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.