CARBON SOLUBILITY IN MID-OCEAN RIDGE BASALTIC MELT AT LOW PRESSURES (250-1950 BAR)

Abstract

The carbon solubility in a Mid-Ocean Ridge Basaltic (MORB) melt was determined by equilibrating natural glass with CO2 produced by decomposition of silver oxalate. These experiments were performed in closed platinum capsules in an internally heated pressure vessel at 1200 and 1300°C, at oxygen fugacity close to the QFM buffer and at pressures between 250 and 1950 bar. Carbon was extracted by incremental heating to fusion and determined by manometry after oxidation to CO2. This method separates small contributions from bubbles trapped in the experimental melts and extracts all the dissolved carbon regardless of its chemical form. Linear correlation between total dissolved C and the total pressure was observed, confirming that carbon solubility obeys Henry's law in the range 0-2000 bar. This yield a best fit minimum solubility of 0.137 +/- 0.004 ppm C/bar which can be applied to MORB at crustal pressures. These experimental glasses, measured by FTIR for carbonate ion absorptions, provide a new determination of the CO2-3 molar absorption coefficient (# = 398 +/- 10 l mol-1 cm-1) in perfect agreement with an independent natural sample calibration (# = 397 +/- 7 l mol-1 cm-1). Comparison to results from literature dissolved C contents of MORB glasses confirms that many are supersaturated at eruption as a result of slow kinetics of degassing, while a few others are undersaturated because of either C loss during degassing of water-rich melts or generation from carbon-poor sources.