HELIUM ISOTOPE CHARACTERISTICS OF ANDEAN GEOTHERMAL FLUIDS AND LAVAS

The first comprehensive helium isotope survey of the Andes is reported here. We have sampled geothermal fluids and phyric lava flows from th e Southern (svz) and Central (cvz) Volcanic Zones, the volcanically ac tive Puna region and the Precordillera, Salta Basin, Longitudinal Vall ey and the aseismic region between the two volcanic zones. Although th e active areas are characterised by significant differences in crustal age and thickness, the svz, cvz and Puna are characterised by a wide and overlapping range in He-3/He-4 ratios (for fluids and phenocrysts) from predominantly radiogenic values to close to the MORB ratio. The measured ranges in He-3/He-4 ratios (R) (reported normalised to the ai r He-3/He-4-R(A),) are: svz (0.18 <R/R(A) <6.9); cvz (0.82 <R/R(A) <6. 0); and Puna (1.8 <R/R(A) <5.4). Modification of magmatic He-3/He-4 ra tios by water/rock interactions (fluids) or post-eruptive grow-in of r adiogenic He-4 or preferential diffusive loss of He-3 (phenocrysts) is considered unlikely; this means that the wide range reflects the heli um isotope characteristics of magma bodies in the Andean crust. The me chanism controlling the He-3/He-4 ratios appears to be a mixing betwee n mantle (MORB-like) helium and a radiogenic helium component derived from radioactive decay within the magma (magma aging) and/or interacti on with He-4-rich country rock: a process expected to be influenced by pre-eruptive degassing of the mantle component. Assimilation of lower crust is also capable of modifying He-3/He-4 ratios, albeit to a much lesser extent. However, it is possible that the highest measured valu es in each zone were established by the addition of lower crustal radi ogenic helium to MORB helium. In this case, the higher 'base level' ra tios of the svz would reflect the younger crustal structure of this re gion. In contrast to helium, there is no overlap in the Sr or Pb isoto pe characteristics of lavas from the active zones: in all areas, there fore, He-3/He-4 ratios appear to vary independently of Sr and Pb isoto pe variations. This decoupling between the lithophile tracers and heli um reflects the different processes controlling their isotopic charact eristics: crust-mantle interactions, alone, for Sr and Pb but for heli um the effects of pre-eruptive degassing and possibly magma aging are possibly superimposed. The presence of mantle helium in the Puna regio n, and to a lesser extent in the Salta Basin, gives an across-arc pers pective to the helium isotope distribution and shows mantle melting to occur significantly to the east of the active arc: this is most proba bly a consequence of lithospheric delamination. The Precordillera hot spring water has the only pure radiogenic helium signal of the entire sample suite and thus marks the western limit of asthenospheric mantle under the Andes.