An Isotopic Investigation Of Early Planetesimal Differentiation Processes

Windmill, Richard Joseph (2021). An Isotopic Investigation Of Early Planetesimal Differentiation Processes. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.00012472

Abstract

The differentiation and early evolution of planetesimals is relatively poorly understood. The Main Group pallasites (PMGs) and IIIAB irons are differentiated meteorite groups from deep planetesimal interiors. They provide a window into the early evolution of rocky planets because of the abundance of samples from these groups and because a common planetary provenance has been proposed. Oxygen isotope analyses are crucial in understanding these relationships. The mineralogy of the PMGs and IIIABs, which seemingly record the magmatic evolution of their parent body, offers a unique opportunity to study early planetary differentiation processes. High-precision oxygen isotope analyses are used in conjunction with petrological characteristics and Cr and W isotope analyses to subdivide these groups and investigate formation processes. Two subgroups are identified in PMGs: PMG-low and PMG-high. The former exhibits an oxygen isotopic disequilibrium between olivine and chromite that is unexplainable through known mass-dependent processes. These minerals therefore either sample multiple isotopic reservoirs mixed during an impact or, less likely, are affected by complex anharmonic or nuclear field shift effects. Further investigation on these effects must be executed to completely discount these latter possibilities. The PMG-high chromite isotope ratios probably record equilibration between these two reservoirs. Chromium and W isotope analyses on PMG samples show no disequilibrium but provide an excellent chronology.

High-precision oxygen isotope analyses of IIIABs has identified three previously unknown subgroups with serious implications for the interpretation of the IIIAB suite of samples. These likely originate from different planetesimals and not from complex core evolution. Finally, chromite in PMGs and IIIABs is shown to be resolvable in Δ17O which precludes a common parent planetesimal. The findings of this study suggest that there may have been many more differentiated planetesimals in the early Solar System than previously thought and necessitate care in future studies linking meteorite groups by parent body.

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