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Chemical and Stable Isotope Compositions for Micro-Crystal Carbonates Suggest Low-Mg/Ca-Pore Water Formation Conditions


Micro-porous limestone reservoirs typically consist of allochems and matrix altered to <10 μm crystals of low-magnesium calcite (LMC). Geochemical data and petrographic observations suggest that these crystals are formed during burial diagenesis of the initially deposited less-stable forms of calcium carbonate such as HMC or aragonite. The small dimensions of the crystals make it difficult to quantify compositions that might help better define details of their origin. Newly developed micro-analysis tools have been applied to this problem. Chemical and cathodoluminescence data were acquired using a field-emission based electron microprobe, which provides a highly-focused beam (<1μm). Select area (10μm2) data on d18O and d13C were acquired using a NanoSIMS, We investigated three typical micro-porous carbonates I (Cretaceous, UAE), II (Jurassic, USA), and III (Cretaceous, UAE). Stable isotope data from this study are similar to previously-collected bulk geochemical data, and are interpreted to reflect a burial trend, with calculated temperatures ranging from about 35–45°C. Utilizing the wavelength-resolved cathodoluminescence capability on our electron probe, we discovered several distinct sub-populations of LMC. Variability in Mg/Ca ratio among these populations suggests that they may represent different stages of formation. The most distinct finding is for sample I where a core and rim zonation was observed for individual crystals, with the core having Mg/Ca=22 mmol/mol and its rim having Mg/Ca=13 mmol/mol. We interpret the core as forming during early marine or very shallow burial diagenesis and the rim as forming during a deeper phase of burial diagenesis. All analyses of Mg/Ca from the three studies are restricted to a range from 5 to 35 mmol/mol. Using the calculated formation temperatures from d18O, the resulting prediction for the range of pore water Mg/Ca is 0.1 to 0.8 mol/mol, which is below widely accepted estimates of mid to late Mesozoic seawater (1–2 mol/mol). NanoSIMS d13C data reveal no trace of negative carbon associated with meteoric diagenesis. Our new high resolution geochemical data reinforce the model that the LMC microcrystals most commonly associated with limestone microporosity form and undergo diagenesis during shallow burial. However, the association with low Mg/Ca pore waters is puzzling, suggesting that advection or sequestration of this Mg may occur during micro-porous limestone formation.