Mass spectrometric analysis of volatiles in fluid inclusions: aliquot calibration valve to simulate inclusion rupture

Abstract

Analysis of nano- to picomole quantities of gas released from microscopic fluid inclusions in minerals can be critical in determining details of the physical and chemical conditions under which mineral deposits formed. Such analyses can routinely be performed using quadrupole mass spectrometry of volatiles released by inclusion decrepitation as minerals are heated or crushed. However, steady-state calibration of mass spectrometric factors has proved inadequate to accurately quantify small amounts of inclusion gas mixtures analyzed under dynamic conditions. An on-line gas aliquot valve mass spectrometer calibrating system has been developed, using adaptation of an high-pressure liquid chromatography valve. By rapidly discharging controlled aliquots of standard gases or gas mixtures, dynamic calibrations simulate nearly all analytical parameters that affect volatiles released during mechanical rupture of fluid inclusions. Analytical sensitivity of the mass spectrometry system is non-linear in its dependence on partial pressure, an effect which could not be deduced using steady-state calibrations. After applying this additional correction, microsample measurements previously wildly in contrast with presumed trapping conditions now accurately reproduce fluid compositions known from geothermal well flow tests or bulk inclusion analyses.