HELIUM, NEON, AND ARGON SYSTEMATICS OF THE EUROPEAN SUBCONTINENTAL MANTLE - IMPLICATIONS FOR ITS GEOCHEMICAL EVOLUTION

In this study we present a comprehensive noble gas study of mantle xen oliths from various European Cenozoic volcanic provinces. The main bod y of samples is from the Massif Central, France, and the Eifel, German y. Smaller subsets of samples are from Spitsbergen and the Graz Basin, Austria. In all the helium; neon, and argon isotopic abundances of a total of forty-five mantle xenoliths, phenocrysts, and xenocrysts were determined. The He-3/He-4-ratios within each volcanic province are ve ry uniform, irrespective of the diverse lithologies and P-T conditions which are represented by our sample suite. Mean He-3/He-4 ratios of t he Massif Central, Eifel, Spitsbergen, and Kapfenstein are 6.53 +/- .2 5, 6.03 +/- .14, 6.65 +/- .25, and 6.1 +/- .7 (1 sigma) times the atmo spheric ratio (R(a)), respectively. The strontium and neodymium isotop ic composition of some of the cpx-separates are highly variable and th erefore in contrast to the uniform He signature. We thus conclude that He and probably also the other noble gases in the xenoliths are effec tively decoupled from the non-volatile elements. Therefore, the He sig nature that is preserved in the xenoliths is actually that of their ho st magmas. Published strontium, neodymium, and lead isotope data of th e unevolved host magmas of the xenoliths correlate well with our xenol ith He data. The position of the fields of the investigated volcanic p rovinces in He-Sr, He-Nd, and He-Pb variation diagrams depict ternary mixtures between DMM-EM-HIMU endmembers as the source the host magmas and their volatiles. The neon isotopic composition of the gases releas ed from the xenoliths is in most cases atmospheric and probably reflec ts atmospheric contamination; only a few samples reveal indications fo r MORE-type Ne or evidence of mass-fractionation. The Ar-40/Ar-36-rati os of the xenoliths are mostly radiogenic, with the highest ratio bein g 17,000. However, all samples have suffered a certain degree of atmos pheric contamination. We calculate that the uncontaminated Ar-40/Ar-36 -ratio Of the European subcontinental mantle is greater than or equal to 30,000. The He-4/Ar-40 ratios have been fractionated, since with va lues around 0.25 they are below both the current production ratio and the MORE ratio. They cannot be explained by atmospheric contamination, as the samples with high Ar-40/Ar-36 ratios also have low He-4/Ar-40 ratios. Helium and Ar were probably fractionated during initial closed system degassing of the host magma. We calculate the original unfract ionated He-4/Ar-40-ratios of the Massif Central and Eifel magma source which are 2.56 +/- .18 and 2.71 +/- .17, respectively. The noble gas systematics are compatible with the notion that addition of crustal ma terial has modified a former MORE-source signature. We use our mean He -3/He-4 ratios and reconstructed He-4/Ar-40-ratios to model the time a nd degree of contamination of a MORE-like source with crustal material . Combining our model with evidence from published oxygen, strontium, neodymium, and lead isotope data we judge that the source of the Massi f Central and Eifel magmas became enriched in K, U, and Th shortly pri or or during the Variscan orogeny (pre-collisional/collisional). In th is case the enrichment in these elements is therefore most probably re lated to subduction of crustal material in a destructive continental m argin setting.