Volatile-rich mantle fluids inferred from inclusions in diamond and mantle xenoliths

Abstract

THE⁴⁰Ar—³⁹Ar technique has been used to unscramble mixtures of radiogenic, parentless (excess) and atmospheric argon components in (hydrothermal) fluid inclusions through correlations with K, Cl and ³⁶Ar respectively¹. These procedures are also applicable to mantle-derived argon isotopes in diamond², previously reported as having anomalously high (6 Gyr) K-Ar ages³. Here we describe ⁴⁰Ar—³⁹Ar measurements on coats and cores of 'coated stones' similar to those analysed for trace elements by Navon et al.⁴, and on fluid inclusions in olivine from an East African mantle xenolith with mid-ocean-ridge basalt (MORB) affinities⁵. Our results prove conclusively that ⁴⁰Ar in the coats of the diamonds, and in the mantle xenolith, is present in a widespread chlorine-rich component. These data, in conjunction with those of Navon et al.⁴, imply the existence of H₂O/CO₂-rich phases with ⁴⁰Ar/Cl ratios which are remarkably uniform over large distances, and which show enrichments of these two incompatible elements by almost four orders of magnitude relative to bulk upper-mantle values. These extreme enrichments imply that the fluid phase must be present at no more than one or two parts in 10⁴ in the region from which the argon and chlorine are extracted.