The Solubility of Water and Effects of Oxygen Fugacity and Water Content on Crystallization in Mafic Magmas

The solubility of water in a basaltic and in an andesitic melt has been determined in the pressure range from approximately 1,000 to 6,000 bars at 1,100° C. The solubility in basaltic melt is 3.1 weight percent at 1,000 bars and 9.4 weight percent at 6,000 bars; in the andesitic melt it is 4.5 weight percent at 1,000 bars and 10.1 weight percent at 5,300 bars. The temperatures of appearance of the primary, secondary, and tertiary phases in the basalt have been determined at 1,000 bars water pressure and at the fo2's of the magnetite+hematite (MH), magnetite+fayalite+quartz (MFQ) and magnetite+wüstite (MW) buffers. Results are as follows: Buffer Pyroxene Plagioclase Ore mineral MH 1,095°C 1,065°C 1,230°C MFQ 1,040° 1,015° 1,010° MW 1,020° 1,010° 995° A comparison of the solubility of water at 1,100°C and up to approximately 6,000 bars pressure in several silicate melts, ranging in composition from granitic to gabbroic, indicates that the spread of solubility is narrower than has been supposed. The marked effect of fo2's on the crystallization sequence in the Columbia River basalt confirms the importance of this factor in determining liquid lines of descent. In experiments with low fo2's (MW buffer) and 1,000 bars water pressure, the basalt was completely liquid at the relatively low temperature of 1,020° C