QUE94201 SHERGOTTITE - CRYSTALLIZATION OF A MARTIAN BASALTIC MAGMA

Basaltic shergottites are commonly described as having crystallized fr om magmas that contained entrained or cumulus pyroxenes. Complexly zon ed clinopyroxenes in QUE94201, a newly recovered Antarctic shergottite , are similar to those formed during continuous growth in some lunar b asaltic melts. Pyroxene cores have nuclei of magnesian pigeonite mantl ed by augite, in turn rimmed by strongly zoned ferroan pigeonite, with outer rims of pyroxferroite. Fe-Ti oxide compositions indicate that t his meteorite crystallized under relatively reducing conditions, more like lunar basalts than other shergottites. REE and other trace elemen t abundances in zoned pyroxenes, maskelynite, and phosphates are consi stent with continuous pyroxene crystallization from an evolving magma, punctuated by the appearance of new phases on the liquidus. The highe r proportion of maskelynite in QUE94201 suggests that its composition may be closer to multiple saturation with pyroxenes + plagioclase than other shergottites, and all pyroxene zones show negative Eu anomalies . Thus, this meteorite may represent the first example of a Martian ma gma unaffected by crystal accumulation. The Fe/(Fe + Mg) ratio and pro portions of incompatible elements (reflected in 4 modal% whitlockite) are high, implying that this is a fractionated melt. The calculated bu lk REE pattern shows extreme LREE depletion, suggesting either less co ntamination with enriched crustal materials or a more LREE-depleted so urce than other basaltic shergottites.