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The growing interest in Li diffusion as a tool to determine timescales of short-time magmatic events, such as magma ascent during eruption, increases the necessity to better understand Li diffusion in common mineral phases. In this context, well-constrained diffusion coefficients and understanding of kinetic processes specific to mineral phases are of crucial importance. To gain further insight especially into the kinetic processes in plagioclase, we investigated the diffusion of Li between natural An61 plagioclase crystals and synthetic glasses of An80 plagioclase composition. Experiments were conducted at 200 MPa in rapid-heat/rapid-quench cold-seal pressure vessels (RH/RQ CSPVs) and internally heated pressure vessels (IHPVs) at temperatures between 606 and 1114 °C. Concentration and isotope profiles of Li were measured using femtosecond laser ablation multicollector inductively coupled plasma mass spectrometry (fs-LA-MC-ICP-MS). We adopted a multispecies diffusion model and specified boundary conditions for plagioclase of labradoritic composition. Using this model, we were able to distinguish between an interstitial (DLii) and a vacancy process (DLiA), with the interstitial process being 0.2–1 orders of magnitude faster than the vacancy process, depending on temperature. D Li i = 10 - 3.76 ± 0.58 exp⁡ - 180.0 ± 12.0 kJ mol - 1 R T m 2 s - 1 D Li A = 10 - 5.53 ± 0.16 exp⁡ - 151.7 ± 3.2 kJ mol - 1 R T m 2 s - 1 Our data indicate charge compensation of Li by Na in both the crystal and the glass. Chemical Li diffusion coefficients in An80 glass are up to 3 orders of magnitude slower compared to Li tracer diffusion in silicate and aluminosilicate glasses and melts, which is attributed to slow Na diffusion at high An content. Our results for chemical diffusion of Li in plagioclase crystals are 1.5–2 orders of magnitude slower than Li tracer diffusion in An- and Ab-rich plagioclase determined in previous studies. This indicates that earlier studies on natural intermediate plagioclase compositions have underestimated timescales by up to 2 orders of magnitude. For accurate determination of timescales from Li diffusion in plagioclase we suggest further exploring the role of Na and a possible dependence on An content. [ABSTRACT FROM AUTHOR]

In this study, we investigate the diffusion of Li and its stable isotopes (6 Li and 7 Li) in flux-rich (1.8 % Li 2 O, 2.6 % B 2 O 3 , 2.3 % P 2 O 5 and 3 % F) pegmatitic melts in order to contribute to the understanding of Li enrichment in such systems. Two glasses were synthesized with a model pegmatitic composition, one of which is highly enriched in Li (> 1 wt %, PEG2-blue) and the other one essentially Li-free (PEG2-Li-free). Diffusion couple experiments were performed to determine the chemical diffusivity of Li in dry pegmatitic melts. Experiments were conducted using rapid-heat and rapid-quench cold-seal pressure vessels in a temperature range of 650–940 ∘ C at 100 MPa with Ar as the pressure medium. We observed rapidly formed diffusion profiles, driven by an interdiffusive exchange of the monovalent alkalis Li and Na, while the other elements are immobile on the timescale of experiments (1–30 min). From these experiments, activation energies for Li–Na interdiffusion were determined as 99 ± 7 kJ mol -1 with a pre-exponential factor of log D0 = - 5.05 ± 0.33 (D0 in m 2 s -1). Li and Na partitioning between the stronger depolymerized PEG2-blue and the less depolymerized PEG2-Li-free leads to a concentration jump at the interface; i.e. Na is enriched in the more depolymerized PEG2-blue. Li–Na interdiffusion coefficients in the studied melt composition are in a similar range as Li and Na tracer diffusivities in other dry aluminosilicate melts, confirming little to no effect of aluminosilicate melt composition on Li diffusivity. Thus, added fluxes do not enhance the Li diffusivity in the same way as observed for H 2 O (Holycross et al., 2018; Spallanzani et al., 2022). Using melt viscosity as a proxy for the polymerization of the melt shows that water has a stronger potential to depolymerize a melt compared to other fluxing elements. Faster diffusion of 6 Li compared to 7 Li leads to a strong Li isotope fractionation along the diffusion profile, resulting in δ7 Li as low as - 80 ‰ relative to the diffusion-unaffected regions. This diffusive isotope fractionation can be quantified with an empirical isotope fractionation factor (β) of 0.20 ± 0.04, similar to previously observed β values for Li diffusion in melts. This suggests in accordance with previously published data that a β value of ca. 0.2 seems to be universally applicable to diffusive Li isotope fractionation in aluminosilicate melts. [ABSTRACT FROM AUTHOR]

Redox processes are ubiquitous in Earth science, and redox transitions often lead to large fractionations of the stable isotopes of many transition metals such as copper. To get insights into the mechanisms of isotope fractionations induced by electrochemical processes, we examine the behavior of copper isotopes during the reduction reaction Cu.sup.2+ + 2e.sup.- = Cu.sup.0. All experiments have been conducted by applying a controlled current between the working electrode and the auxiliary electrode, i.e., the galvanostatic electrodeposition technique, in aqueous CuSO.sub.4 solutions. Controlling parameters were tested by varying electrolyte concentration (0.01-1 mol kg.sup.-1), stirring speed (0-500 rpm), current (0.1-0.5 A), time (35-600 s), and temperature (5-80 °C). In all cases, the plated Cu metal is enriched in the light isotope (.sup.63Cu) with respect to the solution. At room temperature, the Cu isotopic fractionation between the electroplated Cu and electrolyte is found to increase with electrolyte concentration and stirring speed, and to decrease with current and run duration. These trends can be interpreted by three competing processes: copper transport in the solution, kinetics of electrochemical reduction of copper ions and surface diffusion at the electrode, i.e., transport becomes important at low copper concentration, low stirring speed, high currents and large amount of copper precipitation. Copper isotope fractionation has a maximum near 35 °C, decreasing both towards higher and lower temperatures. In the temperature range of 35-80 °C, the dependence of temperature on isotope fractionation can be described by [DELTA]65CuCu0-Cu(II)aq=-0.27±0.04x106T-2+0.16±0.34,where â.sup.65Cu.sub.Cu(0)-Cu(II)aq (â°) represents the copper isotopic composition differences between the product (electroplated copper) and the reactant (electrolyte solution, CuSO.sub.4(aq)), and T is the temperature in K. At low temperature (down to 5 °C), a noticeable deviation from this trend suggests a change in the controlling mechanism, i.e., transport in the solution becomes important. Our findings are best explained by a two-step reduction process including reduction from Cu(II) to Cu(I) and a subsequent reduction of Cu(I) to Cu(0). The good agreement of our high-temperature data with the results from Ehrlich et al. (2004 (See CR33)), who used a different experimental approach to precipitate Cu(I) mineral from CuSO.sub.4 solution, implies that transformation of Cu(II) to Cu(I) dominates the isotope fractionation observed during electrochemical reduction of Cu(II) to Cu(0). These findings support that copper isotopes can be used as effective tracers of redox processes. They may have implications to processes in hydrothermal systems and the formation of ore deposits, e.g., volcanic-hosted massive sulfides, as well as to processes in near surface aquatic environment and related supergene processes., Author(s): Dongmei Qi [sup.1], Harald Behrens [sup.1], Marina Lazarov [sup.1], Stefan Weyer [sup.1] Author Affiliations: (Aff1) 0000 0001 2163 2777, grid.9122.8, Institute of Mineralogy, Leibniz Universität Hannover, , Callinstrasse 3, [...]