PETROGENESIS OF AL-RICH CHONDRULES: EVIDENCE FROM BULK COMPOSITIONS AND PHASE EQUILIBRIA

Abstract

We measured major, minor, and trace-element compositions for eleven Al-rich chondrules from unequilibrated ordinary chondrites to investigate the relationships between Al-rich chondrules, ferromagnesian chondrules, Ca-, Al-rich inclusions (CAIs), and amoeboid olivine aggregates (AOAs). Phase equilibrium considerations show that, for the most part, mineral assemblages in Al-rich chondrules are those expected from melts of the observed compositions. The diversity of mineral assemblages and Al-rich chondrule types arises mainly from the fact that the array of compositions spans both the spinel-saturated anorthite-forsterite reaction curve and a thermal divide defined by where the anorthite-forsterite join crosses the reaction curve. The reaction curve accounts for the two principal varieties of Al-rich chondrule, plagioclase-phyric and olivine-phyric, with or without aluminous spinel. The thermal divide influences the subsequent evolution of each variety. A third variety of Al-rich chondrule contains abundant sodium-rich glass; trace-element fractionation patterns suggest that these glassy Al-rich chondrules could have been derived from the other two by extensive alteration of plagioclase to nepheline followed by remelting. The bulk compositions of Al-rich chondrules (except sodium-rich ones) are intermediate in a volatility sense between ferromagnesian chondrules and type C CAIs. The combined trend of bulk compositions for CAIs, Al-rich chondrules, and ferromagnesian chondrules mirrors, but does not exactly match, the trend predicted from equilibrium condensation at PT ˜ 10-3 atm; the observed trend does not match the trend found for evaporation from a liquid of chondritic composition. We thus infer that the bulk compositions of the precursors to CAIs, Al-rich chondrules, were ferromagnesian chondrules were controlled primarily by vapor-solid reactions (condensation or sublimation) in the solar nebula. Some Al-rich chondrules are consistent with an origin by melting of a compound CAI-ferromagnesian chondrule hybrid; others cannot be so explained. Any hybrid model is restricted by the constraint that the CAI precursor consisted dominantly of pyroxene + plagioclase + spinel; melilite cannot have been a significant component. Amoeboid olivine aggregates also have the inferred mineralogical characteristics of Al-rich chondrule precursors---they are mixtures of olivine with plagioclase-spinel-pyroxene-rich CAIs---but the few measured bulk compositions are more olivine-rich than those of Al-rich chondrules.