Variation in noble gas isotopic composition of gas samples from the Aegean arc, Greece

In contrast to most other arcs with oceanic plate subduction, the Aegean arc is characterized by continent-continent subduction. Noble gas abundances and isotopic compositions of 45 gas samples have been determined from 6 volcanoes along the arc, 2 islands in the back-arc region and 7 sites in the surrounding areas. The He-3/He-4 ratios of the samples ranged from 0.027R(A) to 6.2R(A) (R-A denotes the atmospheric He-3/He-4 ratio of 1.4 x 10(-6)), demonstrating that even the maximum He-3/He-4 ratio in the region is significantly lower than the maximum ratios of most oceanic subduction systems, which are equal to the MORB value of 8 +/- 1 R-A. Regional variations in the He-3/He-4 ratio were observed both along and across the arc. The maximum He-3/He-4 ratio was obtained from Nisyros volcano located in the eastern end of the arc, and the ratio decreased westward possibly reflecting the difference in potential degree of crustal assimilation or the present magmatic activity in each volcano. Across the volcanic arc, the He-3/He-4 ratio decreased with an increasing distance from the arc front, reaching a low ratio of 0.063R(A) in Macedonia, which suggested a major contribution of radiogenic helium derived from the continental crust. At Nisyros, a temporal increase in He-3/He-4 ratio due to ascending subsurface magma was observed after the seismic crisis of 1995-1998 and mantle neon was possibly detected. The maximum He-3/He-4 ratio (6.2R(A)) in the Aegean region, which is significantly lower than the MORB value, is not probably due to crustal assimilation at shallow depth or addition of slab-derived helium to MORB-like mantle wedge, but inherent characteristics of the subcontinental lithospheric mantle (SCLM) beneath the Aegean arc. (C) 2004 Elsevier B.V. All rights reserved.