REACTION BETWEEN ULTRAMAFIC ROCK AND FRACTIONATING BASALTIC MAGMA II. EXPERIMENTAL INVESTIGATION OF REACTION BETWEEN OLIVINE THOLEIITE AND HARZBURGITE AT 1150-1050°C AND 5 KB

Abstract

We present results of experiments on mixtures of olivine tholeiite and mantle harzburgite, at 5 kb and 1050–1150°C, under conditions of controlled hydrogen fugacity. The basalt end-member was Kilauea 1921 olivine tholeiite+3 wt.% H2O, and the harzburgite end-member was a mixture of olivine and orthopyroxene mineral separates made from a mantle-derived lherzolite xenolith. The experiments on mixtures of basalt and harzburgite difl not reach equilibrium in runs ranging from 12 to 200 h duration. Relatively large concentration gradients persisted in both liquid and solid phases in mixed samples, whereas ‘control’ samples containing only basalt were reasonably homogeneous and were probably close to equilibrium. Compositions of solid phases produced, measured by electron microprobe, show a regular increase in Mg/(Mg+Fe) with increasing proportion of harzburgite at constant temperature, but olivine and clinopyroxene in mixed samples were not in Fe-Mg exchange equilibrium. Modes measured for each sample show that the fraction of liquid relative to the amount of basalt in the sample was constant at constant temperature, and independent of bulk composition: reaction between 1921 basalt and harzburgite does not change the mass of liquid in the system. Average experimental liquid compositions for each sample were obtained by mass balance. Using Kds defined by the ‘control’ sample for each temperature, and mass balance constraints, phase assemblages (solid- and liquid-phase compositions and proportions) were calculated for all mixtures. Whether samples included harzburgite or not, all average experimental liquid compositions, and all predicted liquid compositions, for samples run at 1050°C, are high-alumina basalts by the definition of Kuno (1960). By the criteria of Irvine and Baragar (1971), all but two average experimental liquid compositions in basalt-harzburgite mixtures, and all predicted liquid compositions in basalt-harzburgite mixtures, are calc-alkaline basalts and basaltic andesites, whereas liquids in samples containing only basalt are tholeiitic basalts. Combined crystallization and reaction with harzburgite in the upper mantle will produce calc-alkaline derivative liquids from an olivine tholeiite liquid under conditions of temperature, pressure, water and oxygen fugacity, and initial bulk composition which would produce a tholeiitic liquid line of descent by crystallization in a closed system.