Other
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.472h&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 472
Other
10
Abundance, Mars (Planet), Meteoritic Composition, Olivine, Pyroxenes, Temperature Effects, Chemical Composition, Crystallization, Isotopes, Lithology, Mineralogy, Trace Elements
Scientific paper
One of the reported characteristics of the orthopyroxenite ALH 84001 (A84) is the absence of olivine distinguishing it from the other martian meteorites, all of which are olivine-bearing. However, olivine is present in A84, as widely dispersed 5-10 micron grains enclosed within orthopyroxene. This olivine is compositionally homogeneous with a distinctly Fe-rich composition (34.5% Fa). Another newly discovered mineral in martian meteorites is cumulate orthopyroxene in Chassigny. Both observations have profound implications for the magmatic and alteration histories of these meteorites. A84 is dominated by nearly homogeneous cumulus orthopyroxene, and minor chromite found within cumulus orthopyroxene grains and interstitial areas. Olivine and orthopyroxene show disequilibrium compositions. Similar assemblages of olivine, chromite and orthopyroxene can be found in other martian meteorites, such as the nearly identical shergottites LEW 88516 and ALH 77005 (L&A) and the lithology A megacryst assemblage of EETA 79001 (EET-A). The relatively consanguine mineralogy and texture of these lithologies suggests that they could have been derived from similar basaltic parental magmas. Although complex scenarios of mixing or assimilation are required to account for the varying observed trace-element and isotopic compositions for these rocks, the possible parental similarities suggest that a generic brand of Al-poor basaltic magma was (or is) globally prevalent on Mars. Although olivine in A84 may have been resorbed during magmatic evolution, another way to reduce its abundance is metamorphic reaction at much lower temperatures. Substantial olivine may have been present before its removal by magmatic resorption and metamorphic alteration.
Harvey Ralph P.
McSween Harry Y. Jr.
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