Accurate and precise quantification of major and trace element compositions of calcic–sodic fluid inclusions by combined microthermometry and LA-ICPMS analysis

Determination of absolute trace-element concentrations in fluid inclusions using laser-ablation (LA) ICPMS requires an internal standard, i.e., the concentration of one element must be independently known from independent observation. Microthermometric determination of the last melting temperature of ice, hydrohalite or halite is routinely used to calculate apparent salinities in wt.% NaCl equivalent, using phase relations in the binary H2O–NaCl system to estimate Na concentration. Calculating the concentrations of all other elements requires an empirical correction, if additional salt concentrations are of similar magnitude as that of NaCl. If CaCl2 is the main additional salt component, as in many low-temperature basin and basement brines, absolute Na concentrations (wt.% NaCl abs.) can be obtained by observing two melting temperatures (hydrohalite and either ice or halite), uniquely defining the major element composition of the fluid in the ternary model system H2O–NaCl–CaCl2 and allowing Na to be used as internal standard for quantifying all minor and trace elements. Test results for a range of compositions show that calcium concentration can be determined more precisely by microthermometry than by LA-ICPMS analysis, but that both methods agree within error. The combined approach of microthermometry and LA-ICPMS analysis described here permits reliable quantification of major (Ca, Na) as well as trace element concentrations in sodic–calcic brine inclusions, even in Ca-rich host minerals such as fluorite or Ca-bearing carbonates.

► Na–Ca-dominated fluid inclusions (FI) determined by microthermometry and LA-ICPMS.
► Two melting temperatures are measured in each FI, to yield true NaCl concentration.
► Improved accuracy of LA-ICPMS analysis of FI containing CaCl2 as major component
► CaCl2 concentration in FI is more precisely found by microthermometry than LA-ICPMS.
► Accurate determination of Ca-conc. in FI hosted within Ca-bearing host minerals