Petrogenesis of shergottite meteorites inferred from minor and trace element microdistributions

Abstract

As an extension of our previous work on the lherzolitic shergottites (ALHA77005 and LEW88516), ion microprobe measurements of REEs and other selected trace and minor elements were made in individual minerals of the basaltic shergottites (Shergotty, Zagami, and EETA79001). Whole rock REE abundances in these achondrites are dominated by whitlockite, which is usually present only in trace amounts. One of the most significant features is the extensive zoning of trace and minor elements in pyroxenes. This zonation, which provides information on the crystallization histories of these meteorites, is of primary magmatic origin, and not the result of secondary processes such as diffusive reequilibration and/or metasomatic infiltration. Coherent trends in element-element plots for low-Ca pyroxenes indicate that each shergottite formed essentially by progressive (closed-system) fractional crystallization. The calculated REE patterns of the parent melts of all shergottites are LREE-depleted and parallel to their whole rock REE patterns (also indicative of closed-system fractional crystallization). On the basis of observations reported in this study, as well as previously determined petrologic and geochemical (including isotopic) characteristics, we present a model that suggests possible relationships among all five known shergottites.