Crystal uptake into aphyric arc melts: insights from two-pyroxene pseudo-decompression paths, plagioclase hygrometry, and measurement of hydrogen in olivines from mafic volcanics of SW Japan

Abstract

AbstractMinerals of mafic rocks from the SW Japan arc have been studied to deduce P–T–XH2O conditions and their variations in mafic arc magmas. Two-pyroxene thermobarometry of magmas from several volcanoes yields constant temperatures and variable pressures. MELTS fractional crystallization modelling is employed to show that such ‘pseudo-decompression paths’ (PDPs) are artefacts that derive from uptake of pyroxene antecrysts formed at a range of crustal levels by isobaric cooling of previously intruded mafic melts. It is shown that PDPs can be used to constrain oxygen fugacities and initial water contents of the intruded magmas. These constraints, and plagioclase hygrometry, indicate that initial melt H2O contents change systematically along the SW Japan arc. Direct determination of hydrogen in olivine by secondary ion mass spectrometry yields consistently low olivine H2O contents of 11±4 ppm (1σ), with little, if any, along-arc variations. MELTS modelling indicates that crystallization of calcic plagioclase and olivine dominantly occurs during upper crustal differentiation of mafic melts. The combined data indicate that aphyric melts are released from the mantle wedge, taking up most if not all crystals from previously intruded plutonic rocks during rapid magma ascent to the surface.