Insights into Long-standing and Emerging Carbonate Proxies using µ-XRF Imaging and XANES Spectroscopy

The use of synchrotron based techniques (μ-X-ray Fluorescence imaging and X-ray Absorption Near Edge Structure spectroscopy) to understand the source and speciation of geochemical proxies within ancient carbonate rocks is a relatively novel application. The Stanford Synchrotron Radiation Lightsource (SSRL) has several m-XRF imaging and XANES spectroscopy experiments covering a wide range of x-ray spot sizes (0.5 to 250 microns), energies (2 to 23 keV) and sample size (up to 300 x 600 mm). At SSRL, we combine imaging and spectroscopy data to generate individual images of the speciation of the element(s) of interest.

We present new data for the speciation and abundance of sulfur and phosphorus in ancient carbonate rocks. This data provides insights that questions the canonical understanding of carbonate proxies representing global processes. The speciation and abundance of S in a range of facies from Ordovician and Silurian carbonate rocks records the impact of depositional environment and early marine diagenesis. Sulfate (as carbonate-associated sulfate) is the dominant S-species within cements and unaltered fossil fragments, however, sulfate abundance varies with cement morphology (based on the timing and location of precipitation). Sulfate sourced from in situ oxidation of sulfides is prevalent within micrite. This has implications for bulk S-isotope of CAS and pyrite to understand the operation and evolution of the global S-cycle. Preliminary investigations of phosphorus m-XRF imaging, XANES spectroscopy and ³¹P NMR from ~2.5 Ga herringbone calcite (Campbellrand-Malmani platform) indicate the presence of at least 3 PO₄^3- species including micro-inclusions of apatite and monetite, and potentially carbonate lattice bound PO₄^3-. Recent carbonate synthesis experiments that show elevated dissolved PO₄^2- concentrations (at mmol/kg levels) arrests aragonite and calcite nucleation across a range of saturation states, and PO₄^3- incorporation in carbonate causes curvature in the growth step of the mineral phase. We hypothesize that Archean oceans had elevated dissolved PO₄^3- concentrations that controlled the kinetics of Precambrian carbonate precipitation and that phosphate abundance in carbonates may provide an archive to assess ancient marine PO₄^3- concentrations.