GEOCHEMICAL EVOLUTION WITHIN THE TONGA-KERMADEC LAU ARC BACK-ARC SYSTEMS - THE ROLE OF VARYING MANTLE WEDGE COMPOSITION IN SPACE AND TIME

New trace element and Sr, Nd, and Pb isotope data for lavas from the a ctive Tonga-Kermadec arc in the southwest Pacific, tile volcano of Niu a fo'ou in the back-arc Lau Basin, and Pacific Ocean sediments from DS DP Sites 204 and 275, and ODP Site 596, are integrated with existing g eochemical data for lavas from the Lau Basin, Samoa, the Louisville Ri dge Seamount Chain (LR-SMC) and the extinct Lau Ridge are, giving new insights into the petrogenesis of lavas in an active are-back-are syst em. Geochemical variations in Tonga-Kermadec are lavas are the result of (1) differences in the amount and composition of the material being subducted along the arc, and (2) pre-existing heterogeneities in the upper mantle. Differences in the material being subducted beneath the are have an important influence on the chemistry of the are lavas. At the Kermadec Trench, similar to 1 km thick layer of sediment is being subducted beneath the are, compared with similar to 200 m at the Tonga Trench. This results zn the high Th/U and more radiogenic Pb isotope compositions of Kermadec lavas compared with Tonga lavas. The latter h ave Pb isotope compositions intermediate between those of Pacific sedi ments and Pacific mid-ocean ridge basalt (MORB), suggesting that much of the Pb in these lavas is derived Jj-om subducting Pacific Ocean cru st. This is supported blv the Pb isotope signatures of the subducting LR-SMC, which are also observed in lavas from the northern Tongan isla nds of Tafahi and Niuatoputapu. High field strength element (HFSE) and heavy rare earth element (HREE) concentrations are generally lower in Tongan lavas (particularly those from northern Tongan islands) than i n Kermadec lavas. The Tonga Ridge basement, the proto-Tonga are lavas (ODP Site 839) and the older Lau Ridge are lavas are generally less de pleted than the modern are lavas. In the back-arc region, upper-mantle depletion as inferred from HFSE and HREE contents of the lavas broadl y increases eastwards across the Lau Basin, whereas the subduction sig nature and volatile (CO2 and F) contents increase eastwards towards th e modern are. These observations suggest that depletion is due to melt extraction during back-are extension and volcanism, together-with a l ong 'residence time' of mantle material within the mantle wedge. The u pper mantle beneath the northernmost end of the Tonga are and Lau Basi n contains an ocean-island basalt (OIB) component derived from the Sam oa plume to the north. This is reflected in high concentrations of Nb relative to other HFSE in lavas from Niua fo'ou, and Tafahi and Niuato putapu islands at the northern end of the Tonga are. Pb isotopes also suggest an LR-SMC contribution into Tafahi and Niuataputapu. Trace ele ment and isotope modelling is used to investigate the combined effects of varying mantle source depletion and subduction on the geochemistry of the are lavas. The results suggest that the are lava geochemistry can be explained largely by the balance between a relatively constant subduction input of Pb, Th, U, Cs, Ba, Sr, Rb, K and Sc [corresponding to 0.001-0.005 weight faction of the Stolper & Newman (1994, Earth an d Planetary Science Letters, 121, 293-325] 'H2O-rich component' compos ition), into the overlying, but variably depleted mantle wedge.