Order-disorder kinetics in orthopyroxenes of ophiolite origin

The kinetics of the Fe–Mg order-disorder reaction in four low-Fe orthopyroxenes (Fs_7–14) has been studied by use of Mössbauer spectroscopy, to determine cooling rates for the host rocks. The obtained Fe²⁺ distribution data were corrected for the thickness effect. The sample set was selected from rocks expected to have distinctly different cooling histories. Equilibrium data for the exchange reaction were determined for the temperature range 600–800°C, and kinetic data at 650°C. The results indicate that the degree of ordering in low-Fe orthopyroxene is substantially lower than estimated by thermodynamic models. The cooling rates obtained seem realistic for most samples, ranging from 19°C/min at 760°C for a boninite pillow lava to 2°C/million years at 267°C for a regionally metamorphosed olivinite. The errors in obtained cooling rates are estimated to be near one order of magnitude. An Fe³⁺ containing sample yields an unreasonably slow cooling rate which may be due to oxidation of Fe²⁺, occurring after the quenching point of the order-disorder reaction was reached.