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 melthas been determined in the pressure range from approximately1,000 to 6,000 bars at 1,100? C. The solubility in basalticmelt is 3.1 weight percent at 1,000 bars and 9.4 weight percentat 6,000 bars; in the andesitic melt it is 4.5 weight percentat 1,000 bars and 10.1 weight percent at 5,300 bars. The temperaturesof appearance of the primary, secondary, and tertiary phasesin the basalt have been determined at 1,000 bars water pressureand at the fo₂'s of the magnetite+hematite (MH), magnetite+fayalite+quartz(MFQ) and magnetite+w?stite (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 toapproximately 6,000 bars pressure in several silicate melts,ranging in composition from granitic to gabbroic, indicatesthat the spread of solubility is narrower than has been supposed.The marked effect of f_o2's on the crystallization sequence inthe Columbia River basalt confirms the importance of this factorin determining liquid lines of descent. In experiments withlow f_o2's (MW buffer) and 1,000 bars water pressure, the basaltwas completely liquid at the relatively low temperature of 1,020?C.