U-Th dating of vein calcite by LA-MC-ICP-MS: preliminary results from geothermal systems

Abstract:

The measurement of U-series isotopes in precipitated minerals such as calcite holds various challenges, including low U and Th concentrations (in the ppb-ppt range), and the presence of detrital ²³²Th which can lead to age overestimations. Additionally, as yet there does not exist a calcite standard reference material for inter-laboratory accuracy and precision comparison, with most laboratories using their own in-house standard material and focussing largely on application to paleoclimate studies (e.g. corals and speleothems). In actively deforming regions, high-pressure hydrothermal fluids play an important role in faulting and vein formation, and commonly fault rupture is followed by rapid sealing through mineral precipitation. Therefore, precise dating of vein growth is of special importance to our understanding of the complex interplay between seismic events and fluid flow in the upper crust, and opens up a new field of study using U-Th techniques. The ability to accurately date fault-filling calcite within such settings has the power to elucidate the connection between structure and fluid flow in the development of geothermal systems, and provide valuable information on the longevity of the heat/water source, in addition to regional magmatic history. We are developing U-Th measurements and ages of vein calcite from geothermal systems using a Neptune Plus MC-ICP-MS (with 5 CDDs and 3 SEMs) coupled to an excimer 193nm Photon Machines laser. We will be comparing our results with an 189ka in-house flowstone calcite standard previously dated by TIMS, as well as developing a geothermal calcite standard.