U-series disequilibria in subduction zone lavas: Inherited from subducted slabs or produced by mantle in-growth melting?

Abstract

U-series disequilibria of subduction zone lavas have been used to provide temporal constraints on flux transfer in convergent margins. There are debates on whether the disequilibria are inherited from the source modified by slab materials (fluids or sediments) or produced by in-growth melting of the mantle. After filtering out the effect of crustal modification on U-series data, ( 226 Ra)/( 230 Th) is not positively correlated with Sr/Th and Ba/Th in arc basalts with SiO 2 < 52 wt%. This invalidates the key evidence for addition of slab-fluids to the mantle producing 226 Ra excess in subduction zone lavas. Sediment melts with a zircon-free residue may have 231 Pa excess, but it is not consistent with the overwhelming Zr-Hf depletions in subduction zone lavas, which instead require a zircon-saturated residue during sediment melting. application of U-series disequilibria into subduction zone magmatism.

Timescales of magma ascent and degassing and the role of crustal assimilation at Merapi volcano (2006–2010), Indonesia: Constraints from uranium-series and radiogenic isotopic compositions

“…The 2006 and 2010 Merapi volcanic rocks are characterised by 238 U excesses (Fig. 2a), typical of subduction-related volcanic rocks, suggesting that recent fluid addition, likely from the down-going slab (e.g., Condomines et al, 1988;Gill and Williams, 1990;Hawkesworth et al, 1997) or that in-growth melting in the mantle and subsequent crustal modification (e.g., Reubi et al, 2014;Huang et al, 2016), occurred in the last 380 ka. In the past, Th isotopic variation in volcanic rocks (the vertical spread in data on the U-Th equiline diagram) has been used to estimate magma residence or storage times assuming closed-system differentiation (e.g., Heath et al, 1998).…”

“…If the decay chain is affected by chemical fractionation of a parent/daughter elemental ratio, restoration of equilibrium by radioactive decay is determined by the half-life of the daughter nuclide involved. Excess 238 U (( 238 U/ 230 Th)>1) and 226 Ra (( 226 Ra/ 230 Th) >1) in subduction zone volcanic rocks are typically attributed to fluid addition from the subduction slab on timescales of less than ~380,000 years and less than ~8,000 years, respectively (e.g., Condomines et al, 1988;Gill and Williams, 1990;Hawkesworth et al, 1997) although there may be some modification of ratios by crustal-level processes (e.g., Handley et al, 2008;Reubi et al, 2014;Huang et al, 2016). At magmatic temperatures, 234 U is not expected to be fractionated from 238 U, and so fresh igneous rocks should have ( 234 U/ 238 U) = 1.…”