Magnesium transport in olivine mantle: new insights from miniaturized study of volume and grain boundary diffusion in Mg2SiO4 bi-crystals

We report experimental measurements of volume and grain boundary diffusion of 26Mg in Mg2SiO4 bi-crystals at asthenosphere temperatures as a ground reference for olivine. By analysis of literature and combination with previous data, we provide Arrhenius laws D = D0 exp(− E/RT) at ambient pressure for volume diffusion of Mg in Mg2SiO4 in the intrinsic regime along the three crystallographic axes as well as grain boundary diffusion. Parameters for average volume diffusion, calculated as geometrical mean of the three crystallographic axes, are $$D_{0} = 10^{-2.12\,\pm\, 0.91}\,{\text{m}^{2}/{\text{s}}}\, {\text{and}}\, E = 443 \pm 42 {\text{kJ}}/{\text{mol}}.$$ Parameters for average grain boundary diffusion in aggregate are $$D_{0} = 10^{-1.16\, \pm\, 0.49}\,{\text{m}^{2}/{\text{s}}}\, {\text{and}}\, E = 359 \pm 14 {\text{kJ}}/{\text{mol}}.$$ In the asthenosphere, the decrease of both volume and grain boundary diffusion coefficients as a function of pressure should remain negligible up to 1 GPa, and ~ 1 log unit at 10 GPa, while the increase in iron- and hydrogen-bearing olivine should range from ~ 1 to 2 log unit. The equilibration of Mg in olivine grains can be considered instantaneous with respect to geological timescales in the asthenosphere. However, the transport of Mg remains below km-scale even after 1 Gy at 1600 °C of volume or grain boundary diffusion. Long-range transport of major elements in the upper mantle is not possible by solid-state diffusion. Equilibration of long-range heterogeneities and large mass transport is rather controlled by diffusion in intergranular fluid or melt phase, liquid percolation and mantle convection.