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Abstract

This paper constrains fluid flow and chemistry in Miocene dolomites of Spain, where dolomitization has been ascribed to ascending freshwater–mesohaline mixing. End-Miocene dolomite formed as replacement and cement with the same widespread cathodoluminescence. Fluid inclusion final melting temperatures of ice (Tm ice: −0.2 to −2.3 °C) indicate mixing of freshwater and evaporated seawater. δ18O and δ13C data mostly show positive covariation, and only some have variable δ13C and invariant δ18O, arguing that mixing was more important than sulphate reduction. Data range from +0.9 to +6.0‰ for δ18O and from −4.5 to +3.0‰ for δ13C (VPDB (Vienna Pee Dee Belemnite)). Lower stratigraphic units are more depleted isotopically than upper units, suggesting upwards flow of freshwater. 87Sr/86Sr values (0.70866–0.70904) range from less than to greater than late Miocene seawater. δ18O, δ13C and Sr analyses show that freshwater interacted with basement, confirming injection of freshwater from below. Upwards flow of freshwater, driven by low density and hydraulic head, created fluid mixing and CO2 degassing. Comparison of La Molata dolomite to other dolomites of the western Mediterranean suggests that ascending freshwater–mesohaline mixing may be widespread, and that local composition of basement is not the primary driver of dolomitization. The model is broadly applicable to carbonates adjacent to highs, where freshwater discharged into slightly evaporated seawater.

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