U, Th and Ra disequilibria, Sr, Nd and Pb isotope and trace element variations in Sunda arc lavas: predominance of a subducted sediment component

Abstract.

The time scales of magma formation, transfer and storage beneath island arc volcanoes are important because of the insights they provide into the physical processes of magma formation and evolution with possible links to the prediction of eruptive hazards. Here we present new mass spectrometric U, Th and Ra, and Sr, Nd and Pb isotope data, along with major and trace element data, from 19 lavas along the Sunda arc, Indonesia. The lavas range in SiO₂ from 49 to 75% and the combination of high SiO₂ and low ¹⁴³Nd/¹⁴⁴Nd indicates an important role for shallow-level contamination by ancient crustal materials in Sumatra. Diagrams of ¹⁴³Nd/¹⁴⁴Nd versus Th/Ce and U/Th versus ⁸⁷Sr/⁸⁶Sr reveal negative trends which allow contributions from both subducted sediments and fluids to be identified. The fluid component as expressed in elevated U/Th, (²³⁸U/²³⁰Th) and (²²⁶Ra/²³⁰Th)_o ratios is ubiquitous in all of the lavas including near primary lavas from Galunggung. Sr isotope data indicate that the fluids had variable composition and derived from altered oceanic crust, and the U, Th and Ra isotope data suggest that some fluid components must have been added less than 8,000 years ago. Tholeiitic lavas from Flores have the lowest Th contents in the arc and accordingly the highest (²²⁶Ra/²³⁰Th)_o and (²³⁸U/²³⁰Th). For these there is evidence that both U and Ra addition probably occurred less than a few thousand years ago. This suggests that the onset of melting was linked to fluid fluxing and cannot have been due to decompression alone. In contrast, the sediment component may have been added as a partial melt and sediment addition can account for the observed along-arc variations in Sr, Nd, Pb and Th isotopes. There is no requirement in our data for an OIB component in the mantle wedge. Increases in SiO₂ in the Galunggung lavas are accompanied by decreases in both (²³⁸U/²³⁰Th) and (²²⁶Ra/²³⁰Th)_o, which are inferred to reflect mixing with melts of pre-existing mafic lavas. Mixing and differentiation must have occurred faster than could be recorded by either the ²³⁸U–²³⁰Th or ²²⁶Ra–²³⁰Th systems. There is little evidence in our data for any effect of the Australia–Indonesia collision on the composition of the lavas. Across-arc trends in lava composition indicate an increasing relative contribution from subducted sediment, a correspondingly weaker fluid signal and probable decreases in the degree of partial melting. Residual aluminous clinopyroxene or garnet may become important in controlling REE and U/Th fractionation in the far back-arc.