Combining metal and nonmetal isotopic measurements in barite to identify mode of formation

Abstract

Barite (BaSO4) is a highly stable and widely-distributed mineral found in magmatic, metamorphic, and sedimentary rocks of all ages, as well as in soils, aerosol dust, and extraterrestrial material. Barite can form in a variety of settings in the oceans (hydrothermal deposits, cold seeps, water column, or within sediments) and on the continents (soils, sulfidic springs and in the subsurface) when (1) two fluids mix – one containing barium and another containing sulfate, (2) sulfur is oxidized forming sulfate in a barium containing solution, or (3) barium or sulfate is concentrated in microenvironments where either sulfate or barium are already present. Hydrologic and biologic processes can therefore play key roles in the formation of barite and affect its geochemical composition. Characteristics of barite from various modern settings are identified here to serve as analogs for ancient systems, summarizing previous work and adding new details from the pelagic marine, hydrothermal, cold seep and continental setting. Radiogenic strontium in barite clearly identifies the source(s) of fluid forming barite with the most radiogenic values measured in continental sulfidic spring settings associated with a deep fluid component that interacts with ancient crustal rocks. Sulfur and oxygen isotopes can distinguish between sources of sulfate and identify settings where the influence of (bio)chemical processes such as sulfate reduction is prominent. There are no unique stable strontium isotopic signatures for barite formed in any of the settings investigated here, but Holocene coretop marine pelagic barite appears to have a constant offset from seawater of approximately −0.53‰ in coretop samples in contrast to the wide range of values in barite precipitated in other settings. Stable strontium mass dependent fractionation could be useful in understanding post-depositional and diagenetic processes such as authigenic precipitation and recrystallization.