NEW SNC METEORITE ALH 84001:  EVIDENCE FOR SNC METEORITE FROM NOBLE GASES. Y. N. Miura1, N. Sugiura1, and K. Nagao2, 1Dept. of Earth and Planetary Physics, Univ. of Tokyo, Japan, 2Inst. for Study of the Earth's Interior, Okayama University, Japan.

Published in Lunar and Planetary Science XXV, pp.919-920, LPI, Houston, TX.

ALH 84001 was originally classified as diogenite [1]. Small chips of this meteorite were allocated to us under the guise of a diogenite. However, ALH 84001 was recently reclassified as a new type of SNC meteorite according to mineralogical and oxygen isotopic features [2]. We measured noble gases in ALH 84001, which show some characteristic elemental and isotopic compositions similar to those of other SNC meteorites and of the Martian atmosphere.

Experiment and Results:  Noble gas analyses were performed twice using bulk samples weighing 0.513g (#1) and 0.448g (#2), respectively. Noble gases were extracted from the samples stepwisely, and isotopic and elemental compositions of He, Ne, Ar, Kr, and Xe were determined. The extraction temperatures are 700C and 1750C for #1, and 700C, 1000C, 1300C, and 1750C for #2, respectively. The apparatus and procedure of the measurements are about the same as those described in [3] except for some improvements, e.g., installation of an ion counting system.

Isotopic compositions of Ne obtained by all extraction steps show that they are mostly cosmogenic components. We assumed (1) measured Kr is mostly trapped and (2) Ne and Kr are trapped with a ratio similar to a literature value of the martian atmospheric Ne/Kr ratio [4]. Then, trapped 20Ne concentration is only 5% in the measured total 20Ne concentration. On the other hand, Ar consists of cosmogenic, radiogenic, and trapped components. 40Ar/36Ar ratio extracted from 1000C fraction of measurement #2 is about 8000, which is much higher than the martian atmospheric value and also terrestrial atmospheric value. In this fraction radiogenic 40Ar originated from in situ decay of 40K seems to be released. Although the measured bulk 40Ar/36Ar ratios are as low as 1940 and 2220, the 40Ar/36Ar corrected for cosmogenic contribution are 6200 and 6500. They are much higher than the martian and terrestrial atmospheric ratios. The meteorite may contain radiogenic 40Ar. The 129Xe/132Xe ratios except for the lowest temperature fractions of 700C show constant ratio of 2.0 to 2.2, which is close to those obtained from a shergottite EETA 79001 glass sample [5] and from the Viking lander [6]. It seems to contain a large amount of the martian atmospheric Xe. This is one of the strong evidence that ALH 84001 belongs to SNC meteorite group.

Cosmic-Ray Exposure Ages:  Concentrations of cosmogenic 3He, 21Ne and 38Ar and the preliminary calculation of cosmic-ray exposure ages are summarized in Table 1. The concentrations of cosmogenic light noble gases are close to those in Chassigny. The cosmogenic 21Ne/38Ar ratios of these meteorites are about 10, which suggest their higher Mg/Fe ratios than those of L-chondrites and the other SNC meteorites. In fact, 100 Mg/(Mg + Fe2+) of orthopyroxene in ALH 84001 is as high as 19 [2]. In order to calculate the production rates for ALH 84001, equations as a function of chemical compositions presented by [7] and the ordinary production rates for L chondrites [8] were used. Since bulk chemical compositions of ALH 84001 are unknown, the chemical compositions of orthopyroxene reported by [9] were adopted as bulk compositions here. The mean value of cosmic-ray exposure age of T3, T21, and T38 is calculated to be 14 ±: 2 Ma. Although this exposure age is slightly longer than those of Chassigny and nakhlites, they are within error limits considering uncertainties in chemical compositions and shielding effect. The exposure age of ALH 84001 belongs to one of the three clusters for SNC group.

Table 1. Cosmogenic noble gases and cosmic-ray exposure ages.

ALH 84001 # ± 1.8
ALH 84001 #224.83.510.48114.910.116.013.7 ± 3.1

Trapped Noble Gases:  The high 129Xe/132Xe ratio of ALH 84001 suggests that this meteorite contained abundant martian atmospheric Xe and also other noble gases. In previous works on SNC meteorites, it has been suggested that there are at least two components for SNC's trapped noble gases, which are (a) martian mantle and (b) martian atmosphere in origin [e.g., 10]. Their compositions are characterized by (a) low 84Kr/132Xe low 129Xe/132Xe and low 40Ar/36Ar, and (b) high 84Kr/132Xe, high 129Xe/132Xe and high 40Ar/36Ar respectively. Figure 1 shows 129Xe/132Xe vs. 84Kr/132Xe ratios. The data of most shergottites lie on a mixing line between them [10]. However, ALH 84001 data are plotted far above the line. Those of nakhlites are also plotted above the line. Drake et al. [11] proposed an explanation for the high 129Xe/132Xe ratio with relatively lower 84Kr/132Xe ratio for Nakhla that the meteorite interacted with an aqueous fluid after its crystallization, that is, atmospheric Xe was introduced into the meteorite as a sedimentary weathering product on Mars. Our data for ALH 84001 can be also explained by such a process, but there is now no other strong evidence for supporting it.T

The plot for elemental ratios between 36Ar/132Xe and 84Kr/132Xe are presented in Fig. 2. The concentration of trapped 36Ar is calculated assuming 0.188 and 1.55 as trapped and cosmogenic 38Ar/36Ar ratios, respectively, and those of trapped 84Kr and 132Xe are the measured concentrations. In this plot, elemental ratios of trapped Ar, Kr, and Xe for ALH 84001 do not fall on a trend between Chassigny and shergottites. The concentration of trapped Kr in ALH 84001 is depleted compared with the trend of shergottite and Chassigny (Fig. 1), and that of trapped Ar is also depleted (Fig. 2). The fractionated elemental patterns can be attributed to the above process suggested by [11] or other physical processes such as shock implantation or adsorption.

Acknowledgments:  We thank the Meteorite Working Group for providing the sample. This work is supported by Fellowships of the Japan Society for the Promotion of Science for Japanese Junior Scientists.

References:  [1] Antarctic Meteorite Newsletter 8(2). [2] Antarctic Meteorite Newsletter 16(3). [3] Miura Y. et al. (1993) GCA, 57, 1857. [4] Hunten D. M. et al. (1987) Icarus, 69, 532. [5] Becker R. H. and Pepin R. O. (1984) EPSL, 69, 225. [6] Owen T. et al. (1977) JGR, 82, 4635. [7] Eugster O. and Michel Th. (1993) submitted to GCA. [8] Marti K. and Graf T. (1992) Annu. Rev. Earth Planet. Sci., 20, 221. [9] Berkley J. and Boynton N. J. (1992) Meteoritics, 27, 387. [10] Ott U. (1988) GCA, 52, 1937. [11] Drake M. J. et al. (1993) LPSC XXIV, 431. [12] Ott U. and Lohr H. P. (1991) Meteoritics, 27, 271. [13] compiled in Ozima M. and Podosek F. A. (1983) Noble Gas Geochemistry. [14] Bogard D. D. et al. (1984) GCA, 48, 1723.