The parent magma of the second Chassignite NWA 2737: Constraint from trapped melt inclusions

The recently discovered NWA 2737 martian meteorite is the second known chassignite, an olivine-rich igneous cumulate rock. Several forms of trapped liquid within it have been examined, including interstitial melt and magmatic inclusions within large olivine grains. Polyphase melt inclusions have been found within highly equilibrium olivine grains (Fo= 77.7±0.5). Some melt inclusions contain hydrous kaersutite amphiboles and Ti-biotite. Most inclusions are generally spherical with a large range of size from 5µm to100µm in diameter. These inclusions contain prismatic low-Ca pyroxenes (orthopyroxene and pigeonite) and K-rich glass; some contain minor olivine, augite, apatite and chromite crystals. A few grains contain kaersutite laths. Some inclusions even show sub-angular to angular shapes. These inclusions are also very large (~ 200-300µm). They contain low-Ca pyroxenes and glass, and minor augite, chromite, kaersutite, sulfide and phosphate. Parent liquids were calculated from the mineralogy of large vitrophyric inclusions, they are most appropriate for estimating the parental magma composition. Results of representative reconstructed parent melt compositions by Fe-Mg exchange correction are shown in Table 1. MIa most likely represents the primary melt composition of NWA 2737, which resemble those of A* (parental melt composition of Chassigny, Johnson et al., 1991) and whole-rock composition of basaltic shergottites (B-S WR), albeit with a lower Ca and higher Mg content. Some melt compositions (MIb) have relatively higher Mg (MgO=14.95%) and lower Ca and Al contents than that of MIa. While a Mg- and Al-rich composition A’ which was proposed to be parental to the Chassigny and link to the Martian Adirondack class Gusev basalt Humphrey and low-Al liquid can not successfully crystallized the Chassigny mineral assemblages have been suggested by Filiberto, 2008, primary melt composition of NWA 2737 calculated by this study still has lower Al content.Table 1 Reconstructed primary melt compositions from the melt inclusions of NWA 2737. A* from Johnson et al., 1991 A’ from Filiberto, 2008B-S WR: Whole-rock composition average (av) of basaltic shergottites (EETA 79001B (McSween and Jarosewich, 1983; Warren and Kallemeyn, 1997), QUE 94201 (Warren and Kallemeyn, 1997; Kring et al., 2003; Bridges and Warren, 2006), Shergotty (Stolper and McSween, 1979; Bridges and Warren, 2006), Zagami (Stolper and McSween, 1979; Bridges and Warren, 2006), NWA 480 (Barrat et al., 2002a), Los Angeles(Rubin et al., 2000)).