FORMATION AND METAMORPHISM OF GROUP A5 CHONDRULES IN ORDINARY CHONDRITES

Abstract

Group A5 chondrules have FeO-rich olivines (>4 wt% FeO) and albite-normative mesostases and thus are discrete from the major primary chondrule groups. Group A5 chondrules in six LL chondrites have been studied by cathodoluminescence, optical microscopy, electron-micropobe, and, in the case of four chondrules removed from Semarkona (type 3.0) and Krymka (type 3.1), by instrumental neutron activation analysis. Olivine and pyroxene compositions vary with petrographic type in a manner resembling group B1 chondrules (they gain FeO and MnO, lose CaO, MgO, Al2O3, and Cr2O3 and become more homogeneous), but the pyroxene trends are less marked because of lower diffusion rates. There are no metamorphically-related compositional trends in the mesostasis, although compositional zoning is common in the mesostases of two type 3.4-3.7 chondrites. Group A5 chondrules contain unfractionated lithophile element bulk compositions but, as is typical of all chondrules, siderophile, and chalcophile elements are depleted by about an order of magnitude.Group A5 chondrules are abundant in equilibrated (100% by number) but rare in unequilibrated ordinary chondrites (UOCs, 5% by number), but their presence in UOCs is not due to the emplacement of metamorphosed chondrules during brecciation because (1) olivine in group A5 chondrules is heterogeneous, (2) the olivine displays all the compositional properties of olivine in low petrographic type ordinary chondrites, and (3) the mesostasis is not recrystallized. The chondrules display trends that are consistent with chondrule-to-chondrule and chondrule-to-matrix mineral homogenization and equilibration during metamorphism. Like the major chondrule groups A1, A2, and B1 in Semarkona, group A5 is a primary group whose presence provides further insights into chondrule formation process and diversity. Their bulk composition and redox state suggest that the conditions and temperatures of formation for groups A5 and B1 were similar, however, their different mesostasis compositions indicate that their post-solidification histories were different. Either group B1 chondrules experienced slower cooling at low temperatures than group A5, during which albite crystallized in the mesostasis, group B1 suffered a subsequent mild reheating that caused albite crystallization in the mesostasis, or nucleation details for crystallization of albite in the residual melt were different. Group A1 and A2 chondrules could have been derived from group A5- or B1-like material by reduction and evaporative loss during chondrule formation.