Origin of high-An plagioclase in Tongan hIGH-Ca boninites: Implications for plagioclase-melt equilibria at low P(H2O)

Abstract

Detailed petrological, mineralogical and melt-inclusion studies of an unusual plagioclase-phyric high-Ca boninite from the North Tongan forearc demonstrate that phenocrysts of high-An plagioclase (An>90) crystallized at the latest stages of melt evolution from H2O-saturated dacitic melts (64-67 wt.% SiO2, ∼2 wt.% MgO) at ∼1050°C and low pressure (< 1 kbar). These melts contained ∼1.5 wt.% H2O and had low CaO/Na2O (∼3, in wt.%). Our results suggest that the presence of high-Ca (An>90) plagioclase phenocrysts in arc lavas does not necessarily imply either high H2O-contents of the melt (>6 wt.%), or involvement of refractory melts (CaO/Na2O > 8) in magma genesis, as was previously suggested. Established conditions of crystallization [P, T, X, X(H2O)] during evolution of the Tongan boninite contradict those predicted by available models of plagioclase-melt equilibria. The effect of H2O on the activities of plagioclase components in hydrous melts is strongly nonlinear. Extrapolation of experimental results on the effect of H2O on plagioclase-melt equilibria from melt H2O contents of >4 wt% to the low H2O contents (<2 wt%) of the evolved Tongan boninite predicts a less calcic plagioclase than observed, or higher H2O contents than measured in plagioclase-hosted melt inclusions. These observations are in accord with the well-known large effect of small amounts of H2O on mineral melting temperatures, and also with recent results on the effect of H2O on melt viscosities at low H2O contents. Better predictions require new experimental data at low P(H2O) (<1 kbar).