Quantifying Element Fluxes in the Tonga-Lau System
Abstract
Element fluxes in subduction zones between subducting ocean crust and sediment, the overlying mantle and the arc crust are significant over the age of the Earth but are poorly constrained. The proportion of different elements that are released from the subducting plate, and the composition of the residual slab that enters the deep mantle are not well known. Here, we present new data from the Tonga-Kermadec arc which is an 3000 km long arc in the southwestern Pacific along with new and published data from the Valu Fa, Central and Eastern Lau backarc basin spreading centers. The distance between the arc and backarc increases from less than 10 km at ∼23°S to ∼150 km at 19°30‧S. The geochemical composition of glasses and whole rocks along the arc displays strong arc-related signatures with increased ratios of fluid-mobile/immobile elements, e.g. Ba/Nb ratios of >200, Th/Nb ratios of >0.5 and Ce/Pb ratios of <6. Trace element ratios in the backarc lavas range from values comparable to normal mid-ocean ridges (MOR) along the Central Lau Basin in the north to higher Ba/Nb, Th/Nb and lower Ce/Pb ratios that indicate a subduction zone influence in the south. All fluid indices imply a decreasing influence of a subduction related component from south to north in the back-arc and thus with increasing distance to the trench and to the volcanic front. Arc-backarc geochemical gradients such as these are often assumed to result from progressive slab dehydration and vertical mass transfer within the mantle wedge. Assuming that Nb is immobile in fluids, and that Ba/Nb, Th/Nb ratios are not fractionated during melting, then more than 90% Ba and Th in arc lavas is derived from the subducting slab. For backarc lavas 250 km from the trench, 95% Ba and 80% Th is derived from the slab and at slab depth of 350 km and >350 km distant from the trench the contribution is less than 30% Ba and 20% Th respectively. Assuming vertical transfer of Ba and Th, the total mass of subduction-derived Ba in the mantle wedge (up to 350 km distant from the trench) is on the order of 1.6 x 1015kg while the mass of Ba contained in the subducted oceanic crust and sediment is 4.9 x 1014 kg and the total mass of Th in the wedge is 3.6 x 1012kg and the mass subducted is 2.31 x 1012kg. The enrichment in the wedge therefore cannot be explained by simply vertically rising fluids from the dehydrating slab. Flow of material towards the backarc at shallow levels, perhaps driven by upwards flow of subduction-modified mantle in a 'subduction channel' may instead explain the observed cross-arc geochemical gradients. Alternatively, the occurrence of subduction related signatures in the far backarc can potentially be explained by lateral flow of melts from the melting region underneath the arc into the backarc melting regime, if the latter has a width of at least 150-200 km similar to that imaged beneath the flanks of the East Pacific Rise1. 1Forsyth, D.W., et al., Imaging the Deep Seismic Structure Beneath a Mid-Ocean Ridge: The MELT Experiment. Science, 1998. 280(5367): p. 1215-1218.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V31A2757B
- Keywords:
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- 1025 GEOCHEMISTRY / Composition of the mantle;
- 1030 GEOCHEMISTRY / Geochemical cycles;
- 1031 GEOCHEMISTRY / Subduction zone processes