GEOCHEMICAL AND GEODYNAMICAL CONSTRAINTS ON SUBDUCTION ZONE MAGMATISM

The geochemical and geodynamical parameters that may influence the composition of island-arc basalts (IAB's) are evaluated. Systematic correlations amongst high-field strength (HFS) elemental ratios (Zr/Nb, Sm/Nb and TiO2/Zr) relative to Nb abundances, indicate that HFS element systematics are not controlled by the presence of residual Nb-bearing phases in the slab. This provides confirmation of models whereby high-field strength (HFS) and HREE elements remain immobile during slab-fluxing processes and are thus derived from the mantle wedge without additional enrichments from the slab. In contrast enrichment of large-ion-lithophile elements (LIL) such as Rb, Cs, Ba, Sr, Pb, U and LREE (i.e., La, Ce) in IAB's is consistent with slab involvement, with their relative enrichment, being due to a combination of both their high rock-melt incompatibility and slab-"fluid" mobility. As a consequence, the low abundances of HFS elements such as Nb, Ti, Zr, and Hf in IAB's reflect a depleted (relative to MORB source) mantle wedge overlying the subduction slab. Depletion of the arc mantle wedge in HFS elements is attributed to previous melting events in the mantle wedge, and to geodynamic conditions associated with the formation and evolution of coupled island arcs and back-arc basins. These processes ensure a budgetary deficit in the HFS elements relative to those elements derived from the subducted slab (predominantly LILE and LREE). Thus, although in MORB's and OIB's, Nb has a similar incompatibility to U, in subduction zones the main factor controlling its abundance is its highly immobile character, particularly relative to elements like U which are mobile during prograde dehydration reactions in the slab. Based on these observations, a quantitative model has been developed for IAB petrogenesis with the transfer of trace elements from the slab to the mantle wedge being modelled with empirical slab-"fluid" partition coefficients whilst the mantle-wedge to arc-crust transfer is constrained by melt-solid partitioning. The empirically derived slab-"fluid" partition coefficients indicate that the enrichment factors characteristic of slab fluxing processes have a distinctive pattern particularly for the elements Nb, U, Th, and Sr.