Earth and Planetary Sciences ETDs

Publication Date

Fall 12-13-2018

Abstract

The two subgroups of the CV3 chondrites, oxidized and reduced, contain primitive solar system materials that provide many insights into early solar system processes. Both subgroups record significant evidence of secondary alteration that has modified their primary characteristics. In this work, we have studied the petrography, mineralogy, and oxygen isotopic composition of the NWA 2364 CV3OxA chondrite and a large lithic inclusion using SEM, electron microprobe, FIB/TEM, and laser fluorination oxygen isotope analyses in order to characterize their alteration histories in detail. The NWA 2364 host and lithic inclusion consist of chondrules, Calcium-Aluminum-rich Inclusions (CAIs), and fine-grained matrix. In the host, primary minerals in the majority of chondrules have been altered by different types of ferroan olivine. Iron-nickel sulfides are nearly absent from matrix and chondrules. In multiple cases, Ca is absent from chondrule interiors and forms secondary Ca-rich pyroxene aggregates within the matrix. Secondary Na-bearing phases such as nepheline and sodalite are absent. The alteration features found in the lithic inclusion are similar in many ways to the host, but the degree of replacement is more extreme with a few notable differences. In the lithic inclusion, primary minerals have been altered to different types of ferroan olivine and, in some cases, chondrules have been completely pseudomorphed by ferroan olivines. Calcium has been leached from chondrules forming secondary Ca-rich pyroxene aggregates around the peripheries of fine-grained chondrule rims. The lithic inclusion is depleted in the fluid mobile elements, Na, K, and S compared to dark inclusions in Allende. Sodium- and K-bearing secondary phases such as nepheline and sodalite are absent from chondrules and matrix, as are sulfides. Many chondrule olivine phenocrysts contain secondary veins consisting of crystallographically-oriented, elongate, ferroan olivines (a few microns in size), similar in texture to those found in CM chondrites and terrestrial olivine that have undergone serpentinization. Along the exterior of the lithic inclusion there is an abundance of Ca-rich pyroxene aggregates forming a rim along the interface of the host and lithic inclusion. Bulk oxygen isotopic values for the host and lithic inclusion are displaced from the CCAM line, near the compositions of Leoville dark inclusions.

This work provides strong petrographic and geochemical evidence to indicate that the lithic inclusion and the NWA 2364 host were altered by fluid-rock interaction and experienced thermal metamorphism. Based on our observations, we propose the following history for the NWA 2364 chondrite and lithic inclusion. The lithic inclusion underwent aqueous alteration at low temperatures, converting a significant volume of the primary components (matrix, chondrules, etc.) into phyllosilicates and redistributing fluid-mobile elements. Progressive asteroidal heating leading to thermal metamorphism took place, dehydrating the hydrous phases and switching the alteration regime to one of fluid-assisted metamorphism in a lower fluid-rock ratio environment. The fluid released by dehydration was responsible for the fluid-assisted metamorphism, but was ultimately completely lost from the rock. The lithic inclusion was then emplaced in the NWA 2364 host via impact regolith processes. The host was likely still undergoing aqueous alteration at the time of emplacement, causing a geochemical disequilibrium between the two different lithologies. This disequilibrium caused the host and lithic inclusion to interact until the fluids were lost from the rock. Compared to other dark inclusions, the lithic inclusion is highly depleted in fluid mobile elements, suggesting that it has been highly metasomatized by the aqueous fluids. The NWA 2364 lithic inclusion is unique, as it is first lithic inclusion that has definitive evidence of low temperature hydration of primary phases prior to thermal metamorphism, demonstrating that at least some dark inclusions did go through an episode of hydration, not just high temperature interaction with aqueous fluids. This study provides further evidence for parent body alteration, but also adds to our understanding of the dynamic and complex nature of that processes that occurred on the CV3 parent asteroid.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Dr. Adrian Brearley

Second Committee Member

Dr. Charles Shearer

Third Committee Member

Dr. Karen Ziegler

Language

English

Keywords

meteorites, chondrites, secondary alteration, dark inclusions

Document Type

Thesis

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