EXTREME 231PA-ENRICHMENT IN YOUNG CASCADE ARC LAVAS

The Cascade arc is an end member in the global suite of convergent margins, with the young (~ 8–10 Ma) Juan de Fuca slab subducting slowly (~ 3.5 cm a^-1) beneath the North American plate. These features create an unusually hot slab at depth, which is expected to result in shallow slab dehydration, consistent with geophysical interpretations of a highly metasomatized fore-arc mantle. This raises questions about the mechanism of mantle melting and the timing of addition of a slab signature, which is recorded in many Cascade arc lavas. U-series isotope data (²³⁸U-²³⁰Th-²²⁶Ra) for a suite of young mafic lavas from the central Oregon Cascades suggest an origin by dominantly decompression melting, but cannot fully constrain the timing of fluid addition to the mantle wedge (Mitchell & Asmerom, 2011).

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).