Earth and Planetary Sciences ETDs

Publication Date

Spring 2-14-2018

Abstract

This work presents our detailed investigation of textures and major, minor, and trace element compositions of iron sulfides in CM and CR carbonaceous chondrites using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM) from focused ion beam-prepared (FIB) sections, and synchrotron X-ray fluorescence (SXRF) microanalysis. This study represents the first attempt to link micro- to nano-scale textural and chemical characteristics of CM and CR chondrite primary iron sulfides and their alteration products.

The objectives of our studies included determining: 1) if primary sulfides are present in the least-altered CM and CR chondrites, and if so, 2) whether they formed by sulfidization and/or crystallization; 3) whether they are unstable under certain parent body conditions breaking down into alteration products in the moderately- to highly-altered CM and CR chondrites; 4) whether the textural and compositional characteristics of primary and altered primary sulfide grains allow us to determine their formation conditions (e.g., pH, fO2, , fS2 T, fluid composition, etc.) and thermal histories (e.g., cooling rates).

Although iron sulfides are a minor phase in carbonaceous chondrites, we determined that primary sulfide grains are actually a major proportion of the sulfide grains in the weakly-altered CM2 and CR2 chondrites. We argue that pyrrhotite-pentlandite intergrowth (PPI) grains formed from crystallization of monosulfide solid solution melts, whereas sulfide rimmed metal (SRM) grains formed from sulfidization of Fe,Ni metal. Cooling rate calculations of PPI grains yield rates of/hr for CM2 chondrites and 42 to 82 K/hr for CR2 chondrites which pertain to a temperature range not frequently discussed for chondrule cooling (i.e.,

Coordinated FIB-TEM-SXRF microprobe analyses for trace element concentrations of primary pyrrhotite, pentlandite, and associated metal grains from chondrules in CM2 and CR2 chondrites allowed us to measure Co, Cu, Ge, Zn, and Se, in addition to Fe and Ni, at a spatial resolution of 2 µm. The similarity between the CM and CR PPI sulfide trace element patterns provides evidence for similar formation mechanisms and conditions. The similarity between the sulfide and metal trace element patterns provides evidence for a genetic relationship between the two, such as formation by sulfidization. Depletions in Ge and Zn imply that the former experienced volatilization, or else was never incorporated into the metal or sulfide precursor materials, while the latter is partially lithophile during chondrule formation. Trace element concentrations further support a crystallization model of formation for the PPI grains and a sulfidization model of formation for the SRM grains.

In the more aqueously altered CM2 and CR2 chondrites, we observed that primary sulfides were unstable under some parent body conditions and began to break down into different products. The alteration styles consist of primary pyrrhotite altering to secondary pentlandite, magnetite, or phyllosilicates in grains that initially formed by crystallization (PPI grains) and primary metal altering to magnetite, iron carbides, or tochilinite in grains that initially formed by sulfidization (SRM grains). The range of alteration textures and products is the result of differences in conditions of alteration due to the role of microchemical environments and/or brecciation.

The breakdown of primary sulfides is even more apparent in the heavily-altered CM1 chondrites where iron sulfides are largely present as relict primary pentlandite with the associated alteration products of primary pyrrhotite, including secondary pentlandite, magnetite, and serpentine. We argue that several different textural groups of the altered primary sulfides were initially PPI grains. The fact that such different alteration products could result from the same precursor sulfides even within the same meteorite sample further underscores the complexity of the aqueous alteration environment for the CM chondrites. This complexity is likely attributable to a combination of two specific factors; first, that alteration processes were affected locally by microchemical environments and, second, that brecciation mixed together grains with different alteration histories. The different alteration reactions for each textural group place constraints on the mechanisms and conditions of alteration with evidence for variations in pH, variations in fO2, and changing fluid compositions.

Prior to this work, researchers argued that troilite was the dominant primary sulfide in CM and CR chondrites, whereas pyrrhotite and pentlandite were primarily secondary phases, which formed from aqueous alteration on their asteroidal parent bodies. As secondary phases, it was thought that pyrrhotite and pentlandite increased in abundance from the least- to highly-altered samples. From this work, however, we have found that coarse-grained (>10 µm) pyrrhotite and pentlandite are largely primary phases that formed from crystallization in the solar nebula. Pyrrhotite is unstable during alteration and breaks down into several alteration products, resulting in a decrease in abundance from the least- to highly-altered samples. Pentlandite, on the other hand, is resistant to alteration and, in some cases, forms from the breakdown of pyrrhotite; as such, it increases in abundance from the least- to highly-altered samples.

The major new insights into the behavior of sulfides gained from this work include: 1) primary sulfides, in the form of the PPI grains, are relatively common and make up a majority of the sulfides observed in CM and CR chondrites, especially the least-altered samples; 2) PPI grains breakdown into a range of alteration products with increasing degrees of alteration of the host meteorite, which has implications for chalcophile element redistribution; 3) microchemical environments play a major role in the varying conditions responsible for the range of alteration products from the breakdown of the primary sulfides; and 4) sulfides are sensitive indicators of alteration, more so than previously appreciated, making them invaluable in studies of progressive alteration.

Degree Name

Earth and Planetary Sciences

Level of Degree

Doctoral

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Adrian Brearley

Second Committee Member

Rhian Jones

Third Committee Member

Lindsay Keller

Fourth Committee Member

Zachary Sharp

Fifth Committee Member

Charles Shearer, Jr.

Language

English

Keywords

chondrites, iron sulfides, pyrrhotite, pentlandite, aqueous alteration, trace elements

Document Type

Dissertation

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