Igneous and shock processes affecting chassignite amphibole evaluated using chlorine/water partitioning and hydrogen isotopes

Abstract

Abstract Amphibole in chassignite melt inclusions provides valuable information about the volatile content of the original interstitial magma, but also shock and postshock processes. We have analyzed amphibole and other phases from NWA 2737 melt inclusions, and we evaluate these data along with published values to constrain the crystallization Cl and H 2 O content of phases in chassignite melt inclusions and the effects of shock on these amphibole grains. Using a model for the Cl/ OH exchange between amphibole and melt, we estimate primary crystallization OH contents of chassignite amphiboles. SIMS analysis shows that amphibole from NWA 2737 currently has 0.15 wt% H 2 O. It has lost ~0.6 wt% H 2 O from an initial 0.7–0.8 wt% H 2 O due to intense shock. Chassigny amphibole had on average 0.3–0.4 wt% H 2 O and suffered little net loss of H 2 O due to shock. NWA 2737 amphibole has δD ≈ +3700‰; it absorbed Martian atmosphere‐derived heavy H in the aftermath of shock. Chassigny amphibole, with δD ≤ +1900‰, incorporated less heavy H. Low H 2 O/Cl ratios are inferred for the primitive chassignite magma, which had significant effects on melting and crystallization. Volatiles released by the degassing of Martian magma were more Cl‐rich than on Earth, resulting in the high Cl content of Martian surface materials.