Application of low-temperature microthermometric data for interpreting multicomponent fluid inclusion compositions

Abstract

Fluid inclusions are commonly the best available source of information on the compositions of fluids in past geologic environments. Microanalytical data, predominantly from LA-ICPMS, allow assessment of the relative abundances of chemical elements in fluid inclusions. Such data show that geologic fluids commonly contain appreciable concentrations of multiple salts in addition to NaCl, particularly KCl, CaCl 2 , and FeCl 2 as major components. Quantification of absolute salt concentrations generally requires an internal standard concentration, which is typically derived from microthermometric measurements interpreted according to the vaporsaturated liquidus relations of simpler systems such as H 2 O-NaCl or H 2 O-NaCl-CaCl 2. Here, we review and reassess compositional information obtainable from microthermometric measurements in multicomponent chloride-dominated aqueous systems. To do so, we investigate the systematics of vapor-saturated liquidus phase equilibria in complex multicomponent electrolyte solutions through thermodynamic modeling based on Pitzer's equations. We focus on low-to intermediate-salinity chloride-dominated inclusions, in which ice is the liquidus phase, and on the temperature range from subsolidus conditions to <25 °C. On the basis of measured ice and hydrohalite melting temperatures, fluids with predominantly monovalent (Na ± K) chlorides, or mixtures of monovalent and divalent cation chlorides can be identified and a robust value of m Na as a proportion of total cations be calculated. We show that microthermometric measurements laone do not allow unequivocal determination of the identity of salts that are present in addition to NaCl. In combination with microanalytical determination of cation ratios, however, robust compositional results for multi-salt aqueous fluid inclusions can be obtained using microthermometric measurements interpreted with generic H 2 O-(Na,K)Cl-ΣX n+ Cl n phase stability relations.

Fluid Evolution at the Batu Hijau Porphyry Cu-Au Deposit, Indonesia: Hypogene Sulfide Precipitation from a Single-Phase Aqueous Magmatic Fluid During Chlorite–White-Mica Alteration

“…Absolute concentrations were calculated from measured element ratios using Na as internal standard, based on total salinities obtained by microthermometry (wt.% NaCleq.) corrected for the effects of other salts (Heinrich et al, 2003;Steele-MacInnis et al, 2016). This procedure possibly introduces systematic errors, so that the background-corrected element ratios were preferred here for data interpretation (e.g., Seo et al, 2009;Schlöglová, 2018).…”

Hematite Breccia-Hosted Iron Oxide Copper-Gold Deposits Require Magmatic Fluid Components Exposed to Atmospheric Oxidation: Evidence from Prominent Hill, Gawler Craton, South Australia

“…To determine the Na concentration as an internal standard for quantification of LA-ICP-MS analysis, we used the microthermometric approach of Schlegel et al (2012) based on the ternary Na-Ca-H2O model system. We did not use the more recent evaluation of low-temperature phase relations in multi-component fluids by Steele-MacInnis et al (2016), because it introduces variations in calculated Na concentrations that are not real, but primarily reflect random uncertainties in elemental analysis by LA-ICP-MS.…”

Evolution of Ore-Forming Fluids at Azegour Mo-Cu-W Skarn Deposit, Western High Atlas, Morocco: Evidence from Mineral Chemistry and Fluid Inclusions

“…It is evident that L-type inclusions coexist with S-type inclusions within any FIAs of minerals in this stage. The eutectic temperature (Te) of all the inclusion types was between −57.2 and −30.6 • C, indicating a low eutectic due to the presence of salts other than NaCl and KCl, such as CalCl 2 or FeCl 2 [76].…”