Microtextural Studies of Feldspar in Ordinary Chondrites

摘要

Ordinary chondrites contain an important record of events that took place during the earliest period of solar system evolution. These include primary processes, such as chondrule formation, and secondary processes, those that affected asteroids after accretion and modified primary components. Secondary processes include aqueous alteration, thermal metamorphism, and shock effects from impact events. Secondary minerals can provide insight into the chemical and physical conditions that affected their parent asteroids. Feldspar is known to be a secondary mineral that crystallized during thermal metamorphism. The goal of this work is to use the formation and evolution of feldspar to elucidate the conditions of secondary processing on the ordinary chondrite parent asteroids. I show the common occurrence of primary feldspar in chondrules and reveal ubiquitous evidence for widespread metasomatism recorded by feldspar, which has not been fully recognized previously.;Chapter 1 provides an overview of the effects of metamorphism in the L group of ordinary chondrites, as observed in feldspar as well as secondary phosphate minerals. I show that metamorphism of secondary minerals is similar in H, L, and LL groups of ordinary chondrites. Chapter 2 presents a study of the minimally metamorphosed ordinary chondrite Semarkona. I show that primary igneous plagioclase, with a wide range of compositions, is present within chondrules. Chapter 3 then follows the formation and alteration of feldspar in chondrules through the metamorphic sequence. I observe abundant evidence for metasomatism in feldspar, particularly in altered calcic plagioclase, crystallized secondary albite, and exsolved of K-feldspar from primary and secondary albite. I present a three-stage model of metasomatism involving prograde hydrous alteration, dehydration near peak metamorphism, and late-stage infiltration of anhydrous fluids. In Chapter 4, I examine fine-scale exsolution lamellae of K-feldspar in albite to determine cooling rates. I find fast cooling rates at high temperatures and discuss implications for thermal histories of ordinary chondrite parent bodies. Chapter 5 explores the development of porosity in chondrules. I show that pores in chondrules are the result of dissolution of feldspar and mesostasis glass, and that their existence facilitated fluid flow and chemical transport between chondrules and surrounding matrix.