Petrogenesis of the Dar al Gani (DaG) 1.1 Ma ejection‐paired olivine‐phyric shergottites and implications for ~470 Ma Martian volcanism

Abstract

The Dar al Gani (DaG) olivine‐phyric shergottites share mineralogical and geochemical characteristics, which confirm that these meteorites are derived from a single source. Bulk trace elements (La/Yb—0.12), in situ maskelynite 87Sr/86Sr (~0.7014) and redox estimates (FMQ ~ −2) indicate derivation from a depleted, reduced mantle reservoir; identical to all ~470 Ma shergottites ejected at 1.1 Ma. The DaG shergottites have been variably affected by terrestrial alteration, which precipitated carbonate along fractures and modified bulk‐rock fluid mobile (e.g., Ba) elements. Nonetheless, sufficient data are available to construct a multi‐stage formation model for the DaG shergottites and other 1.1 Ma ejection‐paired shergottites that erupted at ~470 Ma. First, partial melting of a depleted mantle source occurred at 1540 ± 20°C and 1.2 ± 0.1 GPa, equivalent to > ~100 km depth. Then, initial crystallization in a staging chamber at ~85 km depth at the crust–mantle boundary took place, followed by magma evolution and variable incorporation of antecrystic olivine ± orthopyroxene. Subsequently, crystallization of olivine phenocrysts and re‐equilibration of olivine antecrysts occurred within an ascending magma. Finally, magmas with variable crystal loads erupted at the surface, where varied cooling rates produced a range of groundmass textures. This model is similar to picritic flood basalt magmas erupted on Earth.

Igneous Diversity of the Early Martian Crust

“…Most Martian meteorites are young (<2.4 Ga) and belong to the shergottite-nakhlite-chassignite (SNC) group. SNCs have mafic compositions (SiO 2 < 52 wt.%), with basaltic and gabbroic sub-groups, as well as olivine and pyroxene cumulates, respectively (e.g., [3][4][5][6][7] and references therein). Yet, the most ancient igneous clasts, discovered in a Martian meteorite called Northwest Africa (NWA) 7034 and its paired rocks dated at 4.41 Ga [8], have basaltic to trachytic and monzonitic compositions [9][10][11][12].…”