Noble Gas Chronology of Meteorites: Brachinites, Ureilites, and Chelyabinsk

Abstract

Noble gas studies offer a glimpse into the history experienced by meteorites, their parent bodies, and the solar system. This thesis reports measurements and interpretations of noble gases of achondrites and the well-known chondrite, Chelyabinsk. Several geochronology dating methods have been used to measure the formation age and impact history of Chelyabinsk. The data from these measurements is generally not well defined and often contradictory. Ar-Ar measurements from this work show well defined impact ages ~30 Ma and ~2700 Ma recorded in different lithologies of Chelyabinsk, which help provide clarity to the complex history proposed by other studies. Chelyabinsk experienced 3-4 impacts after formation at ~4450 Ma, ~2700 Ma, ~30 Ma, and was exposed to cosmic rays ~1.5 Ma. Cosmic-ray exposure (CRE) ages of ureilites are combined with magnesium numbers of olivine and oxygen isotopes to search for common impact events. This technique can also be used to investigate the heterogeneity of the body from which the samples originated. There are 39 ureilites included in this work, and although there is weak evidence of possible clusters, it is clear that most ureilites did not originate in one or two events on a homogeneous parent body. A suite of ultra-mafic achondrites make up the brachinite and brachinite-like achondrites. Assigning an origin to one or multiple parent bodies has proven to be difficult and confusing. This work includes a subset of brachinites, brachinite-like achondrites, and ungrouped achondrites that have some brachinite affinity in some way (oxygen isotopes or mineral abundances). The motivation of this work is to determine and compare their cosmic-ray exposure age distribution, which may indicate common impact and parent body relationships. Noble gases He, Ne, Ar, Kr, and Xe are analyzed to determine their cosmic-ray exposure ages, nominal gas retention ages and trapped compositions. The measured 3He and 21Ne are purely cosmogenic. The 38Ar has a significant trapped component and calculation of the exposure age based on 38Ar is also sensitive to the amount of calcium in the sample, which is contained in minor phases. Brachinite-like samples in this study have three clusters of 21Ne exposure ages ~10Ma, ~26 Ma, and ~ 50 Ma, while the brachinites have possible groupings at ~10 Ma, ~26 Ma, ~40 Ma, and ~50 Ma. Three of the four brachinite clusters coincide with brachinite-like 21Ne ages and Δ17O values. The Δ17O values of the three older (~26 Ma, ~40 Ma, ~50 Ma) clusters are similar, while the younger cluster (~10 Ma) is slightly more depleted in 17O and might suggest a different parent body or reflect heterogeneity in the same parent body. Three of the four clusters contain brachinite-like samples, and suggests that the brachinite and brachinite-like achondrite clusters may originate from the same primitive parent body. Some samples show significant abundances of trapped Kr and Xe of Q composition (36Ar/132Kr ~ 90-150, 84Kr/132Xe ~ 1-8, and 129Xe/132Xe ~ 1.1-1.8) that must have survived thermal metamorphism, illustrating that the brachinites and brachinite-like achondrites are intermediate between chondrites and fully/more degassed differentiated achondrites.