Martian basalt (shergottite) Queen Alexandra Range 94201 and lunar basalt 15555: A tale of two pyroxenes

We present here ion microprobe analyses of rare earth and other selected trace and minor elements in pyroxenes of shergottite Queen Alexandra Range 94201 and lunar basalt 15555. Pyroxene zonation patterns record the crystallization histories of these two basaltic samples from Mars and the Moon, respectively, and allow a comparison of mafic melt evolution on these two planetary bodies. Elemental abundances and trends in pyroxenes of these two rocks indicate that their minerals formed by continuous, closed system fractional crystallization of their respective parent melts. This further supports the idea that QUE 94201 closely represents the composition of a true Martian basaltic melt (McSween et al., 1996). The main differences in pyroxene elemental zonation patterns in these two objects are attributed to earlier crystallization of whitlockite in QUE 94201 (i.e., before the Fe-rich pyroxenes) than in 15555 (after the Fe-rich pyroxenes). The size of Eu anomalies in pyroxenes of QUE 94201 is intermediate between that in pyroxenes of 15555 and the other shergottites and may imply that fO₂ conditions during crystallization of this Martian basalt were significantly more reducing than for other shergottites, although not quite as reducing as for lunar basalts. Cerium anomalies appear to be less prevalent in pyroxenes of QUE 94201 than other Antarctic shergottites and could be indicative of lesser degree of weathering in the Antarctic.