LEW 88516: A Meteorite Compositionally Close to the "Martian Mantle"

Details LEW 88516: A Meteorite Compositionally Close to the "Martian Mantle" LEW 88516: A Meteorite Compositionally Close to the "Martian Mantle"

Jul 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..216d&link_type=abstract

Meteoritics, vol. 27, no. 3, volume 27, page 216

Computer Science

29

Scientific paper

Several samples from a total of 250 mg of the recently discovered Antarctic shergottite LEW 88516 were analysed for major and trace elements by neutron activation techniques, SSMS, and a carbon-sulfur analyser. Results are presented in Table 1, together with data on ALHA 77005 (Wanke et al., 1976). This and earlier results (Boynton et al., 1992; Lindstrom et al.,1992) show the close compositional similarity of Lew 88516 to ALHA 77005. A major difference between the two shergottites is the much lower iodine content of the ALHA 77005 meteorite. The absence of similar variations in Br and Cl confirms earlier suggestions of an Antarctic source for the I excess. In a Mg/Si vs. Al/Si diagram (Fig. 1) the LEW 88516 meteorite plots at the intersection of a Shergotty parent (SPB) body fractionation trend and a line connecting enstatite chondrites and CM chondrites. The position of LEW 88516 and also of ALHA 77005 in the vicinity of ordinary chondrites is indicative of their relatively primitive composition. Lithophile trace elements show some enhancement of Sc and V over heavy REE and depletion of light REE, suggesting either a residual character for the two meteorites or assimilation of a cumulate phase during their formation. Comparatively high Ni and Co also reflect the more mafic character of the two meteorites. The present analysis and the earlier data on ALHA 77005 unambiguously demonstrate the presence of Ir in an abundance range typical for the terrestrial upper mantle. A similar Ir level was found in Chassigny, but the more fractionated Shergotty has 100 times lower Ir contents. The presence of Ir in the martian mantle samples may be the result of sulfide-silicate equilibration. The sulfides in Lew 88516 are small pyrrhotite grains (5-30 micron, 52 atom% S) and occur often together with ilmenite, at grain boundaries of the major silicate minerals. Sulfides contain an average of 1.8% Ni. However, the major fraction of Ni must reside in oxides and/or silicates as the bulk Ni/S-ratio is 0.25 as compared to 0.05 in sulfides. References: Boynton W.V., Hill D.H. and Kring D.A. (1992) Lunar Planet. Sci. (abstract) 23, 147. Lindstrom M.M., Mittlefehldt D.W., Treiman A.H., Wentworth S.J., Gooding J.L., Morris R.V., Keller L.P. and McKay G.A. (1992) Lunar Planet. Sci. (abstract) 23, 783. Wanke H., Dreibus G., Jagoutz E., Palme H., Spettel B. and Weckwerth G. (1986) Lunar Planet. Sci. (abstract) 17, 919. Table 1, which in the hard copy appears here, shows the chemical composition of Shergottite LEW 55816 and comparison with ALHA 77005.

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