Experimental Phase Relations in the System Tonalite-Peridotite-H_2O at 15 kb; Implications for Assimilation and Differentiation Processes near the Crust-Mantle Boundary

Abstract

Experiments at 15 kb in the tonalite-peridotite-H_2O system provide information on some of the phase equilibrium factors that may influence reaction and assimilation processes between quartznormative magmas and ultramafic rocks in the deep crust and upper mantle. Experiments were done with 5 or 10 wt.% H_2O added to powdered natural samples of tonalite, and mixtures of tonalite with 5 or 10 wt.% peridotite added (TP5 and TP10, respectively). The liquidus phase relations of these starting compositions were investigated between 850 and 1100°C at 15 kb, using gold capsules so that iron loss to the sample containers was not a problem and meaningful glass and mineral analyses could be obtained. Experiments on the tonalite alone show either liquidus garnet, for samples with 5% H_2O added, or liquidus hornblende, for samples with 10% H_2O. In contrast, orthopyroxene is the sole liquidus phase, irrespective of water content, in experiments using starting mixtures of 5 or 10 wt.% peridotite added to tonalite. Glass analyses of partially crystallized tonalite define a crystallization path diverging significantly from the calc-alkaline trend towards higher Ca/(Mg + Fe) in the CaO−(MgO + FeO)−¼SiO_2 triangle. In contrast, glasses from partially crystallized mixtures of tonalite with 5 or 10 wt.% peridotite added define a liquid trend close to natural calc-alkaline compositions in terms of Ca/(Mg + Fe). Of more general significance, the proximity of a field of liquidus orthopyroxene on the high (Mg + Fe) side of compositions along the calc-alkaline trend serves to limit the Mgenrichment of such melts by interaction with ultramafic rocks. Unless heat is added to the system, reaction of tonalitic composition melts with ultramafic rocks will produce only slightly Mg-enriched melts: increasing degree of reaction simply results in further precipitation of orthopyroxene + garnet ± clinopyroxene once melt compositions reach the orthopyroxene field boundary.