Noble gas behaviour and composition in the mantle: constraints from the Iceland Plume

Abstract

Current geochemical models envisage that noble gases in the upper mantle source region of Mid-Ocean Ridge Basalts (MORB) arise from three sources; an elementally unfractionated component, introduced by bulk transfer from the deeper mantle via plumes; a radiogenic/nucleogenic component arising from in-situ radiogenic decay of U, Th and K, and accumulated within the upper mantle over a residence time of approximately 1.4 Ga; a subducted atmospheric component, possibly restricted to the heavy noble gases. Our new data from Icelandic sub-glacial basaltic glasses have the highest 20Ne/ 22Ne so far reported in terrestrial samples, indistinguishable from the solar wind value of 13.8. Inferred 4He/ 3He, 21Ne/ 22Ne and 40Ar/ 36Ar ratios in the source are much lower than MORB values, reflecting the presence of a deep mantle component. Measured 22Ne/ 3He ratios are higher than upper mantle values, but correlate with 21Ne/ 4He, implying a recent localised enrichment of Ne in the magma. The implied 22Ne/ 3He ratio prior to this enrichment is identical to MORB estimates. This enrichment and the parallel enrichment in Ne/He appears to require a multi stage process during partial melting, in the magma chamber, and/or during eruption which fortuitously left the ratio of mantle He to Ar and Xe unchanged. 36Ar/ 3He ratios, estimated for the source region, are significantly higher than MORB values implying the introduction of atmospheric 36Ar to the magma at some stage. The samples exhibit only a small 129Xe excess, however, the implied 129Xe */ 3He is indistinguishable from the MORB value. Overall the elemental and isotopic ratios in the Iceland samples support mantle models which predict similar primordial noble gas ratios in the MORB and OIB source regions.