Mineralogy and possible origin of an unusual Cr-rich inclusion in the Los Martinez (L6) chondrite

During a petrological study of the previously unclassified ordinary chondrite Los Martinez a highly unusual Cr-rich inclusion is found which is unique in both extraterrestrial and terrestrial mineralogy. Detailed SEM and TEM studies show that the inclusion consists of a highly zoned single crystal of plagioclase intergrown with chromium-rich spinel which indicates that it is the product of exsolution. The Cr-rich precursors of the inclusion probably have close affinities to the chronite-plagioclase chrondrules observed by Ramdohr (1967) in several ordinary chondrites. Based on the zoning in the inclusion it is suggested that it is the product of fractional crystallization from a melt, which may have formed as a liquid condensate, or by melting of solid condensates, in the solar nebula. Subsequent cooling of this melt condensate resulted in crystallization of the unidentified phase. After crystallization, the inclusion was probably incorporated into a parent body where it underwent metamorphism and was probably shocked to some degree. During this period of parent body metamorphism, exsolution and decomposition of the unknown precursor occurred to produce the observed intergrowth of plagioclase and chromite. Los Martinez is classified as an L6 ordinary chondrite breccia.