HE-3 EVIDENCE FOR A WIDE ZONE OF ACTIVE MANTLE MELTING BENEATH THE CENTRAL ANDES

We report results of a regional survey of helium isotopes measured in water and gas samples in volcanic sulfataras and geothermal springs from the Central Andes of northern Chile and Bolivia between the latitudes 15 degrees S and 23 degrees S. The highest He-3/He-4 ratios (reported as R/R(A) ratios: R = sample 3He/ 4He, R(A) = air He-3/He-4) are associated with the active volcanic are of the Western Cordillera (0.92 < R/R(A) < 5.52) and approach ratios found at other convergent margins in the circum-Pacific region. A significant He-3 component is also present in fluid and gas samples from the high Altiplano plateau (0.48 < R/R(A) < 3.56) and the Eastern Cordillera (0.03 < R/R(A) < 1.2), up to 300 km east of the active are and more than 300 km above the subducting slab. This wide zone of He-3 anomalies is delineated both to the east and the west by regions with low He-3/He-4 ratios (less than or equal to 0.2R(A)), typical of radiogenic helium production in the crust. Studies of the regional groundwater regime suggest that the wide zone of elevated He-3/He-4 values away from the active volcanic are is unlikely to be caused by lateral and shallow transport of magmatic helium and there is no evidence for significant crustal sources of He-3. The high He-3/He-4 ratios are interpreted as reflecting degassing of volatiles from mantle-derived magmas emplaced over an area 400 km wide beneath and into crust up to 75 km thick. The subducting slab is at depths of 100-350 km in this region. In the west, underneath the active volcanic arc, mantle melting is probably largely controlled by mantle hydration and dehydration and the helium isotope data can be used to delineate the extent of the asthenospheric mantle wedge at depth. In contrast, mantle melting behind the are, beneath the Altiplano and Eastern Cordillera, may be a result of convective removal of the base of the lithosphere. The sharp cut-off in the mantle helium signal in the east is interpreted as marking the western edge of thick and relatively cold lithosphere, devoid of mantle melts, which could transport mantle volatiles towards the surface. This may coincide with the limit of underthrusting of the Brazilian shield beneath the eastern margin of the Central Andes.