MARTIAN BASALT (SHERGOTTITE) QUEEN ALEXANDRA RANGE-94201 AND LUNAR BASALT-15555 - A TALE OF 2 PYROXENES

We present here ion microprobe analyses of rare earth and other select ed trace and minor elements in pyroxenes of shergottite Queen Alexandr a Range 94201 and lunar basalt 15555. Pyroxene zonation patterns recor d the crystallization histories of these two basaltic samples from Mar s and the Moon, respectively, and allow a comparison of mafic melt evo lution 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 respect ive parent melts. This further supports the idea that QUE 94201 closel y represents the composition of a true Martian basaltic melt (McSween el al., 1996). The main differences in pyroxene elemental zonation pat terns in these two objects are attributed to earlier crystallization o f whitlockite in QUE 94201 (i.e., before the Fe-rich pyroxenes) than i n 15555 (after the Fe-rich pyroxenes). The size of Eu anomalies in pyr oxenes of QUE 94201 is intermediate between that in pyroxenes of 15555 and the other shergottites and may imply that fO(2) conditions during crystallization of this Martian basalt were significantly more reduci ng than for other shergottites, although not quite as reducing as for lunar basalts. Cerium anomalies appear to be less prevalent in pyroxen es of QUE 94201 than other Antarctic shergottites and could be indicat ive of lesser degree of weathering in the Antarctic.