ORIGIN OF HIGH-AN PLAGIOCLASE IN TONGAN HIGH-CA BONINITES - IMPLICATIONS FOR PLAGIOCLASE-MELT EQUILIBRIA AT LOW P(H2O)

Detailed petrological, mineralogical and melt-inclusion studies of an unusual plagioclase-phyric high-Ca boninite from the North Tongan fore arc demonstrate that phenocrysts of high-An plagioclase (An(>90)) crys tallized at the latest stages of melt evolution from H2O-saturated dac itic melts (64-67 wt.% SiO2, similar to 2 wt.% MgO) at similar to 1050 degrees C and low pressure (<1 kbar). These melts contained similar t o 1.5 wt.% H2O and had low CaO/Na2O (similar to 3, in wt.%). Our resul ts suggest that the presence of high-Ca (An(>90)) plagioclase phenocry sts in are lavas does not necessarily imply either high H2O-contents o f the melt (>6 wt.%), or involvement of refractory melts (CaO/Na2O >8) in magma genesis, as was previously suggested. Established conditions of crystallization [F, T, X, X(H2O)] during evolution of the Tongan b oninite contradict those predicted by available models of plagioclase- melt equilibria. The effect of H2O on the activities of plagioclase co mponents in hydrous melts is strongly nonlinear. Extrapolation of expe rimental results on the effect of H2O on plagioclase-melt equilibria f rom melt H2O contents of >4 wt% to the low H2O contents (<2 wt%) of th e evolved Tongan boninite predicts a less calcic plagioclase than obse rved, 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 a lso with recent results on the effect of H2O on melt viscosities at lo w H2O contents, Better predictions require new experimental data at la w P(H2O) (<1 kbar).