SUBDUCTION ZONE-TO-MANTLE FLUXES OF TRACE ELEMENTS AT INTRAOCEANIC MARGINS AND IMPLICATIONS FOR MANTLE EVOLUTION

Abstract

Chemical and isotopic anomalies in mantle-derived rocks have been explained by invoking the presence of recycled oceanic crust (i.e., material of crustal origin that is incorporated into the mantle at subduction zones) in their mantle sources. Differences between the chemical and isotopic characteristics of the ingoing slab material and those of the inferred slab-derived mantle reservoirs have been attributed to processes within the subduction zone that alter the composition of the ingoing slab. However, there have been few studies that have specifically examined the composition of residual slab material to determine the effects of subduction zone processing. I used a mass-balance approach to calculate residual slab compositions at nine intraoceanic subduction zones. I calculated residual slab trace element ratios and modern isotopic characteristics of ancient subducted slabs in order to examine the feasibility of mantle evolution models that posit a major role for subducted slabs. Using my calculated residual slab compositions, I was unable to reproduce the chemical and isotopic characteristics of the mantle that have been attributed to the incorporation of recycled crustal material. Modifications to my model to account for variations in mantle wedge composition and in crustal addition rates do not significantly alter these conclusions. My results therefore indicate that subduction zone processing alone is probably insufficient to produce the chemical and isotopic characteristics of inferred mantle reservoirs.