3He evidence for a wide zone of active mantle melting beneath the Central Andes
| Autor: | Konrad Hammerschmidt, Simon Lamb, L. Hoke, Hans Friedrichsen, David R. Hilton |
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| Rok vydání: | 1994 |
| Předmět: |
ARC VOLCANIQUE
geography Radiogenic nuclide geography.geographical_feature_category TECTONIQUE DE PLAQUES Volcanic arc Mantle wedge Earth science Geochemistry Crust MAGMATISME Mantle (geology) Geophysics SUBDUCTION Volcano Space and Planetary Science Geochemistry and Petrology Lithosphere Transition zone Earth and Planetary Sciences (miscellaneous) ANALYSE ISOTOPIQUE Geology HELIUM |
| Zdroj: | Earth and Planetary Science Letters. 128:341-355 |
| ISSN: | 0012-821X |
| DOI: | 10.1016/0012-821x(94)90155-4 |
| Popis: | 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°S and 23°S. The highest 3He4He ratios (reported as RRA ratios: R = sample 3He4He, RA = air 3He4He) are associated with the active volcanic arc of the Western Cordillera (0.92 < RRA < 5.52) and approach ratios found at other convergent margins in the circum-Pacific region. A significant 3He component is also present in fluid and gas samples from the high Altiplano plateau (0.48 < RRA < 3.56) and the Eastern Cordillera (0.03 < RRA < 1.2), up to 300 km east of the active arc and more than 300 km above the subducting slab. This wide zone of 3He anomalies is delineated both to the east and the west by regions with low 3He4He ratios ( ⩽ 0.2RA), typical of radiogenic helium production in the crust. Studies of the regional groundwater regime suggest that the wide zone of elevated 3He4He values away from the active volcanic arc is unlikely to be caused by lateral and shallow transport of magmatic helium and there is no evidence for significant crustal sources of 3He. The high 3He4He 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 arc, 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. |
| Databáze: | OpenAIRE |
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