Crustal Input into the Mantle through Subduction Zones, an Isotopic Perspective

Subduction zones are key areas on Earth where material once exposed at the surface is processed: part of the material almost instantaneously finds its way back to the surface in the island arcs but most of it is recycled into the mantle. However, all elements are not transferred in the same way from the surface to the island arc or deep mantle. Most fluid-mobile elements such as B, Li or U escape the subducted slab in arc magmas while others such as Hf and the heavy REE are mainly recycled into the mantle.

Here I revisit the information provided by elements such as HFSE, REE, Pb and Sr whose isotopic compositions constrain their partitioning between arc magmas and the slab. A compilation of data on arcs around the globe shows that two main processes dominate the arc signature: in some cases subducted sediments contribute significant amounts of both fluid-mobile and immobile elements to arc magmas; in other cases they contribute very little, not even for the fluid-mobile elements. While Sr and Pb are highly enriched in lavas produced in fluid-dominated environments, their average Sr isotopic composition is only about 0.7035 and their average ²⁰⁶Pb/²⁰⁴Pb is about 18.5, values that are similar to those of average depleted mantle. This indicates that the fluids come mainly from the basaltic portion of the subducted slab. It also indicates that fluid-mobile elements in the subducted sediments are quantitatively sent back into the mantle unless the sediment melts, an unlikely situation in present-day subduction zones.