Grain boundary diffusion of Si, Mg, and 0 in enstatite reaction rims: a SIMS study using isotopically doped reactants

Diffusion-controlled growth rates of polycrystalline enstatite reaction rims between forsterite and quartz were determined at 1,000 degreesC and 1 GPa in presence of traces of water. Iron-free, pure synthetic forsterite with normal oxygen and silicon isotopic compositions and quartz extremely enriched in O-18 and Si-29 were used as reactants. The relative mobility of O-18 and Si-29 in reactants and rims were determined by SIMS step scanning. The morphology of the rim shows that enstatite grows by a direct replacement of forsterite. Rim growth is modelled within a mass-conserving reference frame that implies advancement of reaction fronts from the initial forsterite-quartz interface in both directions. The isotopic compositions at the two reaction interfaces are controlled by the partial reactions Mg2SiO4 = 0.5 Mg2Si2O6 + MgO at the forsterite-enstatite, and MgO + SiO2 = 0.5 Mg2Si2O6 at the enstatite-quartz interface, implying that grain boundary diffusion of MgO is rate-controlling. Isotopic profiles show no silicon exchange across the propagating reaction interfaces. This propagation, controlled by MgO diffusion, is faster than the homogenisation of Si by self-diffusion behind the advancing fronts. From this, and using D-Si,En(Vol) at dry conditions from the literature, results a D-Si,D-En'delta value of 3 x 10(-24) m(3) s(-1) at 1,000 degreesC. The isotopic profiles for oxygen are more complex. They are interpreted as an interplay between the propagation of the interfaces, the homogenisation of the isotope concentrations by grain boundary self-diffusion of O within the rim, and the isotope exchange across the enstatite-quartz interface, which was open to O-18 influx from quartz. Because of overlapping diffusion processes, boundary conditions are unstable and D(Ox,En)delta cannot be quantified. Using measured rim growth rates, the grain boundary diffusivity sivity D(MgO)delta of MgO in iron-free enstatite is 8 x 10(-22) m(3) s(-1) at 1,000 degreesC and 1 GPa. Experiments with San Carlos olivine (fo(92)) as reactant reveal lower rates by a factor of about 4. Our results show that isotope tracers in rim growth experiments allow identification of the actual interface reactions, recognition of the rate-controlling component and further calculation of D delta values for specific components.