Oxygen isotopic compositions of Allende Type CCAIs: Evidence for isotopic exchange during nebular melting and asteroidal metamorphism

摘要

In situ oxygen isotopic measurements of primary and secondary minerals in Type C CAIs from the Allende CV3 chondrite reveal that the pattern of relative enrichments and depletions of O-16 in the primary minerals within each individual CAI are similar to the patterns observed in Types A and B CAIs from the same meteorite. Spinet is consistently the most O-16-rich (Delta O-17 = -25 parts per thousand to -15 parts per thousand), followed by Al,Ti-dioside (Delta O-17 = -20 parts per thousand to -5 parts per thousand) and anorthite (Delta O-17 = - 15 parts per thousand to 0 parts per thousand). Melilite is the most O-16-depleted primary mineral (Delta O-17 = -5 parts per thousand to -3 parts per thousand). We conclude that the original melting event that formed Type C CAIs occurred in a O-16-rich (Delta O-17 <= 20 parts per thousand) nebular gas and they subsequently experienced oxygen isotopic exchange in a O-16-poor reservoir. At least three of these (ABC, TS26F1 and 93) experienced remelting at the time and place where chondrules were forming, trapping and partially assimilating O-16-poor chondrule fragments. The observation that the pyroxene is O-16-rich relative to the feldspar, even though the feldspar preceded it in the igneous crystallization sequence, disproves the class of CAI isotopic exchange models in which partial melting of a O-16-rich solid in a O-16-poor gas is followed by slow crystallization in that gas. For the typical (not associated with chondrule materials) Type C CAIs as well for as the Types A and B CAIs, the exchange that produced internal isotopic heterogeneity within each CAI must have occurred largely in the solid state. The secondary phases grossular, monticellite and forsterite commonly have similar oxygen isotopic compositions to the melilite and anorthite they replace, but in one case (CAI 160) grossular is O-16-enriched (Delta O-17 = -10 parts per thousand to -6 parts per thousand) relative to melilite (Delta O-17 = -5 parts per thousand to -3 parts per thousand), meaning that the melilite and anorthite must have exchanged its oxygen subsequent to secondary alteration. This isotopic exchange in melilite and anorthite likely occurred on the CV parent asteroid, possibly during fluid-assisted thermal metamorphism. (C) 2008 Elsevier Ltd. All rights reserved.