The Use Of Carbonate Marine Cements In Geochemical Studies At Global And Regional Scales.

Abstract

The presence of fibrous calcite and aragonite in meteoric environments and equant calcite in marine settings suggest that some factor other than fluid Mg/Ca ratio may control the mineralogy and crystal morphology of inorganic calcium carbonate. Regardless of setting, a systematic relationship exists between growth rate, as governed by saturation state, and carbonate mineralogy and morphology, with a progression from equant calcite to fibrous calcite to aragonite with increasing growth rate and higher degrees of saturation. Data on natural magnesian calcites indicate that their composition does reflect seawater chemistry and furthermore suggest that their magnesium content is controlled jointly by Mg/Ca ratio and growth rate. Experimental studies generally support this conclusion, although with a significant degree of ambiguity. The calcitization of formerly aragonite and magnesian calcite marine cements from the Late Guadalupian Permian Reef Complex has involved partial dissolution and subsequent emplacement of a secondary calcite within the marine cement. Remaining marine carbonate recrystallized to calcite in a closed system, preserving the original marine isotopic compositions. Retention of primary isotopic differences between marine aragonite and magnesian calcites supports this scenario. Derivation of the marine signatures indicates that the Delaware Basin was well-mixed and open to the world ocean until the latest Guadalupian. Early diagenesis of marine cements in the Permian Reef Complex was dominated by two processes, calcitization and dolomitization. At topographically higher sites, secondary calcites associated with calcitization are interpreted to have been precipitated in a syndepositional meteoric system as they have invariant oxygen isotopic compositions and variable carbon compositions becoming more enriched basinward. At lower sites analogous calcites were precipitated after burial at greater temperatures. Comparison between dolomite isotopic composition and marine aragonite and secondary calcite signatures indicates that the dolomite was precipitated from shelf-derived hypersaline brines. Petrographic and isotopic evidence suggests that some dolomite was partially recrystallized by later diagenetic fluids.