BORON ISOTOPE EVIDENCE FOR DEEP-SLAP FLUIDS IN ALEUTIAN MAGMAS

Most high-Mg# (>0.5) basalts and andesites from Aleutian Island volcanoes have δ¹¹B (-2 to +5‰) that is uncorrelated with B concentrations (5-27 ppm). These compositions are like arc lavas globally and can be explained by B-rich sources dominated by subducted sediment and altered oceanic crust (AOC). Seafloor lavas from the western Aleutians have more variable and isotopically heavier B with δ¹¹B (0 to +13‰) that is negatively correlated with B concentrations from 2-8 ppm. This pattern includes all western Aleutian seafloor lavas (50-70% SiO₂) but the highest δ¹¹B is observed only in high-Mg# dacites with trace elements and isotopes interpreted to have been produced by melting of subducted MORB under eclogite conditions. The shift toward high δ¹¹B reflects a change in the main source of B west of Buldir Island in the transition from oblique to highly oblique subduction in the westernmost arc. We attribute this change to a loss of B from sources in sediments which have high B concentrations (50-100 ppm) but δ¹¹B from -3 to +5‰ that is much like Aleutian Island lavas. The loss of sediment reveals a source of heavy B that is evident only in western Aleutian samples with <10 ppm B. We interpret this heavy B source to be serpentinite in the mantle section of the subducting plate and we suggest that dewatering of such “deep-slab serpentinite” into an inverted geothermal gradient is very likely the driver of flux melting within the overlying oceanic crust which is hotter than the mantle section of the subducting plate by 200-300° C. A less clearly defined source of isotopically light B is evident in Aleutian island samples with low δ¹¹B (-2.4 to -4.6 ‰) and low B (<11 ppm) which may be from depleted AOC sources that lost ¹¹B in prior episodes of heating and dewatering. This source component will be better defined by additional B isotope work on high-K basalts dredged from seamounts in the Aleutian reararc in locations that lie more than 100 km above the subducting plate. Unique aspects of the Aleutian dataset compared to arcs globally are (1) the common occurrence of B that is sediment-derived implying little need for sources in serpentinite above the subducting plate, and (2) rocks from the hot-slab western Aleutians with isotopically heavy B that appears likely to have been produced by dewatering of serpentinite in the mantle section of the subducting plate.