The Earth's Upper Mantle is dominantly peridotitic but in convergent margin regions, such as the S.W. Pacific, subduction of oceanic crust gives greater chemical and petrological diversity to upper mantle petrology and petrogenesis._x000D_
Examination of currently active, Quaternary and Tertiary volcanism in the S.W. Pacific, particularly seeking information from ocean floor sampling to characterize the basement to island arcs and to investigate active spreading centres, consistently points to parental magmas of basaltic to picritic character. However, these parental magmas are chemically diverse and it is not possible to designate a characteristic 'island arc' or 'convergent margin' petrogenesis in the sense of a characteristic mantle source composition and a charactistic partial melting regime._x000D_
In this respect, magma genesis at convergent margins contrasts with that at oceanic spreading centres. Our studies emphasize the importance of source peridotite ranging from 'fertile lherzolite' (i.e. compositions with 3-4% CaO, A12O3, 0.3-0.5% Na2O) to refractory harzburgite (<1% cao, al 203)-" secondly, our studies emphasize the importance of c- h-o fluids in determining the solidus temperature for mantle peridotite._x000d_>
Thirdly, the process of magma mixing adds considerable complexity to magmatic evolution at convergent margins. End member components for magma mixing range from picrites to magnesian quartz- tholeiites to boninites - all from peridotitic sources.- but also include dacitic to rhyodacitic melts derived from melting of subducted basaltic (eclogitic) -crust. Since each of these 'primary' convergent margin magmas may undergo crystal fractionation and magma mixing, great diversity of magma types in this tectonic setting must be expected.