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Mefeorifics B Planefury Science 32,803-81 1 (1997) 8 Meteoritical Society, 1997. Printed in USA.

Magnetic properties of Martian meteorites: Implications for an ancient Martian magnetic field

D. W. COLLINSON

Department of Physics, The University, Newcastle upon Tyne NEl 7RU, England

(Received 1997 February 18; accepted in revisedform 1997 July 9)

Abstract-The magnetic properties of samples of seven Martian meteorites (EET 79001, Zagami, Nakhla, Lafayette, Governador Valadares, Chassigny and ALH 84001) have been investigated. All possess a weak, very stable primary natural remanent magnetization (NRM), and some have less stable secondary components. In some cases, the latter are associated with magnetic contamination o f the samples, imparted since their recovery, and with viscous magnetization, acquired during exposure of the meteorites to the geomagnetic field since they fell. The magnetic properties are carried by a small content (4%) of titanomagnetite and, in ALH 84001, possibly by magnetite as well. The most likely source of the primary NRM is a thermoremanent magnetization acquired when the meteorite material last cooled from a high temperature in the presence o f a magnetic field. Current evidence is that this was 1.3 G a ago for the nakhlites and Chassigny and 180 M a for shergottites: the time of the last relevant cooling of ALH 84001 is not presently known. Preliminary estimates of the strength of the magnetizing field are in the range 0.5-5 pT, which is a t least an order of magnitude greater than the present field. It is tentatively concluded that the magnetic field was generated by a dynamo process in a Martian core with appropriate structure and properties.

INTRODUCTION

In the early 1980s, a new dimension in meteorite studies emerged when evidence was adduced that a small number of differentiated achondrite meteorites originated on the planet Mars (Wood and Ashwal, 1981; McSween, 1985). Subsequent research has resulted in general acceptance by most meteoriticists of such an origin, and there are currently 12 Martian or "SNC" meteorites, so named after the original type samples Shergotty, Nakhla and Chassigny. A further type, an orthopyroxenite from the Antarctic, ALH 84001, has recently been added (Mittlefehldt, 1994). Research on these meteorites has focused not only on their physical and chemical properties and their relation to other meteorites, but also on the information they can provide on Martian petrology, geophysical processes and evolution (McSween, 1985, 1994). A physical property of some interest in this context is magnetism, in particular any permanent magnetism (natural remanent magnetization, NRM) that the meteorites possess. Natural remanent magnetization with appropriate characteristics is acquired when magnetic minerals are formed or undergo an event such as heating and cooling or shock in the presence of a magnetic field. Thus, following profitable terrestrial and lunar palaeomagnetic studies, there may be evidence from any NRM in SNC meteorites of an ancient Martian magnetic field. Such a field is clearly of considerable interest because of the possibility, or even probability, of its origin by "dynamo" action in a molten, electrically-conducting Martian core with important associated implications for Martian structure and evolution.

Previous magnetic investigations of SNC meteorites (Nagata, 1980; Cisowski, 1986; Collinson, 1986) suggest that at least part of the NRM observed was acquired in an ambient magnetic field on the parent body. In this paper, further studies are reported on seven SNC meteorites, listed below. Where possible, the samples obtained were taken from a region of the main mass remote from any fusion crust, where cooling in the Earth's magnetic field from the high temperature of entry may result in unwanted spurious magnetization. Also where possible, samples of the same meteorite were obtained from different sources to check for uniformity of properties and the possibility of magnetic contamination.

The following meteorites were investigated, with sample masses and their source:

ALH 84001-find, Antarctica, 1984; 14.9 g; two fragments from NASA Johnson Space Center, Houston, Texas.

Shergottites-Zagami: fall, Nigeria, 1962; 6.5 g; two fragments from Natural History Museum, London (NHM).

Elephant Moraine 79001: find, Antarctic, 1979; 2.7 g; two fragments from NASA Johnson Space Center, Houston, Texas.

Nakhlites-Nakhla: fall, Egypt, 191 1; 4.8 g from NHM, 1.0 g from Museum National &Histoire Naturelle, Paris (MNHN).

Lafayette: found in mineral collection at Purdue University, West Lafayette, Indiana, 1931; 2.4 g from NHM; 0.8 g from Center for Meteorite Studies, Arizona State University, Tucson, Arizona; 1.8 g, two fragments from Princeton University, Princeton, New Jersey; 0.15 g from Field Museum, Chicago, Illinois.

Governador Valadares: find, Brazil, 1958; 5.1 g from NHM. Chassignitdhassigny: fall, France, 1815; 2.2 g from NHM;

1.1 g from MNHN; 4.6 g from Naturhistorisches Museum, Vienna, Austria.

Several types of NRM may be acquired by meteorites during their origin and evolution. Thermoremanent magnetization (TRM) is acquired when a magnetic mineral is heated and then cools in a magnetic field. If the Curie temperature is exceeded (i.e., the temperature above which magnetic ordering of domains is lost), the resultant magnetization on cooling is known as the total TRM and is often the primary or most stable component of NRM in igneous rocks. Partial TRM (PTRM) can be acquired on cooling by those particles in which magnetic ordering is lost at a temperature below the Curie point, the "blocking" temperature. Partial TRM is a common source of secondary NRM in rocks, for instance, in rocks that have undergone moderate thermal metamorphism or heating through burial. If magnetic material undergoes sufficiently severe shock, a shock remanent magnetization (SRM) can be acquired, either through partial alignment of domains in a magnetic field under the influence of the shock or through shock-induced heating of the material. Vis- cous remanent magnetization (VRM) can be acquired during exposure of a meteorite to a weak magnetic field for an extended period of time. The VRM, the growth of which is proportional to the logarithm of

803

804 D. W. Collinson

time, can be significant in meteorite finds, which may have been exposed to the geomagnetic field for thousands or millions of years.

EXPERIMENTAL METHODS Remanent magnetization is a vector quantity, with intensity and direc-

tion, the latter being parallel to the direction of the magnetic field in which the magnetization was acquired and the former proportional to the strength of the field. Natural remanent magnetization was measured on a Molspin "Minispin" magnetometer, with a noise level equivalent to a magnetization of 0.4 x 10" Am2 kg-' in a 1 g sample. In general, the initial NRM observed will be the vector sum of two or more magnetization components, and magnetic "cleaning" procedures are applied as an aid in distinguishing different NRMs present and relating them to events in the meteorite's origin and evolution. One of the most common procedures is alternating field demagnetization, in which the rock is subjected to a decreasing alternating field of successively higher initial peak values, resulting in the removal of NRM components of different magnetic stabilities. Changes in NRM direction with increasing peak demagnetizing field, plotted on a stereographic projection, and in intensity then reveal the presence of different NRM components. In this work, a shielded Molspin demagnetizer was used, with two-axis tumbling and two-position orientation of the sample to eliminate spurious magnetizations. The maximum peak demagnetizing field was 100 mT. Anhysteretic remanent magnetization (ARM) was imparted in a direct field of 180 pT and a maximum peak alternating field of 100 mT. The direct field is applied by means of a small magnet array surrounding the sample, fitted into a two-axis tumbler. This ensures that for the palaeointensity estimates, the ARM is acquired and the NRM demagnetized under the same conditions.

Initial susceptibility measurements were carried out on a balanced transformer bridge, in which insertion of a sample into a gap in a transformer core produces a signal proportional to the susceptibility. Anisotropy of susceptibility was measured on a Molspin anisotropy delineator. The sample is rotated in an alternating field of -0.5 mT in a plane perpendicular to the

60

0

Z A l x Z A 2 0 Z A 3

rotation axis, and anisotropy in this plane produces a signal of twice the rotation frequency in a pick-up coil. Measurements about two other axes, combined with a susceptibility measurement along one axis, enables the susceptibility ellipsoid to be determined. Further details of the above and other palaeomagnetic techniques are described in Collinson (1983).

NATURAL REMANENT MAGNETIZATION (NRM)

Zagami

One of the two fragments received was broken into two mutually oriented pieces (ZAl, ZA2). The NRM intensities were 59 and 62 x lo4 A m 2 kg-1, with close agreement between directions of NRM in each fragment, indicating, as expected, uniform magnetization. On alternating field demagnetization, steady decay of intensity was associated with systematic migration of NRM direction, with no stable endpoint being reached (Fig. I). This indicates the presence of two NRM components in the meteorite, a magnetically hard primary component, the coercivity (a measure of its resistance to demagnetization) of which was not reached, and a softer secondary NRM acquired at a later time and removed by the cleaning procedure.

About 10% of the surface of the third fragment (ZA3) is covered with fusion crust. Initial NRM intensity was 22 x 10" Am2 kg-1, and demagnetization behaviour was closely similar to that of ZAl and ZA2 (Fig. 1). This indicates that the local heating and alteration associated with the crust has not significantly modified the bulk NRM of the fragment. The lower intensity in ZA3 prob- ably reflects a smaller content of magnetic mineral compared with ZAl and ZA2.

s

..

I I I I I J 40 80 120

B ( m T ) FIG. 1. Alternating field demagnetization of Zagami samples. In the stereogram, the plane of projection is the reference surface on the sample: the declination of the NRM is measured clockwise from an arbitrary direction on this surface and the inclination is measured inwards from the circumference (0') to the centre (90"). Samples ZAl and ZA2 are mutually oriented.

Magnetic properties of Martian meteorites 805

Elephant Moraine 79001

The magnetic properties of this meteorite have pre- viously been investigated by the author (Collinson, 1986), and the presence of a single, primary magnetization was deduced. A further fragment has been studied, taken from a different region of the 800 g meteorite, giving very similar results to those obtained earlier.

Nakhla

After measurement of the initial NRM of the 4.8 g NHM sample, it was broken into three mutually oriented fragments. Within the limits of the orientation error, the NRM directions in the subsamples agreed with each other and with the NRM of the original sample. The intensities were in the range (40 & 5) x 10" Am2 kg-I. Figure 2 shows alternating field demagnetization of subsamples NAl.l and NA1.3 and NA2, the Paris fragment. A hard, primary NRM of intensity (5-1 0) x 10" Am2 kg-1 is revealed with perhaps minor secondary components, which is indicated by some migration of NRM directions. The Paris sample, NA2, has a much stronger NRM at 310 x 10" Am2 kg-1 and an intensity decay curve of different character to the NHM samples. However, the primary NRM seen in NAI . I and NAI .3 is also present in NA2, and one explanation of the high NRM intensity is that the sample has been exposed to a stray laboratory magnetic field since its collection. To test for this, the demagnetized sample was given an isothermal remanence (IRM, the remanent magnetization remaining after application and removal of a steady magnetic field) equal in magnitude to the initial NRM. The decay of the IRM on alternating field demagnetization can then be compared with that of the NRM, and this is shown in Fig. 2. The similar decay curves suggest that NA2 has been magnetically contaminated.

Lafayette

The investigation of Lafayette gave confusing results, with strong indications of magnetic contamination. The first fragment studied, from NHM, London, possessed an anomalously strong NRM of 30 x Am2 kg-l, which was uniform in the two subsamples (LAI, LA2) into which it was broken. On alternating field demagnetization, LA2 showed a rather rapid decay of RM intensity to -10 x 10-6 Am2 kg-I with no significant change in direction (Fig. 3), which indicated the presence of one dominant NRM component. The intensity was decreasing still at 100 mT de-

45

40

30 r I

-u 0

N

E 4

0 20 I a

0 9 - z

10

0

N A 1 . l o N A 1 . 3 X N A 2 - N R M

I 1 1 1 1 1 + 1 1 1 1

f-+ I 1 I I I I I I I 60 80 100 40 B(rnT) 20

FIG. 2. Alternating field demagnetization of Nakhla samples. For NA2, the NRM is 1Ox the vertical scale values up to 15 mT demagnetizing field and equal to the vertical scale at higher fields. Samples NAI.1 and NA1.3 are mutually oriented.

35 r 1

magnetizing field, implying that any distinct, hard component of NRM was weaker than the above level.

The fragment from Arizona (LA3) gave similar results. On alternating field demagnetization, the strong initial NRM of 25 x Am2 kg-I decayed in a similar way to LA2, except for a small migration of NRM direction at higher demagnetizing fields. However, the two fragments from Princeton, LA4.1 and LA4.2, had much weaker initial NRMs at 1 18 and 116 x 10" Am2 kg-1 and showed intensity and directional changes during demagnetization that suggest the presence of a weak primary NRM of -10 x 10" Am2 kg-1 with a stronger secondary NRM superimposed (Fig. 3). The small 0.15 g chip from Chicago had an NRM intensity of 590 x 10" Am2 kg-1 and decayed steadily and unidirectionally to -30 x 10" Am2 kg-1 at

FIG. 3. Alternating field demagnetization of Lafayette samples LA2 (left- hand scale) and LA4.1 (right-hand scale). The intensity decay of LA2 is not shown after 45 mT because it becomes too weak to record on the given scale.

120 mT demagnetizing field, which indicated a single NRM component (Fig. 4).

The very strong NRM of LAl, LA2 and LA3 is highly anomalous. The small variation in initial susceptibility (the ratio of induced magnetization to inducing field in a sample) and saturated isothermal remanence (the maximum value of the magnetization remaining when a sample is subjected to and then removed from an increasing applied field) indicate only small differences in magnetic mineral content among the five samples (Table 1). A possible explanation is that LAI, LA2 and LA3 have been exposed to a substantial magnetic field at some stage in their history, and the initial NRM is dominated by the resultant isothermal remanence (IRM). Tests show that a field of 50-60 mT is required to impart an IRM equal to the strong

806 D. W. Collinson

- 6001

- N~ 400 f - a

- mary magnetization in Lafayette. Sample LA4 shows a secondary component that is unlikely to be an IRM acquired in a laboratory field.

Governador Valadares

L A 5 O N R M x I R M . NRM

About one-third of the surface of the 5 g fragment is covered with fusion crust. Demagnetization revealed a stable primary NRM with a superimposed secondary com-

%-\ 0,

' 0 -1 \. ' 0

oriented.

TABLE 1. Magnetic properties of Martian meteorites.

Mass (8) NRM, NRM,

EET79001 211A 1.2 211B 1.5

Zagami ZAl 1.8 ZA2 1.5 ZA3 1.3

Nakhla NA1.1 0.3 NA1.2 2.4 NA1.3 1.9 NA2 1.0

Lafayette LA1 1.2 LA2 1.2 LA3 0.8 LA4.1 0.5 LA4.2 1.4 LA5 0.2

Governador GV 5.1 Valadares

Chassigny CH 2.2 CH2 4.6 CH3 1.1

ALH 84001 50.1 4.5 50.2 5.1 152 5.0

= not well defined. t = x 10-3

11.6 11.3

54.7 51.9 21.5

42.3 36.4 41.4

310

23.9t 3 1.6" 25.2t

I I8 147 589

104

0.5 18.0 2.6

2.8 1.8 2.0

~~

- 4

1 o* 5* 5*

8

8 6

<7 <I2

<4 10 7

4 0

9

-

0.3 I 0.6

0.2 0.2 0.2

X O

0.57 0.53

0.50 0.46 0.4 1

1.6 2.1 2.0 1.1

1.7 1.9 1.8 1.5 2.3 2.1

1.5

0.46 0.88 0.38

0.42 0.35 0.35

Jrs ~

-

12.4

8.3 9.1 -

76

82 65

61 61 62 90

71

75

-

-

0.8 15.8 0.5

I .5

1.1 -

AMS

1.012 1.012

1.020 -

-

1.009 1.009 -

-

1.028 1.032 1.042 - -

1.014

1.015 1.005 -

1.008 1.014 1.020

NRM, (initial NRM) and NRM, (primary NRM), units lo4 Amz kg-I; initial susceptibility, x,, units 10" m3 kg-I; saturation remanence (JJ, units Am2 kg-I; susceptibility anistropy (AMS), defined by x,,/xmi,. The primary NRM is the NRM that persists in 90-100 mT demagnetizing fields.

The initial NRM of CHI, the NHM sample, was very weak at 0.9 x lo4 Am2 kg-I, which is only -1Ox the equivalent noise level of the magnetometer. The alternating field demagnetization behaviour is shown in Fig. 6. After removal of a soft component of possibly viscous origin, there is evidence of two other components, stable in the ranges 23-15 mT and 25-50 mT demagnetizing fields, which is indicated by clustering of directions on the stereogram.

The Vienna fragment, CH2, was more strongly magnetized, with an initial NRM of 16 x 10" Am2 kg-I. This may be due to a small area of fusion crust on this sample; susceptibility and saturated IRM are much higher than in CHI and CH3 (Table I ) . De- magnetization reveals ii weak primary NRM and one or possibly two secondary components, although the NRM directional changes are rather different in character to-those seen in CHI. The demagnetization behaviour of the Paris sample (CH3) is more similar to that of CHI, with an initial NRM of 2.6 x 10" Am2 kg-I, a weak, prob- ably viscous component, and a further secondary component, together with a weak primary NRM (Fig. 6).

Allan Hills 84001

The magnetic properties of three fragments of ALH 84001, of which two can be mutually oriented, have been studied. They are weakly magnetized, the initial NRM being in the range (1-2) x lo4 Am2 kg-1. Behaviour during alternating field demagnetization is broadly consistent in the three fragments (Fig. 7), in that a hard primary component is revealed together with two other components, which is indicated by the clustering of points at 20-30 mT and the initial NRM directions. However, there are anomalous features in the results from fragments 50.1 and 50.2. The mutual orientation of the two fragments is estimated to be accurate to lo", and the demagnetization paths on the stereogram should be closely similar on the usual assumption of uniform NRM within the fragments. However, the initial NRM directions are -40" apart, and the primary components show a similar separation, although the clustered intermediate direc-


G V

I I I I I I 0 20 40 60 80 100

B ( m T )

FIG. 5 . Alternating field demagnetization of Governador Valadares.

tions are close together. Laboratory tests show that ALH 84001 acquires significant VRM during a few days exposure to the Earth's magnetic field, and this is a likely cause of the discrepancy in the initial NRM directions since 50.1 and 50.2 were stored at random orientation to the field prior to measurement.

BULK MAGNETIC PROPERTIES

Thermomagnetic measurements for the identification of magnetic minerals from the mode of decay of induced magnetization with temperature could only be carried out on Nakhla and ALH 84001. The curve for Nakhla shows a discontinuity indicating a Curie point of 500-550 "C, possibly due to a low-Ti titanomagnetite. Cisowski (1986) reports Curie points of 150 "C and 500 "C for Nakhla and a range of lower Curie points for other Martian meteorites, which cor-

responds to titanomagnetites of different compositions. Allan Hills 84001 has only a very small magnetic mineral content, and the decay curve is dominated by that due to the paramagnetic iron present. No magnetic mineral is identifiable.

Values of initial susceptibility (xo) and saturated IRM (JrJ are given in Table 1. From a knowledge of the contribution to susceptibility of the titanomagnetite and paramagnetic iron, the titanomagnetite content can be estimated. Using the average amount of paramagnetic iron present, as FeO, of 20 wt% (McSween, 1985), the estimated titanomagnetite content among the samples is 0.02-0.5 wt%. Saturation IRM is acquired in fields of 200-450 mT, which indicates variations in grain size and particle composition among the different samples and possibly grain interaction effects.

Figure 8 shows the acquisition of anhysteretic remanent magnetization (ARM), which is imparted when a sample is subjected to a decreasing alternating magnetic field superimposed on a small direct field, in this instance, 180 pT. The curves of ARM against alternating field indicate a range of grain sizes and coercivities. Zagami is unusual in its shape and acquisition of saturated ARM well above 100 mT. Anhysteretic remanent magnetization acquisition is the basis of a method for estimating the strength of an ancient magnetizing field: its application to the Martian meteorites is described later.

Anisotropy of magnetic susceptibility (AMS) (ix, susceptibility of a material varying according to the axis along which it is measured) arises in meteorites from shaped, aligned or partially aligned parti-

cles. It is defined by the anisotropy ellipsoid, with orth- ogonal axes corresponding to maximum, intermediate and minimum susceptibility. Anisotropy can arise from disc-shaped magnetic particles (foliation) or from needle- shaped magnetic particles (lineation). The present meteorite samples are only weakly anisotropic and both foliation and lineation occur. The ratio of maximum to minimum susceptibility is in the range I .007-1.042 (Ta- ble 1). The cause of particle shape and alignment in meteorites is not clear. Impact shock is a possible cause, but there is no evidence of correlation between shock level and magnitude of AMS in the samples investigated. For instance, no evidence for shock is seen in Lafayette, which has the most marked anisotropy, whereas EET 79001, which is severely shocked, shows only moderate to weak AMS.

ORIGIN OF REMANENT MAGNETIZATION All the meteorite samples investigated show evi-

dence from alternating field demagnetization of a stable, primary remanent magnetization, persisting up to demagnetizing fields in excess of 100 mT. This is the only

component present in EET 79001. In the other meteorites, there are one or more secondary NRMs present, which are indicated by the migration of NRM directions as the secondary components are removed during demagnetization or by evidence for magnetic contamination in Lafayette and the Paris Nakhla sample.

Although magnetic contamination can explain the Nakhla and Lafayette observations, there are some puzzling features. The field strengths required to impart the observed remanence, -60 mT and 20 mT for Lafayette and Nakhla, respectively, are relatively large and seem likely to be associated with some investigative technique rather than a chance exposure. Although it may be coincidental, it is also puzzling that the contaminating field direction is close to that of the primary NRM in each case, as shown by the small change in NRM direction when the IRM is removed. The author encountered

808 D. W. Collinson

h

I I I I I I I I I I 0 20 LO 60 80 100

B ( m T )

FIG. 7. Alternating field demagnetization of ALH 84001 samples. Full and open symbols are on the lower and upper hemisphere, respectively. Samples 50.1 and 50.2 are mutually oriented.

-

and ALH 84001. The most significant feature is the occurrence of a weak but stable primary magnetization in all the meteoritit samples, and three relevant processes may be considered, namely viscous remanent magnetization, shock remanent magnetization and thermoremanent magnetization. Viscous magnetization in igneous rocks, acquired during exposure to a weak magnetic field over a long time period, can be very resistant to alternating field demagnetization (Prevot, 1981), which is consistent with the stability of the primary NRM being considered here. However, if it was acquired in an ancient Martian magnetic field, the VRM would decay substantially during the time (1-1 0 Ma) that the meteorites were in the near-zero interplanetary field in space before impacting the Earth. Alternatively, in the ab- sence of a Martian field, a primary VRM could have been acquired in the geomagnetic field after Earth impact. Be- cause of the much shorter exposure time compared with a Martian origin, a terrestrial VRM is unlikely to have the observed stability against alternating field demagnetization, and the origin of secondary NRM is then also problemati- cal. The intensity decay curve of Zagami (Fig. 1) is quite different to what would be exDected for a VRM acquired at most in a few years.

Magnetization through shock, either in an ambient magnetic field or in a transienl, impact-generated field, has been much discussed as a possible magnetizing process for meteorites (Collinson and Morden, 1994). Unfortunately, SRM has no specific properties to distinguish it from other processes and at present there is no firm evidence for its occurrence in extraterrestrial materials. Although the Martian meteorites are expected to have been shocked, notably

1 84001,50.2 2 L A 3 3 N A 1 . 3 4 G V 5 CH 6 79001 7 Z A 3 a Z A I

FIG. 8 . Acquisition of anhysteretic remanent magnetization with increasing alternating field. The direct (biasing) field is 180,uT.

a similar case of apparent magnetic contamination in studies of the Olivenza chondrite (Collinson, 1987). Although direct evidence of contamination is highly desirable, it currently appears to be the most likely source of the anomalously strong NRMs in Lafayette and Nakhla.

Four main types of alternating field demagnetization behaviour are apparent in the Martian meteorites. Elephant Moraine 79001 and Shergotty (from the results of Cisowski, 1986) possess dominant primary NRM. In Nakhla, NAl.3 could be included in the previous group, but NAI.1 and NA2 show some evidence of primary and secondary components, although in the latter this may be a result of contamination. Governador Valadares, Zagami and Lafayette (LA4.1 and LA4.2) show clear evidence of a secondary component in each, and two secondary components are apparent in Chassigny

during ejection from Mars, the nakhlites and Chassigny are not appreciably shocked but their magnetic properties are comparable with the markedly shocked shergottites. It seems reasonable to suppose that shock may be a modifying influence, imparting weak secondary NRM in the presence of a magnetic field or partially demagnetizing a preexisting NRM in a very weak or zero ambient field.

Since the Martian meteorites have cooled from a high temperature during formation (or, in the case of ALH 84001, also experienced heating and cooling during its history), the author considers that the most likely origin of the primary NRM is a TRM acquired in an ambient magnetic field at the time of cooling. Thermoremanent magnetization is known to impart a primary NRM to many terrestrial and lunar rocks and most likely to other achondrite meteorites. The uncertainties surrounding SRM and VRM must make these processes less likely. Thermal demagnetization tests, in which samples are heated to successively higher temperatures followed by cooling in zero magnetic field and measurement of remaining NRM, could provide further evidence for TRM, which would persist in a sample up to the Curie point or maximum blocking temperature. Difficulties are often encountered with this technique, arising from thermal alteration of magnetic minerals, and only two attempts on Martian meteorites have been reported. Collinson (1986) thermally demagnetized a sample of EET 79001 and found a maximum blocking temperature of 400 OC, which could indicate a TRM. In a sample of Shergotty, Cisowski (1986) found that 90% of the NRM was removed by heating to only 100 "C. However, there was some sus- picion of terrestrial contamination in his sample, and to which temperature the remaining 10% persisted is not reported, although it appears to be much higher. Cisowski also reports difficulties in tests involving the heating of samples of Nakhla and Zagami.


The origin of secondary magnetizations in the Martian meteorites cannot be determined at present. Based on laboratory acquisition tests, viscous magnetization acquired in the geomagnetic field since Earth impact is likely to be a contributory source, particularly in ALH 84001, Governador Valadares and possibly Chassigny. Possible alternative processes are partial thermoremanent magnetization and shock magnetization. The former occurs when a rock is heated to a temperature lower than the Curie point and then cools in a magnetic field. Shock magnetization occurs when a rock is subjected to severe shock in the presence of either an ambient field or possibly a transient field generated during the shock event. Of the meteorites studied, EET 79001 and Zagami are heavily shocked, but the former has no secondary component. Allan Hills 84001 (discussed in more detail below) and Chassigny are moderately shocked, both with secondary components of NRM. Low shock levels occur in Nakhla, Lafayette and Governador Valadares, all of which possess secondary NRM. After the original crystallization, further metamorphic heating during which PTRM might be acquired does not seem to be a feature of the Martian meteorites (except ALH 84001). It is possible that the presence of secondary NRM is evidence for low-temperature secondary heating, as yet undetected by other investigations.

We now discuss the origin of the NRM of ALH 84001 in more detail, since the investigation of this meteorite in connection with possible ancient life on Mars (McKay et al., 1996) has revealed more details of its history and petrology.

The carrier of the NRM is not known with certainty. Allan Hills 84001 is of a different rock type to other Martian meteorites, but until McKay et al.'s paper, no titanomagnetite or indeed any other magnetic mineral had been reported. The weak initial magnetic susceptibility (-0.3 x lo4 m3 kg-I) and saturated remanence (-1 x m2 kg-' indicate a very small content of magnetic mineral. The decay curve of saturated remanence against temperature suggests a 350-400 "C Curie point, possibly titanomagnetite, and one at 550- 600 "C, which is consistent with magnetite. McKay et al.'s discov- ery of magnetite associated with carbonates in ALH 84001 raises the interesting question of whether it is contributing to the NRM. There is currently no estimate of the amount of magnetite present or whether it is uniformly distributed throughout the rock. The grain size of 10-100 nm quoted by McKay et al. covers the range of magnetic states ( i e . , superparamagnetic, single- and multidomain quoted by Butler and Banerjee, 1975), the latter two magnetic states potentially carrying NRM. Because of its strong magnetic properties, a very small amount could account for the magnetism of ALH 84001 ; -0.001% of single domain magnetite, averaged throughout the sample, would give the observed saturated remanence and would be ample to contribute the observed NRM. It is unlikely that the smaller amount of pyrrhotite occurring, which has weaker magnetic properties, is of importance magnetically.

According to Jagoutz et al. (1 994), Treiman (1995) and Ash et al. (1996), ALH 84001 crystallized -4.5 Ga ago, was shocked at 4.1 Ga and then experienced metamorphism (at 900 "C). This was followed by low-temperature alteration before a further shock event. These events could all bear on the acquisition of NRM by the meteorite, together with the mode of formation of the carbonates, with which the magnetite is associated. There is currently some contro- versy concerning the temperature of formation of the carbonates, whether "hot" (>650 "C) (Harvey and McSween, 1996) or "cold" (-400 "C) (Romanek, 1994; Treiman, 1995). If the former is the case, and assuming the whole rock is heated to the temperature, any TRM acquired by the low Curie point mineral prior to carbonate formation would be lost and remagnetization would occur if a field was present. The time of formation of the carbonates is also uncertain

but is believed to be sometime during the period 3.9-1.4 Ga (Knott et al., 1995; Wadwha and Lugmair, 1996). It is also not presently known whether magnetite and carbonate formation were contem- poraneous or whether magnetite formation was associated with later carbonate dissolution (McKay et aL, 1996). A chemical formation process for the magnetite is possible, or precipitation through bacterial activity, if there was primitive life on the planet, could be re- sponsible. The latter could occur through the anaerobic reduction of ferric oxide by dissimilatory microorganisms, as described by Lovley et al. (1987). The conditions would not appear to be appropriate for magnetotactic bacteria, which produce and utilise magnetite to guide them along magnetic field lines to sites of optimum environment (Blakemore and Frankel, 1981). Magnetite from this source is known to contribute NRM to some terrestrial sediments (Petersen et al., 1986). Magnetite of either chemical or biogenic origin could become magnetized in an ambient Martian field as the magnetite formed. The process, chemical remanent magnetization (CRM), is known to occur on Earth, for example in the hematite pigment precipitated in red sandstones and in the in situ oxidation of magnetite to hematite.

The following scenario is tentatively proposed for the magnetic history of ALH 84001. A Martian magnetic field is unlikely to have existed at the time of original crystallization, and it is unlikely that a TRM would have been acquired in a weak and highly irregular interplanetary field. However, during the later thermal metamorphism, any TRM would be replaced by a new TRM during cooling, if there was an ambient magnetic field. If there was later low-temperature carbonate formation, this new TRM would be the primary component revealed at high demagnetizing fields, although the divergent NRh4 directions in 50.1 and 50.2 are puzzling. Scattered primary NRM directions occur in brecciated achondrite meteorites (Collin- son and Morden, 1994) when the constituent clasts are magnetized prior to their inclusion in the breccia, but this situation is not appropriate in the present context. If there was high-temperature carbonate formation, the titanomagnetite, assuming it was also heated, would be remagnetized on cooling and the magnetite, if formed at the same time, would also acquire a TRM, both minerals then carrying the primary NRM component. The presence of a primary component of NRM strongly suggests that a Martian magnetic field existed at the time. The secondary magnetizing event, indicated by the clustering of NRM directions at 15-30 mT demagnetizing fields, could be carried by magnetite if it was formed after carbonate formation, through a uniform CRM acquired at the time of chemical or biogenic precipitation. In view of the ease with which ALH 84001 acquires viscous magnetization, it is likely that there is also a component of VRM in this secondary NRM, acquired in the Earth's magnetic field during the -1 3 000 years that the meteorite was on or in the Antarctic ice sheet. It is possible that the secondary NRM is entirely due to VRM acquisition by magnetite or titanomagnetite or both. The divergent initial NRM directions can reasonably be ac- counted for by short-term VRM acquisition. Since their formation by breaking of the original fragment, the two chips were stored with random orientations relative to the geomagnetic field.

It seems unlikely that shock magnetization (SRM) is important in ALH 84001. The earlier shock event at -4.0 Ga was before the primary magnetizing event, and if the later one was that which ejected the rock from Mars -15 Ma ago, it is unlikely that the magnetic field existed as recently. Shock in zero magnetic field can result in some demagnetization.

It must be stressed that the above scenario is provisional, and an improved version must await more information on the role played by magnetite and tighter constraints on the chronology of the relevant events in the meteorite's history.

810 D. W. Collinson

THE ANCIENT MAGNETIZING FIELD

The present surface magnetic field on Mars has an upper limit of -0.1 p T (Dolginov, 1987; Russell, 1987), which is intuitively too weak to account for the magnetization of the Martian meteorites. If the observed NRM is a TRM, it is possible in principle to determine the strength of the field in which the TRM was acquired, essentially by giving a sample a TRM in a known laboratory field and com- paring the laboratory TRM intensity with that of the natural TRM. Efforts to do this with the Martian samples have not proved suc- cessful. Cisowski (1986) attempted this method on a sample of Shergotty, with a result that was difficult to interpret but suggested an ancient magnetizing field strength (paleointensity) in the range 0.5-2.0 pT. A similar attempt on Nakhla was unsuccessful, as was one by the present author on EET 79001. The problems encountered with these samples were almost certainly associated with the thermal alteration of magnetic minerals, and the present author has investigated an alternative technique involving only room temperature measurements. This utilises anhysteretic remanent magnetization (ARM), instead of TRM, the magnitudes of which are related when the biasing field imparting ARM is the same as the ambient field imparting TRM (Stephenson and Collinson, 1974). After alternating field demagnetization of the NRM of a sample, ARM is imparted in steps of increasing alternating field up to 100 mT with a constant direct field of 180 pT. The ratio of NRM removed to ARM gained, is measured in a series of alternating field intervals (usually 10 mT) up to the maximum. This ratio is proportional to the ratio of the ancient magnetizing field to the direct field used, 180 pT. The method has been used with some success on the rocks returned from the Moon by the Apollo missions, where similar problems associated with thermal alteration were encountered in paleointensity determinations (Collinson, 1984).

Because of such features as magnetic contamination, unsuitable demagnetization curves and marked secondary NRM components, not all the meteorites are suitable for paleointensity estimates. Mea- surements have been carried out on appropriate fragments of Nakhla, Governador Valadares, Lafayette and EET 79001. The results are shown in Table 2. It is necessary, however, to regard these values with caution. Although the internal consistency of each determination was reasonable, there is considerable uncertainty associated with the ratio of TRM to ARM acquired in the same ambient field. This ratio enters into the calculations of paleointensities and differs for different magnetic minerals. Facey (1952) has carried out appropriate measurements on titanomagnetites and finds values of 3-5, and a value of 4.0 has been adopted for the results in Table 2.

Another method of estimating paleointensities is to calculate the ratio of initial NRM intensity to saturated remanent magnetization in a sample, the latter being a measure of magnetic mineral content. On certain assumptions, the normalized NRM intensities will indicate relative paleointensities among different samples, and if absolute values can be determined for one or more samples to calibrate the method, absolute paleointensities can be estimated for all normalized samples. This technique was also developed for the Apollo rocks (Cisowski et al., 1984). In the present meteorite samples, the agreement between ARM and normalization methods is not always good, but paleointensities in the range 0.2-3 pT are obtained, which is a similar range to that obtained with the ARM technique. Cisowski (1987) reports a comparable range of values for Shergotty and Nakhla samples.

Although the paleointensity data are not of high quality, they are of sufficient consistency to suggest the existence of an ancient mag-

TABLE 2. Paleointensity estimates (in pT) for Martian magnetic field.

ARM Normalization method method

EET 79001 ,167 3.6 ,169 3.0 ,211 1.8

Nakhla NAI.1 2.2 NA1.3 3.2

Lafayette LA4.1 4.0 Governador GV 4.5

Chassigny CH - Valadares

2.6 2.2 0.8 0.8 0.7 1.3 1.3

0.5

Present Mars field: 0.03-0.1 pT (Dolginov, 1987); 0.01 pT (Russell, 1987).

netic field on Mars of strength in the approximate range 0.5-5.0pT. This is significantly stronger than the present Martian field or any likely interplanetary field, and it seems reasonable to propose its generation by a dynamo process in a molten, electrically conducting Martian core.

DISCUSSION AND CONCLUSIONS

The age of original crystallization is now generally accepted to be 1.3 Ga for nakhlites and Chassigny and 180 Ma for shergottites (McSween, 1994). If the primary NRMs were acquired by thermo- remanence at these times, the magnetic field was then in existence. If this field was of internal, dynamic origin, then questions arise concerning the existence of a suitable core and whether dynamo action occurred. It may be noted here that even if the primary NRM is of viscous or shock origin on Mars, these processes also require an ambient magnetic field (although SRM could conceivably be acquired in a transient, impact-generated field).

A solid inner core and liquid outer core are thought to be necessary for dynamo field generation. Laul ef al. (1986) and Treiman ef al. (1987) have considered Mars core compositions and emphasised the importance of S content in determining whether the core would be solid or liquid. Whether the appropriate structure was present in the past is still controversial. If it was, the occurrence of dynamo- generated magnetic fields in many planetary bodies, the Earth, the giant planets, possibly in Mercury, 10, Ganymede, and Europa, and the parent bodies of the asteroids, Gaspra and Ida, and almost certainly in the ancient Moon, must favour the past existence of a dynamo- generated Martian field.

If the crystallization ages of 1.3 Ga for the nakhlites and Chas- signy and 180 Ma for the shergottites are correct, there is some dif- ficulty in accepting the existence of a significant field and active dynamo at the latter date and an inactive dynamo at present. It is true that the paleointensities estimated from Shergotty and ALH 77005 (Table 2) are weak, but EET 79001 gives estimates comparable with the nakhlites. The paleointensity data are at present of insuffi- cient precision to allow a decrease in field strength between 1.3 Ga and 180 Ma to be detected, which might indicate a decaying dynamo. It is conceivable that the shergottites, which are heavily shocked, acquired a shock magnetization in transient, impact-generated magnetic fields, but this process is yet to be confirmed as viable on the scale of a meteorite impact.

Information about a Martian magnetic field prior to 1.3 Ga may become available if the chronology of events in the evolution of ALH 84001 is elucidated. Of particular interest is the timing of the

Magnetic properties of Martian meteorites 8 1 1

thermal metamorphic event or carbonate formation, (if "hot"), when the primary NRM would have been acquired. Stevenson et a/. (1983) have considered a Mars core model in which dynamo action com- menced very early and ceased by -3.5 Ga ago. I f large areas of the crust had become magnetized in this field, then subsequent to ces- sation of dynamo action, magnetic fields due to this magnetization could be postulated, in which the meteorites later acquired their NRM. However, the crustal magnetization would be of unlikely intensity t o provide the fields indicated b y the paleointensity estimates and is inconsistent with the near-zero present field.

As noted earlier, the origin and significance of the secondary NRMs observed is not clear, although viscous magnetization acquired in the geomagnetic field is likely to be a contributory source. This applies particularly where the meteorite is a find (e .g . , Lafayette, Governador Valadares and ALH 84001) and may have impacted the Earth many hundreds or thousands of years before recovery. It is also possible that a V R M component could have been acquired in the Martian field. T h e different direction of secondary N R M s of viscous origin can be explained, since there is no reason to expect that the geomagnetic or Martian field will be directed along the axis of NRM. If a secondary NRM is a PTRM or SRM acquired in an ambient Martian field, this implies a change in direction of the field relative to that in which the primary N R H was acquired. This could be caused by ambient magnetic field changes involving reorien- tation of the global field or rotation and tilting o f geological blocks.

From the results described in this paper, it is tentatively concluded that a weak global magnetic field of internal origin with strength in the approximate range 0.5-5 pT existed on Mars 1.8 G a ago and possibly earlier. If the shergottite crystallization age was 180 Ma, a field was also present then, but the paleointensity data is of insuf- ficient quality to indicate any decrease in strength from that at 1.3 Ga, which might be expected in view o f the very weak or zero present field. Although we now have better knowledge of. the Martian in- terior, the structure and evolution of the core remains uncertain. The results of paleomagnetic studies on the lunar rocks now provide some of the strongest evidence for a lunar core and an active dynamo in the past. It is not unreasonable to hope that the data presented in this paper will form a basis for similar information about Mars, which hopefully can be consolidated by future investigations.

Acknowledgments-The author is indebted to the following Institutions for the loan of meteorite samples: Natural History Museum, London, U.K.; Museum National d'Histoire Naturelle, Paris, France; Center for Meteorite Studies, Arizona State University, Tucson, Arizona, USA; Department of Geology and Geophysics, Princeton University, Princeton, New Jersey, USA; Field Museum, Chicago, Illinois, USA; Naturhistorisches Museum, Vienna, Austria; NASA Johnson Space Center, Houston, Texas, USA. The author is also indebted to anonymous reviewers for constructive comments.

Editorial handling: R. P. Binzel

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