Refining the sedimentary pyrite proxy: Applications for understanding past ocean chemistry

本提案资助开发用于海洋化学研究的黄铁矿代理工具。海洋是地球上生命起源和进化的关键。古海洋研究不仅可以提供地球上生命如何进化的线索，还可以提供如何识别可能支持生命的系外行星的线索。研究成果还将纳入经济地质学，应用于矿产勘探和采矿实践。使用LA-ICPMS对黄铁矿进行海洋化学分析比传统的散装样品代理具有以下几个优点：1)原位分析只允许检查一种矿物，从而排除了共存矿物的混合地球化学特征；2)该技术允许根据矿物结构选择样品，这意味着只考虑早期黄铁矿颗粒；3)由于绿片岩相中黄铁矿TE含量可能被保留，因此我们的研究可以包括更多的盆地，特别是在前寒武纪。

This proposal funds the development of the pyrite proxy tool for ocean chemistry studies. Our oceans hold the key to the beginning and evolution of life on Earth. Paleo-ocean studies can provide not only clues on how life evolved on Earth, but also how to identify exoplanets that may also support life. The research outcomes will also feed into economic geology, with applications in mineral exploration and mining practices. The use of LA-ICPMS analysis of pyrite for ocean chemistry offers several benefits over traditional bulk sample proxies: 1) the in-situ analysis allow only one mineral to be examined, thus excluding the mixed geochemical signatures from coexisting minerals; 2) the technique allows sample selection by mineral texture, which means only the early stage pyrite grains are considered; and 3) since pyrite TE content may be preserved up greenschist facies, more basins can be included in our investigations, especially during the Precambrian. While this pyrite proxy tool is an enticing breakthrough, there are still several aspects that must be assessed before it can be widely implemented in both paleo-ocean. First, the degree to which TEs vary within a sedimentary basin and under different local redox conditions is an important aspect that is poorly understood. We will systematically investigate the TE content of pyrite along transects across well studied sedimentary basins to see how TE content of pyrite varies from near shore to deep water settings. We will also conduct Fe speciation, S-isotope, and whole rock trace and major element analyses on the same samples as the pyrite analyses to unravel how depositional redox conditions affect the pyrite TE content. The ability of pyrite to preserve its TE content through metamorphism means the pyrite TE proxy has the potential to allow additional (metamorphosed) sedimentary basins to be considered for ocean chemistry studies, especially important in the Precambrian. However, it has not been thoroughly tested whether the TE content of pyrite in the unaltered, innermost core of a metamorphosed sample is indeed the same as unmetamorphosed sedimentary pyrite. Using pyrite from sedimentary basins that have experienced contact metamorphism, we will compare the TE content of sedimentary pyrite and cores of metamorphosed pyrite to determine whether the TEs are preserved through metamorphism. The project will then be expanded to examine regional metamorphism. We will use a series of nano- and micro-analytical techniques (LA-ICPMS, TEM, APT, nanoSIMS, and XANES) to detail how TEs are hosted in pyrite. The use of APT to characterise how TEs are held in pyrite in 3D space at the atomic scale will provide invaluable data for other pyrite-related studies, such as metallurgy and mine site reclamation. The data generated through this study with further support mineral exploration studies in developing geochemical targeting tools by establishing a TE range for background, unmineralized pyrite.