Pergamon Geochimica el Cosmochimica Acta, Vol. 59, No. 19, pp. 3953-3959, 1995 Copyright 0 I995 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/95 $9.50 + .oo 0016-7037(95)00258-8 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJ Synthetic fluid inclusions: XIII. Experimental determination of PVT properties in the system Hz0 + 40 wt% NaCl + 5 mol% CO2 at elevated temperature and pressure* C. SCHMIDT, K. M. Rosso, and R. J. BODNAR Fluids Research Laboratory, Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0420, USA (Received October 5, 1994; accepted in revisedform April 5, 1995) Abstract-The location of the liquid + vapor --, liquid phase boundary and the P-T slopes of iso-Th lines were determined for a constant composition of 40 2 0.1 wt% NaCl and 5 + 0.15 mol% COT (both relative to H20) at high density. Synthetic fluid inclusions with this composition were formed in cold-seal pressure vessels at pressures of 2 and 4 kbar and temperatures between 3.50” and 700°C. The inclusions were analyzed on a gas-flow heating/cooling stage to determine the temperatures of halite dissolution [ Tm(H+L+v-L+v,] and total homogenization [ 7’hcL+v_LJ. Addition of 40 wt% NaCl to an aqueous solution containing 5 mol% CO2 causes a significant shift of the liquid + vapor + liquid boundary towards higher pressures. The slopes of the iso-Th lines decrease from 29.5 bars/“C for ThcL+v-.L) of 400°C. to 6.4 bars/V for Th,L+v-L) = 600°C. Addition of 5 mol% CO2 to an aqueous solution containing 40 wt% NaCl results in halite dissolution temperatures that are slightly higher (Tm(H+L+V-L+V) = 332°C) than the literature value of 323°C for the vapor-saturated liquidus of an HZ040 wt% NaCl mixture. Calculated molar volumes for 40 wt% NaCl + 5 mol% CO1 solutions at 2 and 4 kbar show trends that are similar to those of other compositions in the ternary system H20-C02-NaCl at the same pressures and temperatures. In the P-T range of this study, all excess volumes are negative and lie between the values for the compositions H20-5 mol% CO1 and HZ040 wt% NaCl. 1. INTRODUCITON The system H,O-NaCl-Ca is one of the most important fluid systems in geochemistry. Fluid inclusions approximated by this ternary system are abundant in metamorphic rocks of all grades (except perhaps very low grade rocks and zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA eclogites) , particularly if carbonates were involved in the metamorphic process. Carbon dioxide concentrations of metamorphic fluids vary over wide ranges and the salinity can exceed 50 eq. wt% NaCl (Roedder, 1984). Furthermore, stability fields in P-T space of reactions which involve Hz0 and COZ may be shifted considerably owing to the fact that addition of NaCl to a HzO- CO2 mixture significantly affects the fugacities of both HZ0 and CO* (Bowers and Helgeson, 1983). Inclusions containing predominantly aqueous salt solutions and CO2 are also fre- quently reported from igneous rocks, especially granites, peg- mat&es, carbonatites, and alkaline rocks, and in many hydro- thermal deposits, e.g., Sn-W vein deposits, greisens, skams, and other replacement deposits (Roedder, 1984). Owing to various experimental difficulties, few studies have been conducted to determine the PVZX properties of H@NaCl-CQ mixtures at high temperatures and pressures. In fact, the only extensive dataset was given by Gehrig ( 1980) for the pseudobinary H20-C02-6 wt% NaCl for pressures up to 3 kbar and temperatures between 150°C and 5OO”C,along with some data for 10 and 20 wt% NaCl relative to H20. Kotelnikov and Kotelnikova ( 1990). Frantz et al. ( 1992), Johnson ( 1992), and Shmulovich and Plyasunova (1993) * Presented at the fifth biennial Pan-American Conference on Re- search on Fluid Inclusions (PACROPI V) held May 19-21, 1994, at the Instituto de Investigaciones Electricas in Cuemavaca, Morelos, Mexico. utilized synthetic fluid inclusion techniques to determine liq- uid-vapor immiscibility limits, a few lines of equal homoge- nization temperature (iso-Th lines) and isochores. Joyce and Holloway ( 1993) measured the activity of HZ0 in the ternary mixture at 850°C and 2 kbar, and at 700°C and 5 and 6 kbar. Data for NaCl-rich compositions are scarce. However, Chou (1988) determined the halite liquidus temperature in H20- C02-NaCl mixtures at 2 kbar using differential thermal anal- ysis. Based on experimental results, mainly those of Gehrig ( 1980), and on theoretical considerations, Bowers and Hel- geson ( 1983) presented a modified Redlich-Kwong equation of state, which is applicable over restricted ranges of pres- sures, temperatures, and salinities. Brown and Lamb (1989) approximated isochore locations by assuming ideal geomet- rical mixing between H20-NaCl and CO* as endmembers. In this study, synthetic fluid inclusions are used in con- junction with Raman spectroscopy to obtain molar volumes and excess volumes for an aqueous solution of 40 wt% NaCl and 5 mol% CO*, both relative to water, for temperatures between 350” and 700°C and pressures of 2 and 4 kbar. Fur- thermore, the slopes of iso-Th lines were calculated from mi- crothermometric data and the liquid + vapor + liquid phase boundary location in P-T space was estimated. 2. DETERMINATION OF SOLWS AND ISO-Th LINES 2.1. Methoddogy The synthetic fluid inclusion technique has been described in detail by Bodnar and Sterner ( 1987) and Sterner and Bodnar ( 1991); there- fore, only the general procedure and experimental aspects specific to this study are reported here. Platinum capsules were loaded with a fixed composition of 40 2 0.1 wt% NaCl and 5 2 0.15 mol% CQ. 3953