Studying diffusion of hydrogen in nominally anhydrous minerals (NAMs), like clinopyroxene, at low temperatures is a challenging task due to experimental and analytical difficulties. We applied a combination of hydrogen implantation to produce concentration gradients in natural diopside crystals with Nuclear Resonance Reaction Analysis (NRRA) measurements of nanoscale diffusion profiles. Thereby, we were able to conduct experiments at temperatures between 195 – 400 °C. Obtained diffusion rates show a consistent Arrhenius relation Ｄн = 5.47 (± 13.98) ·10⁻⁸ · exp (-115.64 (±11.5) kJ mol⁻¹/RT) m²s⁻¹. Notably, our results lie well within the range of extrapolations from high temperature experiments (≥ 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 geotherms (oceanic and continental) from 600 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 P-T-𝘧O₂, that control the concentration gradient at the crystal’s rim. The results from this model allow an assessment when re-equilibration in dependence of crystal size and cooling rate must be considered. Fast ascent (e.g., kimberlitic melt) preserves initial hydrogen contents even in 0.5 mm size clinopyroxene crystals. However, dwelling at low temperatures (e.g., 300 °C) for several thousands of years (e.g., serpentinization) leads to extensive re-equilibration in 2 mm crystals.