Insights into the Earliest Magmas from Inclusions and Trace Elements in Zircon

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Earth's first few hundred million years remains one of the last frontiers for fundamental discovery in the earth sciences but lacks a rock record. Grains of the mineral zircon deposited in younger sediments are the best known resource for investigating this period. Zircons from the Jack Hills locality in Western Australia have yielded great insights into the character of Hadean (i.e., 4 billion years ago, or Ga) magmas through investigation of both their trace chemistry and exotic phases trapped within the zircons as they grew (i.e., mineral inclusions). However, fully exploiting the information recorded in these zircons and their inclusions has been hampered by a lack of foundational studies. This project constitutes a series of investigations into various aspects of zircon trace element chemistry and inclusion character, which will allow for better understanding of the geologic processes acting on Hadean Earth. Fundamental observations on the trace element chemistry and mineral inclusion character of the Hadean Jack Hills zircons have revealed several aspects of the chemistry and mineralogy of the magmas from which they grew. Quartz and muscovite dominate thelikely primary inclusion assemblages of the zircons, suggesting shale-like precursors. The zircons also contain rare oxide, sulfide, and carbonaceous phases relevant to various under-explored aspects of their host magma chemistry, especially volatiles. However, questions remain as to how to distinguish primary inclusion assemblages, their relationship to magma chemistry, and the relevance of various aspects of host zircon trace chemistry to magmatic volatile contents.This project constitutes several allied investigations of Hadean zircon trace chemistry and inclusion character which can reveal important aspects of source melt chemistry and define objective criteria for distinguishing primary versus altered mineral inclusions. The discovery of carbonaceous inclusions in Hadean zircons allows for the documentation of carbon isotopic variations during the Hadean and early Archean. In addition, the relationship of source magma trace element content and redox conditions with zircon chemistry and inclusion assemblages will be investigated using a suite of ~400 million year old granitoids of known composition and origin, and the results applied to better understand the last dark age of Earth history.