Beryllium diffusion in olivine: A new tool to investigate timescales of magmatic processes

The diffusion of beryllium (Be) in pure synthetic forsterite (fo₁₀₀) and San Carlos olivine (fo₉₀) was studied between 950–1475 °C at atmospheric pressure, as a function of silica activity (aSiO₂), crystallographic orientation, oxygen fugacity (fO₂, for diffusion in San Carlos olivine) and water fugacity (fH₂O, at 1.15 GPa pressure (P)). The diffusivity of Be in olivine is faster than that of Mg²⁺ or Fe²⁺ but slower than that of H⁺, and appears to be insensitive to aSiO₂, fH₂O and P, but is highly anisotropic, with diffusivities described by:[001]:log⁡D₀=−5.82 (±0.15),E_a=227.6 (±4.1) kJmol⁻¹;[010]:log⁡D₀=−4.64 (±0.38),E_a=285.8 (±11.6) kJmol⁻¹;[100]:log⁡D₀=−4.20 (±0.27),E_a=326.1 (±7.9) kJmol⁻¹. Diffusion of Be²⁺ in natural San Carlos olivine was determined at 1160 to 1350 °C and was found to be similar to that in forsterite. The exception was one experiment in natural olivine in relatively oxidising conditions, where diffusion was slightly faster than its lower fO₂ or pure forsterite counterparts. The equilibrium solubility of Be in forsterite in equilibrium with BeO (bromellite) and MgO (periclase) is lower than when in equilibrium with BeO and Mg₂Si₂O₆ ((proto)enstatite). This shows that Be²⁺ substitutes into olivine forming a defect with the stoichiometry Be₂SiO₄. The sensitivity of Be diffusion in olivine to only temperature and crystal orientation means that only these two factors need to be known to extract timescales from natural diffusion profiles (our preliminary experiments show that the dependence of diffusion on fO₂ is minor or negligible in geologically relevant conditions). This represents a considerable advantage over using diffusion profiles of other cations (e.g., Fe²⁺–Mg²⁺, Ni²⁺ or Ca²⁺), where all of the above mentioned variables must be constrained before accurate timescale determinations can be made. Two examples of the potential for using Be diffusion profiles to determine timescales in natural olivine xenocrysts are presented.