Magma genesis in the New Britain island arc: further insights into melting and mass transfer processes

Abstract

Quaternary volcanic rocks from the New Britain island arc display a wide range in chemical compositions. The source of the lavas shares isotpic characteristics with Indian Ocean type mid-ocean ridge basalt (MORB). In contrast, the high field strength elements (HFSE) are extremely depleted in the volcanic front rocks compared with MORB. We propose that this results from a previous melt-extraction event - hypotheses invoking residual phases in either the mantle wedge or subducting slab cannot account for the depletion relative to MORB. In addition, elements other thanthe HFSE are also affected. Chemical signatures in parts of the New Britain arc and Manus Basin may relate to a previous subduction episode along the now inatctive Vitiaz-West Melanesian trench. Ultra-depleted volcanic front basalts invariably have strong 'fluid'-related trace element signatures, including high Sr/Nd and U/Th (and 238U disequilibrium), together with positive Eu anomalies that can be related to the mobility of Eu2+ in the slab-serived flux. Negative Ce anomalies are attributed to a minor sedimentary component. Across-arc geochemical profiles record a decrease in the degree of partial melting and diminishing influence of a slab-derived fluid with depth, superimposed upon the depleted mantle composition beneath the volcanic front. Element partitioning into (and not necessarily the source of) the fluid is considered to exert strong control on the chemistry of volcanic front magmas, a feature that may go some way to explaining the contradictory estimates of the slab flux derived from isotope vs trace element data in many subduction suites.