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

摘要

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 H_2O-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 CaCl_2 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 H_2O-NaCl-CaCl_2 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.