The source for the volatile-poor end-member is the pre-impact, non-altered portion of the basalt, and is most likely the nominally anhydrous ARQ 621 (NAMs, i.e., olivine, pyroxene, and plagioclase) in the original basalt, because there is no report of primary hydrous minerals (apatite in particular) in Tissint (Balta et al., 2015). It is well known that NAMs on Earth can store up to a few hundred ppm of H2O as OH (e.g., Bell and Rossman, 1992). However, analysis for H2O in NAMs in martian meteorites is limited, and the data vary from 0 to ∼1000 ppm (Boctor et al., 2003, McCubbin et al., 2014 and Usui et al., 2015). The volatile-poor end-member inferred from the impact melts could serve as a proxy for H2O in the bulk sample before impact (detail in Section 4.4), which in turn could be an indicator of the magmatic-water content. Such an approach is diaphragm independent and complementary to the other magmatic-water proxies, such as melt inclusions and hydrous phases (amphibole and apatite; Boyce et al., 2014, Greenwood et al., 2008 and Pernet-Fisher et al., 2014).
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