Quantitative Digital Tapho-geochemistry of Upper Permian Petrified Wood from Paraguay: Multi-phase Permineralisation Resolved via High-Resolution μCT and Molecular Spectroscopy

Abstract The reconstruction of the three-dimensional mineralogical and anatomical architecture of fossil plants is essential for understanding Palaeozoic taphonomic processes. This study presents a high-resolution characterisation of Baieroxylon cicatricum (Ginkgoales) from the Upper Permian Tacuary Formation of the Independencia Group, exposed at Arroyo Vino (Gauirá Departament, eastern Paraguay), within the Guaireña Basin. Utilising X-ray micro-computed tomography (μCT) with an isotropic voxel size of 10 μm, the secondary xylem was digitally reconstructed to evaluate mineral density gradients through 16-bit attenuation mapping. These data were cross-validated using SEM-BSE-EDS and Confocal Raman Spectroscopy (CRS) to establish a comprehensive paragenetic sequence. The results reveal a multi-phase permineralisation history. The primary stage involved the volumetric replacement of woody tissues by stable alpha-quartz and hematite clusters (55.5 wt% Fe), stabilising the cellular morphostructure. A subsequent metasomatic event introduced metastable silica polymorphs, specifically monoclinic moganite and opal microspheres, into residual void spaces. The spatial distribution of opal within the 3D quartz framework, confirmed by Raman shifts at 787, 978, and 1587 cm-1, indicates that the specimen remained geochemically active post-lithification. Furthermore, the detection of chromium (7.56% Cr) and uranium suggests that mineralising fluids were influenced by tectonic faulting within the Gondwanan Paraná Basin. The preservation of vestigial carbonaceous matter (14.1 wt%) confirms the efficiency of silica replacement in encapsulating primary biological relicts. This study marks a shift from descriptive palaeoxylology to quantitative digital tapho-geochemistry, establishing a new diagnostic framework for interpreting complex mineralisation processes and reinforcing the palaeoenvironmental and palaeobiogeographical context of Gondwanan Permian floras.