The Sharps Chondrite - New Evidence on the Origin of Chondrules and Chondrites

Abstract

Chemical, petrographic and mineralogical studies of the Sharps chondrite reveal that this meteorite is a complex agglomerate of chondrules and lithic fragments in a matrix similar in composition to carbonaceous chondrites. One type of fragment resembles Type II carbonaceous chondrites in composition, but contains igneous glass and has been depleted in sulphur (0.5%) and enriched in carbon (6%). The change in carbon and sulphur contents is ascribed to heating after the agglomeration as is also indicated by the textures. The heating episode has also indurated the whole rock but not equilibrated the major silicate phases. Among the chondrules are a few conspicuous ones with two generations of olivine. Evidence is given to show that pre-existing chondritic, relatively iron-rich (7–15% Fe) and nickel-poor (<0.01% Ni) olivine was caught in a second chondrule- forming process producing reduced silicates, iron-poor olivines (1–2% Fe) and ‘glass’ which reacted with the earlier olivine. Thus, several cycles of igneous events and deposition can be recognized. The secondary chondrules provide strong arguments against direct condensation of chondrules. The evidence, rather, favors the so-called ‘ignimbritic’ model of chondrule and chondrite formation. Textural relations, e.g., welding of different components, are also consistent with the latter alternative.