EXTREME 231PA-ENRICHMENT IN YOUNG CASCADE ARC LAVAS
Here we report the first (231Pa/235U) data from the Cascade arc for this same suite of lavas from central Oregon to further investigate the conditions and processes of mantle melting. Measured (231Pa/235U) ratios range from 1.15–8.67, with initial, age-corrected ratios ranging from 1.18–9.14. The five most primitive basalts have initial (231Pa/235U) ratios ranging from 5.02–8.06. These new results are significantly higher than any previously published data, which reach maximum values of ~ 2–2.5 in arcs and ocean island basalts, and ~ 2.5–3.5 in mid-ocean ridge and continental basalts.
Dynamic melting models of a peridotitic mantle source cannot reproduce these data using reasonable model parameters. Addition of a highly Pa-enriched sediment melt to the mantle wedge immediately prior to melting could allow for preservation of this sediment melt signature in the erupted lavas, although generating a sufficiently enriched melt is problematic. Dynamic melting of rutile ± apatite-bearing mélange diapirs ascending into the mantle wedge beneath the arc (cf. Marschall & Schumacher, 2012) could potentially generate magmas with the requisite high (231Pa/235U).