FINGERPRINTING THE TEMPERATURE AND FLUID SOURCE OF CALCITE IN GEOTHERMAL SYSTEMS USING CLUMPED ISOTOPES

Abstract

Geothermal exploration relies on locating sufficient permeability and high temperature. Traditionally, minerals can be used to map the thermal structure of paleo or active geothermal systems via temperature dependent hydrothermal mineral deposition (e.g., smectite?illite-smectite?illite) or methods such as fluid inclusion microthermometry.
Clumped isotopes offer a new way of characterising the temperature and fluid source from which calcite is deposited. This technique is based on the thermodynamic relationship between carbonate mineral growth temperature and the abundance of chemical bonding (“clumping”) between 13C and 18O isotopes (expressed as ?47) within single carbonate ions. In the gas phase, isotopic exchange between CO2 molecules and water is continuous (e.g., Eiler, 2007) and so CO2 gas will record the ambient fluid temperature. When the CO2 is trapped in a solid mineral phase, the isotope ratio is fixed. Thus, clumped isotopes will record the temperature of crystallisation, enabling the application of clumped isotope paleothermometry to a range of geological problems (e.g., Dale et al., 2010; Huntington et al., 2010; Henkes et al., 2013; MacDonald et al. 2015).
As a geothermal test case in New Zealand, samples from the Kawerau Geothermal Field have been analysed for clumped isotopes. The selected samples have been previously constrained with fluid inclusion and stable isotopes analyses. We present petrography, ?13C and ?18O, fluid inclusion temperatures, and clumped isotope data to show how clumped isotopes can fingerprint the temperature and fluid source of calcite in geothermal systems.