U-series, Sr-Nd-Pb isotope and trace-element systematics across an active island arc-continent collision zone: Implications for element transfer at the slab-wedge interface

We present U-series, Sr-Nd-Pb isotope and trace-element results of a regional study of geochemical systematics across an island arc-continent collision zone in the East Sunda Arc of Indonesia. Samples from four active volcanoes exhibit a striking compositional range from low-K tholeiitic to ultrapotassic, but all are characterised by high ⁸⁷Sr/⁸⁶Sr (0.7053-0.7067), radiogenic lead isotope ratios (²⁰⁶Pb/²⁰⁴Pb = 18.99-19.15), low (²³⁰Th)/(²³²Th) (0.66-0.85), and low ¹⁴³Nd/¹⁴⁴Nd (0.51255-0.51272), except for high ¹⁴³Nd/¹⁴⁴Nd (>0.51286) at the volcanic front. Low (²³⁰Th)/(²³²Th) ratios are also found in terrigenous sediments in front of the arc, which, in combination with Sr-Nd-Pb isotopic constraints, indicates that subducted continental material contributes to magma sources in this arc sector. The volcanoes close to the trench show a large excess of ²³⁸U over ²³⁰Th (up to 80%) and of ²²⁶Ra over ²³⁰Th (up to 800%). In addition, they are enriched in elements thought to be mobile in hydrous fluids during slab-wedge transfer, such as Ba, Pb, and Sr. In contrast, U-Th-Ra systematics are close to equilibrium in the volcanoes behind the front. Abundance patterns of incompatible trace elements in these rocks are similar to those of the terrigenous sediments, so that, in comparison with the arc-front lavas, they possess low Ba/La, Ba/Th, La/Th, Pb/Ce, and Zr/Nb. Higher concentration levels and less interelement fractionation form conspicuous differences with the front volcanics. Our combined isotopic and trace element data are consistent with three-component mixing whereby a slab-derived hydrous fluid and a siliceous melt are both added to the sub-arc mantle source. The hydrous fluid largely controls the input in the shallow part of the subduction zone, whereas the siliceous melt dominates the flux at deeper levels. Sedimentary material is considered to be the primary source of both. The large U-Th-Ra disequilibria at the front suggests that element transfer is a currently active process associated with present-day subduction of continental material.