title: From protoplanetary dust to asteroidal heating: a mineralogical study of the CO3 chondrites
Creators: name: Bonato, Enrica
Publisher: name: University of Glasgow
Year: 2020
Subjects: subject: Carbonaceous chondrites, CO chondrites, meteorites, thermal metamorphism, matrix mineralogy, organic matter.

description: Carbonaceous chondrites are among the most primitive extra-terrestrial materials available for study. These meteorites provide a detailed record of the geological processes and events that have shaped our solar system over the last 4.5 billion years. "Ornans-like" carbonaceous chondrite meteorites, also referred to as CO3 chondrites, comprise pristine, primitive mineralogy that has undergone no or minimal aqueous alteration. CO3 chondrites are also known to contain up to 3.5% carbon in the form of insoluble and soluble organic matter, graphite and carbonates. The CO3 chondrites form a suite of samples that have experienced increasing degrees of thermal metamorphism, from weakly heated CO3.0s such as Colony and DOM 08006, to strongly meta-morphosed CO3.8s such as Isna. Detailed studies of this suite of CO3 chondrites enables firstly a determination of the most primitive and earliest formed aggregates of crystalline, amorphous and organic solids, and their textural relationships from which inferences can be made regarding the nature and composition of the protoplanetary disk; and secondly quantification of the effects of parent body metamorphism on these early solar system solids. In this thesis I have studied 12 CO3 chondrites that cover the whole metamorphic sequence, namely Colony and DOM 08006 (CO3.0), NWA 7892 (CO3.05), MIL 090010 (CO3.1), Kainsaz (CO3.2), Felix (CO3.3), Ornans (CO3.4), Lancé (CO3.5), Moss and ALHA77003 (CO3.6), War-renton (CO3.7) and Isna (CO3.8). Bulk mineralogy and chemical compositions were quantified using X-ray powder diffraction (XRD) and electron probe microanalysis, and were contextualised with in-situ, spatially resolved scanning and transmission electron microscopy (TEM), and synchrotron-based scanning transmission X-ray microscopy (STXM) combined with X-ray ab-sorption near edge structure (XANES) spectroscopy. XRD was used to quantify the bulk modal mineralogy of the CO3 chondrites. I found that the most primitive samples mostly comprise Mg-rich olivine and pyroxene, and Fe-bearing amor-phous silicates. Samples between CO3.0 and CO3.1 contained ~35% forsterite, ~13% Fo60 olivine, ~26% pyroxene, ~2.5 % sulphide, ~0.7 % metal, ~5 % magnetite and ~14 % amorphous material. On heating, forsterite within the primitive samples was systematically replaced by Fe-rich olivine such that all the olivine in the CO3.8 was Fo60. This transformation was linear and could be used for rapidly and accurately defining the petrologic grade of a CO chondrite. The amorphous Febearing silicates were fully crystallised by CO3.2, magnetite had disappeared by CO3.3 and nepheline appeared at CO3.3 and gradually increases in abundance up to CO3.8. Changes in the modal mineralogy are reflected in the bulk chemistry with the matrix becoming depleted in Fe and enriched in Mg, due to equilibration with chondrules, and there is a small linear increase in Cr with increasing metamorphism. Bright-field TEM images show the fine grain size and heterogeneous texture of the matrix in the most primitive CO3 chondrites, which consists of an amorphous groundmass within which is embedded ~ 0.1 μm silicate, sulphide, metal and phyllosilicate grains. TEM imaging also revealed a systematic change in the porosity of the matrix as a function of metamorphic grade. Recrystallization and equilibration of the low porosity, low permeability matrix in the CO3.0-3.1 chondrites caused by metamorphic heating, progressively increased the porosity and average grainsize of the minerals up to CO3.8. Fe L-edge XANES analysis of the STXM data revealed that the amorphous Fe-bearing silicates and the matrix of the most primitive CO3.0 chondrites are almost fully oxidized with the Fe3+/ΣFe ratio close to 1.0. On heating the Fe becomes rapidly reduced with Kainsaz containing only about 10 % Fe3+ and Moss being dominated by Fe2+. Limited spatial variation in the Fe L-edge X-ray absorption spectra was observed in DOM 08006, most likely related to the proximity of metal and sulphides to the amorphous silicates. No significant variation in the Fe L-edge X-ray absorption spectra was observed in the silicate fraction of Moss even down to the 40 nm scale. STXM and XANES at the C, N, and O K-edges reveal spatial variations in the functional chemistry of the organic matter in the most primitive CO3 chondrites. This variation was most evident in the intensity of the aromatic, ketone and carboxyl spectral features. The presence of carbonate was also occasionally observed most particularly in and close to a ~1.3 μm wide carbonate vein in a sample of NWA 7892. As a function of increased metamorphic heating I found that the aromatic group persists while the ketone and carboxyl groups disappear such that in Moss CO3.6 only aromatic carbon was observed (with a potential trace of carbonate). Graphite was not definitively identified in any of the samples. Spectral features on the O K-edge show the progressive crystallisation of the amorphous silicate into olivine with metamorphic heating. The effects of metamorphic heating on the primitive CO3 chondrites is to crystallise the amorphous Fe-bearing silicates, systematically modify the modal mineralogy, increase the porosity of the matrix and homogenise the molecular speciation in the organic matter. Furthermore, the hydrated amorphous silicates dehydrate within a narrow temperature interval of about 100°C and there is a concomitant reduction of the Fe3+ to Fe2+ as the amorphous Fe-silicates transform into crystalline minerals. This reduction of the Fe is facilitated by the changing redox conditions likely due to the removal of oxidizing H2O and the initial presence of reducing agents such as H and C. I conclude that CO meteorites formed from anhydrous parent bodies in which minimal aqueous alteration took place and the main source of water was hydrated amorphous silicates. I propose that these amorphous silicates were hydrated in the nebula prior to accretion onto the CO parent bodies. Water within the amorphous silicates contributed to the oxidation of the Fe to Fe3+. Low porosity and limited permeability in the primitive materials restricted any fluids from circulating within the parent body. Changes from metamorphic heating released water and increased permeability such that organic matter became homogenized and subsequently partially dissociated generating a reducing environment. It is possible that the CO parent bodies had an onion shell like structure with high petrologic type COs concentrated in the inner part of the asteroid and low petrologic types closer to the surface.