Serpentinite fluids and slab-melting in the Aleutian arc: Evidence from molybdenum isotopes and boron systematics.

Mo isotope compositions have been increasingly used as a source tracer in magmatic systems. Here, we investigate the relative role of subducting oceanic crust, fluid-rich sources (e.g., serpentinite, altered oceanic crust – AOC), subducted sediments and upper mantle in the chemical composition of Aleutian arc magmas with Mo and B systematics. We present elemental and Mo isotope compositions (δ 98 / 95 Mo) and B concentrations on Aleutian lavas (n = 59), from Okmok Volcano in the east to the westernmost seamount Piip, showing absence of Mo isotope fractionation during magmatic differentiation. Additionally, we report Mo isotope systematics for serpentinized peridotites from the South-West Indian Ridge (SWIR) (n = 6), AOC (n = 2) and Pacific sediments (DSDP 183, ODP 886) (n = 5) outboard the Aleutian arc. Molybdenum isotope composition and B enrichment (e.g., B/Ce) patterns display a step-function increase along the arc, with low, MORB-like, δ 98 / 95 Mo and low B/Ce values in the western section of the arc (B/Ce = 0.15–1.07; δ 98 / 95 Mo =−0.38 to +0.01‰), that abruptly increase in the central-eastern volcanoes Korovin, Seguam and Yunaska (B/Ce = 1.20–2.60; δ 98 / 95 Mo = +0.03 to +0.30‰) near the intersection of the Amlia Fracture Zone (AFZ) with the trench, but decrease again farther east at Okmok (B/Ce = 0.76 and δ 98 / 95 Mo = −0.12‰ on average). These data patterns are interpreted to reflect an along-arc changing source in the Aleutian magmas. AOC and Pacific sediments have predominantly low δ 98 / 95 Mo (−0.47 to −0.32 and +0.17 to −1.9‰, respectively), while serpentinites have extremely high δ 98 / 95 Mo (up to +1.09‰) and high B/Ce (∼22000). Based on the low δ 98 / 95 Mo in sediments and AOC, and lack of correlation between along-arc δ 98 / 95 Mo and radiogenic sediment tracers, subducted sediments and AOC do not exert first-order controls on the observed Mo isotope compositions. Rather, low, MORB-like, δ 98 / 95 Mo but high Mo enrichments (e.g., Mo/Ce) in the western samples are consistent with slab melting under rutile-bearing eclogitic facies with near absent Mo isotope fractionation from the slab to the arc sources. In turn, the abrupt increase of δ 98 / 95 Mo and B/Ce in lavas near the AFZ are best explained by a serpentinite endmember (likely dehydration fluids) at the AFZ that is not evident elsewhere along the arc. Results from this study provide evidence for serpentinites as an additional heavy Mo isotope signature component in subduction zones and demonstrate that high δ 98 / 95 Mo coupled with B enrichments are a useful proxy for tracing serpentinite fluids in subduction zones. • Mo isotope compositions trace source characteristics in magmatic systems. • Serpentinites are a heavy-Mo isotope and high B/Ce source in subduction magmas. • Slab melting under eclogitic facies leads to nearly absent Mo isotope fractionation. [ABSTRACT FROM AUTHOR]