MINERAL CHEMISTRY AND THE ORIGIN OF ENSTATITE IN UNEQUILIBRATED ENSTATITE CHONDRITES

A comprehensive study of the petrography, cathodoluminescence (CL), ra re earth element (REE) abundances, and magnesium and silicon isotopic compositions of enstatite, the major mineral in unequilibrated enstati te chondrites (UECs), is reported. For comparison, observations were a lso made in equilibrated enstatite chondrites (EECs) and aubrites, In UECs, there are no REE patterns or abundances that uniquely characteri ze the CL color and intensity, grain size, or occurrence of enstatite. Red enstatite is dominant in UECs and blue enstatite in EECs. Three R EE patterns represent the diversity observed in UEC enstatite; many gr ains are depleted in one or more of the three most volatile REEs under reducing conditions (i.e., Yb, Eu, and Sm). All REE patterns are comp atible with the fractional crystallization (during chondrule formation ) of melts whose precursors either had flat REE patterns or were deple ted in the most volatile REEs. No evidence was found to suggest that e nstatite grains require nebular condensation or multiple reservoirs, a s suggested by others. Magnesium and silicon isotopic ratios are norma l, therefore excluding the role of evaporation in separating the REEs. This fractionation was most likely accomplished during condensation o f the chondrule precursors. Our data, combined with synthetic experime nts, support the idea that coexisting red and blue CL enstatites forme d by crystallization from a single melt. Most of the red enstatite was converted to blue by diffusion and recrystallization to form the more equilibrated E chondrites. It is not clear why enstatite is depleted in the most volatile REEs while oldhamite, a highly refractory mineral which is the major carrier of REE in E chondrites, commonly has enric hments of these elements. Copyright (C) 1998 Elsevier Science Ltd.