Improvement of the determination of element concentrations in quartz-hosted fluid inclusions by LA-ICP-MS and Pitzer thermodynamic modeling of ice melting temperature

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has become an essential analytical tool for the study of paleofluid chemistry through the analysis of individual fluid inclusions. The calculation of major and trace element concentrations in fluid inclusions is usually based on empirical equations whose significance and accuracy are questionable. In addition, methods for estimation of analytical uncertainties element concentration in individual fluid inclusions are lacking. This study describes a method based upon Pitzer's thermodynamic model for the calculation of major element (Na, K, Mg and Ca) concentrations in low-to moderate-salinity fluid inclusions. A signal processing protocol, used in combination with the new method is also developed to calculate the concentration, for each inclusion, and uncertainty for each major and trace element. In order to validate the proposed method, synthetic and natural fluid inclusions (from Alpine quartz veins) were ablated with a 193 nm ArF excimer laser and analyzed with a quadrupole ICP-MS, equipped with an octopole collision-reaction cell. The difference between the calculated and actual element concentration (i.e. accuracy) does not exceed 20% and the calculated relative standard deviation (i.e. precision) for all element concentrations is similar to 10% in standards (glasses, solutions in capillary tubes and synthetic fluid inclusions). The element concentrations obtained with this new method for the Alpine fluid inclusions are in good agreement with those previously measured using Laser Induced Breakdown Spectroscopy (LIBS) or crush-leach methods. Finally, the calculated concentrations and associated uncertainties determined for each element in individual fluid inclusions show that the sensitivity of LA-ICP-MS analysis is high enough to reflect small variations of major and trace element concentrations in the Alpine paleofluid, initially considered to have a constant chemistry. The new approach presented in this paper highlights small but significant variations in the paleofluid chemistry that were not previously detected with conventional LA-ICP-MS data processing. (C) 2012 Published by Elsevier Ltd.