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

The carbon solubility in a Mid-Ocean Ridge Basaltic (MORE) melt was de termined by equilibrating natural glass with CO2 produced by decomposi tion of silver oxalate. These experiments were performed in closed pla tinum capsules in an internally heated pressure vessel at 1200 and 130 0 degrees C, at oxygen fugacity close to the QFM buffer and at pressur es between 250 and 1950 bar. Carbon was extracted by incremental heati ng to fusion and determined by manometry after oxidation to CO2. This method separates small contributions from bubbles trapped in the exper imental melts and extracts all the dissolved carbon regardless of its chemical form. Linear correlation between total dissolved C and the to tal pressure was observed, confirming that carbon solubility obeys Hen ry's law in the range 0-2000 bar. This yields a best fit minimum solub ility of 0.137 +/- 0.004 ppm C/bar which can be applied to MORE at cru stal pressures. These experimental glasses, measured by FTIR for carbo nate ion absorptions, provide a new determination of the CO32- molar a bsorption coefficient (epsilon = 398 +/- 101 mol(-1) cm(-1)) in perfec t agreement with an independent natural sample calibration (epsilon = 397 +/- 71 mol(-1) cm(-1)). Comparison to results from literature diss olved C contents of MORE glasses confirms that many are supersaturated at eruption as a result of slow kinetics of degassing, while a few ot hers are undersaturated because of either C loss during degassing of w ater-rich melts or generation from carbon-poor sources.