Composition of chondrule silicates in LL3-5 chondrites and implications for their nebular history and parent body metamorphism

Abstract

Our petrologic studies of 75 type IA and type II porphyritic olivine chondrules in nine selected LL group chondrites of type 3.3 to type 5 and comparisons with published studies of chondrules in Semarkona (LL3.0) show that compositions of silicates and bulk chondrules, but not overall chondrule textures, vary systematically with the petrologic type of the chondrite. These compositional trends are due to diffusive exchange between chondrule silicates and other phases (e.g., matrix), such as those now preserved in Semarkona, during which olivines in both chondrule types gained Fe 2+ and Mn 2+ and lost Mg 2+, Cr 3+, and Ca 2+. In a given LL4-5 chondrite, the olivines from the two chondrule types are identical in composition. Enrichments of Fe 2+ in olivine are particularly noticeable in type IA chondrules from type 3.3–3.6 chondrites, especially near grain edges, chondrule rims, grain boundaries, and what appear to be annealed cracks. Compositional changes in low-Ca pyroxene lag behind those in coexisting olivine, consistent with its lower diffusion rates. With increasing petrologic type, low-Ca pyroxenes in type IA chondrules become enriched in Fe 2+ and Mn 2+ and depleted in Mg 2+, Cr 3+, and A1 3+. These compositional changes are entirely consistent with mineral equilibration in chondritic material during metamorphism. From these compositional data alone we cannot exclude the possibility that chondritic material was metamorphosed to some degree in the nebula, but we see no evidence favoring nebula over asteroidal metamorphism, nor evidence that the chondrule reacted with nebular gases after crystallization. Modelling of the equilibration of chondrule olivines suggests that heterogeneous FeO concentrations in olivine could be preserved after cooling from 600°C at rates of 1–10°C/Ma for at least tens to hundreds of millions of years. This is consistent with published estimates for the maximum metamorphic temperatures in type 3 chondrites, thermal histories derived from metallographic and fission-track cooling rates, and 4.4 Ga 40Ar- 39Ar ages for ordinary chondrites. Since the lifetime of the solar nebula was not more than 10 6 years and there is abundant evidence that meteorite parent bodies were heated, some even above their melting point, we confidently conclude that the formation of type 3.3–5 ordinary chondrites from type 3.0 material by metamorphism occurred in parental asteroids, not the solar nebula.