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The parental melt of lherzolitic shergottite ALH 77005: A study of rehomogenized melt inclusions

Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, U.S.A.

Correspondence: ^* E-mail: Christina_Calvin{at}brown.edu

Lherzolitic shergottite ALH 77005 is one of the most primitive martian meteorites. To characterize the parental melt of this primitive meteorite, olivine and chromite-hosted melt inclusions have been experimentally rehomogenized. The rehomogenization was performed with hydrostatic pressures (800–1000 bars) of CO₂ + CO gas along with finely powdered graphite at temperatures of 1150–1185 °C. Equilibrium between the host and inclusion melt was determined based on the lack of zonation in the host surrounding the melt inclusion, equilibrium K_D values of host and melt inclusions, and textures of the melt inclusion. Chromite-hosted melt inclusions, where chromite is poikilitically enclosed by olivine, contain ~7.5 wt% MgO. This composition most closely reflects the parental melt of ALH 77005. The melts trapped in Fo₇₅ olivine contain ~7.1 wt% MgO when brought to equilibrium with the host. This olivine-hosted melt inclusion composition has lower SiO₂ (~50 vs. 53.9 wt%) and higher Cr₂O₃ (~1.2 vs. 0.2 wt%) and P₂O₅ (~1.2 vs. 0.5 wt%) than previous estimates for ALH 77005. In addition, compared with the chromite-hosted inclusions, the olivine-hosted ones have higher Al₂O₃ and lower CaO than can be explained through crystallization of phases known to be on the liquidus. This finding suggests that magma mixing occurred between chromite and olivine crystallization or olivine-hosted inclusions were contaminated by secondary minerals such as phosphate. Both olivine-and chromite-hosted melt inclusions in ALH 77005 have slightly higher Al₂O₃ than olivine inclusions in Chassigny but significantly higher Al₂O₃ than nakhlites such as MIL 03346 and Nakhla at similar values of MgO.

Key Words: ALH 77005 • SNC meteorite • melt inclusion • parent melt • lherzolitic shergottite • olivine • chromite