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

Abstract

Determination of absolute trace-element concentrations in fluid inclusions using laserablation (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.

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

“…NaCl and CaCl2 concentrations defined the quantity of the total Cl-salinity (Cltot) as a measure of the conservative salinity component. 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.…”

“…The concentrations of major and trace elements in Ca-bearing fluid inclusions were quantified using the microthermometrically calculated absolute Na concentrations as internal standard, which was then applied to individual LA-ICP-MS analyses. The use of absolute Na concentrations increases the accuracy of the element concentration data compared to quantification methods that use NaCl equivalent salinity (Schlegel et al, 2012). In some FIA, individual inclusions analyzed by LA-ICP-MS did not allow determination of their Na-Ca salinity, in which case the average Na content obtained from other fluid inclusions in the same FIA was used as internal standard.…”

“…Sodium was then used as internal standard for quantification of elemental concentrations, with an empirical salt correction based on the LA-ICP-MS signals for potassium (Heinrich et al, 2003). The reported CaCl2 concentrations and Ca/Na ratios are therefore mostly based on the two microthermometric phase transitions, being more accurate than the determination of the Ca concentration by LA-ICP-MS (Schlegel et al, 2012).…”

“…This paper integrates new fluid inclusion results with previous documentation of detailed geology, mineral paragenesis and alteration geochemistry (Schlegel and Heinrich, 2015) and a microanalytical study of texturally-constrained sulfur isotope variations (Schlegel et al, 2017). Rigorous petrography of fluid inclusion assemblage(s) (FIA), microthermometry and quantitative fluid inclusion analysis by laser-ablation inductivelycoupled plasma-mass spectrometry (LA-ICP-MS), and improved quantification methods for Ca-Na brines (Schlegel et al, 2012) constrain the process of economic ore formation at Prominent Hill. By integration of these data, we show how essential characteristics of hematite-rich IOCG breccia deposits can be explained by a distinct process that combines genetic elements of porphyry-type Cu and sediment-hosted Cu deposits.…”

Capabilities of sequential and quasi-simultaneous LA-ICPMS for the multi-element analysis of small quantity of liquids (pl to nl): insights from fluid inclusion analysis

“…At the present time, sequential scanning QMS typically allows registering 20 isotope signals with m/Q from 7 to 238 in less than 260 ms in a single mass scan. [3][4][5]29,30 The signals generated in QMS from this type of tiny samples are usually of relatively low intensity for minor to trace elements and last only for several seconds due to the rapid release of the uid during the ablation process. [3][4][5]29,30 The signals generated in QMS from this type of tiny samples are usually of relatively low intensity for minor to trace elements and last only for several seconds due to the rapid release of the uid during the ablation process.…”