SIDEROPHILE TRACE-ELEMENTS IN ALH84001, OTHER SNC METEORITES AND EUCRITES - EVIDENCE OF HETEROGENEITY, POSSIBLY TIME-LINKED, IN THE MANTLE OF MARS

We report neutron activation analyses, including radiochemical determi nation of trace siderophile elements (Au, Ge, Ir, Ni, Os and Re), for three SNC/martian meteorites, and Os and Re results for numerous eucri tes. Ratios such as Ga/Al in the SNC orthopyroxenite ALH84001 confirm its martian affinity-its many distinctive characteristics, most notabl y its near-primordial age, notwithstanding. To the list of ALH84001's idiosyncrasies can now be added extraordinarily low concentrations of Au, Ni and, especially, Re (17 pg/g), for a martian meteorite. We cons ider several possible origins for the anomalously low Re content in AL H84001, including metasomatism or alteration. The pyroxene-cumulate na ture of this rock probably does not account for its low Re content. Ot her SNC meteorites are also cumulates. An examination of Re-Nd variati ons among terrestrial basalts and komatiites suggests that Re is compa tible with mantle minerals in general and only incompatible with olivi ne (however, olivine dominates the mantle residuum, especially during komatiite genesis). Our preferred model is that the ALH84001 parent me lt formed in a mantle source region that was far more Re-depleted, and /or at a substantially lower oxygen fugacity, than the sources of the young SNC meteorites. Such a contrast is consistent with models that r eplenish siderophile elements in planetary mantles by gradual admixtur e of late-accreting matter and similarly derive most planetary water ( which serves as an oxidant) very late in accretion. According to this model, ALH84001 formed before the siderophile-rich matter and water ha d been mixed well into the martian interior. Possibly the martian mant le never became generally as Re-rich and/or oxidized as the source reg ion(s) of the younger SNCs.