USING IN-SITU CHEMICAL DATA TO DETERMINE TEMPERATURES IN FOSSIL AND MODERN GEOTHERMAL SYSTEMS: A NEW APPLICATION OF INTERSECTOR ELEMENT PARTITIONING IN A SINGLE TOURMALINE CRYSTAL

Geothermal systems are an important component in the nation’s low-carbon energy portfolio. As more geothermal options are considered, additional geothermal systems have the potential to be utilized for energy production. One challenge in characterizing these systems is ascertaining their temperatures (T). Typically, the compositions of select minerals, specific mineral assemblages or dissolved fluid components are used as geothermometers of active geothermal systems. This technique requires the presence of specific minerals or compositions.

The mineral tourmaline, a borosilicate, occurs in numerous geothermal systems and is extremely sensitive to its host rock environment. With low volume diffusion, tourmaline retains chemical fingerprints of its previous thermal conditions. Tourmaline can exhibit sector zoning and the chemical differences between it’s a and c sectors have been calibrated as a geothermometer in medium-temperature settings (van Hinsberg & Schumacher, Am Min, 2006). Consequently, intersector element partitioning makes tourmaline ideal as a single mineral thermometer which records T conditions in a geo/hydrothermal system. Such an approach provides an alternative method for T determinations in these systems.

To evaluate tourmaline’s geothermometric potential, a sector-zoned tourmaline was analyzed from a fossil hydrothermal system, a tin deposit in southwest Bolivia. Associated minerals include quartz, plagioclase, muscovite, monazite, zircon and rutile (with Sn). Detailed imaging coupled with in-situ chemical data using the electron microprobe (EMPA) were obtained from a highly zoned single tourmaline grain. Oscillatory growth zoning develops in each sector. Twelve pairs of analytical points were selected such that the same growth zone was analyzed in the corresponding a and c^- crystallographic sectors. Tourmaline compositions include schorl, oxy-schorl and oxy-dravite. Using partitioning data for Ca and Ti measured for each sector in the 12 pairs, temperatures calculated range from 300°C to 420°C ±50°C. These data compare, within uncertainty, of Ts obtained for this locality using other techniques. These results provide a proof of concept for using sector-zoned tourmalines to determine Ts in fossil and active geothermal systems with boron-bearing fluids.