Magnetic properties of a freshly fallen LL ordinary chondrite: the Bensour meteorite

Physics of the Earth and Planetary Interiors 140 (2003) 343–358

Magnetic properties of a freshly fallen LL ordinary chondrite: the Bensour meteorite Jérˆome Gattacceca a,∗ , Pierre Rochette a , Michèle Bourot-Denise b,1 a

CEREGE, BP 80, 13545 Aix-en-Provence Cedex 4, France b MNHN, Paris, France

Received 10 February 2003; received in revised form 30 September 2003; accepted 6 October 2003

Abstract A comprehensive rock magnetic, magnetic anisotropy and paleomagnetic study has been undertaken in the brecciated LL6 Bensour ordinary chondrite, a few months only after its fall on Earth. Microscopic observations and electronic microprobe analyses indicate the presence of Ni-rich taenite, tetrataenite and rare Co-rich kamacite. Tetrataenite is the main carrier of remanence. Magnetization and anisotropy measurements were performed on mutually oriented 125 mm3 sub-samples. A very strong coherent susceptibility and remanence anisotropy is evidenced and interpreted as due to the large impact responsible for the post-metamorphic compaction of this brecciated material and disruption of the parent body. We show that the acquisition of remanent magnetization postdates metamorphism on the parent body and predates the entering of the meteorite in Earth’s atmosphere. Three components of magnetization could be isolated. A soft coherent component is closely related to the anisotropy of the meteorite and is interpreted as a shock remanent magnetization acquired during the same large impact on the parent body. Two harder components show random directions at a few mm scale. This randomness is attributed either to the formation mechanism of tetrataenite or to post-metamorphic brecciation. All components are likely acquired in very low (≈␮T) to null ambient magnetic field, as demonstrated by comparison with demagnetization behavior of isothermal remanent magnetization. Two other LL6 meteorites, Kilabo and St-Mesmin, have also been studied for comparison with Bensour. © 2003 Elsevier B.V. All rights reserved. Keywords: Rock magnetism; Magnetic anisotropy; Ordinary chondrite; Shock magnetization; Tetrataenite

1. Introduction Despite numerous studies of their magnetic properties (e.g. Sugiura and Strangway, 1987), the interpretation of the natural remanent magnetization (NRM) of the most common type of meteorites—ordinary ∗ Corresponding author. Tel.: +33-442971508; fax: +33-442971595. E-mail addresses: [email protected] (J. Gattacceca), [email protected] (P. Rochette), [email protected] (M. Bourot-Denise). 1 Fax: +33-140793524.

chondrites (OC)—remains a puzzle. In particular the NRM vectors, even when of high stability during demagnetization, are widely dispersed at the mm scale (Funaki et al., 1981; Collinson, 1987; Morden and Collinson, 1992) even for meteorites with high metamorphic grade (5 or 6) and homogeneous fabric. These observations led Morden and Collinson (1992) to propose a formation of the meteorite parent body without post-accretion metamorphic reheating, at odds with accepted models. More recently Wasilewski and Dickinson (2000) highlighted the various bias and spurious effects on NRM that can occur in meteorite collections, and Rochette et al. (2003) evidenced a

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systematic decrease of metal content due to weathering in found meteorites, even those from Antarctica. Among OC the LL class has been utilized more frequently for paleomagnetic studies (e.g. Brecher and Ranganayaki, 1975) because they show the larger coercivity and remanence stability and because their low metal content (1–5%) minimize problems linked to magnetic interactions and sample shape. Thanks to the fall of a new LL meteorite in 2002, named Bensour, we had the opportunity to obtain a material of optimal freshness. Indeed the metallic iron responsible for the magnetic properties of OC meteorites start to rust as soon as the meteorite enters the Earth’s atmosphere, greatly helped by the presence of water, even in a museum drawer. Bensour meteorite fell the 11 February 2002 in Sahara at the Morocco–Algerian border and was soon after collected by nomads, well aware of the commercial value of such artifacts, and sold to a meteorite dealer. We acquired a 10.3 g angular fragment, devoid of fusion crust. The sample had not seen any drop of water and any magnetic fields due to magnet exposure since its fall on Earth. The sample reached our laboratory the 17 April 2002, and was since stored in a magnetically shielded room with a field intensity <50 nT (apart from sawing session) and rapidly measured. Our sample was thus exposed to the terrestrial magnetic field and atmosphere for only 3 and 4–5 months before magnetic measurements, respectively. This contrast with the residence time on Earth in uncontrolled magnetic and atmospheric conditions of previously studied LL chondrites, e.g. St-Severin, fallen in France in 1967 and studied (Brecher and Ranganayaki, 1975) to 20 (Morden and Collinson, 1992) years later, or Olivenza fallen in Spain in 1923 and measured 60 years later (Collinson, 1987), not talking about found meteorites (Nagata, 1979; Funaki et al., 1981) with unknown residence time of several kyrs. To compare Bensour results with an old fall of the same type (LL6), we also studied samples from the St-Mesmin meteorite fallen in France in 1866. We obtained 11.4 g of St-Mesmin meteorite in two fragments from the Paris Natural History Museum that curates this meteorite since its fall. Besides its old terrestrial age the motivation to study St-Mesmin meteorite is that Brecher et al. (1977) gave for this meteorite a value of remanent magnetization at saturation (Mrs ) at odds with the observed range for other LL meteorites

(Rochette et al., 2003). We thus intended to check the reproducibility of this anomaly. For a preliminary study, we also lately acquired 5.1 g of the Kilabo meteorite, fallen in Nigeria the 21 July 2002, and classified as LL6 with a shock stage S3 (Russell et al., 2003). Revisiting the LL chondrite magnetism is interesting in order to check the reliability and reproducibility of the conclusions of Morden and Collinson (1992)— based on a repeated manual fragmentation and measurement with Molspin spinners, without principal component analysis of NRM demagnetization—with a different sampling scheme and more modern instruments and data processing. Moreover, in agreement with early measurements (Weaving, 1962), Morden and Collinson (1992) highlighted the strong homogeneous magnetic anisotropy of OC, based on measurements of anisotropy of magnetic susceptibility (AMS), but did not discuss the relationship between anisotropy and NRM directional scatter. On the other hand, Brecher (1976) suggested that in some cases the NRM may be confined to an easy plane of magnetization and controlled by the petrographic texture of the rock. Therefore to implement the understanding of the magnetic fabric effect we conducted for the first time a remanence anisotropy study on a meteorite using anhysteretic remanence technique (Jackson, 1991).

2. Petrography As Bensour meteorite has not been described previously, a petrographic investigation was undertaken on a polished section of the magnetically studied sample (see Section 3) as well as a thin section from an independent sample using optical and scanning electronic microscopy. Thanks to rapid collection after its fall, Bensour meteorite has not suffered any terrestrial weathering. Minerals, especially the opaques (metal and sulfides) are intact. Oxides particularly sensitive to this kind of alteration such as phosphates (apatite and merrillite) and plagioclases are also in optimal state. Preliminary petrological investigation on Bensour identifies this meteorite as a LL ordinary chondrite with high metamorphic grade 6 corresponding to a peak metamorphism temperature between 800 and 960 ◦ C (Mc Sween and Patchen, 1989). Since submission of this work, Cole and Sipiera (2003) published a short description of Bensour, reaching

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Fig. 1. (a) Transmitted light optical microscope picture of a thin section showing Bensour breccia structure with angular light colored clasts of variable size wrapped in darker material. (b) Backscatter SEM image. On the left side is a light colored clast showing fissured plagioclase (dark), pyroxene (gray) and olivine (light gray). The clast is separated from the dark material on the right side by a vein filled with opaque minerals (white on the picture). Minerals are the same on both side but granulometry is more heterogeneous on the right one where the fissure network is also denser. (c) Reflected light optical microscope image after etching. The lower grain on the left is cloudy taenite (CT) with a thin rim of tetrataenite (Te). In the center a smaller cloudy taenite grain with tetrataenite rim is associated to a large troilite (Tr) grain. Native copper (C) is found at the edge of the cloudy taenite grain.

the same conclusions. Like most LL ordinary chondrites Bensour is a breccia with easily visible clasts (Fig. 1a). Light gray clasts, with a mean grain size of 50–200 ␮m, contain minerals with sharp crystalline shapes despite cracking. Dark areas that surround these clasts have a more complex mineral organization and contains mostly micron-sized troilite grains homogeneously scattered. The limit between the two lithologies is not always sharp but in most cases the limit of clasts is underlined by veins filled with opaque minerals (Fig. 1b). Such textures are typical of clast formation after post-metamorphic cooling. Coarser clasts are richer in sulfides (troilite) and impoverished in metal compared to finer clasts where the relative proportion of metal and sulfide correspond to what is commonly encountered in LL chondrites. Under the microscope, the metal in all type of clasts has a very homogeneous texture. Etching with nital (alcohol + 2% HNO3 ) show that almost all metal grains are cloudy taenite with a rim of tetrataenite (Fig. 1c). Some rare kamacite grains can also be observed. Chemical analyses were performed with an electronic microprobe (SX100, Cameca). Nine hundred forty analyses for 150 grains were obtained as follows: point analysis at the center of small grains and 10 ␮m spaced point analyses in large grains. The analyses with 50% of Ni correspond to the tetrataenite border of the grains. No zonation was evidenced in taenite grains, corroborating microscope observations. Taen-

ite grains have a mean Ni content of 45.23 and 1.87% of Co. Nickel poor grains (4.11% Ni) correspond to a peculiar form of kamacite with a remarkable 8.21% of Co. Bensour meteorite is also remarkable for the equal proportion of metallic iron and nickel whereas metallic iron is twice as abundant as nickel in most LL chondrites (Jarosewich, 1990). Such high Ni content is also observed in Bandong (LL6) and Olivenza (LL5) meteorites, this latter also having kamacite with 5% of Co (Sears and Axon, 1976). Bensour displays abundant petrographic evidence of shock. Troilite is always deformed: it contains stress twins inside the clasts and is polycrystalline at the border of the clasts or when filling the separation veins (Fig. 2a). Some of the metallic grains underwent plastic deformation well evidenced by etching that underlines the presence of deformed Neuman bands in kamacite grains (Fig. 2b) and shear and elongation of some cloudy taenite grains (Fig. 2c and d). Phosphate minerals, characterized in back-scatter SEM observations, are scattered in the clasts but apatite is often at the border of the clast, associated with veins, whereas merrillite is mostly present as rounded grains inside the clasts. Silicates were fissured but not heated to high temperatures as relics of chondrules are still well visible. These features plus the absence of melt pockets point to a shock stage S3 corresponding to shock pressure between 5 and 20 GPa and temperature increase between 20 and 150 ◦ C (Stöffler et al., 1991).

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Fig. 2. (a) Transmitted and reflected light optical microscope image with crossed nicols showing a large grain of troilite. Stress twins oriented along the principal crystalline planes are well visible. The twins are slightly deformed. (b) Reflected light optical microscope image of a cloudy taenite grain after etching with nital. The grain was stretched then deformed by shock. (c) Reflected light optical microscope image of a metallic grain etched with nital. The upper light grain is tetrataenite. The lower grain is kamacite in which Neuman bands formed by shock are in turn deformed. (d) Reflected light optical microscope image after etching of a sheared cloudy taenite grain.

In conclusion Bensour is an LL ordinary chondrite with a large proportion of oxidized iron present in ferro-magnesian silicates (olivine and pyroxene), which explains the high Ni and Co content of the metal and the quasi-absence of kamacite and zoned taenite that are usually present in most OC. Brecciation took place on a already cooled metamorphosed (type 6) material with non-homogenous granulometry. Shocks caused the formation of open cracks in which the most volatile elements (S, Cl) circulated: polycrystalline troilite concentrated in these veins and apatite crystallized nearby. Shocks also caused

deformation of some metallic grains and numerous dislocations in silicate as well as troilite twins.

3. Sampling and magnetic measurements The original sample (10.33 g) was mounted with bee wax on a glass slide. It was then cut with a diamond wire saw in an alcohol bath, first in two partial slices perpendicular to the shortest axis. Then two perpendicular sets of saw cuts produced 13 partial cubes of approximately 5 mm size, with five cut sides, and 10 edge

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Fig. 3. (a) Schematic view of Bensour meteorite sample with indication of saw cuts (thick lines) that followed the first saw cut in the ZY plane (cut plane facing downward on this representation). Numbers (respectively ∗) refer to mutually oriented cubic (respectively non-cubic) sub-samples. The shaded area is a large metallic grain. The dotted line is the magnetic foliation. (b) Backscatter SEM image with the same orientation and scale of the smaller slice of Bensour sample obtained after the first saw cut in the ZY plane. In the following stereographic projections the X-axis is oriented toward the north, the Z-axis points downward. (c) Rose diagram of metallic grains orientation obtained by image analysis of 10 SEM images of parts of the polished section shown on (b). The dotted line is the mean orientation of the grains.

samples with three or four cut sides (Fig. 3a). Thirteen precisely mutually oriented cubic sub-samples were obtained, for a total mass of 4.833 g. These cubes were in turn mounted into plastic boxes. We estimate that this procedure ensured a better precision in relative orientation, together with more regular sample size than the fragmentation procedure of Morden and Collinson (1992). Samples 4 and 12 each contain large (2–3 mm) cloudy taenite grains and sample 9 contains a smaller one. Another 10 non-cubic fragments for a total mass of 4.044 g could be oriented with respect to the cubes. Finally, 0.328 g of the meteorite was recovered as non-oriented fragments. About 10% of the original sample was lost during the sawing process. NRM measurements were performed with a 2G-Enterprise DC Squid cryogenic magnetometer with online AF demagnetization in a shielded room. AMS and high temperature measurements were performed with a KLY2-CS2 Agico apparatus. Isothermal remanent magnetization (IRM) was imparted with a pulse magnetizer at 3 T and measured with a

JR-5 Agico spinner magnetometer. Hysteresis cycles were performed with a Micromag VSM apparatus. Anisotropy of anhysteretic remanent magnetization was evaluated using a three-position scheme described in Gattacceca and Rochette (2002).

4. Magnetic properties and mineralogy Specific magnetic susceptibility (χ) measured on the whole 10.3 g sample as well as on another independent 3 g sample gave a log χ of 3.71 ± 0.02 (in 10−9 m3 kg−1 ), typical of other LL6: from 3.56 to 4.29 with an average of 3.97 ± 0.23 based on 18 meteorites (Rochette et al., 2003). Similarly the log Mrs values of 2.50 ± 0.13 (in 10−3 Am2 kg−1 ) obtained from different samples are in the range of other LL chondrites: 1.99–2.78 for eight meteorites. The two St-Mesmin samples gave log χ = 4.12 and 4.30 (Table 1) similar to the log χ = 4.21 of Rochette et al. (2003). log Mrs of 2.54 and 2.44 for St-Mesmin suggests that Brecher

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Table 1 Magnetic properties of the LL ordinary chondrite samples Sample 1 2 3 4 5 6 7 8 9 10 11 12 13

Mass (mg) 378 381 220 258 394 382 364 506 490 314 448 418 280

Mean∗ σ∗

χ (10−8 m3 kg−1 )

NRM (10−6 Am2 kg−1 )

Mrs (10−3 Am2 kg−1 )

ARM (10−6 Am2 kg−1 )

NRM/Mrs (10−3 )

P

T

Pa

Ta

487 375 448 125 419 571 421 386 558 318 270 804 429

736 229 301 1970 278 660 214 278 1200 108 245 4490 187

330 204 285 1780 279 332

332 164 265 2410 241 349

2.23 1.12 1.06 1.11 0.99 1.99

−0.110 0.570 0.522 0.588 0.520 0.286

240 409 158 145 963 187

1.26 3.96 0.53 1.33 8.79 0.76

0.028 0.713 0.609 0.285 0.549 0.716 0.58 0.913 0.594 0.103 0.099 0.523 0.386

1.535 1.788 1.394 3.460 1.405 1.751

271 475 205 220 910 246

1.115 1.128 1.192 1.329 1.152 1.26 1.149 1.112 1.171 1.091 1.096 1.346 1.118

1.368 1.570 1.361 1.158 2.617 1.315

0.390 0.134 0.340 −0.020 −0.211 −0.460

518 258

838 1216

461 458

489 644

2.094 2.297

1.174 0.086

0.469 0.271

1.727 0.663

0.212 0.345

1.76 0.41

1.368 1.300

0.684 0.646

1.737 1.589

0.639 0.49

0.41 1.16 3.21

1.113 1.135 1.247

0.413 0.303 0.527

1.521 1.312 1.890

m368 m369

9755 1697

2000 1310

610 115

346 278

kil1 kil2 kil3

3788 1020 300

509 514 642

92 642 610

228 552 190

149 233 175

0.58 −0.47 0.43

Samples 1–13 are from Bensour, samples m368 and m369 from St-Mesmin, samples kil1–kil3 from Kilabo meteorite; χ: magnetic susceptibility; NRM: natural remanent magnetization; Mrs : isothermal remanent magnetization at saturation; ARM: anhysteretic remanent magnetization (AF field 80 mT, steady field 50 ␮T); P and T: shape parameter and anisotropy ratio for AMS; Pa and Ta : shape parameter and anisotropy ratio for anisotropy of anhysteretic remanent magnetization. (∗ ) Mean values do not include measurements from St-Mesmin and Kilabo.

et al. (1977) log Mrs value of −0.15 was biased or based on a highly anomalous sample. The three Kilabo samples gave a mean log χ = 3.74 and log Mrs = 2.46, very close to Bensour values. For the 13 cubic sub-samples, χ ranges from 2.7 × 10−6 to 1.25 × 10−5 m3 kg−1 and log χ is 3.68 ± 0.09 in 10−9 m3 kg−1 (Table 1). In detail, the mean susceptibility is (4.25 ± 0.54) × 10−6 m3 kg−1 for 11 of the samples but is two to three times higher for samples 4 and 12 that contain large cloudy taenite grains. The mean intensity of ARMs acquired at 80 mT for a steady field of 50 ␮T is (2.49±0.56)×10−4 Am2 kg−1 for 10 of the samples but is about 10 times stronger for samples 4 and 12. The mean intensity of Mrs acquired at 3 T is (4.61 ± 2.59) × 10−1 Am2 kg−1 for 12 samples. Samples 4 and 12 have Mrs intensities ∼5 times higher (1.78 and 0.91 Am2 kg−1 ) than the other 10 samples ((2.60 ± 0.50) × 10−1 Am2 kg−1 ). Therefore, while Bensour meteorite appears magnetically homogeneous at the cm3 scale, like other OC

(Rochette et al., 2003), it becomes quite heterogeneous at the 0.1 cm3 scale, consistent with the observation of mm size metallic grains. A low and high temperature thermomagnetic curve performed under argon atmosphere indicates a Curie point around 580 ◦ C with an irreversible cooling path with an increase of susceptibility (Fig. 4). This feature is a good indicator of the presence of tetrataenite (Nagata and Funaki, 1982). Non-zero susceptibility at 650 ◦ C indicate the presence of a small amount of kamacite representing 10% of the initial susceptibility. Hysteresis loops (Fig. 5) indicate a high coercive force (Hc ) of ∼60 mT. Hysteresis loops after heating at 650 ◦ C shows a drastic decrease of hysteresis and coercive force (∼4 mT). Mrs is also reduced by a factor of 10. These are good indicators of the transition from tetrataenite to taenite after heating above 600 ◦ C (Wasilewski, 1988). Saturation isothermal remanence AF demagnetization curves indicate median destructive fields of ∼30 mT and the persistence of

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Fig. 4. Typical low field susceptibility vs. temperature curve recorded from −185 to 650 ◦ C and back to room temperature under argon atmosphere.

∼5% of the original remanence after demagnetization at 150 mT. Tetrataenite mass abundance with respect to taenite can be deduced from the comparison of hysteresis parameters before and after heating (Nagata and Funaki, 1982). This method indicates about 40% of tetrataenite and 60% of taenite and kamacite. Therefore it is very likely that tetrataenite form not only the rims observed around cloudy taenite grains but is also present as microcrystals inside cloudy taenite grains. In conclusion, Bensour meteorite has bulk magnetic properties well in agreement with other studied LL6 meteorites. Tetrataenite is the main remanence-carrying mineral, with minor contribution of taenite and kamacite. The latter minerals have a stronger contribution to susceptibility and saturation magnetization.

5. Magnetic fabric We measured the AMS of the 13 cubic samples and additional six oriented edge samples (Tables 1, Fig. 6a). AMS ratios P range from 1.091 to 1.346 with a mean value of 1.17. The distribution of the anisotropy direction is very oblate (T = 0.97 for the mean normed tensor), K3 -axes show a good directional grouping (mean tensor K3 axis D = 273◦ , I = 59◦ ) whereas K1 -axes are scattered along a great circle (pole D = 266◦ , I = 65◦ , α95 = 7.5◦ ). Nevertheless the use of bivariate statistics (Henry and Le Goff, 1995) allows the definition of a significant lineation. Samples 12 and 4 that contain large metallic grains are also the most anisotropic. Re-measurement of AMS after repeated AF demagnetization along a

Fig. 5. Typical corrected hysteresis loop of Bensour meteorite sub-samples before (thick dashed line) and after heating at 650 ◦ C (thin line).

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Fig. 6. Equal area stereographic projection plots. (a) K1 (boxes) and K3 (dots) axes of AMS ellipsoid of the 19 measured samples. The star is the Fisherian mean K3 -axis (D = 268◦ , I = 59◦ , α95 = 6.6◦ ). The great circle is the best fit great circle to the K1 -axes. The black box is the mean K1 -axis computed with bivariate statistics (D = 7◦ , I = 84◦ ), with associated 95% confidence limit. (b) K1 (boxes) and K3 (dots) axes of AARM ellipsoid of the 13 measured samples. The star is the Fisherian mean K3 -axis (D = 278◦ , I = 63◦ , α95 = 14.7◦ ). The great circle is the best fit great circle to the K1 -axes.

single axis gave identical result. The anisotropy is thus not affected by remanence state and domain alignment (Rochette et al., 1992). The good directional grouping of the AMS data confirms the results of Morden and Collinson (1992) on several ordinary chondrites. AMS measurements for St-Mesmin samples provided slightly higher values for P (1.368 and 1.300) and similar oblate fabric (T = 0.68 and 0.65). For Kilabo samples, the fabric is also oblate (mean T = 0.41) and P is between 1.11 and 1.25 (Table 1). The anisotropy ratios we obtain (1.09 < P < 1.35) are in good agreement with other published measurements on LL chondrites (e.g. Weaving, 1962). We performed image analysis on 10 back-scatter SEM images obtained on the polished section shown in Fig. 3b, i.e. in a plane perpendicular to the magnetic foliation. Clear grains, corresponding to metallic phases, were counted with a circular window and data were evaluated using the method described in Launeau and Robin (1996). The sum of the 10 analyses indicates a preferential shape orientation of the grains precisely in the magnetic foliation plane (Fig. 3c) with a shape anisotropy ratio of 1.09. To evaluate the anisotropy of remanence, we measured anisotropy of anhysteretic remanence of eight samples (Table 1). ARM was acquired in a 80 mT peak alternating field and a steady 50 ␮T field. Rema-

nence anisotropy ratios Pa range from 1.158 to 3.460 and the fabric is oblate. Once again, the metallic grain bearing samples are the most anisotropic. The directional data (Fig. 6b) are quite similar to the AMS data with a good grouping of K3 -axes whereas K1 -axes are scattered along a great circle. The mean K3 -axis is undistinguishable from AMS mean K3 -axis. The high anisotropy ratios pinpoint the fact that a single measurement of ARM or IRM can be significantly different (with up to a factor 3) from another single measurement and that anisotropy of remanence must be addressed to precisely compare magnetization intensities. In Table 1, all the values for ARM or IRM are mean values derived from tensorial calculation or from three measurements along the three perpendicular axes of the samples, one of which being by chance sub-parallel to K3 . The same method is used for susceptibility. The two St-Mesmin samples also have strong anisotropy of remanence with oblate fabric (Table 1). The relation between P and Pa (Fig. 7) is different from Pa = P 2 expected if both susceptibility and ARM are carried by multidomain grains (e.g. Nagata, 1961): the susceptibility may be carried by multidomain grains whereas ARM is in part carried by smaller grains. An alternative explanation is that taenite and tetrataenite contribute differently to suscepti-

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Fig. 7. Pa vs. P plot for Bensour sub-samples. The theoretical curve for multidomain grains is also represented.

bility and remanence and that their respective fabric is different. Repeated hysteresis loops performed along different directions in the K1 K3 -plane (Fig. 8) on two cubic samples show an evolution of saturation magnetization Ms dominated by the sin 2θ term as indicated by Fourier analysis of the curve. This behavior indicates that the anisotropy of tetrataenite is principally controlled by crystalline anisotropy and not shape anisotropy. The ratio between maximum and minimum Ms is equal to Pa measured independently. The same experiment was conducted on the same sample after heating above 600 ◦ C. Some evolution of Ms with orientation is still visible but drastically reduced compared to the pre-heating experiment. The Fourier analysis of the Ms curve show that sin θ term (caused

Fig. 8. Hysteresis loop parameters remanent magnetization at saturation (Mrs ) and saturation magnetization (Ms ) of Bensour sub-sample 5 measured along different direction in the foliation plane.

by wobbling of the sample during rotation of the sample handler) become as important as sin 2θ term variations. This indicates that the anisotropy of taenite is dominated by shape anisotropy. For the same sample AMS measurements before and after heating give identical anisotropy axes directions and higher P after heating (from 1.266 to 1.513), the shape parameter being almost unchanged: the orientation of primary tetrataenite crystalline axes was the same as the shape preferential orientation of secondary taenite grains. This could be a key to understand the formation mechanism of tetrataenite by ordering of taenite and will require further investigation.

6. Natural remanent magnetization NRM of the entire sample (10.33 g) before sawing has a moment of 1.02 × 10−5 Am2 or an intensity of 9.88 × 10−4 Am2 kg−1 , and did not change after 1 month in the shielded room of the laboratory. The vectorial sum of the NRMs of all the oriented recovered fragments (8.88 g) has a moment of 8 × 10−6 Am2 , which is close to the original total NRM, suggesting that the whole preparation process did not modify significantly the magnetization of the meteorite. The specific intensity of magnetization of the 13 cubic samples (Table 1) range from 1.08 × 10−4 to 4.49 × 10−3 Am2 kg−1 , the mean value being (8.38 ± 6.61) × 10−4 Am2 kg−1 . Samples 4 and 12 have natural remanent magnetization intensities notably higher than the other samples. Let apart these 2 samples, the

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mean intensity for the other 11 samples is (4.03 ± 1.93) × 10−4 Am2 kg−1 . The magnetization of the entire sample is thus dominated by the magnetization of the two large cloudy taenite grains it contains. The ratio of NRM to IRM at saturation (REM ratio) is generally close to 10−3 with a few higher values between 10−3 and 10−2 (Table 1). However, as advocated in Verrier and Rochette (2002), the REM ratio is not adapted to evaluate multicomponent magnetizations with different coercivity spectra. It is much more relevant to discuss “derivative” REM ratio (REM ) based on the slope of NRM versus AF field, normalized to

the same slope for IRM, in the AF range at which a given directional component is demagnetized. The NRM directions before demagnetization are scattered over the entire sphere (Fig. 9a). No particular grouping is observed with a precision parameter (Fisher, 1953) k = 2.5. Cubic samples were demagnetized in alternating fields up to 150 mT peak field. Demagnetization data were represented on orthogonal demagnetization diagrams (Zijderveld plots) and on equal-area projections (Fig. 10). They were evaluated using principal component analysis (Kirschvink, 1980).

Fig. 9. Equal area stereographic projection plots. (a) Natural remanent magnetization of the 13 Bensour cubic sub-samples before demagnetization. (b) Medium coercivity components. (c) Medium coercivity components after correction for anisotropy of remanence. (d) Low coercivity components of magnetization. The star is the Fisherian mean direction excluding samples 8, 9 and 12 (D = 5◦ , I = −8◦ , α95 = 23.6◦ ). The great circle is the magnetic foliation.

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Fig. 10. Representative orthogonal vector plots and equal area stereographic projections of demagnetization data of some Bensour cubic sub-samples. Data from one St-Mesmin sample and one Kilabo sample are also shown. Black circles: pojections into the horizontal XY-plane; open circles: projections into the vertical plane.

Almost all Zijderveld plots present three components of magnetization: a low coercivity component (0–25 mT), a medium coercivity component (25–150 mT) and a high coercivity component that

cannot be isolated up to peak fields of 150 mT, resulting in Zijderveld plots not converging towards the origin (Fig. 10). The remaining intensity of magnetization after demagnetization at 150 mT range from 10

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to 65% (mean 35%) of the initial intensity. Median destructive fields range from 30 to more than 150 mT. The REM ratios of the low coercivity component is between 5 × 10−4 and 2 × 10−3 . For the medium coercivity component, the REM ratios are between 1.5 × 10−3 and 5 × 10−3 . We tried to estimate REM

of the high coercivity component by computing the ratio between residual NRM and residual IRM after demagnetization at 150 mT and obtained values between 1.2 × 10−3 and 1.3 × 10−1 . This large range is due to the chaotic behavior of magnetizations during AF treatment with high fields (Fig. 10) and these values cannot be regarded as indicative of a particular mode of magnetization acquisition. For the two St-Mesmin samples, the demagnetization behavior is very different (Fig. 10) with a median destructive field of 1 and 4 mT indicating the presence of an intense low coercivity component of magnetization. This component has a REM ratio of 1.5 × 10−2 , which indicates a different acquisition mechanism compared to the low coercivity component of Bensour meteorite: it is probably an IRM (although REM is quite low) or a weathering or viscous magnetization acquired on Earth. Between 10 and 100 mT, a noisy high coercivity component can be isolated and has a derivative REM ratio of about 2.5 × 10−4 . For Kilabo samples, the demagnetization behavior is in turn rather different (Fig. 10). The median destructive field is ∼35 mT, and a stable component of magnetization can be isolated between 10 and 100 mT, with REM ratios between 0.5 × 10−3 and 8 × 10−3 , similar to the REM ratios of the high coercivity component of Bensour meteorite. For the three mutually oriented samples, these components are randomly oriented. For Bensour samples, if one discards the two anomalous samples 9 and 12 that contain metallic grains and sample 8, the low coercivity directions (Fig. 9d) present a significant grouping with a precision parameter k = 5.3 (α95 = 19.9◦ , 13 directions). Taking into account all samples except 12 that contains the largest metallic grain, the directions are scattered along a great circle whose pole D = 277◦ , I = 53◦ (α95 = 24◦ ) is undistinguishable from the magnetic foliation pole. The medium coercivity components are widely scattered over the entire sphere (n = 11, k = 1.8, Fig. 9b) and do not show any particular relation with the magnetic fabric. A possible

high coercivity component may be accessible through the great circles method but the 10 great circles that can be defined for the medium coercivity components do not define a clear intersection. We corrected the measured magnetization direction for the magnetic anisotropy of the samples determined by anisotropy of anhysteretic magnetization using the following simplified formula: Ic = atan(Pa tan(Im )) where Ic and Im are the corrected and measured inclination with respect to the maximum anisotropy plane, and Pa the remanence anisotropy ratio. This assumes a purely planar fabric. As will appear below, a full triaxial treatment would not give significantly different results. For the low coercivity components, the corrections were indeed minor (a few degrees at most) because the uncorrected directions lie close to the maximum anhysteretic magnetization plane. For the medium-coercivity components and great circles, the corrections are up to 20–30◦ but directions after correction are still widely scattered (n = 11, k = 2.5, Fig. 9c).

7. Discussion The anisotropy measurements confirm the previous work by Morden and Collinson (1992), i.e. the fabric of ordinary chondrites is planar and the foliation is homogeneous over a volume of at least a few cm3 . Moreover, using adapted statistical tools, we identify a magnetic lineation that probably reflects a petrofabric lineation. The origin of anisotropy of meteorites is still debated. It seems however that the petrofabric (and AMS) is not acquired during metamorphism on the parent body as there is no correlation between the grade of metamorphism and the degree and shape of anisotropy of chondrites (Sugiura and Strangway, 1987; Martin and Mills, 1982; Morden and Collinson, 1992). AMS, linked to the preferential orientation of magnetic grains, is probably due to an impact related deformation as indicated by the correlation between the intensity of foliation and the shock facies (Sneyd et al., 1988). NRM directions of sub-samples before demagnetization are scattered as indicated by previous studies. However, principal component analysis of the demagnetization plots evidences magnetically soft components (2–25 mT) that are not randomly scattered, but

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lie along the magnetic foliation plane, most of them being clustered more or less along the magnetic lineation (compare Figs. 8 and 14). The correspondence between magnetization direction and magnetic fabric for shocked rocks had already been noted by, e.g. Brecher (1976), in particular for lunar samples. On the contrary, higher coercivity components (>25 mT) are randomly distributed. The significantly higher REM

ratio for the higher coercivity component points toward either a more efficient magnetization process or a higher acquisition field intensity. Thermal history of the Bensour meteorite parent body is a key point for the interpretation of its natural remanent magnetization. It is now accepted that parent bodies accreted cold and were heated after accretion presumably by short-lived radionuclides (e.g. Keil, 2000 for a review). For the LL6 meteorites parent body, this led to peak temperature between 800 and 960 ◦ C (Mc Sween and Patchen, 1989). The second major event was repeated disruption of the parent body by hypervelocity impacts within the asteroid belt. A post-brecciation compaction, likely responsible for the homogenous fabric and observation of deformed clasts, is needed. The shock stage of 3 indicates an increase of temperature between 20 and 150 ◦ C and pressure shock wave between 5 and 20 GPa (Stöffler et al., 1991). The inner temperature of the parent body before the impact is an important parameter and can be evaluated between ∼130 and 165 K for an asteroid orbiting at three astronomic units from the Sun according to asteroids thermal models (Spencer et al., 1989). Therefore the peak temperature reached by Bensour meteorite after the impact is between 150 and 315 K, which is well below the Curie temperature of taenite (∼820 K) and kamacite (∼1030 K) and below the disordering temperature of tetrataenite (∼820 K). It means that the thermal effect of the impact did not erase the original magnetization. This conclusion remains valid even if one consider a higher shock stage of 4 that would give a peak temperature between 230 and 465 K. Another possible mechanism that could affect the original magnetization of the meteorite is the cosmic-ray exposure (Butler and Cox, 1971). However, with a probable exposure age of 15 Ma that is the exposure age of most of LL6 chondrites (Graf and Marti, 1994), the effect of cosmic-ray would not be significant if compared to the already moderate effect obtained with a much

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higher time-integrated neutron flux by Butler and Cox (1971). The magnetic carrier of the middle and high coercivity components is tetrataenite. Tetrataenite is formed during the slow cooling of the meteorite parent body in space by Ni solid state diffusion below 320 ◦ C (Clarke and Scott, 1980), favored in Bensour case by a high Ni content. Tetrataenite thus predates brecciation. The relation between the magnetization of tetrataenite and that of the taenite parent mineral is largely unknown (Wasilewski, 1988; Wasilewski et al., 2002). The randomness of the medium–high coercivity components reflects either an original randomness of magnetization directions carried by taenite/kamacite or the randomizing effect of tetrataenite formation if this latter does not preserve the original magnetization direction of its parent mineral. Collinson (1987) explains the NRM randomness by the accretion of already magnetized grains that have not been subsequently heated above the Curie temperature of the original main magnetic carrier (kamacite: 760–780 ◦ C). The scale of these grains is below 100 mm3 according to our measurements and below ∼mm3 according to Collinson (1987). But this is at odds with the metamorphic grade 6 of Bensour meteorite that implies temperature above 800 ◦ C: no remanent magnetization could survive the post-accretion metamorphism on the LL6 parent body. Clear petrographic evidence of post-metamorphic brecciation invalidates Collinson’s scenario (at least in Bensour case) and could provide an explanation for our high coercivity component: it can be a post-metamorphic TRM, randomized in direction by brecciation. An alternative explanation for the randomness of the high coercivity components is that the formation of tetrataenite does not preserve the magnetization of the parent mineral. However this second possibility cannot be invoked as a general mechanism as tetrataenite free HED meteorites also show scattered directions of NRM between oriented sub-samples (Collinson and Morden, 1994). Various hypotheses can be made for the origin of the soft component of magnetization. Over a few cm3 , these component directions lie along a plane and are even clustered for most of the samples. Therefore this component was acquired by the grains after brecciation. It is noteworthy that the low coercivity components are related to the anisotropy axes direction. They

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lie precisely in the maximum anisotropy plane and most directions (except samples with large metallic grains) are close to the magnetic lineation (Fig. 9d). This magnetization is post-brecciation and closely correlated with the compaction responsible for the coherent anisotropy. This meteorite should not have been exposed to stray magnetic fields, such as exposure to a magnet. This is confirmed by the low REM ratios for the low coercivity component, lower than 2 × 10−3 , that precludes an acquisition of this component as an IRM (Verrier and Rochette, 2002). Moreover this kind of contamination would have certainly led to better grouped low coercivity components with an orientation independent with respect to anisotropy directions. Our sample shows no fusion crust and the temperature increase when entering the Earth atmosphere was negligible a few mm inside the meteorite (Melcher, 1979). Therefore, no TRM acquisition took place while the meteorite was entering the atmosphere. Even if the meteorite’s stay in the Earth magnetic field was brief (3 months) a viscous magnetization can be suspected. But viscous magnetization acquisition experiment performed during 2 months on samples 2, 6, 10 and 13 in a geomagnetic field of 46 ␮T led to maximum variation of the magnetization of between 2 × 10−6 and 5×10−6 Am2 kg−1 . The mean intensity of the low coercivity components is (2.14±1.71)×10−4 Am2 kg−1 and therefore cannot be explained by a viscous magnetization acquired in the Earth field. In order to simulate the heating in the Earth field from the original temperature of the meteorite to ambient temperature following the fall of the meteorite, we cooled samples 2, 10 and 11 in liquid nitrogen (77 K) and let them warm up to ambient temperature in the geomagnetic field (46 ␮T). Maximum variation of magnetic intensity was between 0.7 × 10−6 and 4×10−6 Am2 kg−1 and cannot account for the low coercivity component of Bensour meteorite that is about 100 times stronger (Table 1). Conversely, no demagnetization (variation less than 10−6 Am2 kg−1 ) was observed for sample 7 that was cooled in liquid nitrogen and heated to ambient temperature in zero magnetic field, confirming that the process of heating between 77 K and room temperature does not affect significantly the magnetization of the samples. Therefore the low coercivity component of the Bensour meteorite appears to be a pre-fall magnetization. Its low REM

ratio would fit with a shock induced magnetization or a partial TRM acquired at moderate temperature. Impacts of a few GPa can impress a low coercivity magnetization to rocks even in a null magnetic field environment (e.g. Cisowski and Fuller, 1978; Dickinson and Wasilewski, 2000). Therefore the impact leading to disruption of the parent body (5–20 GPa) could have imparted the observed low coercivity magnetization of Bensour meteorite. This interpretation is strengthened by the relation between low coercivity magnetization directions and AMS, this latter being very probably impact related. The impact is likely to be the primary impact that disrupted the LL6 meteorite parent body into smaller bodies from which the meteorite itself was extracted much later by a secondary, weaker impact. Even if Bensour is not dated yet, it is likely that this major impact has, like for most of LL chondrites, an age of ∼1.2 Ga (Bogard, 1995) that would also be the age of the low coercivity component of magnetization of Bensour meteorite. In another shocked LL6 meteorite, Rubin (2002) advocates a moderate impact-related metamorphism postdating brecciation. This could correspond to the “welding” and ductile compaction of Bensour, responsible for a thermal rather than shock pressure origin for the soft component. The low REM value (of the order of 10−3 ) suggests a very low ambient field (less than a few ␮T, Cisowski, 1991; Kletetschka et al., 2003) if any during the soft NRM acquisition. Its close confinement to the magnetic lineation direction support the Brecher (1976) suggestion of a textural remanence acquired in quasi-absence of ambient field. If the fact that one part of NRM has a coherent direction can be extrapolated to much larger scale, the hypothesis that random directions leads to negligible asteroid scale NRM (Wasilewski et al., 2002) could be reconsidered.

8. Conclusion The fall and timely recovery in Sahara of Bensour LL6 meteorite provided the opportunity to work on an optimally fresh chondrite. Magnetic measurements confirm that the anisotropy of magnetic susceptibility of such meteorites is homogeneous at cm3 scale. Despite the planar fabric, a magnetic lineation could also be defined. Anisotropy of remanence is strong (up

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to a factor 3.5) and its principal axes have the same orientation as the axes of anisotropy of susceptibility. The strong anisotropy of remanence can bias the evaluation of magnetic parameters such as isothermal remanence at saturation or intensities of anhysteretic magnetization. As stressed by Selkin et al. (2000) for igneous rocks, this can also be an important bias for the numerous paleointensities studies on chondrites (e.g. Brecher and Ranganayaki, 1975; Brecher et al., 1977). For a sample of a few cm3 , the bulk magnetic susceptibility and remanence signals are dominated by 2 mm-long metallic grains. AF demagnetization of mutually oriented sub-samples allows to discriminate three components of magnetization. Medium and high coercivity components carried by tetrataenite are randomly distributed down to a scale of 125 mm3 . They represent the magnetization acquired by tetrataenite during its formation by low-temperature atomic ordering of taenite, but the link with the original magnetization of taenite and kamacite grains, acquired during cooling of the parent body after metamorphism, is unknown. Alternatively this randomness can be due to a post-metamorphic TRM subsequently randomized by brecciation. A non-random low coercivity component is evidenced during stepwise AF demagnetization between 2 and 20–25 mT. Its origin clearly predates the fall of the meteorite on Earth and postdates the last major shock on the meteorite parent body, responsible for the homogeneous magnetic fabric. The coercivity spectrum of this component as well as the strong relation with the impact-related petrofabric indicate that the low coercivity component is likely related to this major impact, possibly in quasi-absence of ambient magnetic field. Lastly, comparison of NRM demagnetization behaviors between freshly fallen (Bensour, Kilabo) and other OC (St-Mesmin and former studies) indicates that the freshness of the meteorite may be a crucial parameter for the existence of a stable and interpretable original NRM component.

Acknowledgements We thank S. Nitsche at CRMC2 (CNRS, Luminy) for the sawing facility, M. Farmer and E. Twelker for providing the Bensour and Kilabo samples and corresponding information, and J.-L. Bouchez (LMTG, Toulouse) for his help with image analysis for shape

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anisotropy. We thank two anonymous referees for their reviews. References Bogard, D.D., 1995. Impact ages of meteorites: a synthesis. Meteoritics 30, 244–268. Brecher, A., 1976. Textural remanence: a new model of lunar rock magnetism. Earth Planet. Sci. Lett. 29, 131–145. Brecher, A., Ranganayaki, R.P., 1975. Paleomagnetic systematics of ordinary chondrites. Earth Planet. Sci. Lett. 25, 57–67. Brecher, A., Stein, J., Fuhrman, M., 1977. The magnetic effects of brecciation and shock in meteorites, the LL-chondrites. Moon 17, 205–216. Butler, R.F., Cox, A.V., 1971. A mechanism for producing magnetic remanence in meteorites and lunar samples by cosmic ray exposure. Science 172, 939–941. Cisowski, S.M., 1991. Remanent magnetic properties of unbrecciated eucrites. Earth Planet. Sci. Lett. 107, 173–181. Cisowski, S.M., Fuller, M., 1978. The effect of shock on the magnetism of terrestrial rocks. J. Geophys. Res. 83, 3441–3458. Clarke Jr., R.S., Scott, E.R.D., 1980. Tetrataenite-ordered FeNi, a new mineral in meteorites. Am. Mineral 65, 624–630. Cole, K.J., Sipiera, P.P., 2003. Kilabo and Bensour: a comparative study of two recent LL6 falls from Africa. In: Proceedings of the 34th Lunar and Planetary Science Conference. Abstract #1135. Collinson, D.W., 1987. Magnetic properties of the Olivenza meteorite-possible implications for its evolution and an early solar system magnetic field. Earth Planet. Sci. Lett. 84, 369–380. Collinson, D.W., Morden, S.J., 1994. Magnetic properties of howardite, eucrite and diogenite (HED) meteorites: ancient magnetizing fields and meteorite evolution. Earth Planet. Sci. Lett. 126, 421–434. Dickinson, T.L., Wasilewski, P., 2000. Shock magnetism in fine iron particle iron. Metor. Planet. Sci. 35, 65–74. Fisher, R., 1953. Dispersion on a sphere. Proc. R. Soc. A (London) 217, 295–305. Funaki, M., Nagata, T., Momose, K., 1981. Natural remanent magnetizations of chondrules, metallic grains and matrix of an Antarctic chondrite. ALH-769. Mem. Natl. Inst. Polar Res. Spec. Issue 20, 300–315. Gattacceca, J., Rochette, P., 2002. Pseudopaleosecular variation due to remanence anisotropy in a pyroclastic flow succession. Geophys. Res. Lett. 29, doi:10.1029/2002GL014697. Graf, T., Marti, K., 1994. Collisional record in LL-chondrites. Meteoritics 29, 643–648. Henry, B., Le Goff, M., 1995. Application de l’extension bivariante de la statistique de Fisher aux données d’anisotropie de susceptibilité magnétique : intégration des incertitudes de mesure sur l’orientation des directions principales. C. R. Acad. Sci. Paris 320, 1037–1042. Jackson, M., 1991. Anisotropy of magnetic remanence: a brief review of mineralogical sources, physical origins, and geological applications, and comparison with susceptibility anisotropy. PAGEOPH 136, 1–28.

358

J. Gattacceca et al. / Physics of the Earth and Planetary Interiors 140 (2003) 343–358

Jarosewich, E., 1990. Chemical analyses of meteorites: a compilation of stony and iron meteorites analyses. Meteoritics 25, 323–337. Keil, K., 2000. Thermal alteration of asteroids: evidence from meteorites. Planet. Space Sci. 48, 887–903. Kirschvink, J.L., 1980. The least-square line and plane and the analysis of paleomagnetic data. Geophys. J. R. Astron. Soc. 62, 699–718. Kletetschka, G., Kohout, T., Wasilewski, P.J., 2003. Magnetic remanence in the Murchison meteorite. Metor. Planet. Sci. 38, 399–405. Launeau, P., Robin, P.Y., 1996. Fabric analysis using the intercept method. Tectonophysics 267, 91–119. Martin, P.M., Mills, A.A., 1982. Preffered orientation of chondrules orientation in meteorites. Earth Planet. Sci. Lett. 51, 18–25. Mc Sween Jr., H.Y., Patchen, A.D., 1989. Pyroxene thermobarometry in LL-group chondrites and implications for parent body metamorphism. Meteoritics 24, 219–226. Melcher, C.L., 1979. Kirin meteorite: temperature gradient produced during atmospheric passage. Meteoritics 14, 309–316. Morden, S.J., Collinson, D.W., 1992. The implications of the magnetism of ordinary chondrite meteorites. Earth Planet. Sci. Lett. 109, 185–204. Nagata, T., 1961. Rock Magnetism, second ed. Maruzen, Tokyo, 350 pp. Nagata, T., 1979. Meteorite magnetism and the early solar system magnetic field. Phys. Earth Planet. Int. 20, 324–341. Nagata, T., Funaki, M., 1982. Magnetic Properties of Tetrataenite-Rich Stony Meteorites. In: Proceedings of the Seventh Symposium on Antarctic Meteorites. Mem. Natl. Inst. Polar Res. 25, 222–250. Rochette, P., Jackson, M.J., Aubourg, C., 1992. Rock magnetism and the interpretation of anisotropy of magnetic susceptibility. Rev. Geophys. 30, 209–226. Rochette, P., Sagnotti, L., Bourot-Denise, M., Consolmagno, G., Folco, L., Gattacceca, J., Osete, M.L., Pesonen, L., 2003. Magnetic classification of stony meteorites. 1. Ordinary chondrites. Meteor. Planet. Sci. 38, 251–268.

Rubin, A.E., 2002. Post-shock annealing of Miller Range 99301 (LL6): implications for impact heating of ordinary chondrites. Geochim. Cosmochim. Acta 18, 3327–3337. Russell, S.S., Zipfel, J., Folco, L., Jones, R., Grady, M.M., Grossman, J.N., 2003. The Meteoritical Bulletin No. 87. Meteor. Planet. Sci. 38. Sears, D.W., Axon, H.J., 1976. Metal of high Co content in LL chondrites. Meteoritics 11, 97–100. Selkin, P.A., Gee, J.S., Tauxe, L., Meurer, W.P., Newell, A.J., 2000. The effect of remanence anisotropy on paleointensity estimates: a case study from the Archean Stillwater Complex. Earth Planet. Sci. Lett. 183, 403–416. Sneyd, D.S., McSween, H.Y., Sugiura, N., Strangway, D.W., Nord, G.L., 1988. Origin of petrofabrics and anisotropy in ordinary chondrites. Meteoritics 23, 139–149. Spencer, J.R., Lebofsky, L.A., Sykes, M.V., 1989. Systematic biases in radiometric diameter determinations. Icarus 78, 337– 354. Stöffler, D., Keil, K., Scott, E.R.D., 1991. Shock metamorphism of ordinary chondrites. Geochim. Cosmochim. Acta 55, 3845– 3867. Sugiura, N., Strangway, D.W., 1987. Magnetic studies of meteorites. In: Meteorites and the Early Solar System. University Arizona Press, Arizona, pp. 595–615. Verrier, V., Rochette, P., 2002. Estimating peak currents at ground lightning impacts using remanent magnetization. Geophys. Res. Lett. 29, doi: 10.1029/2002GL015207. Wasilewski, P., 1988. Magnetic characterisation of the new magnetic mineral tetrataenite and its contrast with isochemical taenite. Phys. Earth Planet. Int. 52, 150–158. Wasilewski, P., Dickinson, T., 2000. Aspects of the validation of magnetic remanence in meteorites. Meteor. Planet. Sci. 35, 537–544. Wasilewski, P., Acuña, M.H., Kletetschka, G., 2002. 433 Eros: problems with the meteorite magnetism record in attempting an asteroid match. Meteor. Planet. Sci. 37, 937–950. Weaving, B., 1962. Magnetic anisotropy in chondritic meteorites. Geochim. Cosmochim. Acta 26, 451–455.