T120 Melt and Fluid Inclusion Analysis in Resource Investigations (Posters)

Monday, 19 October 2009
9:00 AM - 6:00 PM
Hall A

INTERPRETATION OF H2O/CO2/NaCl PHASE EQUILIBRIA IN EXPERIMENTS AT MAGMATIC TEMPERATURES AND PRESSURES

Paper number: T120-3
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Cold-seal hydrothermal experimentation relevant to geological systems such as magmatic-hydrothermal Cu-Au ore environments requires the variation and control of many intensive variables, including initial H2O/CO2/NaCl(eq.) ratios, run fO2, and the estimation of fluid inclusion (FI) pressure, final salinity and H2O/CO2 ratios. CO2 production via oxalic acid (OA) is common in experiments, but H2 gas is generated during OA decomposition and must diffuse across the Pt capsule wall. The initial internal fH2i (e.g., fH2i ~9.7 MPa; log fO2 ~ -16.5) can be calculated upon OA decomposition from mass balance and phase relations (Duan et. al. 1995, GCA). By using the diffusion equation of Chou (1987: Hydrothermal Techniques), an instantaneous diffusion rate (dH2/dt) is obtained. Iterative calculation of dH2/dt and internal fH2 yields the time required for equilibration with the external fH2 (~0.32 MPa, or log fO2 ~ -14). Experiments performed at 800oC, 100 MPa, and H2O/CO2 ~ 10 in Pt capsules (0.125 mm thick) require ~30 minutes for H2 equilibration. Raman spectroscopy on the trapped vapor inclusions confirms the absence of CO and CH4, which would be expected with prolonged elevation of fH2i at 800oC.

The determination of pressure and salinity is also vital to characterizing FI. Inclusion pressure for vapor inclusions (P) can be estimated using an isochoric quench model. Given an initial estimate of inclusion salinity, the volume of condensed aqueous liquid (L) and CO2 (V) are calculated. The CO2 density is converted to P given a suitable equation of state. Clathrate melting temperature Tmclath is a function of salinity and pressure (Diamond 2003, MAC, SC 32) for inclusions containing (V). Given a Tmclath, P and salinity are then iterated until they converge at the clathrate stability boundary. Run product vapor inclusions containing L and V, and which nucleate clathrate (Tmclath~ 4oC), have calculated P ~ 25-26 bars @ 1-3 wt% salinity, consistent with observed phase relations. In summary, H2 generated from OA decomposition has minimal effect on fO2, at the above experimental conditions. Given published phase equilibria, mass balance calculations, an equation of state for CO2, and empirical confirmation, an isochoric quench model for FI may provide a useful tool in evaluating salinity in CO2/clathrate-bearing vapor inclusions.