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Diffusional isotope fractionation has been widely used to explain lithium (Li) isotope variations in minerals and rocks. Isotopic mass dependence of Li diffusion can be empirically expressed as , where is the diffusivity of a Li isotope. The knowledge about temperature and compositional dependence of the factor which is essential for understanding diffusion profiles and mechanisms remains unclear. Based on the potential energy and interatomic forces generated by deep neural networks trained with ab initio data, we performed deep potential molecular dynamics (DPMD) simulations of several Li pseudo-isotopes (with mass = 2, 7, 21, 42 g/mol) in albite, hydrous albite, and model basalt melts to evaluate the factor. Our calculated diffusivities for 7Li in albite and model basalt melts at 1800 K compare well with experimental results. We found that in albite melt decreases from at 4000 K to at 1800 K. The presence of water appears to slightly weaken the temperature dependence of , with decreasing from to in hydrous albite melt. The calculated in model basalt melt takes much smaller values, decreasing from at 4000 K to at 1800 K. Our prediction of in albite and hydrous albite melts is in good agreement with experimental data. More importantly, our results suggest that Li isotope diffusion in silicate melts is strongly dependent on melt composition. The temperature and compositional effects on can be qualitatively explained in terms of ionic porosity and the coupled relationship between Li diffusion and the mobility of the silicate melt network. Two types of diffusion experiments are suggested to test our predicted temperature and compositional dependence of . This study shows that DPMD is a promising tool to simulate the diffusion of elements and isotopes in silicate melts. 

Diffusional isotope fractionation occurs in geochemical processes (such as magma mixing, bubble growth, and crystal growth), even at magmatic temperatures. Isotopic mass dependence of diffusion is commonly expressed as Di Dj ¼ mj mi
 b , where Di and Dj are diffusion coefficients of two isotopes whose masses are mi and mj. How the dimensionless empirical parameter b depends on temperature, pressure, and composition remains poorly constrained. Here, we conducted a series of first-principles molecular dynamics simulations to evaluate the b factor of Mg isotopes in MgSiO3 and Mg2SiO4 melts using pseudo-isotope method. In particular, we considered interactions between Mg isotopes by simultaneously putting pseudo-mass and normalmass Mg atoms in a simulation supercell. The calculated b for Mg isotopes decreases linearly with decreasing temperature at zero pressure, from 0:158  0:004 at 4000 K to 0:121  0:017 at 2200 K for MgSiO3 melt and from 0:150  0:004 at 4000 K to 0:101  0:012 at 2200 K for Mg2SiO4 melt. Moreover, our simulations of compressed Mg2SiO4 melt along the 3000 K isotherm show that the b value decreases linearly from 0:130  0:006 at 0 GPa to 0:060  0:011 at 17 GPa. Based on our diffusivity results, the empirically established positive correlation between b and solvent-normalized diffusivity (Di/DSi) seems to be applicable only at constant temperatures or in narrow temperature ranges. Analysis of atomistic mechanisms suggests that the calculated b values are inversely correlated with force constants of Mg at a given temperature or pressure. Good agreement between our first principles results with available experimental data suggests that interactions between isotopes of major elements must be considered in calculating b for major elements in silicate melts. Also, we discuss diffusion-controlled crystal growth by considering our calculated b values. 

Silicate melts have served as transport agents in the chemical and thermal evolution of Earth. Molecular dynamics simulations based on a deep neural network potential trained byab initiodata show that the viscosity of MgSiO₃melt decreases with increasing pressure at low pressures (up to ∼6 GPa) before it starts to increase with further compression. The melt electrical conductivity also behaves anomalously; first increasing and then decreasing with pressure. The melt accumulation implied by the viscosity turnover at ∼23 GPa along mantle liquidus offers an explanation for the low‐velocity zone at the 660‐km discontinuity. The increase in electrical conductivity up to ∼50 GPa may contribute to the steep rise of Earth's electrical conductivity profiles derived from magnetotelluric observations. Our results also suggest that small fraction of melts could give rise to detectable bulk conductivity in deeper parts of the mantle.