The effect of sediment recycling in subduction zones on the Hf isotope character of new arc crust, Banda arc, Indonesia

Abstract

A large portion of Earth's crust is formed at convergent plate boundaries that are accompanied by the subduction of sediments that can contain evolved crust-derived detritus. Partial melting of such sediments can strongly affect the trace element and isotope geochemistry of new arc rocks. Here, we present high-precision Lu-Hf-Zr concentration data and Hf isotope compositions for a series of volcanic rocks from the Banda arc, East Indonesia, to quantify the transfer of subducted Hf to the Banda arc crust and address the influence of recycled Hf in subduction zones on the Hf isotope systematics of arc rocks. Along-arc from NE to SW, the 176Hf/177Hf decreases from 0.28314 to 0.28268 ranging from predominantly mantle-like ratios towards more crustal signatures. Hf-Nd isotope co-variations require low Nd/Hf in the arc magma source, inconsistent with fluid addition to the arc melts, but in agreement with the involvement of partial sediment melts. The systematic decrease in Hf-Nd isotopes infers a NE-SW along-arc increase in the involvement of subducted continental material (SCM) in the arc magma source, consistent with δ18O and Nd-Sr-Pb isotope constraints. The along-arc decrease in Hf isotopes coupled with increasing Zr/Hf (from 31.9 to 36.1) and decreasing Lu/Hf suggests that the newly produced arc lavas contain crustal-derived Hf as a result of partial melting of SCM associated with the breakdown of zircon. Based on recent experimental estimates of temperatures required to achieve zircon breakdown, we infer that slab surface temperatures in the Banda arc region need to be as high as 925°C. As a consequence of the inheritance of non-radiogenic Hf from SCM, the juvenile Banda arc crust exhibits Hf isotope model ages biased by hundreds of millions of years. We conclude that crust-formation ages derived from Hf isotope ratios of convergent margin rocks and their constituent minerals (such as zircon) can be geologically meaningless mixing ages, even when they readily preserve low δ18O values (i.e., <6.5). These findings are discussed with respect to the inferred origin of Hadaean zircons at convergent plate boundaries, which appear consistent with an origin in a convergent margin setting.