Oxygen-isotopic compositions of minerals from the Baszkówka chondrite

Abstract

Oxygen isotopic compositions of two picotite-bearing chondrules from Baszkówka L5(S1) equilibrated ordinary chondrite have been determined by secondary ion mass spectrometry.  The O-isotopic compositions of the minerals in the picotite-bearing chondrules are slightly more 16O-rich than the bulk O-isotopic composition of Baszkówka.  The petrological properties and the O-isotopic compositions of these chondrules suggest that they have closer relationship to ferromagnesian chondrules than CAIs. The 16O-enrichment of the picotite-bearing chondrules may result from incomplete O exchange reaction between 16O-rich chondrule precursors and the 16O-poor solar nebular gas.  This implies that the picotite-bearing chondrules are more primitive than ordinary ferromagnesian chondrules in Baszkówka.  The O-isotopic compositions of the minerals in one of these chondrules relate to the crystallization sequence.  These O-isotopic variations of minerals within the chondrule may result from incomplete O exchange between the chondrule melt droplet and the nebular gas.  The heterogeneity of the O-isotopic composition in Baszkówka indicates that Baszkówka has not been homogenized completely during thermal metamorphism and/or secondary alteration.