Hydrogen diffusion in clinopyroxene at low temperatures (195°C–400°C) and consequences for subsurface processes

Studying diffusion of hydrogen in nominally anhydrous minerals, like clinopyroxene, at low temperatures is a challenging task due to experimental and analytical difficulties. In this study, to overcome these problems we have produced H concentration gradients in single crystals of natural diopsidic clinopyroxene by ion implantation and measured the nanoscale profiles before and after diffusion anneals using Nuclear Resonance Reaction Analysis. These steps allowed us to conduct experiments at temperatures between 195°C and 400°C. Obtained diffusion rates show a consistent Arrhenius relation \(D_{H}\)= 5.47(\(\pm\)13.98) · \(10^{−8}\) · exp (−115.64(\(\pm\)11.5) kJ \(mol^{−1}\)/RT) \(m^{2}s^{−1}\). Notably, our results lie well within the range of extrapolations from high temperature experiments (\(\geq\)600°C) of previous studies. This implies that fast diffusion of hydrogen (compared to other elements) extends to low temperatures. We used these results in a non-isothermal diffusion model that simulates the ascent of crystals (0.5, 1.0, and 2.0 mm) along two representative \(\textit {P-T}\)-paths from 600°C to 100°C, to assess potential re-equilibration of H contents in clinopyroxene at low temperatures. Our model highlights the need to carefully consider boundary conditions, which are a function of \(\textit {P-T-f}\)\(O_{2}\), that control the concentration gradient at the crystal's rim. The results from this model help to assess, as a function of crystal size and cooling rate, when re-equilibration must be considered