Implications of thorium excesses and high 230Th in seafloor lavas of the western Aleutian Arc

Seafloor lavas of the western Aleutian arc define a unique dataset for island arcs globally in 238U-230Th isotope space, with 230Th excesses up to 79% and 230Th/232Th activity ratios [(230Th/232Th)] up to ∼2.27. Basic modeling to produce the observed 230Th excesses is consistent with low Yb and fractionated Dy/Yb in end-member high-Mg# dacites reflecting a role for residual garnet. The models require use of partitioning data with DU ∼0.2 and DTh ∼0.1 in garnet as measured under experimental conditions appropriate for equilibration of hydrous and high-silica melts with eclogite at 2.0 points to a source with a modest but measureable contribution of U-enriched AOC. Most melting of the subducting plate is in Pacific MORB that has not been strongly affected by seawater alteration (Yogodzinski et al., 2017 EPSL). Key mixing relationships can only be observed in arc volcanic rocks lacking subducted sediment in their source which adds significant quantities of radiogenic Sr. This criterion is met in the western Aleutians but not by common arc rocks globally. An important caveat for the dataset is that the ages of most western Aleutian samples are unknown, so measured (230Th/232Th) and 230Th-excess are considered minimums. We cannot rule out a role for 230Th ingrowth to explain high (230Th/232Th), but strong correlations of (230Th/232Th) with long-lived isotopes suggest that elevated (230Th) in island-arc volcanic rocks globally (compilation of Huang et al., 2016 Chem Geol) is primarily an indicator of source composition, and not a product of ingrowth during melt transport. In the global dataset, we find that mixing of (230Th/232Th) and (238U/230Th) with 87Sr/86Sr, 208Pb/204Pb, 143Nd/144Nd, and with trace element ratios such as Th/U and Lu/Hf are all consistent with a role for an eclogite-melt source component (similar to that observed western Aleutian seafloor lavas) in island-arc volcanic rocks throughout the Aleutian arc and in arc volcanic rocks globally.