Abstract
Reversed experiments have been performed to better constrain the equilibrium partitioning of Fe between the Ml and M2 sites in orthopyroxene and the kinetics of the ordering process. Two samples (M32b, XFe = 0.388; PX4-TM2, XFe = 0.813) were experimentally disordered at 900 °C and ordered at 500, 600, 700, and 800 °C for various times between 3 × 102 and 4 × 106 s. Reversals were obtained by heating naturally ordered materials that had not been disordered in advance in order to better constrain the equilibrium ordering state. Ordering and redox states of the pyroxenes were determined using Mössbauer spectroscopy. Results suggest that the equilibrium ordering state for a given composition and temperature is more disordered than predicted by the model of Ganguly (1982). Fitting the data to the equation of Saxena and Ghose (1971) yields In K = 1203.30/T + 0.1272 (T in kelvins), WM1 = 5449 J/mol, and WM2 = 2083 J/mol.
A strong dependence of the kinetic data on composition is suggested by the combination of all available rate data. Calculations performed using these data yield cooling rates for many metamorphic terranes of 0.1 to 100 °C/m.y., which are significantly slower than those previously obtained from pyroxene-ordering studies. These rates are within the range suggested by other approaches. The relatively large error ranges obtained, however, suggest that additional calibration is necessary at lower temperatures and with Fe-, Ca-, and Al-rich compositions. Studies of ordering in orthopyroxenes may be a useful method for obtaining cooling rates in igneous and contact-metamorphic environments.
