Trace elements in mineral separates of the Peña Blanca Spring aubrite: Implications for the evolution of the aubrite parent body

Abstract

Abstract— The origin of the aubrite parent body (APB) and its relation to the enstatite chondrites is still unclear. Therefore we began a detailed chemical study of the aubrite Peña Blanca Spring. Bulk samples and mineral separates (oldhamite, troilite, alabandite, pyroxene) of Peña Blanca Spring were analyzed for major and trace elements by instrumental neutron activation analysis (INAA). In addition, a leaching experiment was performed on a powdered bulk sample to study the distribution of trace elements in aubrite minerals.The elemental abundances in Peña Blanca Spring are compared to abundances in EH‐chondrites and EL‐chondrites in an attempt to distinguish volatility related fractionations (evaporation, condensation) from planetary differentiation (melting and core formation). Low abundances of siderophile (e.g., Ir) and chalcophile (e.g., V) elements in bulk samples indicate that 25% (by mass) metal and about 6% (by mass) sulfide separated from an enstatite chondrite like‐parent body to form a core and a residual mantle with aubrite composition. We argue that the high observed rare earth element (REE) abundances in oldhamite (>100 × EH‐chondrite normalized) reflect REE incorporation into oldhamite during nebular condensation. Thus, oldhamite in aubrites is, at least in part, a relict phase as originally proposed by Lodders and Palme (1990). Some re‐equilibration of CaS with silicates has, however, occurred, leading to partial redistribution of REE, as exemplified by the uptake of Eu by plagioclase.The distribution of the REE among aubritic minerals cannot be the result of fractional crystallization, which would occur if high degrees of partial melting took place on the APB. Instead, the REE distributions indicate incomplete equilibrium of oldhamite and other phases. Therefore, a short non‐equlibrium melting episode led to segregation of metal and sulfides.