[1] Recycled ancient oceanic crust with variable amounts of aging, or inclusion of sediments of differing types and origins has often been invoked as a source for present-day ocean island basalts (OIB), but the current evidence remains largely qualitative. Previous quantitative modeling has shown that much has to be learned in order to better understand the implications of crustal recycling on mantle heterogeneity. Here, we present new model calculations incorporating recent constraints on subduction-zone processes and the composition of subducted sediments. Modeled compositions of the recycled oceanic crust vary widely as a function of the recycling age and composition of the oceanic crust. HIMU-type sources can only be created by recycling igneous oceanic crust if it has undergone substantial modification during subduction. Although the required modifications are qualitatively consistent with dehydration processes in subduction zones, the many uncertainties prevent a precise estimate of the isotopic composition of ancient recycled igneous crust. Inclusion of sediments increases the isotopic variability and although the resulting Sr and Nd isotopic signatures can be similar to enriched mantle (EM) signatures, the Pb isotopic composition of EMtype OIB is difficult to reconcile with the presence of sediment in their sources. The large variability of modeled compositions of the subducted crust suggests that if mantle heterogeneity is largely formed by crustal recycling, each OIB is likely to have a unique isotopic composition resulting from specific combinations of composition, age and subduction modification of the subducted crust. Given the variability of the recycled components, a small number of relatively well-defined enriched compositions can only be explained if either the subduction processing of oceanic crust is a far better defined process than observation would seem to indicate, or, the intramantle disaggregation and mixing of compositionally diverse recycled materials is surprisingly efficient.

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[1] We present new experimental partitioning data for a range of petrogenetically important elements at pressures up to 3.4 GPa. The experiments are designed to mimic low degrees of anhydrous melting beneath mid-ocean ridges. The available data indicate that the partition coefficients are pressure, temperature, and composition dependent. Therefore partitioning behavior over the appropriate range of pressure, temperature, and composition must be quantified, in order to model continuous extraction of melt during the adiabatic rise of mantle material. For this purpose, we have parameterized the partitioning behavior of the REE, Hf, Zr, U, and Th based on a simple thermodynamic model. Although these parameterizations cannot be used for retrieving thermodynamic constants yet, they do yield accurate descriptions of the partitioning behavior that are useful for modeling decompression melting. Our parameterizations show that the partitioning of trace elements is strongly dependent on the Ca and Alcontent of the clinopyroxene (cpx) and REE are always incompatible in cpx on the peridotite solidus at pressures up to 3.4 GPa. For garnet the data indicate that the heavy REE partition coefficients decrease with increasing pressure. Our data also indicates that Pb is more incompatible than Ce in clinopyroxene; Ce and Pb have similar partition coefficients in garnet. Therefore the presence of a residual phase with high Pb partition coefficients is required to produce the near-constant Ce/Pb ratios in MORB and OIB. Sulfides are the most likely phase to buffer the Pb content in the melt. Except at small porosities (<0.3%), clinopyroxene on the peridotite solidus is unable to fractionate U from Th significantly (15% 230 Th-excess), whereas garnet can fractionate U from Th effectively at porosities up to 1%. Therefore if the 230 Th-excesses in midocean ridge basalts are melting phenomena, then melting with garnet residual is required in order to be compatible with physical observations on porosities and upwelling rate at mid-ocean ridges. New model calculations that include the compositional dependent partitioning of the trace elements show that the predicted physical characteristics (depth and extent of melting, upwelling rate, porosity) of the MORB melting regime are similar for the Lu/Hf, Sm/Nd, and U-Th systems.

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