Petrological and noble gas features of Lascar and Lastarria volcanoes (Chile): inferences on plumbing systems and mantle characteristics

Abstract

Lascar (5592 m a.s.l.) and Lastarria (5697 m a.s.l.) are Chilean active stratovolcanoes located in the Central Volcanic Zone (CVZ; 16 degrees S to 28 degrees S) that have developed on top of a 71 km thick continental crust. Independently of the similarities in their Plinian/Vulcanian eruptive styles, their complex magmatic feeding structures and the origins of their magmatic fluids still necessitate constraints in order to improve the reliability of geochemical monitoring. Here we investigate the petrography, bulk-rock chemistry, and mineral chemistry in products from the 1986-1993 explosive eruptive cycle at Lascar and from several Holocene eruptive sequences at Lastarria. These data are integrated with measurements of the noble gas isotopes in fluid inclusions (Pis) of minerals from the same products as well as in fumarole gases. The geochemistry of minerals and rocks shows that the studied products belong to high-K-calc-alkaline series typical of subduction-related settings, and provide evidence of differentiation, mixing, and crustal assimilation that are higher at Lastarria. The contribution of slab sediments and fluids to magma genesis in the wedge is limited, suggesting a homogeneous mantle beneath CVZ. The deepest crystallization processes occurred at variable levels of the plumbing systems according to the lithostatic equivalent depths estimated with mineral equilibrium geobarometers at Lascar (15-29 km) and Lastarria (similar to 20-40 km). The( 40)Ar/Ar-36 and He-4/Ne-20 ratios in [Is and fumarole gases indicate the presence of some degree of air contamination in the fluids from both volcanoes. The He-3/He-4 values at Lascar (6.9-73 Ra) are relatively homogeneous and comparable to those of fumaroles, suggesting a main zone of magma crystallization and degassing. In contrast, the He-3/He-4 values at Lastarria (5.31-8.01 Ra) vary over a wide range, suggesting various magma storage levels and providing evidence of crustal contamination, as indicated by the rock chemistry. We argue that mantle beneath the two volcanoes has a MORB-like signature of He-3/He-4, while local crustal contamination explains the lower ratios measured at Lascar.