On the relationship between isolated and chondrule olivine grains in the carbonaceous chondrite ALHA77307

Abstract

The origin of isolated olivine grains in carbonaceous chondrites has been suggested to be either by direct condensation or by fragmentation of chondrules. In an attempt to resolve this debate, isolated olivine grains in the carbonaceous chondrite ALHA77307 have been studied in detail. Zoning characteristics and minor element compositions of olivines from all isolated and chondrule occurrences are described. The data show that a strong relationship exists between isolated olivines and the chondrule population. Chondrule olivine compositions and zoning properties are consistent with olivine crystallization from the molten chondrules. Because properties of isolated grains are very similar to chondrule olivines, an igneous origin for most, if not all, isolated grains is proposed. Large isolated forsterite grains may be derived from fragmentation of “macroporphyritic” type I chondrules which contain one or very few large olivine grains. Relic forsterite grains in type II chondrules, and complex isolated olivine grains with forsteritic cores and fayalite-rich rims, provide evidence that fragmentation and recycling of chondrules and their components was a common and efficient process in the chondrule-forming region.