Petrologic-kinetic studies of meteorites.

Abstract

This dissertation presents the results of six published and submitted papers covering various petrologic and kinetic studies of meteorites. The principal topic was the formation of mineralogically layered structures by diffusion-controlled reactions during metamorphism or metasomatism. Previous steady-state models for forming mineral layers by metasomatism were modified to make such models more versatile. These models were used in conjunction with chemical and petrographic data obtained with microprobe, scanning-electron-microscopy (SEM), and optical microscopy techniques to study (1) olivine coronas in mesosiderites and (2) layers associated with Ca-Al-rich inclusions (CAIs) in carbonaceous chondrites. CAI layers occur both near the margins of coarse-grained CAIs as "rims", and throughout the interiors of fine-grained CAIs, and the data strongly suggest that the processes for forming rims and fine-grained CAIs were essentially the same. A wide variety of otherwise puzzling textural, mineralogical, and phase composition data for olivine coronas and CAI layers can be explained if these layer structures formed by coupled reaction-diffusion processes. Olivine coronas formed by the reaction of olivine mineral clasts with surrounding mesosiderite-like matrix during high-temperature metamorphism in the near-surface region of the mesosiderite parent body. CAI layers appear to have formed in part by the reaction of melilite-rich CAIs with a surrounding environment that consisted mainly of Mg-Si-rich gas and forsteritic olivine in a probable nebular setting. There is evidence that steady-state growth models for the CAI layers are oversimplified, and that a steady-state condition was not fully attained. Unrelated petrologic studies were also performed for (1) a large, distinctive Si-rich clast in the Bovedy (L3) ordinary chondrite, and (2) a "new" L6 chondrite, Nullarbor 018, that was discovered in Australia. The Si-rich clast was studied with SEM, microprobe, and neutron activation analysis techniques in addition to a collaborative effort to determine its O-isotopic composition. These data suggest that the clast is a complex igneous differentiate that formed on a parent body that was initially similar to, but distinct from, ordinary chondrites.