Source and parental melts of poikilitic shergottites: Implications for martian magmatism

Abstract

Martian poikilitic shergottites are cumulate rocks that can help advance the understanding of magmatic evolution from near the base of the crust (∼10 kbar) to near-surface conditions. Through a comprehensive petrographic and geochemical study, we aim to better understand poikilitic shergottite formation and the evolution in the martian interior. A suite of poikilitic shergottites, including Northwest Africa (NWA) 7755, NWA 11043, NWA 11065, NWA 10618 and Alan Hills (ALHA) 77005, were investigated for their major, minor, and trace element compositions of olivine-hosted melt inclusions (MI). The MI occur within both the early-evolutional stage textural and late-evolution stage textural domains in olivine. Major element compositions of MI indicate fractional crystallization between the early and late-crystallizing domains. Calculated parental melt compositions from these MI data yielded results that also petrogenetically link the poikilitic shergottites with the olivine-phyric shergottite subgroup. Trace element compositions of MI show that the later-crystallizing MI could have undergone open-system processes, such as fluid exsolution. Lutetium-Hf and Sm-Nd isotopic analyses were performed on NWA 7755 and NWA 11043 to constrain their age and source isotopic compositions. Northwest Africa 7755 shows a 176Lu/177Hf crystallization age of 223 ± 46 Ma, which fits into the expected range for enriched shergottites of ∼165 Ma to 225 Ma. A similar crystallization age and 176Lu/177Hf and 147Sm/144Nd source composition of NWA 7755 to the other enriched shergottites suggest that this specimen likely shares a long-lived geochemical source with these samples that has lasted for at least 60 Ma. Northwest Africa 11043 shows scatter throughout the Lu-Hf and Sm-Nd isotopic data, suggesting that this sample is not in isotopic equilibrium. This sample was possibly inherited from high-temperature processes, such as incomplete magmas mixing from a similar, but distinct, source. We conducted in situ U-Th-Pb isotope analyses of Ca-phosphate minerals for NWA 11043 and found an unreliable crystallization age of 59.2 ± 138.4 Ma: phosphates are likely recording the period of shock metamorphism related to the ejection event. Consistent crystallization ages and magmatic histories support previous work that suggest there is a common magmatic system on Mars that is responsible for the formation of enriched shergottites.