Incompatible trace element geochemistry of the anorthositic achondrite Northwest Africa (NWA) 15118: Relation to diogenites and the Vestan magma ocean

Abstract

Petrogenetic models for the howardite–eucrite–diogenite (HED) clan of achondrites have been challenged by the lack of substantial plagioclase in the HED record, which is at odds with the chemical composition of diogenites. Northwest Africa (NWA) 15118, an anorthositic achondrite, displays strong isotopic affinities with HEDs and has been proposed as a lunar-style primary flotation crust of the Vestan magma ocean. Nevertheless, a geochemical link with known HEDs, particularly diogenites, remains to be demonstrated. We present major, minor, and trace element data for plagioclase and orthopyroxene in NWA 15118. Despite textural evidence for post-crystallization shock and thermal metamorphism, transect major and minor element data reveal that igneous crystallization trends are preserved. Normalized trace element data reveal depletion in Ti, Nb, Hf, Zr in plagioclase and corresponding enrichment in orthopyroxene. Orthopyroxene in NWA 15118 does not plot on the Y versus Ti array formed by diogenite orthopyroxenes, which have a higher Ti/Y ratio. The calculated melt composition in equilibrium with NWA 15118 plagioclase has lower Ti/Y, Ti/Yb, and La/Sm ratios than melts in equilibrium with diogenite orthopyroxenes; differences in the melt compositions cannot be accounted for by the choice of partition coefficients or by single-stage magmatic processes. Therefore, we argue that NWA 15118 and diogenites are not complementary cumulates that crystallized simultaneously from a global Vestan magma ocean. Furthermore, the modeled evolution curve of such a magma ocean does not produce the composition of NWA 15118 plagioclase equilibrium melts in Ti-Y-Yb space, indicating that NWA 15118 is unlikely to have been a primary flotation crust of a global magma ocean. Our findings suggest that the incompatible trace element composition of NWA 15118 likely reflects more complex, multistage magmatic processes and/or source heterogeneities than envisioned in geochemistry-based HED petrogenetic models proposed to date.