238U-230Th equilibrium in arc magmas and implications for the time scales of mantle metasomatism

- Volcán de Colima magmas are in (230Th/238U) equilibrium.
- (230Th/238U) equilibrium but significant 231Pa and 226Ra excesses.
- Metasomatised mantle returned to secular equilibrium before melting.
- Metasomatised mantle in secular equilibrium common in subduction zones.
- Time interval between metasomatism and melting variable in subduction zones.

Large excesses of 238U and 226Ra relative to 230Th characterize many arc magmas and are commonly interpreted to represent recent addition of slab-derived fluid to the mantle wedge beneath the arc. A significant proportion of arc magmas are, however, in 238U-230Th radioactive equilibrium. This is generally thought to result from “buffering” of the young slab fluid U-series signal by a sediment component in secular equilibrium. Here we present new 238U-230Th-226Ra and 235U-231Pa measurements for historic andesites from Volcán de Colima, Mexico. In all lavas (230Th/238U) are in equilibrium, whereas (231Pa/235U) and (226Ra/230Th) are significantly greater than one. These data demonstrate that arc magmas with (230Th/238U) equilibrium can have significant 231Pa and 226Ra excesses, precluding ageing of the magmas in the crust as the cause of 230Th/238U equilibrium. Quantitative modeling of metasomatic and melting processes further indicates that addition of sediment melts to a depleted mantle wedge produces significant 230Th excesses and that 238U excesses induced by recent addition of fluids derived from the altered oceanic crust are not sufficient to compensate these 230Th excesses. U-series activity ratios in Colima magmas are best explained by models in which the metasomatised mantle returns to secular equilibrium before melting, implying a time lag ⩾350 kyr, with subsequent production of 231Pa and 226Ra excesses by in-growth during melting rather than by addition of slab fluids. Investigation of a global compilation of U-series data in arc magma indicates that our model proposed for Colima applies to most arc magmas in or near (230Th-238U) equilibrium. The time lag between mantle metasomatism and melting appears to vary between hundreds years to more than 350 kyr in subduction zones. We posit that the absence of U/Th elemental fractionation during melting of arc sources in (230Th/238U) equilibrium reflects a higher fO2 compared to MORB sources that yield magmas with 230Th excesses.