Strontium Isotopic Variations of Authigenic Calcite in Clastic Strata Record Its Sediment Provenance and Fluid−Rock Interactions

Abstract

Strontium isotopes of authigenic carbonate potentially record sediment provenance, fluid sources, and fluid–rock interactions, little was studied on this topic in clastic strata. This study investigated clastic rocks containing authigenic calcite in the Lower Triassic Baikouquan Formation in the Junggar Basin, northwestern China. Mineral compositional and fluid inclusion analyses were conducted to constrain the precipitation processes of authigenic calcite, and the Sr contents and isotope ratios of the calcite were also measured. The authigenic calcite was precipitated at 80–140°C as the final product of thermochemical oxidation of hydrocarbons and thus has high Mn contents and highly negative δ13CVPDB values (as low as −70‰). The calcite also exhibits anomalously low 87Sr/86Sr values (0.704827, 0.706612), which are lower than contemporaneous seawater and published 87Sr/86Sr values of carbonate cements in clastic sediments, and also much lower than 87Sr/86Sr values (0.722027, 0.736750) of alkali feldspar in the strata. These low 87Sr/86Sr values record the low 87Sr/86Sr of the dominant rocks in the provenance area, such as volcanic rocks. During diagenesis, especially mesodiagenesis, the charging of hydrocarbon-bearing fluids promoted abundant dissolution of orthoclase in the alkali feldspar detritus, releasing radiogenic 87Sr into the pore waters, and eventually increasing the 87Sr/86Sr values in the late-stage calcite that precipitated after this reaction. This inference is consistent with the positive correlation between the calcite 87Sr/86Sr ratios and the dissolution intensity of orthoclase. In regions that do not undergo hydrocarbon-charging and where orthoclase remains stable, the lower 87Sr/86Sr ratios of the calcite generally record the provenance. For authigenic calcite associated with intense fluid–rock interactions, the higher 87Sr/86Sr ratios reflect the enhanced dissolution intensity of 87Sr-rich minerals such as orthoclase. Therefore, combined with a petrological study, Sr isotopes of authigenic carbonate in clastic sediments can trace sediment provenance and intensity of fluid–rock interactions.