Petrology and origin of the shergottite meteorites

Abstract

Shergottites contain cumulus pigeonite and augite, probably without cumulus plagioclase and crystallized under relatively oxidizing conditions. Shergotty and Zagami may differ in the relative proportions of cumulus pyroxenes and crystallized intercumulus liquid, but the compositions of pyroxenes and liquid are similar in both meteorites. Absence of olivine in melting experiments suggests that the shergottites crystallized from fractionated derivatives of primary liquids. Low-Ca pyroxene and augite apparently began to crystallize from these primary liquids prior to plagioclase. Shergottites can be readily distinguished from other achondrite groups by their mineralogies, crystallization sequences and inferred source region compositions. However, the source regions of the shergottites may be related to those of other achondrite types by addition or loss of volatile components. The bulk composition of the Earth's upper mantle overlaps that of permissible shergottite source regions. Shergottites and terrestrial basalts display similarities in oxidation state and concentrations of trace and minor elements with a wide range of cosmochemical and geochemical affinities. Accretion of similar materials to produce the terrestrial upper mantle and the shergottite parent body or accretion of the Earth's upper mantle from planetesimals similar to the shergottite parent body may account for many of their similarities. Models of the origin of the Earth's upper mantle which attribute its oxidation state, its siderophile element abundances and its volatile element abundances to uniquely terrestrial processes or conditions, or to factors unique to the origin and differentiation of large bodies, are unattractive in light of the similarities between shergottites and terrestrial basalts.