Boron isotopes identify deep-slab serpentinite in the source of Aleutian arc magma

Seafloor lavas of the Western Aleutian arc have isotopically heavy boron (δ¹¹B to +13.4‰) that is negatively correlated with B content (ppm). Endmember samples are primitive dacites and rhyodacites (δ¹¹B > +10‰, SiO₂ = 63%−70%, Mg# > 0.60) with adakitic trace-element and isotopic characteristics that require roles for residual garnet and rutile in their formation. The source of isotopically heavy B is likely serpentinite in the mantle section of the subducting plate, which dewaters into an inverted geothermal gradient and drives melting within the overlying volcanic section at depths where prior effects of seawater alteration were minimal. Most volcanic rocks from the Aleutian Island locations have 10−30 ppm B with an average δ¹¹B of ∼+1.0‰ ± 1.3‰, reflecting a mixed source dominated by subducted sediment. A subset of island samples has B that is isotopically light (δ¹¹B < −2.4‰) and at low concentrations (<11.0 ppm), which is typical of arc lavas globally from rear-arc settings where depth-to-slab is high, and where δ¹¹B may be interpreted to reflect a source in dehydrated (isotopically light) altered oceanic crust. Mass balance modeling indicates that isotopically heavy B from deep-slab serpentinite is present in the Aleutian source arc-wide but is typically masked by sediment-derived B at volcanic centers outside of the westernmost segment of the arc.