EXPERIMENTS AND MODELS OF ANHYDROUS, BASALTIC OLIVINE-PLAGIOCLASE-AUGITE SATURATED MELTS FROM 0.001 TO 10 KBAR

A new method for modeling fractional crystallization processes that in volve olivine (ol), plagioclase (plag) and augite (aug) is presented. This crystallization assemblage is the major control on the chemical v ariations in mid-ocean ridge basalts. The compositional and temperatur e variations in ol-plag-aug saturated basalts over a range of pressure s are described using empirical expressions. A data base of 190 experi ments in natural and basalt-analog chemical systems is used to describ e temperature, Al, Ca and Mg molar fractions as functions of Si, Fe, N a, Ti and K molar fractions and pressure. Increases in the abundances of Na and K cause Ca and Mg abundances to decrease and Al abundance to increase in ol-plag-aug saturated melts. The equations can be used to predict pressure and temperature and thus provide a useful thermobaro meter. A model is described to calculate ol-plag-aug fractional crysta llization as a function of pressure and melt composition, using melt a nd augite models developed here, combined with existing models for oli vine-melt and plagioclase-melt equilibria. We compare the fractional c rystallization sequence of ALV-2004-3-1 predicted from the models pres ented in this paper, Langmuir et al. (1992) modified by Reynolds (1995 ), Ghiorso and Sack (1995) and Ariskin et al. (1993) at 0.001 and 4 kb ar. As an example the model is applied to estimate pressure of crystal lization of glasses from the east flank of the East Pacific Rise at 11 degrees 45'N.