MAGMA GENESIS IN THE NEW BRITAIN ISLAND-ARC - FURTHER INSIGHTS INTO MELTING AND MASS-TRANSFER PROCESSES

Quaternary volcanic rocks from the New Britain island arc display a wi de range in chemical compositions. The source of the lavas shares isot opic characteristics with Indian Ocean type mid-ocean ridge basalt (MO RB). In contrast, the high field strength elements (HFSE) are extremel y depleted in the volcanic front rocks compared with MORB. We propose that this results from a previous melt-extraction event-hypotheses inv oking residual phases in either the mantle wedge or subducting slab ca nnot account for the depletion relative to MORB. In addition, elements other than the HFSE are also affected. Chemical signatures in parts o f the New Britain arc and Manus Basin may relate to a previous subduct ion episode along the now inactive Vitiaz-West Melanesian trench. Ultr a-depleted volcanic front basalts invariably have strong 'fluid'-relat ed trace element signatures, including high Sr/Nd and U/Th (and U-238 disequilibrium), together with positive Eu anomalies that can be relat ed to the mobility of Eu2+ in the slab-derived flux. Negative Ce anoma lies are attributed to a minor sedimentary component. Across-arc geoch emical profiles record a decrease in the degree of partial melting and diminishing influence of a slab-derived fluid with depth, superimpose d upon the depleted mantle composition beneath the volcanic front. Ele ment partitioning into (and not necessarily the source of) the fluid i s considered to exert strong control on the chemistry of volcanic fron t magmas, a feature that may go some way to explaining the contradicto ry estimates of the slab flux derived from isotope vs trace element da ta in many subduction suites.