HELIUM, NEON AND ARGON ISOTOPE SYSTEMATICS IN KERGUELEN ULTRAMAFIC XENOLITHS: IMPLICATIONS FOR MANTLE SOURCE SIGNATURES

Abstract

New noble gas data are presented for ultramafic mantle xenoliths, occurring in 10-20 Ma old volcanics from the Kerguelen Archipelago. Fusion results indicate that neon is isotopically primitive, whereas helium shows considerable isotopic variations, tending towards more radiogenic values. These fusion results of mantle xenoliths are the first report of a difference between helium and neon isotope systematics, which in most other studies show correlated isotope systematics. Helium isotopic compositions obtained by crushing the same xenoliths yield values which are identical to or slightly lower than the theoretical values predicted by the neon data. Comparison of helium with mantle neon and argon reveals that the samples have experienced considerable helium loss, making the helium isotope systematics of the xenoliths prone to secondary disturbances. Caution is clearly required when interpreting helium isotopic compositions of mantle-derived samples, particularly in cases where helium abundances are low (also cf.); combined noble gas studies provide a means of evaluating the helium isotopic signature in such cases. Our neon fusion results, together with helium and neon obtained by crushing, preserve evidence for a primitive mantle component in these Kerguelen xenoliths. This primitive component in the Kerguelen xenoliths may have been metasomatically introduced from the Kerguelen plume into upper mantle peridotite by CO2-rich, silica-rich melts, now represented by cogenetic melt and fluid inclusions in the xenoliths.