The oldest carbonate minerals on Earth: insights into the early history of the solar system

Sofe, Mahmood (2013) The oldest carbonate minerals on Earth: insights into the early history of the solar system. PhD thesis, University of Glasgow.

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CM carbonaceous chondrites display a range of alteration (aqueous alteration) from slightly altered (CM2.5) to most altered (CM2.0). They are of particular interest in understanding early solar system evolution because they are made of a mixture of mineral grains that formed at high temperatures in the solar nebula with secondary minerals including phyllosilicates and carbonates whose origin is controversial. Previous studies suggest that these secondary minerals were produced by low temperature water-mediated alteration of anhydrous minerals within the asteroidal parent body of carbonaceous chondrites. Others have suggested that phyllosilicates and other secondary minerals may have formed from a nebula gas or within an earlier-formed asteroid. This study has focused on answering these questions by the study of carbonates. Results from this study are consistent with an origin of all the secondary minerals by parent body aqueous alteration.

In addition to aragonite, calcite and dolomite, CM chondrites contain breunnerite and Ca-poor dolomite. This study has found a very complex sequence of mineralisation in CM chondrites, which was: aragonite ≥ (i.e. formed before or at the same time) calcite free of rims > calcite rimmed with tochilinite and/or Fe-sulphide > phyllosilicate pseudomorphs after calcite ≥ calcite replacing Mg-rich olivine > calcite veins > dolomite > calcite replacing dolomite > dolomite veins > breunnerite > Ca-poor dolomite > calcite cement.

Aragonite grains record a period relatively early in the aqueous alteration history of the parent body of CM chondrites. This initial parent body water crystallized aragonite with high oxygen isotope values (average δ18O of 39.9 ± 0.57 ‰ and δ17O of 20.4 ± 1.1‰), these values falling as the water obtained oxygen from reaction with anhydrous silicates. Aragonite was probably present in all CM chondrites, but was dissolved or replaced by calcite in some moderately and highly altered CM chondrites as aqueous alteration progressed. Aragonite in less altered CM chondrites crystallized from fluids with higher concentrations of Fe than those from which the aragonite grains in the moderately altered chondrites crystallized, and whether aragonite or calcite formed was determined by the fluid Mg/Ca ratio. The majority of aragonite grains in CM chondrites have a preferred orientation, which is most likely to be the result of compression in the parent body during crystallisation. Calcite free of rims and inclusions is comparable in texture and petrographic appearance to most aragonite grains, but it exhibits more complex CL patterns, which suggests that this calcite precipitated from fluids that were less compositionally stable than those from which aragonite crystallized.

Calcite grains rimmed with tochilinite and/or Fe-sulphide are present in all CM chondrites studied, and are more abundant than aragonite and calcite free of rims and inclusions. CL patterns of this calcite are more complex than those of the other carbonate generations. The most complicated CL patterns have been found in grains from less altered CM chondrites, and CL characteristics become simpler towards the highly altered CM chondrites. Rimmed calcite has lower δ18O values (average δ18O of 37.5 ± 0.65‰ and δ17O of 20.9 ± 1.3‰) than aragonite grains, and so probably formed after aragonite. It was partially replaced by phyllosilicates to leave serpentine-tochilinite rimmed pseudomorphs, which in some cases comprise 3.15 vol% of a meteorite. Tochilinite-serpentine pseudomorphs after calcite are present only in moderately altered CM chondrites, whereas Mg-rich serpentine pseudomorphs after calcite occur in the moderately and highly altered CM chondrites, indicating that solutions at later stages in the alteration sequence were richer Mg than Fe, a result of the dissolution of Mg-rich silicates; consequently new minerals rich in Mg were formed.
Calcite replacing Mg-rich olivine is found in all the studied CM chondrites, but it is more abundant and coarser grained with increasing degree of alteration. Results of this study suggest that calcite after Mg-rich olivine, and Mg-rich serpentine or serpentine-tochilinite intergrowth with calcite, probably formed at the same time or immediately after each other. Fe, Mg and Si were released from Mg-rich olivine, and were used for the replacement of calcite by Mg-rich serpentine.
Calcite veins in CM chondrites formed in late stages of aqueous alteration as a result of dissolution and recrystallisation of earlier formed Ca-carbonate grains. A large calcite vein (millimetres in size) in the Antarctic CM chondrite LON 94101 has average δ18O of 18.4 ± 0.3‰ and δ17O of 9.0 ± 0.5‰, and low concentrations of Fe in comparison with other calcite and aragonite grains. Petrographic observations show that shock post dated the crystallization of rimmed calcite grains and the LON 94101 vein, suggesting this calcite vein was formed by water in the parent body rather than by Antarctic weathering.
Dolomite is found mainly in the highly altered CM chondrites. These rocks also contain calcite intergrown with dolomite and inclusions of dolomite within calcite. Dolomite has probably been completely replaced by calcite in the most altered CM chondrites. In common with calcite veins, dolomite veins in highly altered CM chondrites have very low concentrations of Fe and Mn, and more Mg than dolomite grains in the meteorite matrix, and may have been sourced from dissolution of earlier dolomite grains. The Antarctic CM chondrite QUE 93005 contains complex carbonate minerals consisting of bimineralic grains (i.e. dolomite-breunnerite) and polymineralic grains (i.e. breunnerite-(Ca-poor-dolomite)-calcite cement). The presence of breunnerite in this meteorite suggests an evolutionary link between CM and CI chondrites (CI chondrites are more aqueously altered than CM chondrites).
Based on the results of this study, CM chondrites were probably derived from the same parent body, but from different regions that vary in ice content, porosity and permeability, and source of heating (26Al with a half-life 717,000 years).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Carbonate minerals, CM chondrites
Subjects: Q Science > QE Geology
Colleges/Schools: College of Science and Engineering > School of Geographical and Earth Sciences
Supervisor's Name: Lee, Prof. Martin and Smith, Dr. Caroline
Date of Award: 2013
Depositing User: Mr Mahmood Sofe
Unique ID: glathesis:2013-4107
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Mar 2013 10:24
Last Modified: 19 Mar 2013 10:24

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