This thesis presents cosmochemical discoveries coupled with improvements of analytical techniques focused on deepening our understanding of the formation of rocky planetary bodies during the early evolution of the solar system. Chondrite meteorites and their components have been utilized to study the early stages of planetary body formation since they are thought to be samples of these bodies. Previous attempts to construct a comprehensive model for planetesimal evolution that explains cosmochemical signatures have relied on the well-known chondrite groups, such as ordinary and carbonaceous chondrites, that had been studied for over a hundred years. In this thesis, I primarily focused on studies of a relatively new and unstudied meteorite group, the Rumuruti (R) chondrites. The bulk chemistry, petrology and O-isotope studies presented in this thesis have revealed evidence that the chondrite parent object(s) are formed through chaotic accretionary processes. In addition to the studies of R chondrites detailed here, another study designed to further our understanding of the petrology of iron meteorite inclusions is presented. The main thesis findings include the following:
The quantification of oxygen isotope SIMS matrix effects in olivine samples (from Mg-rich to Fe-rich) were investigated by using the UCLA CAMECA ims 1270 and 1290 instruments in order to improve the accuracy of in situ O-isotope measurements in geochemical/cosmochemical olivine samples. One of the main findings was that oxygen isotope SIMS matrix effects are reproducible. With this knowledge in hand, a model curve was developed that can be used for correcting observations of instrumental mass fractionation in olivine samples of intermediate chemical composition. The model curve was calibrated by utilizing data from SIMS analysis on a San Carlos olivine, which was chosen to be the primary standard in O-isotope studies in the R chondrite chondrules.
R chondrite parent body evolutionary processes were explored by analyzing the bulk chemistry and petrology of R chondrites from petrologic types 3 to 6. I confirmed that R chondrites have a monolithic bulk composition and that R chondrites are closely related in terms of composition to ordinary chondrites. However, R-chondrite volatile abundances are much higher in comparison to those found in ordinary chondrites. And, it was found that metamorphosed R chondrites recorded different degrees of oxidation within their olivine and spinel components. These results suggest that local environment conditions on the parent asteroid were not uniform but instead were diverse.
The oxygen isotope composition of different chondrite components in one of the least equilibrated R chondrites, PRE 95404, was examined in order to investigate the apparent heterogeneity, as evidenced just above, in the parent body formation conditions. The results show that the different chondrite components present in PRE95404 experienced a variety of metamorphic processes suggesting that material from different proto bodies were incorporated into the fine grain matrix before they were finally lithified in the R parent bodies. These results conflict with the conventional ordinary chondrite accretionary model, which starts from chondrule formations followed by immediate accretion yielding the chondritic asteroids that subsequently underwent in situ metamorphism. Instead, the thesis results support a model that the R chondrite parent body formed by accretion of pre-existing planetesimal materials. The studies presented in the thesis also clarify the relationships between the R chondrites and other chondrite groups.
Lastly, a new a mineral, MnCr2S4, was discovered during an investigation of Cr-bearing inclusions found in iron meteorites. The new mineral has been given the name Joegoldsteinite.
