Along-strike trace element and isotopic variation in Aleutian Island arc basalt: Subduction melts sediments and dehydrates serpentine

摘要

Trace element and Sr-Nd-Pb isotope compositions of basaltic lavas from 11 volcanoes spanning 1300 km of the Aleutian Island arc provide new constraints on the recycling of elements in melts and fluids derived from subducted oceanic crust and sediment. Despite a nearly twofold variation in the flux of sediment subducted along the Aleutians, proxies indicating the presence of sediment melt in the magma source, including Th/La and Th/Nd, do not vary systematically along strike. In contrast, ratios including B/La, B/Nb, B/Be, Cs/La, Pb/Ce, and Li/Y suggest that the quantity or composition of fluid transferred from the slab into the mantle wedge varies an order of magnitude along strike and is apparently correlated with sediment flux. However, the most distinctive fluid addition corresponds spatially with subduction of the Amlia Fracture Zone (AFZ), a likely repository for H2O-rich serpentinite. Sr, Nd, and Pb isotope ratios, together with Th/Nd and B/La ratios, show that the majority of these basalts reflect a common baseline metasomatism of the mantle that accumulated, perhaps over millions of years, via small additions of both slab fluids and partially melted sediment. The paradox of requiring slab surface temperatures high enough to melt a layer of sediment, while lower-temperature dehydration reactions that supply water occur sufficiently deep to flux melting >80 km beneath the volcanoes is reconciled in a four-stage model: (1) as sediment and altered ocean crust is carried to ～60 km depth and temperatures increase to ～650°C, metamorphic dehydration reactions release most of the fluid and B to the shallow mantle wedge beneath the fore arc, but some of this mantle is metasomatized and flows downward; (2) the uppermost layer of sediment begins to melt at ～750°C and >60 km depth; this small volume of melt physically mingles with the overlying metasomatized mantle wedge as it flows further downdip; (3) below the sediment veneer, the uppermost 1 km of ocean crust reaches 650°C at ～90 km depth where antigorite breaks down, releasing B-rich H2O; and (4) this fluid infiltrates the layer of residual unmelted metasediment leaching the remaining inventory of fluid mobile elements and ascends into the modified mantle lowering its solidus and inducing partial melting. Where antigorite is likely abundant at the surface of the Pacific plate along the AFZ, deep H2O flux to the mantle is enhanced. This is reflected in higher B and lower incompatible element contents in the magmas, a relationship that links the amount of serpentine subducted to the extent of partial melting and the major element composition of arc basalt.