AN EXTENDED EQUATION TO CALCULATE NACL CONTENTS FROM FINAL CLATHRATE MELTING TEMPERATURES IN H2O-CO2-NACL FLUID INCLUSIONS - IMPLICATIONS FOR P-T ISOCHORE LOCATION

This report informs fluid inclusion researchers of an extended equation to calculate wt% NaCl from final CO2-clathrate melting temperatures in H2O-CO2-NaCl fluid inclusions. This equation is valid only if melting occurs in the presence of CO2-liquid, CO2-vapor, and salt solution. Previous workers used the equation of BOZZO et al. (1973, 1975) for salinity calculations; however, their equation is limited to clathrate melting temperatures between 10 and about 0-degrees-C (0-16 wt% NaCl). Application of this equation to final clathrate melting temperatures from 0-degrees-C to the peritectic temperature of -10.0-degrees-C (16-24.2 wt% NaCl) yields significant errors in calculated NaCl content. Experimental data over the entire range of 10 to -10.0-degrees-C is provided by CHEN (1972b) and his best fit equation is W = +0.00098241 (10 - T)(T2 + 45.385T + 1588.75), where W is the wt% NaCl and T is the final clathrate melting temperature (degrees-C). The values of W calculated by this equation are relative only to the salt solution (independent of CO2 content) and have an average and maximum deviation of 1.33 and 5.61%, respectively (from the experimentally determined values of W). This equation should be used instead of the equation of BOZZO et al. (1973, 1975) in fluid inclusion data reduction programs such as HALWAT and CHNACL described by NICHOLLS and CRAWFORD (1985) and FLINCOR described by BROWN (1989). Without such correction, calculation of isochore position in P-T space using these programs may yield significant errors if fluid inclusions have final clathrate melting temperatures between 0 and -10.0-degrees-C.