IMPLICATIONS OF THORIUM EXCESSES AND HIGH 230TH IN SEAFLOOR LAVAS OF THE WESTERN ALEUTIAN ARC (Invited Presentation)

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 D_U ∼0.2 and D_Th ∼0.1 in garnet as measured under experimental conditions appropriate for equilibration of hydrous and high-silica melts with eclogite at <1050° C and 2-4 GPa (Skora & Blundy, 2010 J Petrol; Kessel et al., 2005 Nature). Mixing relationships indicate that (²³⁰Th/²³²Th) is more sensitive to the presence of AOC (altered oceanic crust) in the source than is ⁸⁷Sr/⁸⁶Sr, so the coupling of unradiogenic Sr (⁸⁷Sr/⁸⁶Sr <0.7029) with (²³⁰Th/²³²Th) >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.