ELSEVIER Earth and Planetary Science Letters 128 (1994) 341-355 EPSL 3He evidence for a wide zone of active mantle melting beneath the Central Andes L. Hoke a, D.R. Hilton b,1 S.H. Lamb a, K. Hammerschmidt b, H. Friedrichsen a Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK b Institut fiir Mineralogie, FR Geochemie, Freie Universitiit Berlin, Boltzmannstra~e 18-20, D-14195 Berlin, Germany Received 20 May 1994; accepted 24 August 1994 Abstract 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 3He/4He ratios (reported as R/R A ratios: R = sample 3He/4He, R A = air 3He/4He) are associated with the active volcanic arc 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 3He 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 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 3He/4He ratios (< 0.2RA), typical of radiogenic helium production in the crust. Studies of the regional groundwater regime suggest that the wide zone of elevated 3He/4He 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 3He/4He 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. I. Introduction Considerable controversy surrounds the for- mation and uplift of areas of unusually thick 1 Present address: Vrije Universiteit Amsterdam, Faculty of Earth Sciences, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands. continental crust (< 75 km) in active plate con- vergence zones, such as the high plateau regions of the Central Andes and Tibet. The debate is about the mechanisms of both crustal thickening and uplift. For instance, crustal thickening and uplift may be predominantly a result of homoge- neous crustal shortening, or some other mecha- nism, such as the addition of substantial volumes 0012-821X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0012-821X(94)00186-3342 L. Hoke et al. /Earth and Planetary Science Letters 128 (1994) 341-355 of new material to the base of the crust. Dynamic models of such regions predict that parts of the mantle lithosphere exert an important control on the overall pattern of deformation [1-6]. Of par- ticular interest is the idea that convective removal of the base of the thickened lithosphere may cause surface uplift [3,6] in addition to that caused by crustal thickening. The thermal consequences of convective removal of the lower (cold) litho- sphere and replacement with (hot) asthenosphere should be melting at depths of less than ~ 120 km [7]. This model has been used to explain volcanism with mantle characteristics in both the Tibetan plateau [6-8] and the Puna region of northwest Argentina [9,10]. Segregating and as- cending mantle melts would be expected to trans- port mantle volatiles into the crust, degassing during emplacement. Geothermal and deeply cir- 14~,S 16' 18' 70:'W 68" I I~ : :i :?'11~,~ I PERU 66' l 64 ° CI!NOZOIC STRATA 1} 1,{ 1 ';-TER TI AI,?.Y - 14" 1,OS FF.AII+I5S IGNIMBI~.rI+E A ACTIVI'2 VOI+{TAN() ~ I+AKI~/SAI,AR • {; l';{ Yll II!RMAI+ ()R • SI[IIA;ATAF, ASAMH,I! SITE 1 I:{)R III!LIUM IS{ }TOPI'++ 16+' ANAI.YSES X ~ MAJ{)R I:AULT S all t a Crtlz tS" 2{} ~' !- (~- I~i~)::!::!:;::i~ ~'j~ Q ~ ~!i::f:ii~i~::~ili~:;:~:!:i:;i::ii~:ii::;::;2::~::!::!::!::i:;::!::l f;:!if:iilI~ '~ "-t 20 ° , ........ .... ............................................. Uyuni ! : )::!::!i~: :;il ~i~ :::iz:.i): :zii:.i i ~ ! ! ! ! ! !:;:!:: ))ii~i::i~)::i i 22" 22:' ID • GENTINA 2 i~: A[ ucam a / (a) 70" 68' 66' 64' Fig. 1. (a) Geological map of the Central Andes of Bolivia and northern Chile showing the localities of 70 geothermal and sulfatara sample sites listed in Table 1. The pre-Tertiary and Cenozoic outcrop pattern and all active volcanoes along the arc of the Western Cordillera are also shown.(b) 18°S 67°W 66°W 19°S 18°S 20°S 21 °S 22°S L. Hoke et al. /Earth and Planetary Science Letters 128 (1994) 341-355 69°W 68°W (c) 68°w 17°S 19°S 20°S 21 °S 17°S 18°S 19°S 68%V 67°W 67°W 66 °w 65 °W 343 16 °S 17°S 18°S 19°S Fig. 1. (b) Helium isotope ratios for sample sites along the volcanic arc and adjacent areas, including the Precordillera in the west and the Altiplano in the east (see box in a). (c) Helium isotope ratios of the sample sites in the Eastern Cordillera and along the transition zone between the Eastern Cordillera and the Altiplano in the west (see box in a). LF = the Miocene-Pliocene Los Friales ignimbrite. Darker shading = the pre-Tertiary outcrop pattern; lighter shading = the Cenozoic outcrop pattern. eulating groundwaters would finally