Experimental study of a low-alkali tholeiite at 1-5 kbar: optimal condition for the crystallization of high-An plagioclase in hydrous arc tholeiite

We conducted melting experiments on a low-alkali tholeiite (SiO2 similar to 52 wt%, MgO similar to 6.5 wt%, CaO/ Na2O similar to 4.4, Al2O3/ SiO2 similar to 0.33) under both H2O-undersaturated and H2O-saturated conditions to investigate the effect of H2O on the Ca - Na partitioning between plagioclase and melt. Experiments were performed in the temperature and pressure ranges of 1,000 - 1,300 degrees C and 1 - 5 kbar, respectively, with varying H2O contents of 0-12wt%. Redox condition was 0 - 2 log unit above NNO ( nickel - nickel oxide) buffer. Temperature-bulk H2O diagrams for the low-alkali tholeiite are constructed at 1, 2, and 5 kbar, and compositions of near-liquidus plagioclase and coexisting melt are determined. To exclude the effect of melt composition (CaO/Na2O and Al2O3/ SiO2 ratios) on plagioclase composition and to reveal the effect of H2O on An (= 100 x Ca/( Ca+ Na)) content and K-D(Ca-Na) (=(Ca/Na)(pl)/(Ca/Na)(melt)), we focused on the composition of near-liquidus plagioclases which crystallized from melts with nearly constant CaO/ Na2O and Al2O3/ SiO2 ratios. Our experimental results show that, at each experimental pressure, An content of the near-liquidus plagioclase and the K-D(Ca-Na) almost linearly increases as H2O content in melt increases. Each of the An content and the K-D(Ca-Na) variations in a low-alkali tholeiitic system (CaO/ Na2O similar to 4.0 - 4.5, Al2O3/ SiO2 similar to 0.27 - 0.33) can be described by one equation using temperature, pressure, and melt H2O content as parameters. An content and K-D(Ca-Na) of liquidus plagioclase increases with increasing melt H2O and with decreasing pressure, elucidating that nearly H2O-saturated conditions of 2 - 3 kbar is optimal for the crystallization of the most An-rich plagioclase (> An(88)). We suggest this pressure condition of 2 - 3 kbar, corresponding to depth of 7 - 11 km, plays an important role for the origin of An-rich plagioclase in H2O-rich low-alkali tholeiite. At pressures more than ca. 4 kbar, crystallization of liquidus Ca-rich clinopyroxene decreases the CaO/Na2O ratio of liquid, thus prohibiting the crystallization of high-An plagioclase from hydrous tholeiite.