COMPARISON OF THE LEW88516 AND ALHA77005 MARTIAN METEORITES - SIMILARBUT DISTINCT

By mineral and bulk compositions, the Lewis Cliff (LEW) 88516 meteorit e is quite similar to the ALHA77005 martian meteorite. These two meteo rites are not paired because their mineral compositions are distinct, they were found 500 km apart in ice fields with different sources for meteorites, and their terrestrial residence ages are different. Minera ls in LEW88516 include: olivine, pyroxenes (low- and high-Ca), and mas kelynite (after plagioclase); and the minor minerals chromite, whitloc kite, ilmenite, and pyrrhotite. Mineral grains in LEW88516 range up to a few mm. Texturally, the meteorite is complex, with regions of olivi ne and chromite poikilitically enclosed in pyroxene, regions of inters titial basaltic texture, and glass-rich (shock) veinlets. Olivine comp ositions range from Fo64 to Fo70, (avg. Fo67), more ferroan and with m ore variation than in ALHA77005 (fo69 to Fo73). Pyroxene compositions fall between En77Wo4 and En65Wo15 and in clusters near En63 Wo9 and En 53Wo33, on average more magnesian and with more variation than in ALHA 77005. Shock features in LEW88516 range from weak deformation through complete melting. Bulk chemical analyses by modal recombination of ele ctron microprobe analyses, instrumental neutron activation, and radioc hemical neutron activation confirm that LEW88516 is more closely relat ed to ALHA77005 than to other known martian meteorites. Key element ab undance ratios are typical of martian meteorites, as is its nonchondri tic rare earth pattern. Differences between the chemical compositions of LEW88516 and ALHA77005 are consistent with slight differences in th e proportions of their constituent minerals and not from fundamental p etrogenetic differences. Noble gas abundances in LEW88516, like those in ALHA77005, show modest excesses of Ar-40 and Xe-129 from trapped (s hock-implanted) gas. As with other ALHA77005 and the shergottite marti an meteorites (except EETA79001), noble gas isotope abundances in LEW8 8516 are consistent with exposure to cosmic rays for 2.5-3 Ma. The abs ence of substantial effects of shielding from cosmic rays suggest LEW8 8516 spent this time as an object no larger than a few cm in diameter.