Fe–Mg interdiffusion rates in clinopyroxene: experimental data and implications for Fe–Mg exchange geothermometers

Chemical interdiffusion of Fe–Mg along the c- axis [001] in natural diopside crystals ( X Di = 0.93) was experimentally studied at ambient pressure, at temperatures ranging from 800 to 1,200 °C and oxygen fugacities from 10 −11 to 10 −17 bar. Diffusion couples were prepared by ablating an olivine ( X Fo = 0.3) target to deposit a thin film (20–100 nm) onto a polished surface of a natural, oriented diopside crystal using the pulsed laser deposition technique. After diffusion anneals, compositional depth profiles at the near surface region (~400 nm) were measured using Rutherford backscattering spectroscopy. In the experimental temperature and compositional range, no strong dependence of D Fe–Mg on composition of clinopyroxene (Fe/Mg ratio between Di 93 –Di 65 ) or oxygen fugacity could be detected within the resolution of the study. The lack of f O 2 -dependence may be related to the relatively high Al content of the crystals used in this study. Diffusion coefficients, D Fe–Mg , can be described by a single Arrhenius relation with D Fe - Mg = 2.77 ± 4.27 × 10 - 7 exp( - 320.7 ± 16.0 kJ / mol / RT)m 2 / s . D Fe–Mg in clinopyroxene appears to be faster than diffusion involving Ca-species (e.g., D Ca–Mg ) while it is slower than D Fe–Mg in other common mafic minerals (spinel, olivine, garnet, and orthopyroxene). As a consequence, diffusion in clinopyroxene may be the rate-limiting process for the freezing of many geothermometers, and compositional zoning in clinopyroxene may preserve records of a higher (compared to that preserved in other coexisting mafic minerals) temperature segment of the thermal history of a rock. In the absence of pervasive recrystallization, clinopyroxene grains will retain compositions from peak temperatures at their cores in most geological and planetary settings where peak temperatures did not exceed ~1,100 °C (e.g., resetting may be expected in slowly cooled mantle rocks, many plutonic mafic rocks, or ultra-high temperature metamorphic rocks).