Melting of refractory mantle at 1 center dot 5, 2 and 2 center dot 5 GPa under, anhydrous and H2O-undersaturated conditions: Implications for the petrogenesis of high-Ca boninites and the influence of subduction components on mantle melting

Boninites are an important 'end-member' supra-subduction zone magmatic suit e as they have the highest H2O contents and require the most refractory of mantle wedge sources. The pressure-temperature conditions of boninite origi ns in the mantle wedge sources. The pressure-temperature conditions of boni nite origins in the mantle wedge are important to understanding subduction zone initiation and subsequent evolution. Reaction experiments at 1.5 GPa ( 1350-1530 degrees C), 2 GPa (1400-1600 degrees C) and 2.5 GPa (1450-1530 de grees C) between a model primary high-Ca boninite magma composition and a r efractory harzburgite under anhydrous and H2O-undersaturated conditions (2- 3 wt % H2O in the melt) have been completed. The boninite composition was m odelled on melt inclusions occurring in the most magnesian olivine phenocry sts in high-Ca boninites from the Northern Tongan forearc and the Upper Pil low Lavas of the Troodos ophiolite. Direct melting experiments on a model r efractory lherzolite and a harzburgite composition at 1.5 GPa under anhydro us conditions (1400-1600 degrees C) have also been completed. Experiments e stablish a P, T 'melting grid' for refractory harzburgite at 1.5, 2 and 2.5 GPa and in the presence of 2-3 wt % H2O. The effect of 2-3 wt % dissolved H2O produces a liquidus depression in primary boninite of similar to 112 +/ - 19 degrees C at a given temperature. The H2O-bearing melts, recalculated to 100 wt % anhydrous, are similar to 2-6 wt % higher in MgO, similar to 1- 2 wt % higher in SiO2 and similar to 1-1.5 wt % lower in FeO, compared with nominally anhydrous melts at the same P and T. These differences are consi stent with a change in the melting reaction, resulting in a higher contribu tion of orthopyroxene to the melt phase, compared with anhydrous conditions . We conclude that high-Ca boninite petrogenesis requires temperatures as h igh as similar to 1480 degrees C at depths of similar to 45 km in the mantl e wedge; these are constraints for any proposed model of intra-oceanic subd uction zones. A comparison of the results from the boninite-harzburgite rea ction experiments with the direct melting experiments on refractory lherzol ite and harzburgite indicates that the influence of subduction components ( included in the composition of the added model boninite) is to cause high-p ressure melting cotectics to move towards the olivine apex (i.e. to relativ ely higher pressures) of the molecular normative projection from diopside o nto the base of the 'basalt tetrahedron' [Jd + CaTs + Lc-Qz-Ol] compared wi th anhydrous melting of normal mantle in the absence of a subduction compon ent. The subduction component involved in high-Ca boninite petrogenesis in addition to H2O has relatively high Al2O2 and Na2O contents. The experiment al data from this and other studies empirically quantify the absolute effec t of dissolved H2O (0.2-21 wt %) on the liquidus depression of olivine-satu rated basaltic melts with similar to <3 wt % total alkalis as follows: olivine liquidus depression (degrees C) = 74.403 x H2O wt %)(0.352). The equation that describes this empirical relationship in non-linear with an error of similar to 9 relative percent.