Potassium distribution and isotope composition in the lithospheric mantle in relation to global Earth's reservoirs

Recent analytical advances have provided means to measure potassium (K) isotopes in various terrestrial materials, but little is known about K distribution and stable isotope composition in the lithospheric mantle because of: (a) common low K abundances, (b) potential contamination and alteration, (c) diversity of mantle rocks and minerals hosting K in different tectonic settings. We report K abundances and delta K-41 values for well-studied whole-rock (WR) mantle xenoliths (spinel and garnet peridotites and pyroxenites) from mobile belts, a craton, a subduction zone, as well as for K-rich phases (mica, amphibole, silicate glass) and xenolith-bearing volcanic materials (67 samples). The xenolith materials show extremely broad ranges of K content (7 mu g/g to 6.6 wt.%) and delta K-41 (-2.77 parts per thousand to 0.62 parts per thousand). They contrast with the narrow delta K-41 range for host volcanic materials (-0.53 parts per thousand to -0.27 parts per thousand) and literature data on oceanic basalts (melting products of upwelling asthenosphere: -0.43 +/- 0.17 parts per thousand, 2sd). Amphibole-bearing subduction zone peridotites show the highest WR delta K-41 values (0.40-0.62 parts per thousand) likely inherited from fluids released into the mantle wedge from subducted oceanic crust. All other WR samples yield negative delta K-41: -0.06 parts per thousand to -2.77 parts per thousand for peridotites and -0.17 parts per thousand to -0.52 parts per thousand for pyroxenites. The delta K-41 in K-rich mantle phases range from positive values (0.16-0.57 parts per thousand) for phlogopite in strongly metasomatized peridotites to negative values (-0.27 to -0.94 parts per thousand) for phlogopite, amphibole and glass pockets from other samples, which cannot be explained by equilibrium inter-mineral fractionation inferred from ab initio calculations. We attribute the broad delta K-41 variations to (a) isotope fractionation during fluid-rock interaction in the mantle, and (b) distinct sources of K-bearing fluids. Kinetic isotope fractionation during fluid percolation and diffusion is inferred for composite xenoliths (phlogopite and pyroxenite veins in peridotites) that have lower delta K-41 in the hosts than in the veins due to slower migration of K-41 than K-39 from the veins (former fluid channels) to host mantle. Overall, the K isotope fractionation in the lithospheric mantle appears to be greater than for magmatic fractionation in the crust. The average delta K-41 of normal off-craton continental lithospheric mantle calculated from the least altered fertile and lightly metasomatized lherzolites is -0.57 +/- 0.28 parts per thousand (2sd). This value is within error (though a little lower) of estimates for both continental crust and MORB and OIB mantle sources indicating that these major silicate Earth reservoirs have similar bulk delta K-41 values, apparently due to low or negligible K isotopic fractionation during their formation by magmatic differentiation and melting. By contrast, K isotopes in modern and fossil subduction zones are fractionated via fluid-related equilibrium and kinetic processes. (C) 2021 Elsevier Ltd. All rights reserved.