Geochemistry of the Oceanic Lithosphere Being Subducted Beneath the Tonga-Kermadec Arc

The Tonga-Kermadec trench (T-K), part of an intra-oceanic volcanic arc system, marks the westward subduction of the Pacific plate beneath the Indo-Australian Plate at a maximum rate of 240mm yr-1. Previous studies have assumed that the subducting lithosphere is old and fairly homogeneous in composition with only the amount and type of subducted sediments varying along strike of the arc system. It has been suggested that the latitudinal variation seen in the Tonga-Kermadec arc lavas may be caused by the sediment input and variable depletion of the mantle wedge (e.g., Ewart and Hawkesworth, J. Petrol., 1987; Gamble et al., J. Petrol., 1996). Basaltic samples dredged from the trench were found to be mid- to late-Cretaceous in age. Our data show that the chemistry of the previously subducted Pacific plate was not homogeneous along strike of the volcanic arc. Although the samples are mainly MORB, there is a range from tholeiitic to alkalic basalts with definite compositional heterogeneity. Rare earth element data show depleted to enriched chondritic-normalized patterns with La/Sm (N) values ranging from 0.38 to 3.02. Concentrations of incompatible trace elements also show high variability (e.g., Ba = 5 to 260 ppm, Rb = 0.4 to 39 ppm, and Sr = 60 to 625 ppm). Sr and Nd isotopic data also show variability with trends similar to those of the Tonga-Kermadec arc lavas. Most of the Mesozoic Pacific crust is geochemically enriched in terms of incompatible trace elements and Sr, Nd, and Pb isotopes due to a volcanic event that contaminated the upper mantle and lithosphere in the Pacific during the mid-Cretaceous (Janney and Castillo, J. Geophys. Res., 1997). The mid-Cretaceous volcanic event could be a possible source of the compositional variability seen in the subducting lithosphere. The fabric of the incoming plate, as indicated by magnetic data, is nearly perpendicular to the trench axis. Thus, varied crustal age, alteration, sediment cover, crustal and mantle wedge composition provide a heterogeneous feed material to the T-K. These components must play a role in generating observed along strike variations in the arc magmas.