Ionov, D.A., Doucet, L.S., Pogge von Strandmann, P.A.E., Golovin, A.V., Korsakov, A.V., Links between deformation, chemical enrichments and Li-isotope compositions in the lithospheric mantle of the central Siberian craton, 2017, Chemical Geology, 475: 105–121.
We report the concentrations ([Li]) and isotopic compositions of Li in mineral separates and bulk rocks obtained by MC-ICPMS for 14 previously studied garnet and spinel peridotite xenoliths from the Udachnaya kimberlite in the central Siberian craton as well as major and trace element compositions for a new suite of 13 deformed garnet peridotites. The deformed Udachnaya peridotites occur at > 5 GPa; they are metasomatized residues of melt extraction, which as a group experienced greater modal and chemical enrichments than coarse peridotites. We identify two sub-groups of the deformed peridotites: (a) mainly cryptically metasomatized (similar to coarse peridotites) with relatively low modal cpx (< 6%) and garnet (< 7%), low Ca and high Mg#, sinusoidal REE patterns in garnet, and chemically unequilibrated garnet and cpx; (b) modally metasomatized with more cpx and garnet, higher Ca, Fe and Ti, and equilibrated garnet and cpx. The chemical enrichments are not proportional to deformation degrees. The deformation in the lower lithosphere is caused by a combination of localized stress, heating and fluid ingress from the pathways of ascending proto-kimberlite melts, with metasomatic media evolving due to reactions with wall rocks. Mg-rich olivine in spinel and coarse garnet Udachnaya peridotites has 1.2–1.9 ppm Li and δ7Li of 1.2–5.0‰, i.e. close to olivine in equilibrated fertile to depleted off-craton mantle peridotites from literature data, whereas olivine from the deformed peridotites has higher [Li] (2.4–7.5 ppm) and a broader range of δ7Li (1.8–11.6‰), which we attribute to pre-eruption metasomatism. [Li] in opx is higher than in coexisting olivine while Δ7LiOl-Opx (δ7LiOl − δ7LiOpx) ranges from −6.6 to 7.8‰, indicating dis- equilibrium inter-mineral [Li] and Li-isotope partitioning. We relate these Li systematics to interaction of li- thospheric peridotites with fluids or melts that are either precursors of kimberlite magmatism or products of their fractionation and/or reaction with host mantle. The melts rich in Na and carbonates infiltrated, heated and weakened wall-rock peridotites to facilitate their deformation as well as produce high [Li] and variable, but mainly high, δ7Li in olivine. The carbonate-rich melts preferentially reacted with the opx without achieving inter-mineral equilibrium because opx is consumed by such melts, and because of small volumes and uneven distribution of the metasomatic media, as well as short time spans between the melt infiltration and the capture of the wall-rock fragments by incoming portions of ascending kimberlite magma as xenoliths. Trapped inter- stitial liquid solidified as cryptic components responsible for high [Li] and the lack of δ7Li balance between olivine and opx, and bulk rocks. Unaltered δ26Mg values (0.20–0.26‰) measured in several olivine separates show no effects of the metasomatism on Mg-isotopes, apparently due to high Mg in the peridotites.
Hin, R.C., Coath, C.D., Carter, P.J., Nimmo, F., Lai, Y-J., Pogge von Strandmann, P.A.E., Willbold, M., Leinhardt, Z.M., Walter, M.J., Elliott, T., Magnesium isotope evidence that accretional vapour loss shapes planetary compositions, 2017, Nature, 549: 511–515.
It has long been recognized that Earth and other differentiated planetary bodies are chemically fractionated compared to primitive, chondritic meteorites and, by inference, the primordial disk from which they formed. However, it is not known whether the notable volatile depletions of planetary bodies are a consequence of accretion1 or inherited from prior nebular fractionation2. The isotopic compositions of the main constituents of planetary bodies can contribute to this debate3–6. Here we develop an analytical approach that corrects a major cause of measurement inaccuracy inherent in conventional methods, and show that all differentiated bodies have isotopically heavier magnesium compositions than chondritic meteorites. We argue that possible magnesium isotope fractionation during condensation of the solar nebula, core formation and silicate differentiation cannot explain these observations. However, isotopic fractionation between liquid and vapour, followed by vapour escape during accretionary growth of planetesimals, generates appropriate residual compositions. Our modelling implies that the isotopic compositions of magnesium, silicon and iron, and the relative abundances of the major elements of Earth and other planetary bodies, are a natural consequence of substantial (about 40 per cent by mass) vapour loss from growing planetesimals by this mechanism.
Perez-Fernandez, A., Berninger, U.-N., Mavromatis, V., Pogge von Strandmann, P.A.E., Oelkers, E.H., Ca and Mg isotope fractionation during the stoichiometric dissolution of dolomite at temperatures from 51 to 126° C and 5 bars CO2 pressure, 2017, Chemical Geology, 467: 76–88.
Natural polycrystalline hydrothermal Sainte Colombe dolomite was dissolved in stirred titanium closed system reactors in aqueous 0.1 mol/kg NaCl solutions at 51, 75, 121, and 126 °C and a pressure of 5 bars CO2. In total, 52, 27, 16, and 12%, respectively, of the dolomite placed in the reactors dissolved into the fluid phase during these experiments. Each experiment lasted from 12 to 47 days and the fluid phase in each evolved towards, but did not exceed, ordered dolomite equilibrium at a pH of 5.9 ± 0.3. All aqueous reactive fluids were under- saturated with respect to all potential secondary phases including calcite and magnesite. The reactive fluid compositions at the end of the experiments had a molar Ca/Mg ratio equal to that of the dissolving dolomite, and the dolomite recovered after the experiments contained only pure dolomite as verified by scanning electron microscopy. The Ca and Mg isotopic ratios of the reactive fluids remained within uncertainty equal to that of the dissolving dolomite in the experiments performed at 51 and 75 °C. In contrast, the Ca isotopic composition of the reactive fluid in the experiment performed at 121 and 126 °C was significantly greater such that Δ44/42Casolid- fluid = −0.6 ± 0.1‰, whereas that of Mg was within uncertainty equal to that of the dissolving mineral. The equilibrium fractionation factors for both divalent cations favor the incorporation of isotopically light metals into the dolomite structure. Our results at 121 and 126 °C, therefore, are consistent with the one-way transfer of Mg from dolomite to the fluid but the two-way transfer of Ca from and to dolomite as equilibrium is approached during its stoichiometric dissolution. The lack of Mg returning to the dolomite structure at these conditions is attributed to the slow dehydration kinetics of aqueous Mg. As more than 12% of the dolomite dissolved during the 121 and 126 °C closed system experiments, our observations indicate a significant change in the Ca isotopic signature of the dolomite during its stoichiometric dissolution. Moreover, as there is no visual evidence for dolomite recrystallization during this experiment, it seems likely that the resetting of Ca isotopic signatures of carbonate minerals can be readily overlooked in the interpretation of natural systems.