THE PRESSURE AND TEMPERATURE-DEPENDENCE OF CARBON-DIOXIDE SOLUBILITY IN THOLEIITIC BASALT MELTS

The solubility of carbon dioxide in tholeiitic melt (1921 Kilauea basalt) was determined under experimental conditions of 1 kbar, 1200-degrees-C; 10 and 15 kbar and 1300-1600-degrees-C. We examined the solubility at pressure and temperature conditions intermediate to those reported in previous studies, and, in particular, we addressed the effect of temperature on carbon dioxide solubility. Two different carbon sources were used in the experiments, silver oxalate and a mixture of carbonate minerals, to examine the effects of dissolved silver on carbon dioxide solubility. Three analytical methods were employed to measure accurately and precisely the dissolved carbon in the run products: (1) Fourier transform micro-infrared spectroscopy, (2) secondary ion mass spectrometry, and (3) bulk carbon analysis with a Perkin Elmer Elemental Analyzer. The first two methods are micro-beam techniques which allowed for assessment of sample homogeneity. Consistent with previous solubility studies, infrared analyses showed that carbon is dissolved in basaltic melt in the form of carbonate. However, our experimental results differ from the previous solubility study in that we demonstrate carbon dioxide solubility is temperature independent. At 1 kbar and 1200-degrees-C, carbon dioxide solubility is 543 ppm; at 10 kbar and 1300, 1400, and 1500-degrees-C, carbon dioxide solubility is approximately 0.77 +/- .07 wt%; and at 15 kbar and 1400, 1450, 1500, 1550, and 1600-degrees-C, the solubility is approximately 1.21 +/- .13 wt%. Dissolved silver does not appear to affect the solubility. These results invalidate previous models for carbon dioxide solubility. We have developed a new model which describes the pressure and temperature dependence of carbon dioxide solubility for tholeiitic basalts. Regression of the solubility data for the reaction CO2vapor + O2-melt = CO3(2-melt) gives a heat of solution (DELTA-H0 at 1 kbar and 1473 K) of 5.20 +/- 4.30 kJ/mol and the change in partial molar volume (DELTA-V0[CO3(2-melt) - O2-melt]) of 23.14 +/- 1.03 cm3/mol. Application of this model suggests that fluid-saturated partial melting of the MORB source region cannot be supported.