Crustal controls on light noble gas isotope variability along the Andean Volcanic Arc

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 varia-tions in helium isotope compositions. We find He-3/He-4 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 He-3/He-4 ratio of 1.39 x 10(-6)). These distinct isotope com-positions cannot be explained by variable radiogenic He-4 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 approximate to 0.08-0.13). Instead, the progressively more radiogenic He-3/He-4 signatures in Ecuador and Peru reflect He-4 addition upon magma ascent/ storage in the crust, this being especially thick in Peru (>70 km) and Ecuador (>50 km) relative to Colombia (similar to 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.