Diffusion of phosphorus in olivine and molten basalt

Abstract

The diffusivity of phosphorus in San Carlos olivine (SCO) was measured at near-atmospheric pressure and 650-850 °C by in-diffusion of P from a surface powder source consisting of pre-reacted SCO and AlPO4. The experiments were conducted in evacuated silica-glass ampoules at oxygen fugacities fixed by solid-state buffers, generally Ni-NiO but also including two experiments buffered at wüstite-magnetite. Phosphorus uptake profiles were characterized by Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA). The temperature dependence of P diffusion in SCO conforms to the expected Arrhenius relation D = D0exp(-Ea/RT), where the constants are as follows: log(D0, m2/s) = -10.06 ± 0.80 and Ea = 229 ± 16 kJ/mol. These values characterize P as a relatively slow diffuser in olivine-slower by about an order of magnitude than Cr and Ca at basalt near-liquidus temperatures-but substantially faster than Si.