In-situ laser ablation of mineral fluid inclusions: Evaluating the effects of solid and liquid matrices, laser repetition rate and fluence on data quality

Abstract

Fluids play an important role in many geological/ore genetic processes. The composition of the hydrothermal fluid is a critical but often unknown parameter that controls the mobilization of elements and their concentration into mineralized zones. In-situ analyses of fluid inclusions trapped in minerals using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) is a powerful tool to constrain the composition of the mineralizing fluid. The LA-analysis of fluid inclusions is analytically challenging due to potential mismatch of matrix between fluid inclusions and solid NIST reference glasses commonly used as calibrating standards. We conducted experiments to evaluate the effects of using solid and liquid as calibrating standard on data quality during ablation of liquid/mineral fluid inclusions, as well as the effects of variable energy fluence and repetition rate on the time-dependent elemental fractionation. The LA-analyses of liquids (with NaCl matrix) when standardized against another solution has lower precision and accuracy compared to standardization with NIST reference glasses. The 193 nm laser has little matrix effect and causes negligible elemental fractionation. The trace element concentrations of liquids retrieved by external standardization with NIST reference glass has better accuracy and precision. Higher laser fluence (8.5 J/cm2) produces better sensitivity, lesser time-dependent elemental fractionation, lower detection limits, and better accuracy/precision and is recommended for ablation of natural fluid inclusions or liquids. We tested our preferred analytical protocol by analyzing the composition of quartz-hosted fluid inclusions from the Malanjkhand copper deposit in central India. The LA-ICPMS measurements of the fluid inclusions help to establish that the mineralizing fluid had high concentrations of K, Rb, and Cs. Fluid inclusions from the Malanjkhand copper deposit are trapped from fluids that are dominantly of magmatic origin, sourced from highly differentiated granitic magmas. The NIST glasses are appropriate calibrating standards for ablation of synthetic liquids as well as natural fluid inclusions.