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

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 Sr-87/Sr-86 (similar to 0.7014) and redox estimates (FMQ similar to -2) indicate derivation from a depleted, reduced mantle reservoir; identical to all similar to 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 similar to 470 Ma. First, partial melting of a depleted mantle source occurred at 1540 +/- 20 degrees C and 1.2 +/- 0.1 GPa, equivalent to > similar to 100 km depth. Then, initial crystallization in a staging chamber at similar to 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.