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

地球最初的数亿年仍然是地球科学基础发现的最后前沿之一，但缺乏岩石记录。沉积在较年轻沉积物中的矿物锆石颗粒是研究这一时期最著名的资源。来自西澳大利亚杰克山地区的锆石通过研究其痕迹化学和生长过程中捕获在锆石中的奇异相(即矿物包裹体)，对Hadean(即40亿年前或Ga)岩浆的性质产生了深刻的见解。然而，由于缺乏基础性研究，充分利用这些锆石及其包裹体中记录的信息一直受到阻碍。该项目对锆石微量元素化学和包裹体特征的各个方面进行了一系列研究，这将有助于更好地了解作用于Hadean地球的地质过程。对Hadean Jack Hills锆石的微量元素化学和矿物包裹体特征的基本观察揭示了它们生长的岩浆化学和矿物学的几个方面。锆石的原生包裹体组合以石英和白云母为主，显示出类似页岩的前驱物质。这些锆石还含有稀有氧化物、硫化物和碳质相，与其宿主岩浆化学中未被开发的各个方面有关，特别是挥发物。然而，如何区分原生包裹体组合，它们与岩浆化学的关系，以及寄主锆石痕迹化学的各个方面与岩浆挥发分含量的相关性仍是一个问题。该项目包括几个联合的Hadean锆石痕迹化学和包裹体特征研究，可以揭示源岩熔体化学的重要方面，并确定区分原生矿物包裹体和蚀变矿物包裹体的客观标准。Hadean锆石中碳质包裹体的发现可以记录Hadean和早太古宙的碳同位素变化。此外，还将利用一套成分和成因已知的约4亿年前的花岗岩类，研究源岩浆微量元素含量和氧化还原条件与锆石化学和包裹体组合的关系，并将结果用于更好地理解地球历史上的最后一个黑暗时代。

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.