.y. old, is much larger than thatfor the young basalts from the ridge axis (Lecaille et al.,

504

MAGNETIC VISCOSITY OF SUBMARINE BASALTS

TABLE 1Main Characteristics of the Viscous Magnetizations

of DSDP Samples from Leg 37 (Hole 332B)

Sample(Interval in cm)

27-2, 4-635-1,4-735-1,46-4935-1, 104-10735-1, 136-13935-2, 29-3235-2, 33-3635-2, 91-9435-3, 28-3135-3, 81-8436-1, 104-10636-2,3-1236-2, 113-11536-2, 153-16136-3, 143-14536-6, 66-6837-1, 126-12837-2, 43-4537-2, 102-10544-4, 24-26

°i

3.4821.4

6.084.513.572.733.024.263.224.114.862.772.973.972.665.64

21.85.75

51.915.6

Viscous Inducedσ̂ •

σi

0.130.170.300.320.780.810.710.160.38

0.140.100.080.13

0.180.070.390.080.12

ki

0.900.850.730.730.550.440.460.570.43

0.700.460.580.68

0.780.880.710.591.1

Magnetization

ri

1.21.31.91.93.24.03.72.13.5

1.72.21.71.7

1.51.22.11.70.82

σ/(extr.)

6.8555.128.522.555.853.850.516.432.4

13.99.266.88

11.4

17.134.445.3

12221.0

Comparison ofNRM and Induced

Magnetization

Q

28.70.476.695.68

15.09.12

23.911.355.942.828.037.550.836.850.420.9

5.0033.4

9.9612.8

σNRM

σ/(extr.)

14.60.181.431.140.960.471.432.935.56

9.7811.221.912.8

6.90'3.174.304.249.52

V

0.383.891.791.542.552.302.340.731.410.630.760.150.180.392.392.071.732.124.542.09

Viscous RemanentMagnetization

<y

V

0.660.550.550.670.690.550.560.64

0.66

0.60

0.760.750.710.670.66

V

1.11.70.730.800.810.810.910.91

0.97

1.1

1.21.11.20.951.2

mdf

8548281821162724389292931859753395755054

Comparison"Trainages"

ΔOj

1.41.81.71.71.41.71.71.4

1.3

2.3

1.31.21.51.31.4

Note: Magnetizations σ and σ are given in 10 ^ emu/g for h-0.5 oe; σ is the induced (σ̂ ) or remanent (σ^') magnetization measured

5 sec after the field is applied or removed; σ? and Aσr' are the variations of these two kinds of magnetization when the time

increases from 5 sec to 20 min; k is defined in the text;rz is the ratio of the induced magnetization calculated from the parabolic

function to the induced magnetization calculated from the linear function, both for t = 0.7 m.y.; σ zv e x t r \ is the value obtained

σNRMfrom the parabolic function; Q is the Konigsberger ratio, given by ; the median destructive field (mdf) is in oe peak, the

°iparameters characterizing the "trainage" have not been calculated when σ is smaller than 0.2 X 10 emu/g.

1974). This seems to indicate that the magnetic viscosityof submarine basalts results from some alterationprocess.

3) The behavior of VRM with respect to AF treat-ment is quite different from one sample to another one(Figure 2 and Table 2).

However the median destructive field (mdf, Table 1) isalways smaller than the mdf for the NRM of nonviscoussubmarine basalts. As the growth of the mdf of VRMsseems to correspond only to a factor of 2 or so when / in-creases from 1 month to 0.7 m.y. (Prévot, 1975), the AFtechnique must be a quite efficient method for themagnetic cleaning of most of the submarine lavas.

4) If the VRM is due to thermal fluctuations, thegrains carrying this magnetization would be large(multidomain ?) for the lowest mdf values and small(single domain ?) for the highest mdf values.Theoretically, this may be verified from the σ./ σj,ratio which is equal to 1 for single-domain grains' (Néel,1949) and to 2 for multidomain grains (Néel, 1951). Forsubaerial lavas it has been found (Prévot, 1975) that thisratio varies indeed from 1 for high mdf values (150 oefor a weak field VRM acquired for 32 months) to 2 forlow mdf values (20 oe for a similar VRM). For sub-marine basalts, it can be seen in Table 1 that no evidentrelationship appears between the mdf and the AσJAσr,ratio. This may indicate that the magnetic viscosity ofsubmarine basalts is not only a thermally activated

viscosity, but results also from a diffusion aftereffectprocess.

CONCLUSIONS

1. The VIM carried by submarine basalts is notablylarger than the corresponding VRM measured just afterthe removal of the field. For the samples studied herethe mean ratio of the intensities is equal to five, the ex-treme values being 2 and 20.

2. The growth of the VIM with time is important.The VIM for t = 0.7 m.y. extrapolated from themeasurements carried out when t increases from 5 secto 20 min, is about four times larger than the VIM for t= 20 min (extreme values: 1.2 and 11). For about onequarter of the 20 samples studied, the VIM carried insitu is probably comparable in intensity to the naturalremanent magnetization.

3. Because of the systematic effect of the VIM, wecalculate that the global magnetization carried in situby normally magnetized blocks must be statisticallytwice larger—or so—than the magnetization carried bythe blocks with reverse NRM.

4. For most of the samples, the VRM is notablysmaller than the stable NRM. Moreover the VRM isnot difficult to erase by AF treatment. AF cleaningseems therefore an efficient method to get reliablepaleomagnetic results from most of these rocks.

505

C PLESSARD, M. PREVOT

TABLE 2Magnetic Measurements for Determination

of the Viscosity Index

Sample(Interval in cm)

Hole 332A

21-1, 131-13429-1, 74-7733-2, 92-9537-1, 116-119

Hole 332B

2-2, 86-892-6, 129-1323-4, 10-159-2, 104-10614-2, 81-8322-1, 57-6025-4, 47-4927-2, 4-628-2, 23-2633-1,62-6535-1, 4-735-1,46-4935-1, 104-10735-1, 136-13935-2, 29-3235-2, 33-3635-2,91-9435-3, 28-3135-3, 81-8436-1, 104-10636-2, 3-1236-2, 113-11536-2, 153-16136-3, 143-14536-5, 40-4236-6, 66-6837-1, 126-12837-2, 43-4537-2, 102-10543-2, 94-9644-1, 75-7744-4, 24-26

Site 335

5-2, 36-389-2, 74-7610-5, 101-10312-3, 115-117

Weight

(g)

39.042.646.343.9

57.952.453.944.743.337.942.715.545.245.614.416.414.914.434.916.616.218.312.017.417.712.622.417.546.916.518.321.213.940.432.616.1

47.452.448.546.4

σNRM

66.563.2

608155

58.079.924.1

14528.4

29560.2

10011133.210.040.725.653.524.972.148.0

18017613610415114613473.9

118109192517177

99.1200

12474.2

139194

σVRM

6.415.05

18.22.39

1.385.929.095.155.541.325.625.103.325.706.256.115.027.656.159.666.195.941.232.860.420.455.840.271.491.773.822.308.790.909.363.60

2.531.240.821.94

V(%)

9.68.02.81.5

2.67.4

37.83.5

19.50.49.35.13.0

17.262.515.019.614.324.713.412.9

3.30.72.10.40.34.00.22.01.53.51.21.70.59.41.8

2.01.70.61.0

Note: The specific magnetizations σ are given in 10 °emu/g. v is the viscosity index (defined in the text).

5. There is some evidence that the magnetic viscosityof these submarine basalts is not only a thermally ac-tivated viscosity but results also from a diffusion aftereffect process. The diffusion processes may be easier intitanomaghemites (because of imperfections in thecrystal structure) than in fresh, nonoxidized titano-magnetites. This hypothesis would explain the increasein magnetic viscosity as the submarine basalts becomeolder and their titanomagnetites more maghemitized.

REFERENCESBiquand, D. and Prévot, M., 1970. Sur la surprenante

resistance à la destruction par champs magnétiques alter-

(0e peak)

Figure 2. Alternating field demagnetization curves forVRMs obtained in a 3-oe field applied for 1 month.Circles: Sample 332B-36-2, 3-12 cm; squares: sample332B-37-2, 102-105 cm; asterisks: Sample 332B-35-1,136-139 cm; triangles: Sample 332B-35-2, 29-32 cm.

natifs de 1'aimantation rémanente visqueuse acquise parcertaines roches sédimentaires au cours d'un séjour, mëmebref, dans le champ magnétique terrestre: C.R.Aca. Sci.Paris, v. 270, p. 362-365.

Lecaille, A., Prévot, M., Tanguy, J-C, and Francheteau, J.,1974. Intensité d'aimantation de basaltes dragués dans lerift médio-atlantique vers 36°50'N) C.R.Acad. Sci. Paris,v. 279, p. 617-620.

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.

Néel, L., 1949. Théorie du trainage magnétique desferromagnétiques en grains fins avec application aux terrescuites: Ann. Géophys., v. 5, p. 99-136.

, 1951. Le trainage magnétique: J. Phys. Radium,v. 12, p. 339-351.

Plessard, C, 1971. Modification des propriétés magnétiques,en particulier du trainage, après réchauffement d'uneroche préalablement stablisée thermiquement: C.R. Acad.Sci. Paris, v. 273, p. 97-100.

Pozzi, J-P. and Thellier, E., 1963. Perfectionnements récentsapportés aux magnétomètres de très haute sensibilitéutilises an minéralogie magnétique: C.R. Acad. Sci. Paris,v. 257, p. 1037-1041.

Prévot, M., 1975. Magnétisme et minéralogie magnétique deroches néogènes et quaternaires: These, Paris.

Prévot, M. and Grommé, S., 1975. Intensity of magnetizationof subaerial and submarine basalts and its possible changewith time: Geophys. J. R. Astron. Soc, v. 40, p. 207-224.

Thellier, E., 1967. A "big sample spinner magnetometer". InMethods in paleomagnetism; Amsterdam (Elsevier),p. 150-154.

Thellier, E. and Thellier, O., 1944. Recherches géomagné-tiques sur des coulees volcaniques d'Auvergne: Ann. Geo-phys., v. 1, p. 37-52.

506