Boron and lithium isotopic variations in a hot subduction zone—the southern Washington Cascades

Abstract

New B and Li isotopic data are presented for well-characterized basalts from a transect across the Cascades volcanic arc, S. Washington, for a sediment core on the incoming Juan de Fuca plate, and for dacites from Mt. St. Helens. The basalts define two compositional groups, one (I) having affinities to those from intraplate settings and the other (II) resembling typical volcanic arc calcalkalic lavas. These groups are inferred to have distinct sources. Group I lavas have low Li/Y (0.25–0.6) and B/Nb (∼0.1–0.45), near-constant Ba/Nb (7–18), and show little cross-arc variation in fluid-mobile/immobile element ratios. In contrast, Group II lavas have slightly elevated Li/Y (∼0.5) and higher Ba/Nb (21–100) regardless of location; B/Nb is enriched (to ∼2.2) in the frontal arc, but toward the backarc drops to lower values (<0.4) overlapping with intraplate basalts. δ 11B values are among the lowest reported for volcanic arcs. In detail, δ 11B exhibits spatial variation, being highest (∼−3‰ to −0.4‰) in frontal arc lavas then decreasing sharply to OIB-like values (−8‰ to −9‰) in the backarc region; no systematic variation with magma types is apparent. δ 7Li values (2.5‰ to 5‰) are MORB-like and show no systematic spatial variation. Combined with Sr–Nd–Pb isotopic and other trace element data, these results are consistent with weak slab contributions beneath the frontal arc ranging to negligible inputs below the backarc. This pattern is attributed to progressive dehydration of the subducting slab in an unusually warm subduction zone (SZ); initial inventories of fluid-mobile elements (FMEs) apparently were reduced extensively before the slab reached subarc depths, and essentially depleted as it moved beneath the backarc. Slab contributions appear to be superimposed on a compositionally layered mantle structure consisting of two domains sampled by the distinct basalt groups. Group I lavas appear to form by decompression melting of convectively upwelling deeper mantle that is little modified from an OIB–MORB source composition, except for the relatively small effect on B composition beneath the frontal arc. Group II lavas may form by melting of enriched domains within shallower lithospheric mantle. Because both groups have similar δ 11B, production of Group II magmas need not involve modern inputs of slab fluid. This interpretation differs from the standard paradigm for arc magmatism in that flux melting is not invoked as the dominant cause for partial melting; efficient dehydration of the slab may remove the impetus to drive this mechanism.