Geochemistry and Crystal Structure of Recrystallized Dolomites
Abstract
Shallow marine to peritidal carbonates of the Triassic Csukma Formation in the Mecsek Mts. and Villany Hills of SW Hungary are made up of dolomites, limestones, and dolomitic limestones that show evidence of a complex diagenetic history. Integration of petrographic, conventional stable oxygen and carbon isotope, clumped isotope, and strontium isotope data with the paleogeography, paleoclimate, and burial history of the region revealed four major diagenetic stages in the Mecsek Mts. Stage 1: Reflux dolomitization of the peritidal carbonates during the Middle Triassic. Stage 2: Recrystallization of the reflux dolomites in intermediate burial setting. Stage 3: High-temperature, fault-controlled dolomitization and renewed recrystallization of the reflux dolomite during the Early Cretaceous by seawater drawn down and circulated through rift-related faults in a deep burial setting. Stage 4: Saddle dolomite cementation related to tectonic expulsion of basinal fluids during the Late Cretaceous and Cenozoic. Due to striking differences in the burial history of the Mecsek Mts. and Villany Hills, evidence of diagenetic phases resulting from deep burial and rift-related faulting, such as high-temperature dolomite recrystallization and considerable saddle dolomite cementation, are missing in the Villany Hills. Here dolomitization likely occurred penecontemporaneously via geothermal convection of normal to slightly modified seawater in a near-surface to shallow burial setting followed by partial recrystallization of the dolomites in an intermediate burial setting with low water to rock ratios. Geochemical data and unit cell parameters obtained by Rietveld refinement of conventional and synchrotron X-ray and neutron diffraction data collected on recrystallized dolomites of the Csukma Formation suggest that recrystallization with high water to rock ratio versus recrystallization in low water to rock ratio may be reflected in the unit cell dimensions of recrystallized dolomites. Recrystallization processes with low water to rock ratio result in a noticeable trend between crystal size and unit cell parameters that cannot be explained by Ostwald ripening. In samples recrystallized with high water to rock ratios compositionally indistinguishable overgrowth cement on the matrix dolomite may have a significant effect on the overall unit cell dimensions determined from powder diffraction analyses. This study is the first to apply combined synchrotron X-ray diffraction and neutron diffraction to investigate crystal structural changes in sedimentary dolomites due to recrystallization in various diagenetic environments.
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- OSU Dissertations [11222]