Natural remanent magnetization and thermomagnetic properties of the Allende meteorite

EARTH AND PLANETARY SCIENCE LETTERS 17 (1972) 120-128. NORTH-HOLLAND PUBLISHING COMPANY

NATURAL

REMANENT

MAGNETIZATION

AND THERMOMAGNETIC

OF THE ALLENDE

PROPERTIES

METEORITE

Robert F. BUTLER* Department of Geophysics, Stanford University, Stanjbrd, Calif. 94305, USA Received 5 May 1972 Revised version received 16 August 1972 Paleomagnetic examination of a l-kg specimen of the Allende meteorite has revealed the existence of natural remanent magnetization of constant direction and moderate intensity (3.0 × 10 -4 emu/g). Cooling of the fusion crust in the Earth's magnetic field following atmospheric entry has remagnetized the outer 1 mm of the specimen. Comparison of AC demagnetization spectra of NRM with spectra of laboratory-produced remanence suggests that the natural remanence is thermal or thermochemical remanence formed during original accretion or post-accretion reheating of Allende. Paleointensity determinations on three samples show that the NRM was formed in a field of approximately 1.10e. Thermomagnetic analysis indicates that the ferromagnetic minerals in Allende consist of 95 wt% taenite containing 67% Ni plus 5 wt% taenite with 36% Ni. These iron-nickel minerals amount to only 0.46 wt% of the'whole rock.

1. Introduction A recent investigation by Banerjee and Hargraves [ 1] has established the existence of natural remanent magnetization (NRM) in 1 Type I and 3 type II carbonaceous chondrites. These meteorites have undergone no reheating since accretion and may serve as recorders of the magnetic field present in the primary solar nebula. Type III carbonaceous chondrites may have suffered some reheating after accretion [2], and could record the magnetic field present during cooling from the peak reheating temperature. Alternatively, Type III carbonaceous chondrites could have accreted from slightly differentiated material. Thus, they could act as recorders of the magnetic field in the solar nebula at a different time or place than recorded by Type I and Type II carbonaceous chondrites. The Allende meteorite shower occurred in February 1969 over northern Mexico. Preliminary petrologic investigation by King et al. [3] and more detailed examination by Clarke et al. [4] have established Allende * Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minn. 55455, USA.

as a Type Ill carbonaceous chondrite. The K - 4 0 A r age of the chondrules in Allende is (4.44 -+ 0.14) X 109 yr [5]. The existence of stable NRM in Allende would constitute evidence that the meteorite cooled in the presence of a magnetic field. This field could have been produced by the parent body from which Allende was derived. An alternative possibility is the existence of an interplanetary magnetic field at this early stage in the history of the solar system. The existence of such an interplanetary field has been postulated by Sonnet et al. [6]. As suggested by Banerjee and Hargraves [1] a determination of the intensity of the field responsible for the natural remanence could help to distinguish between these alternatives and could also aid in determining the required size and number o f meteorite parent bodies. Thermomagnetic analysis of Allende was performed in order to supplement the optical and electron microprobe examination by Clarke et al. [4] in which taenite (or possibly awaruite) with 6 7 - 6 8 % Ni was identified as the dominant ferromagnetic mineral.

121

R.F. Butler, Natural remanent magnetization and properties o f Allende meteorite

2. Natural r e m a n e n t m a g n e t i z a t i o n The specimen of Allende e x a m i n e d was a l-kg stone c o m p l e t e l y covered by a fusion crust o f approximately 1 m m in thickness. The fusion crust is developed by heating during atmospheric entry. However, removal o f m o l t e n material by ablation during entry is more rapid than inward diffusion of heat and theoretical calculations by Lovering, Parry, and Jaeger [7] indicate that only the outer 1/2 cm of iron m e t e o r i t e s is significantly heated. An even smaller layer o f chondritic m e t e o r i t e s would be affected because of their lower thermal conductivity.

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Fig. 2. Stereogram showing motion of the direction of remanence during progressive AC demagnetization. The numbers indicate the peak demagnetizing field (in Oe), and the mean direction for the interior samples is shown by the square.

I

/ /

/

/

/

Fig. 1. Equal area stereogram of natural remanent magnetization directions. The solid figures indicate directions in the lower hemisphere while open figures are in the upper hemisphere. Circles are NRM directions for interior samples while triangles are directions of natural remanence for samples from the exterior of the specimen. The mean of the interior directions is shown by the square. Five cores were drilled c o m p l e t e l y through the specim e n and the cores were carefully oriented w i t h respect to each other. Each core was then cut into at least three samples suitable for m e a s u r e m e n t on a spinner m a g n e t o m e t e r . The samples f r o m the ends o f each core contained some of the fusion crust.

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Fig. 3. Stereogram illustrating movement in direction of natural remanence during removal of fusion crust from an exterior sample. The thickness of material removed is shown to the right of each direction, and the square indicates the mean of the NRM directions for interior samples.

122

R.F. Butler, Natural remanent magnetization and properties o f A llende meteorite

The directions of natural remanent magnetization are shown in fig. I. NRM directions for samples from the interior of the specimen are tightly grouped while directions for samples from the exterior (i.e., samples containing fusion crust) show a larger scatter. The intensity of NRM for interior samples shows only a slight variation and averages to 3.0 × 10 _4 emu/g. A significantly higher average intensity of 5.5 × 10 4 emu/g was found for the exterior samples. The tight grouping of directions for interior san> pies along with scattered directions of NRM in exterior samples suggests that a secondary component of lnagnetization has been produced by cooling of the fusion crust and baking of a thin layer of underlying material in the Earth's magnetic field. The results of progressive alternating-current (AC) demagnetization of one of the exterior samples is shown in fig. 2. Removal of the secondary component moves the direction of remanence along a great circle toward the mean direction for the interior samples. As a further test that the scattering of NRM directions is due to the fusion crust, the crustal layer was removed from another of the exterior samples in a stepwise manner and its NRM direction change observed. Fig. 3 shows the results of this experiment in which removal of only 1 mm of crustal material was sufficient to move the direction of remanence to the interior group mean. The intensity of remanence in the fusion crust was 1.4 X 10- 3 emu/g. Thus, Allende possesses a natural remanent magnetization of moderate intensity and constant direction in its interior along with a remagnetization of the external mm by cooling of the fusion crust in the Earth's magnetic field. 3. Stability of natural remanence The natural remanent magnetization in Allende could have been produced by three different mechanisms: (1) isothermal magnetization acquired in the Earth's field during the 2 years between atmospheric entry and measurement of NRM; (2) isothermal magnetization by a field in space; or (3) formation of thermoremanent or thermochemical remanent magnetization by heating and cooling in the presence of a magnetic field in space. Resistance of isothermal ternanent magnetization (IRM) to AC demagnetization is less than the resistance of thermoremanence or ther-

mochemical remanence. Comparison of AC demagnetization spectra of different forms of laboratoryproduced remanence with the spectrum o f the NRM can thus help to differentiate between the possible origins of the natural remanence. Fig. 4 shows the AC demagnetization spectrum of the natural remanence and the spectra of two types of laboratory-produced remanence. A 4 ° change in direction of NRM was observed upon demagnetization in 1 2 . 5 0 e but no further change occuried. Almost all the natural remanence is contained in grains having coercive forces of > 200 Oe and only a slight scatter of direction was noted during repeated treatment at 800 Oe. The AC demagnetization spectrum shows that the NRM is very stable and, in fact, is comparable in stability to terrestrial basalts used for paleomagnetic research. Tile same sample was subsequently exposed to a 20-Oe field at room temperature for 30 sec and the resulting remanence was AC demagnetized. An intensity of only 5.2 X 10-5emu/g was produced by this exposure. Comparison of the AC demagnetization spectrum of this 20-Oe IRM with the natural remanence spectrum shows that this low field isothermal remanence is very much softer than the NRM. Both the low intensity of this laboratory produced remanence and its lack of resistance to demagnetization indicate that the natural remanence observed in Allende could not have been produced by isothermal exposure to any realizable magnetic field in space. Nor could the NRM have been produced simply by exposure to the Earth's magnetic field. Exposure of the same sample to a 14-kOe magnetic field for 10 sec at room temperature was used to produce saturation isothermal remanence (SRM) of 4.4 X 10 - 2 emu/g. SRM is more resistant to AC demagnetization than any isothermal remanence produced by a smaller field. However, comparison of the demagnetization spectrum of this remanence with the natural remanence spectrum shows that even saturation isothermal remanence is not as strongly held as is the NRM. This comparison can best be made by noting the relative amounts of remanence contained in grains having coercive force > 200 Oe. The fact that no isothermal remanence can account for the stability of the NRM strongly suggests that the natural remanence is thermoremanent or thermochemical remanent magnetization. Radiometric

R.F. Butler, Natural remanent magnetization and properties of Allencle meteorite 80 F

NATURAL

REMANENCE

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12 5

25

50

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200

400

800

>800

I00

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60

ISOTHERMAL

REMANENCE

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12 5

25

50

I00

200

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No comparison of natural remanence with laboratory-produced thermoremanence is shown because of chemical changes which occur in some ferromagnetic constituents during heating to high temperature (>500°C). Such comparisons are only valid when no changes in the possible carriers of remanence take place during the production of the different types of remanence. Alteration of the AC demagnetization,~pectrum of the natural remanence in Allende by the cosmicray exposure mechanism of Butler and Cox [8] is unlikely to be important for two reasons. First, this mechanism has proven to be important only in cases where the original remanence is quite soft (e.g., with median microscopic coercivity of 25 Oe, as opposed to > 400 Oe in Allende). Secondly, the cosmicray exposure age of Allende is only 5 my [9] and radiation damage would not be very significant for this short exposure.

>800

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4. Thermomagnetic analysis SATURATION

ISOTHERMAL

REMANENCE

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0

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12 5 PEAK

25

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200 FIELD

400

800

>800

( oersteds )

Fig. 4. Histogram of percent of remanence contained in each AC demagnetization interval. For example, approximately 45% of the natural remanence is contained in grains having coercive force between 400 and 800 Oe. The intensities of remanence before demagnetization were 3.4 X 10 -4, 5.2 X 10-5 , and 4.4 X 10 -2 emu/g for natural remanence, 20 Oe isothermal remanence, and saturation isothermal remanence, respectively. dating of Allende by Fireman, DeFelice, and Norton [5] indicates that the reheating event took place early in the meteorite's history. Therefore, the natural remanence was almost certainly produced by the presence of a magnetic field during the original accretion or during the post-accretion heating. This field could have been produced either by the parent body from which Allende was derived or by an interplanetary magnetic field at this early stage in the history of our solar system.

Polished section examination and electron microprobe analysis by Clarke et al. [4] indicates that i r o n nickel alloys are the dominant ferromagnetic constituents in Allende. Thermomagnetic analysis was undertaken in order to identify the number, composition, and concentration of these ferromagnetic minerals. A typical example of saturation magnetization-vs.temperature ( J s - T ) data is shown in fig. 5. The instrument used was the continuously recording magnetic balance described by Doell and Cox [ 10]. The magnetizing field was 4 kOe while the maximum temperature reached was 800°C. Although all J s - T runs were made in a continuously monitored vacuum of < 2 X 10 -4 torr, the possibility still exists that outgassing of the samples during heating could provide sufficient local oxygen pressure for oxidation of the ferromagnetic grains. The heating curve in fig. 5 shows Curie points at 320 and 610°C while the cooling curve shows only one Curie point at approximately 590°C. Curie temperatures were determined by the method of Gromm~ et al. [11]. The thermal cycling has also produced an increase in the room temperature saturation. Repeated J s - T runs on the same sample show only the

R.F. Butler, Natural remanent magnetization and properties of Allende meteorite

124

590°C Curie point and no further increase in room temperature magnetization was observed. 0.8

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by the curve labeled "Y to ct cooling". Thus, iron nickel alloys with < 30% Ni display a large thermal hysteresis in Js-T curves. Iron-nickel alloys with > 30% Ni are "y-phase at all temperatures and have Curie points as indicated in fig.6. The 320°C Curie point in the heating curve is that of a "),-phase containing 36% Ni. Although pyrrhotite (Fe St+x) has a Curie point of 320°C when within the composition range 0.10 < X < 0.14 [13], the electron microprobe examination of AIlende by Clarke et al. [4] indicates that the troilite present is pure FeS. Stoichiometric troilite is antiferromagnetic and would not produce a Curie point in thermomagnetic curves.

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400

800 ~\\

600

TEMPERATUG'E (°C)

Fig. 5 . J s - T curve in vacuum for a typical sample. The heating and cooling curves are distinguished by arrows, while the vertical lines indicate the Curie temperatures observed. The m a x i m u m temperature of heating was 800°C and the magnetizing field was 4 kOe.

Oxidation of metal grains to magnetite (Fe304) or trevorite (NiFe204) cannot be responsible for the Curie point at 590°C upon cooling. Hysteresis loops measured with a vibrating sample magnetometer in fields up to 14 kOe before and after thermal cycling show a 16% increase in saturation magnetization of the ferromagnetic constituents. Oxidation would lead to a decrease in the saturation moment. An acceptable explanation of the thermomagnetic data must account not only for the changes in Curie temperatures during thermal cycling but also for the large increase in room temperature saturation magnetization. Phase transition and Curie point temperatures of iron-nickel alloys determined by Pickles and Sucksmith [12] are shown in fig.6. Alloys with < 30% Ni are body-centered cubic (s-phase) at room temperature and transform to a face-centered structure (3`phase) at a temperature given by the curve labeled "~ to 1' heating". The "/-phase is paramagnetic at the phase transition temperature and thus the phase transformation gives the appearance of a Curie point. Upon cooling from the "),-phase region, the reverse (3` to ~) transition does not occur until a much lower temperature is reached. This temperature is indicated

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~ 600 400

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20 40 60 80 PERCENT NICKEL IN IRON

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Fig. 6. Phase transformation and Curie point temperatures for i r o n - n i c k e l alloys. Fine lines indicate the compositions of the 3,-phase alloys responsible for the Curie points observed in the thermomagnetic data. See text for details (after Pickles and Sucksmith [ 13 ] ).

The 610°C transition could be either the Curie point of a 3`-phase with approximately 67% Ni or the c~ to 3' phase transformation of a 17% Ni or-phase. However, the lack of a large thermal hysteresis indicates that the high temperature transition is a Curie point rather than a phase transformation. Therefore, the dominant ferromagnetic phase is a 67% Ni "),-phase This determination is in agreement with the published results of Clarke et al. [4] and Banerjee and Hargraves [14]. On the basis of electron microprobe examination, Clarke et al. determined that the dominant iron-nickel mineral contained 67% Ni and 1.6% Co. Addition of small amounts of cobalt will raise the Curie point by 1 or 2°C but will not affect the thermomagnetic analysis to follow. By using saturation induction values for 36 and 67% Ni alloys given by Bozorth [15] and subtracting the paramagnetic contribution from the room temperature saturation magnetization, a starting compo-

R.F. Butler, Natural remanent magnetization and properties o.f Allende meteorite

sition of the ferromagnetic constituents in Allende can be calculated. Paramagnetic susceptibility was determined by examination of room temperature hysteresis loops in fields of up to 14 kOe. Analysis of the sample whose thermomagnetic curve is shown in fig. 5 shows the starting iron-nickel content to be: 90 wt% 7-phase containing 67% Ni and 10 wt% 7-phase containing 36% Ni. The ferromagnetic constituents comprise only 0.45 wt% of the whole rock. Mixing of these two starting phases would yield an alloy with 59% Ni. As can be seen in fig. 6, this composition has a Curie point at 590°C. Thus, high temperature homogenization of the original twophase system could produce the Curie point changes observed during thermal cycling. Examination of saturation induction (Bs) as a function of % Ni shown in fig. 7 suggests that the increase in room temperature saturation magnetization produced by heating is a natural consequence of high temperature homogenization of the ferromagnetic minerals. The starting phases of 36 and 67% Ni have B s = 13.0 × 103 gauss andB s = 13.3 X 103 gauss respectively, while mixing to produce a 59% Ni alloy would increase saturation induction to 14.5 × 103 gauss.

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70

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PER CENT NICKEL IN IRON

Fig. 7. Saturation induction as a function of percent nickel in iron (after Bozorth [15]). Saturation magnetization (as) of the pure ferromagnetic phases can be calculated from saturation induction by:

125

o s = B s / 4 n p emu/g

where: p = density in g/cm. Application of the above formula using the values o f B s previously cited and densities given by Bozorth [ 15] yields saturation magnetization values of 104 and 106 emu/g for 36 and 67% Ni phases respectively. For 59% Ni, the saturation magnetization would be 138 emu/g. The calculated starting composition would have an average saturation magnetization per g of ferromagnetic material of 106 emu/g. However, homogenization would produce an increase to 138 emu/g. The calculated increase in room temperature saturation magnetization of the rock (paramagnetic contribution removed) would be from 0.44 emu/g to 0.57 emu/g. This agrees with the observed increase of from 0.44 to 0.56 emu/g. These results are summarized in table 1. Therefore, high temperature homogenization of the original two phase system can account for both the changes in Curie points and the increased room temperature saturation magnetization produced by heating. Thermomagnetic analyses of four additional samples show the same characteristics with varying content of the two starting phases. The average starting compositions of the ferromagnetic constituents are: 95 wt% "y-phase with 67% Ni and 5 wt% y-phase containing 36% Ni. The average total iron-nickel content of Allende is calculated to be 0.46 wt%. In order to determine the temperature at which mixing of the starting phases occurs, one sample was heated in 100°C steps and its saturation magnetization accurately determined after each heating. Most of the expected increase took place between 600 and 700°C with a smaller increase between 400 and 600°C. An alternative explanation of the thermomagnetic properties of Allende is given by Banerjee and Hargraves [14]. They attribute the 320°C Curie point in the heating curve to the presence of pyrrhotite. Reactions between pyrrhotite, troilite, and 67% Ni "r-phase are thought to account for the changes in Curie points during thermal cycling.

126

R.F. Butler, Natural remanent magnetization and properties of Allende meteorite

Table 1 Comparison between observed and calculated values of saturation magnetization before and after homogenization of the starting composition. Composition of starting phases

Js (emu/g)

Js'(emu/g) Calculated

10 wt% 7-phase with 36% Ni + 90 wt% 7-phase with 67% Ni

0.44

Observed

0.57

0.56

Js = saturation magnetization before homogenization while Js' = saturation magnetization after homogenization.

5. Paleointensity determination The natural remanence of Allende is of thermal or thermochemical origin and, therefore, presents an opportunity for determination of the field strength in which it was magnetized. Such a determination would yield an estimate of the field strength present during the cooling of the meteorite from its peak reheating temperature. Three samples were examined according to the procedures outlined by Coe [ 16]. The samples were heated and cooled in vacuum of 10-5 tort in a continuously monitored field-free space with total field < 1007. A solenoid surrounding the oven was used to apply a 0.59-Oe field during formation of thermoremanence. Two of the samples were AC demagnetized in 25-Oe peak field before each measurement of remanence while the remaining sample was measured without demagnetizing. Results of five heating steps from room temperature to 200°C are shown in fig. 8. Results for the samples which were AC demagnetized are shown as (a) and (b) in fig. 8. The least-squares best fit straight lines to data points from 30 to 150°C indicate paleointensities of 1.31 -+ 0.42 Oe, 1.08 -+ 0.15 Oe, and 0.93 -+ 0.20 Oe for samples a, b, and c, respectively (error intervals are at the 90% confidence level). The average paleointensity for the three samples is approximately 1.10e. Banerjee and Hargraves [14] have obtained an average paleointensity of 1.09 Oe for two samples of Allende. Heating to greater than 150°C causes deviation from the theoretically expected linear behavior of NRM-vs.TRM. The exact reason for this behavior is not known. However, the deviation from linearity at higher temperatures suggests that the natural remanence is at least partly thermochemical remanence rather than simple thermoremanent magnetization.

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Fig. 8. Natural remanent magnetization-vs.-thermoremanent magnetization. Data points shown were obtained at room temperature, 80°C, 110°C, and 150°C. Results for two samples which were AC demagnetized in 25-Oe peak field before each measurement of remanence are shown in (a) and (b), while results for the sample which was not demagnetized are shown in (c).

Reliability of paleointensity determinations on samples containing thermochemical remanence is uncertain. However, the well-behaved nature of the lower temperature data shown in fig. 8 suggests that the chemical change responsible for formation of the thermochemical remanence was not operative below 150°C. Thus, the natural remanence with blocking temperatures below 150°C is almost certainly thermoremanence and will yield a valid estimate of paleointensity. 6. Conclusions Examination of a 1-kg specimen of the Allende meteorite has established the existence of a stable natural remanent magnetization with an average intensity of 3.0 X 10 -4 emu/g. AC demagnetization

R.F. Butler, Natural remanent magnetization and properties o f Allende meteorite

and progressive removal of the fusion crust has shown that deviation of the direction of NRM for exterior samples from the mean internal direction is due to a remagnetization of the fusion crust following atmospheric entry. The AC demagnetization spectrum of the NRM shows that the natural remanence is very stable. No significant changes in direction occur during demagnetization. Comparison of the demagnetization spectra of NRM and 20 Oe isothermal remanence indicates that the natural remanence cannot be accounted for by viscous magnetization in a small isothermal field. Thus, the original magnetization must be of extra-terrestrial origin. Comparison of spectra of the original magnetization and saturation isothermal remanence suggests that the natural remanence must be thermal or thermochemical remanence. Therefore, the natural remanence was most likely produced by cooling in the presence of a magnetic field following accretion or following post-accretion reheating. Thermomagnetic curves on samples of Allende show changes in Curie points and saturation during thermal cycling in vacuum. Curie points of 320 and 610°C during heating are replaced by a single Curie point at 590°C upon cooling. Thermal cycling yields a 16% increase in saturation magnetization at room tempera, ture. These unusual characteristics can be accounted for by the existence of two phases of taenite (')'-phase iron nickel) which mix upon heating to high temperature ( > 600°C). The average composition of the ferromagnetic constituents was found to be: 95 wt% T-phase with 67% Ni and 5 wt% ")'-phase containing 36% Ni. The total iron nickel content of Allende is calculated to be 0.46 wt% of the whole rock. Examination of three samples by a variation of Thellier's method yields paleointensity values of 1.31 + 0.42 Oe, 1.08 + 0.15 Oe, and 0.93 + 0.20 Oe. Although the natural remanence is probably partly thermochemical remanence, the average paleofield of approximately 1 . 1 0 e is thought to be a good estimate of the magnetic field intensity present at the time of formation of the NRM in Allende. This paleointensity determination represents an important constraint on the magnitude of the magnetic fields present 4.4 by ago during the early history of our solar system. If Ailende was magnetized by the body from which it was derived, this parent planet must have possessed a strong magnetic moment.

127

The alternative to magnetization by the parent body would be magnetization by an interplanetary field. This field would have to be many orders of magnitude stronger than the presently observed 1O's of gammas. The existence of such interplanetary fields during early stages of the solar system has been postulated by Sonnet et al. [6].

Acknowledgements Dr-. Robert Folinsbee of the University of Alberta kindly provided the sample of Allende. 1 thank Dr. C.S. Gromm~ of the Rock Magnetics Laboratory, U.S. Geological Survey in Menlo Park, California for use of the magnetic balance and field-free space. I am also indebted to Dr. Subir Banerjee for helpful discussions of this project and to my advisor Dr. Allan Cox for his encouragenaent and guidance. This work was supported by NSF grant GA 30685.

References [ 1 I S.K. Banerjee and R.B. Hargraves, Natural remanent magnetization or" carbonaceous chondrites, Earth Planet. Sci. Letters I0 (1971) 392. [2 ] J.A. Wood, Meteorites and the Origin of Planets (McGrawHill, Maidenhead, 1968) 128 pp. [3] E.A. King Jr., E. Schonfeld, K.A. Richardson and J.S. Eldrige, Meteorite fall at Pueblito de Allende, Chihuahua, Mexico: preliminary information, Science 163 11969) 928. [4 ] R.S. Clarke Jr., E. Jarosewich, B. Mason, J. Nelen, M. Gomex and J.R. Hyde, The Allende, Mexico, meteorite shower, Smithsonian Contrib. Earth Sci. 5 (1971) l. [51 E.L. Fireman, J. DeFelice and E. Norton, Ages of the Allende meteorite, Geochim. Cosmochim. Acta 34 (1970) 873. [6] C. Sonnet, D. Colburn, K. Schwartz and K. Keil, The melting of asteroidal-sized bodies by unipolar dynamo induction from a primordial T Tauri sun, Astrophys. Space Sci. 7 (1970) 446. [7] J.F. Lovering, L.G. Parry and J.C. Jaeger, Temperaturc and mass losses in iron meteorites during ablation in the earth's atmosphere, Geochim. Cosmochim. Acta 19 (1960) 395. [8] R.F. Butler and A.V. Cox, A mechanism for producing magnetic remanence in meteorites and lunar samples by cosmic-ray exposure, Science 172 ( 1971) 939. [9] E.L. Firemen and R. Goebel, Argon 37 and Argon 39 in recently fallen meteorites and cosmic-ray variations, J. Geophys. Res. 75 (1970) 2115.

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R.F. Butler, Natural remanent magnetization and properties o f Allende meteorite

{10] R.R. Doell and A.V. Cox, Recording magnetic balance, in: Methods in Paleomagnetism, ed. D.W. Collinson, K.M. Creer and S.K. Runcorn (Elsevier, Amsterdam, 1967) 440. [11] C.S. Grommg, T.L. Wright and D.L. Peck, Magnetic properties and oxidation of iron-titanium oxide minerals in Alae and Makaopuhi lava lakes, Hawaii, J. Geophys. Res. 74 (1969) 5277. [ 12] A.T. Pickles and W. Sucksmith, A magnetic study of the two-phase iron-nickel alloys, Proc. R. Soc. London 175 (1940) 331.

[ 13 ] T. Nagata, Rock Magnetism (Maruzen, 1961). [141 S.K. Banerjee and R.B. Hargraves, Natural remanent magnetizations of carbonaceous chondrites and the magnetic field in the early solar system, Earth Planet. Sci. Letters, this issue. [15] R.M. Bozorth, Ferromagnetism (Van Nostrand, 1951) 986 pp. [16] R.S. Coe, Paleo-intensities of the earth's magnetic field determined from Tertiary and Quaternary rocks, J. Geophys. Res. 72 (1967) 3247.