Oxygen isotope evidence for slab melting in modem and ancient subduction zones

We measured oxygen isotope compositions of 34 adakites, high-Mg andesites, and lavas suspected to contain abundant slab and sediment melts from the Western and Central Aleutians, the Andes, Panama, Fiji, Kamchatka, Setouchi (Japan), and the Cascades. This suite covers much of the diversity of arc lavas previously hypothesized to contain abundant 'slab' melts. Measured and calculated values of 6180 for olivine phenocrysts in these samples vary between 4.88 parts per thousand and 6.78 parts per thousand, corresponding to calculated melt values of 6.36 parts per thousand to 8.17 parts per thousand. Values of 6180 for these samples are correlated with other geochemical parameters having petrogenetic significance, including Sr/Y, La/Yb, Sr-87/Sr-86, and Nd-143/Nd-144. Archetypical adakites from Adak Island (Central Aleutian) and Cook Island (Andean Austral zone), previously interpreted to be nearly pure melts of basaltic and gabbroic rocks in subducting slabs, have values of 8180 slightly higher than those of normal mid-ocean-ridge basalts, and in oxygen isotope equilibrium with typical mantle peridotite (i.e., their subtle O-18 enrichment reflects their Si-rich compositions and low liquidus temperatures, not O-18-rich sources). Other primitive adakites from Panama and Fiji show only subtle sub-per mil enrichments in the source. This finding appears to rule out the hypothesis that end-member adakites are unmodified partial melts of basaltic rocks and/or sediments in the top (upper 1-2 km) of the subducted slab, which typically have delta O-18 values of ca. 9-20 parts per thousand, and also appears to rule out them being partial melts of hydrothermally altered gabbros from the slab interior, which typically have delta O-18 values of ca. 2-5 parts per thousand. One explanation of this result is that adakites are mixtures of partial melts from several different parts of the slab, so that higher- and lower-delta O-18 components average out to have no net difference from average mantle. Alternatively, adakites might be initially generated with more extreme delta O-18 values, but undergo isotopic exchange with the mantle wedge before eruption. Finally, adakites might not be slab melts at all, and instead come from diffierentation and/or partial melting processes near the base of the arc crust in the over-riding plate. High-Mg andesites and Setouchi lavas are commonly higher in delta O-18 than equilibrium with the mantle, consistent with their containing variable amounts of partial melts of subducted sediments (as we conclude for Setouchi lavas), slab-derived aqueous fluid (as we conclude for the Cascades) and/or crustal contaminants from the over-riding plate (as we conclude for Kamchatka). (c) 2005 Elsevier B.V. All rights reserved.