Untangling the effects of crustal and source processes on the Mo isotope systematics of oceanic arcs: insights from the Tonga-Kermadec subduction system

Abstract

The Mo stable isotope system is a promising tracer of crustal recycling. As subduction zones play a major role in returning crustal material back to the Earth’s mantle, understanding the Mo isotope behavior during processes operating at these settings is paramount. The Mo isotope compositions of arc lavas are highly variable and despite recent advances in this field, questions remain about Mo isotope fractionation and mobilization during slab processes and the relative effect of magmatic differentiation on the Mo isotope compositions (δ^98/95Mo) of arc lavas. The Tonga-Kermadec arc is well suited to assess and disentangle crustal and source processes in intra-oceanic arc settings owing to widespread felsic volcanism and along-arc source variations, characterized by increasing proportions of a metasedimentary component from the northern Tonga to the southern Kermadec arc. In the pursuit of these objectives, we analyzed a set of basaltic to dacitic tholeiitic arc lavas from volcanoes covering the whole length of the Tonga-Kermadec arc and sediments from DSDP site 204. Tonga-Kermadec arc lavas have highly variable Mo concentrations (0.21 and 2.35 µg/g) and δ^98/95Mo values (−0.29 to +0.34 ‰). At some volcanoes, considerable δ^98/95Mo variations in samples as primitive as basalt are correlated with indices of differentiation and Sr-Nd radiogenic isotopes, indicating an impact of crustal assimilation on Mo isotope compositions. However, the δ^98/95Mo of the most primitive lavas analyzed for each volcano appear to have preserved primary melt signatures, since they form an along-arc trend mirroring those of Pb isotope averages, a proxy that is controlled by source processes in the Tonga-Kermadec arc. Using these data, a north–south decrease in δ^98/95Mo of the sources of arc magmas is resolved. This is interpreted to reflect the competing signals of isotopically heavy slab-fluids (δ^98/95Mo higher than D-MORB values) with that of an increasing contribution of a metasedimentary component that is lighter than the DMM toward the south. Our data suggest that crustal assimilation can significantly distort primitive δ^98/95Mo even in mafic magmas from simple intra-oceanic arcs. The induced intra-volcano δ^98/95Mo scatter has the potential to obscure along-arc variability as well as systematics with typical chemical and isotopic proxies used to characterize arc lava sources (e.g., Ba/Th, radiogenic isotopes). However, we also show that a careful evaluation of these effects can, at least in the Tonga-Kermadec arc, allow the assessment of arc lava source compositions.