Crustal controls on light noble gas isotope variability along the andean volcanic arc

Abstract

This study combines new noble gas data from fluid inclusions in minerals from Sabancaya, Ubinas, and El Misti (CVZ, Peru) and Villarica (South Chile, SVZ) with a revised noble gas compilation in the Andes, to identify systematic along arc variations in helium isotope compositions. We find 3He/4He ratios varying from 8.8 RA (Colombia) to 7.4 RA (Ecuador) within the NVZ, and only as high as 6.4 RA in the CVZ (RA is the atmospheric 3He/4He ratio of 1.39 × 10-6). These distinct isotope compositions cannot be explained by variable radiogenic 4He production via slab fluid transport of U and Th in the mantle wedge, since both NVZ and CVZ share similar slab sediment inputs (Th/La ≈ 0.08-0.13). Instead, the progressively more radiogenic 3He/4He signatures in Ecuador and Peru reflect 4He addition upon magma ascent/ storage in the crust, this being especially thick in Peru (>70 km) and Ecuador (>50 km) relative to Colombia (∼30-45 km). The intermediate compositions in the North (8.0 RA) and South (7.9 RA) Chile, both high sediment flux margins, mostly reflect a more efficient delivery of radiogenic He in the wedge from the subducted (U-Th-rich) terrigenous sediments. Our results bring strong evidence for the major role played by crustal processes in governing noble gas compositions along continental arcs.