Quantitative analysis of major, minor and trace elements in fluid inclusions using laser ablation–inductively coupled plasmamass spectrometry

Abstract

Microscopic fluid inclusions in minerals are the main source of information about the chemical composition of fluids associated with large-scale material transport in the Earth's interior. Hydrothermal transport processes are responsible for the natural enrichment of metal resources in many ore deposits. For the multi-element analysis of the microscopic fluid inclusions (typically 5–50 µm in diameter), LA-ICP-MS has become one of the most promising techniques owing to the recent progress in laser optics design and the development of high-sensitivity ICP mass spectrometers. The quantitative analyses of 19 major, minor and trace elements covering a concentration range of five orders of magnitude were carried out on 39 single natural fluid inclusions, together with a number of experiments to optimise controlled ablation and to test the calibration procedure. A modified commercial ICP-MS instrument was used together with a prototype ablation system based on a 193 nm excimer laser. In a stepwise opening procedure for complex polyphase inclusions, a small hole (4 µm pit) was first drilled for the partial release of liquid and vapor, followed by complete drilling out using a pit covering the entire inclusion. Controlled ablation improves the reproducibility of element ratios to less than 20% for most major, minor and trace elements measured in an assemblage of cogenetic inclusions (including elements that are initially present as solid precipitates within the inclusion), provided that the entire transient ICP-MS signal is integrated. Element ratios were calculated from integrated intensity ratios using an external standard, either a NIST SRM glass or an aqueous standard solution ablated directly through a plastic film. Absolute concentrations were calculated from the element ratiosviaan internal standard element, whose concentration was determined prior to ablation. Microthermometric measurements of phase transitions were used to determine total salinity from known phase diagrams, by measuring either the depression of ice-melting temperature, or the temperature of dissolution of NaCl crystals. Salinity can be related to the concentration of Na (or in some cases Cl), which serves as the internal standard element for the quantification of trace element concentrations. Calculated limits of detection are in the ng g^–1to µg g^–1region, depending on the volume of the inclusions. The accuracy of the overall analysis, including internal and external calibration, is typically between 5 and 20%, as demonstrated on alkali elements in synthetic fluid inclusions of known composition.