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

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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.