Geochemical, and stable and radiogenic isotope records in Devonian and Early Carboniferous carbonates from Valle de Tena, central Pyrenees (Spain): evidence for their diagenetic environments

Mineralogical, textural and geochemical investigations were made to determi ne the post-depositional evolution of Devonian and Early Carboniferous carb onates from Valle de Tena. The carbonate association is made up of low-Mg c alcite, which occurs as micrite, spar cements, neomorphic patches and spar filling veinlets. Non-stoichiometric dolomite and ankerite occur as cements (dolomite also as replacements) in the Middle Devonian, post-dating calcit e types. All these phases pre-date tectonic stylolites, indicating compacti on after stabilization of the carbonate minerals. Strontium concentrations indicate that Early Devonian and Early Carboniferous micrites initially pre cipitated as aragonite; Middle and Late Devonian micrites precipitated as h igh-Mg calcites. Both precursors were diagenetically stabilized to low-Mg c alcites through interaction with meteoric waters in phreatic environments. Trace elements in dolomite and ankerite indicate precipitation from Sr-enri ched meteoric water. All studied carbonates, except Middle Devonian limesto nes, precipitated in reducing environments, which favoured incorporation of Fe and Mn. Late calcite generations precipitated from more saline waters t han micrites. Light delta(18)O values in micrites suggest alteration mainly in meteoric-phreatic environments. The dolomites and ankerites precipitate d from more O-18-depleted fluids than the calcites, suggesting a greater co ntribution from meteoric waters. Variations in delta(13)C of micrites repre sent primary secular trends, according to published delta(13)C variations. The delta(13)C oscillations within each succession probably relate to sea-l evel oscillations. Strontium isotopes also point to a meteoric origin of di agenetic fluids. Model calculations suggest that O and Sr isotopes equilibr ated between calcites and fluid at relatively low water/rock ratios, wherea s C isotopic signatures are inherited from limestones.