Earth and Planetary Sciences ETDs


Mark Tyra

Publication Date



I carried out a petrologic and Mn-Cr isotopic study of carbonates in the paired CM1 chondrites, ALH 84049 and ALH 84051, in an effort to understand the origin and chronology of formation of carbonates in the most heavily altered CM chondrites. Dolomite is strongly compositionally zoned (Ca, Mg, Fe, Mn), indicating very heterogeneous formation conditions, yet all carbonate Mn-Cr analyses form individual isochrons. In this study, I also analyzed the Mn-Cr isotope systematics of the CR1 chondrite GRO 95577 and determined that siderite is the youngest secondary mineral yet observed in carbonaceous chondrites. This has implications for the CR parent body as it either was large enough to retain heat for long periods of time or was heated by impact after most aqueous alteration in carbonaceous chondrites had ceased. This study also presents analyses of carbonates in the same samples (ALH 84049 and GRO 95577) in-situ for their oxygen, and, in ALH 84049, carbon isotope composition to constrain aqueous alteration. The results show that multiple generations of carbonates must have occurred in ALH 84049 from a carbon source with either heterogeneous carbon isotopes or with changing carbon isotope compositions from ongoing methane formation. Furthermore, in GRO 95577, the oxygen isotope values suggest that calcite precipitated before siderite if CR chondrite fluids followed a closed system oxygen isotope evolution path similar to CM chondrites.

Degree Name

Earth and Planetary Sciences

Level of Degree


Department Name

Department of Earth and Planetary Sciences

First Advisor

Brearley, Adrian

First Committee Member (Chair)

Hutcheon, Ian

Second Committee Member

Sharp, Zachary

Third Committee Member

Jones, Rhian

Fourth Committee Member

Shearer, Charles

Project Sponsors

National Aeronautics and Space Administration, United States Department of Energy, New Mexico Space Grant, Geological Society of America, Meteoritical Society, The Moore Foundation, University of New Mexico RPT grant, New Mexico Geological Survey Beverly Wellnitz Memorial Scholarship, University of New Mexico Department of Earth & Planetary Sciences Alumni scholarship




meteorites, asteroids, Mn-Cr isotope systematics, carbon isotopes, oxygen isotopes, water in asteroids, petrology, CM chondrite, CR chondrite, aqueous alteration, carbonate, siderite, calcite

Document Type