CARRIER PHASES OF Xe IN ALH 84001.  J. D. Gilmour, J. A. Whitby, and G. Turner, Department of Earth Sciences, University of Manchester, Manchester M13 9PL, UK.

Published in Meteoritics, 31, p. A51.

The ancient martian orthopyroxenite, ALH 84001, contains Xe with the elevated 129Xe/132Xe ratio characteristic of the martian atmosphere, while the Kr/Xe ratio is similar to that of the nakhlites (depleted in Kr relative to the shergottites; 36Ar is also depleted) [1,2]. Here we report the results of an extended study combining stepped heating (laser and filament furnace) and laser probe analyses of irradiated and unirradiated aliquots of this meteorite in an attempt to determine the host phase of the 129Xe.

Neutron irradiation produces excesses of 128Xe from 127I, 131Xe from 130Ba (and possibly 130Te), and fission isotopes (131-136Xe) from 235U. Excesses of 131Xe were corrected for a fission contribution using the measured 134Xe/132Xe ratios on the assumption of two-component mixing between martian atmospheric Xe (SPB) and neutron-induced fission of 235U. (No significant evidence for in situ fission of 238U was found in any unirradiated sample.) The Xe contents of the various phases are discussed below.

Carbonate:  The temperature step in which carbonate decrepitated was determined by monitoring the CO2 release from the sample. Analysis of an irradiated sample showed releases of 128Xe (I content in carbonate ~425 ppb) and 131Xe with traces of 136Xe excess and 126Xe (spallation from Ba). Excess 129Xe was present at less than the 2-sigma level: (3.2 ± 1.7) × 10-11 ccSTP g-1 in carbonate. This phase is not a major carrier of trapped martian atmosphere.

Apatite:  Laser probe analysis of a single mineral grain of apatite in an unirradiated polished section yielded a martian atmosphere component but was most notable for significant excesses of the spallation-derived isotopes 124Xe and 126Xe (Fig. 1). Subsequent electron probe analysis of the melt showed a 25% contribution from the apatite grain, the remainder having originally been orthopyroxene. Combined with estimates of the grain size and the published apatite REE abundances [3], this is consistent with the known cosmic-ray exposure age (15 Ma [4]).

Maskelynite:  During laser stepped heating of an irradiated sample there was a significant evolution of Ba-derived 131Xe at intermediate temperatures, while 128Xe, 129Xe, and 136Xe excesses were chiefly concentrated in the higher temperature releases (Fig. 2; maximum releases are 128Xe, 1.2 × 105 atoms; 129Xe, 1.5 × 104 atoms; 131Xe, 2 × 104 atoms; 136Xe, 1.6 × 104 atoms). We attribute the excess 131Xe to the presence of maskelynite since laser probe analyses of maskelynite-rich areas yielded large 131Xe excesses. Comparison of the release patterns of 131Xe and 129Xe excesses eliminates maskelynite as the major carrier of martian atmospheric Xe in this meteorite.

Orthopyroxene:  A series of laser probe analyses of spots with diameters in the range 50-200 Ám failed to show a dramatic concentration of gas in any one phase. Rather, estimated gas contents from 19 pits were all in the range 10-11-5 × 10-10 ccSTP g-1, 132Xe with a distribution centered close to the accepted bulk value of 2.8 × 10-11 ccSTP g-1. This homogeneity strongly suggests that the major carrier of 129Xe in this meteorite is orthopyroxene. The similarity in elemental and isotopic abundances of this component to that found in the nakhlites suggests that they may have similar host phases.

References:  [1] Swindle T. D. et al. (1995) GCA, 59, 793-801. [2] Miura Y. N. et al. (1995) GCA, 59, 2105-2113. [3] Wadhwa M. and Crozaz G. (1995) LPS XXVI, 1451. [4] Eugster O. (1994) Meteoritics, 29, 464.