A unified model for terrestrial rare gases

Abstract

A steady state upper mantle model for the rare gases has been constructed which explains the available observational data of mantle He, Ne, Ar, and Xe isotope compositions and provides specific predictions regarding the rare gas isotopic compositions of the lower mantle, subduction of rare gases, and mantle rare gas concentrations. The model incorporates two mantle reservoirs; an undegassed lower mantle (P) and a highly degassed upper mantle (D). Chemical species are transferred into D within mass flows from P at plumes and from the atmosphere by subduction. Rare gases in D are derived from mixing of these inflows with radiogenic nuclides produced in situ. The upper mantle is degassed at mid‐ocean ridges and hotspots. Flows of each isotope into D are balanced by flows out of D, so that upper mantle concentrations are in steady state. Rare gases with distinct ^3He/^4He, ^(20)Ne/^(22)Ne, ^(129)Xe/^(130)Xe, and ^(136)Xe/^(130)Xe are stored in P and are transferred into D. In P, isotopic shifts are due to decay of U‐ and Th‐ decay series nuclides, ^(40)K, ^(129)I, and ^(244)Pu over 4.5 Ga. Radiogenic ^(136)Xe in P is dominantly from ^(244)Pu. In the well‐outgassed D reservoir, additional isotopic shifts are due to decay of U‐ and Th‐ series nuclides and ^(40)K over a residence time of ∼1.4 Ga. Since ^4He, ^(21)Ne, ^(40)Ar, and ^(136)Xe are produced in proportions fixed by nuclear parameters, the resulting isotopic shifts are correlated. The model predicts that the shift in ^(21)Ne/^(22)Ne in D relative to that in P is the same as that for ^4He/^3He in the respective mantle reservoirs. This is compatible with the available data for MORB and hotspots. The minimum ^(40)Ar/^(36)Ar, ^(129)Xe/^(130)Xe, and ^(136)Xe/^(130)Xe ratios in P are found to be substantially greater than the atmospheric ratios. The range in Ne, Ar, and Xe isotopes measured in MORB are interpreted as reflecting contamination of mantle rare gases by variable proportions of atmospheric rare gases. Subduction is not significant for He and Ne, but may account for a substantial fraction of Ar and Xe in D. The rare gas relative abundances in P are different than that of the atmosphere and are consistent with possible early solar system reservoirs as found in meteorites. The ^3He/^(22)Ne and ^(20)Ne/^(36)Ar ratios of P are within the range for meteorites with ‘solar’ Ne isotope compositions. The ^(130)Xe/^(36)Ar ratio of the lower mantle is greater than that of the atmosphere and may be as high as the ratio found for meteoritic ‘planetary’ rare gases. In the model, atmospheric rare gas isotope compositions are distinct from those of the mantle. If the Earth originally had uniform concentrations of rare gases, degassing of the upper mantle would have provided only a small proportion of the nonradiogenic rare gases presently in the atmosphere. The remainder was derived from late‐accreted material with higher concentrations of rare gases. However, radiogenic ^(129)Xe and ^(136)Xe abundances imply a substantial loss of rare gases up to ∼10^8 years after meteorite formation either from the early Earth or from late‐accreting protoplanetary materials. Rare gases must have been lost during accretion and the moon‐forming impact, so that nonradiogenic rare gases in the atmosphere must have been supplied by subsequently accreted material with nonradiogenic Xe, possibly from comets. Fractionation of atmospheric Xe isotopes relative to other early solar system components occurred either on late‐accreting materials or during loss from the Earth.