Combined microthermometric and Raman spectroscopic technique to determine the salinity of H 2O–CO 2–NaCl fluid inclusions based on clathrate melting

Abstract

Fluid inclusions approximated by the system H 2O–CO 2–NaCl are common in many geologic environments. In order to apply microthermometric data from these inclusions to infer P–T (pressure–temperature) trapping conditions, the composition of the inclusions, including the salinity, must be known. Normally, salinities of aqueous inclusions are determined from ice-melting temperatures obtained during microthermometry. However, when CO 2-bearing aqueous fluid inclusions are cooled they often form a hydrate that incorporates H 2O into the structure, and salinities estimated from ice-melting temperatures are therefore higher than the actual salinity. A technique that combines data from Raman spectroscopic and microthermometric analyses of individual inclusions was developed to determine the salinity of CO 2-bearing aqueous inclusions based on measured clathrate melting temperatures and CO 2 pressures obtained from Raman analyses. In this study, the pressure within inclusions was determined using Raman spectroscopy based on the splitting of the Fermi diad of CO 2, measured at the clathrate melting temperature. The CO 2 densities (and pressures) predicted by the equation developed in this study are in relatively good agreement with previously published equations, except for very low densities and correspondingly low pressures. The combined Raman spectroscopy – microthermometry technique thus provides both the temperature and the pressure in the inclusion at clathrate melting. For inclusions in which the clathrate melts in the presence of CO 2 liquid, the salinity can be determined with a precision of a few tenths of a wt% NaCl, whereas for inclusions in which clathrate melts in the presence of CO 2 vapor the salinity error may be a few wt% NaCl. Applying the method to synthetic fluid inclusions with known salinity suggests that the technique is valid for determining salinity of H 2O–CO 2–NaCl fluid inclusions in which clathrate melts in the presence of liquid CO 2 only or vapor CO 2 only.