Date of Degree

2-2021

Document Type

Dissertation

Degree Name

Ph.D.

Program

Earth & Environmental Sciences

Advisor

Denton S. Ebel

Committee Members

Michael K. Weisberg

Haydee Salmun

Edward D. Young

Subject Categories

Biological and Chemical Physics | Cosmochemistry | Fluid Dynamics | Geochemistry | Hydrology | Mineral Physics

Keywords

carbonaceous chondrites, cosmochemistry, petrology, modeling, aqueous alteration, oxygen isotopes, mineralogy

Abstract

Icy planetesimals are significant objects of study for meteoritics, planetary science, and astrobiology due to their connections to the origins of life and liquid water on Earth. An existing closed system aqueous alteration model was adapted to simulate several scenarios involving early Solar System geologic processes occurring in an icy planetesimal interior. The model described in this work has been developed not only to test the validity of constraints currently thought to apply to CM1 parent bodies, but to directly compare the implications of these constraints for the isotopic composition and the modal mineralogy of carbonaceous chondrites. Isotopic ratios of the ungrouped C chondrite Acfer 094 were used to simulate those of the anhydrous precursors of CM chondrite parent bodies. Predictions of the model for the isotopic and mineralogical compositions of the altered parent body were then compared to properties of the highly altered CM1 chondrite Moapa Valley, which is described here in mineralogical and infrared-spectroscopic detail for the first time.

Results from this model indicate that: 1) CM1 chondrite alteration temperatures can be achieved on parent bodies ≥ 20 km assuming a relatively “late” instantaneous accretion time of ~3.0 Ma after CAIs formed in a forsterite parent body and an earlier accretion time of ~2.36 Ma years in a forsterite/enstatite parent body; 2) Aqueous alteration is highly dependent on the thermodynamic properties of the anhydrous silicates the parent body is composed of before alteration has begun; and 3) For phyllosilicate abundances of > 30 vol% to be produced by the model in the large quantities observed in CM chondrites (> 70 vol %), it is likely that vulnerable anhydrous silicates and opaque minerals would have to be altered in the early stages of alteration, and carbonates must form at the last stages of alteration. Using an oxygen isotope fractionation model we simulated the 18O-affinity in CM chondrite tetrahedral Al-bearing Mg,Fe-serpentines and predict that between 120-180˚C, Δ18Owater−serpentine≈ 1-4 ‰, a substantial ~2-3 ‰ less than the Δ18Owater−serpentine of Mg,Fe-serpentines without significant substitutions under the same temperature conditions. We also report oxygen isotopic fractionation factors for CM1 chondrite serpentines, standard cronstedtite, and Acfer 094 enstatite.

These results indicate that the in situ alteration that occurred on CM parent bodies was not only profoundly influenced by the thermodynamic properties of its anhydrous pre-cursors, but that the alteration process itself likely influenced the chemical and isotopic gradients between chondritic components as it progressed. The relatively 16O-rich oxygen isotopic ratios of some highly altered CM chondrites may be explained in the context of a single CM parent body system by the distribution of Al3+ cations and subsequent lowering of 18O affinity as in situ alteration proceeded.

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