New Insights into Silicification in Carboniferous and Permian Brachiopods: Implications for shell microstructure and geochemistry

Silicified fossil shells are well-known for their exquisite preservation of morphological features; however, because they have undergone such diagenetic process, they have generally not been used for geochemical proxy-based palaeoenvironmental and palaeoclimate reconstructions. However, silicification is a variable and often incomplete process that frequently results in portions of fossil shells being preserved with morphologically pristine microstructures. Here, we assess the potential of such partially-silicified brachiopod shells to preserve structurally and chemically pristine calcite fabrics. We combine petrographic analyses, scanning electron microscopy (SEM), cathodoluminescence (CL), energy dispersive spectroscopy (EDS), Raman spectroscopy, and in situ boron (δ 11 B) and oxygen (δ 18 O) isotope analyses to characterize silica-replaced shell domains and well-preserved calcite relics in fossil brachiopod from the Carboniferous Mobarak Fm. of Iran and from the Permian Qarari Unit of Oman. We show that silicification is a highly spatially and temporarily heterogenous process. We demonstrate that original fabric relics preserved within partially silicified shells retain pristine chemical compositions, faithfully recording primary biomineralisation signatures along shell growth. We show that partial silicification can act as a protective shield, preventing further alteration from affecting the shells. Our findings reveal that silicified fossil shells hold untapped potential for geochemical analyses.