Fe-Mg order-disorder in orthopyroxenes

Published data on the distribution of Fe and Mg between the two distinct octahedral sites (M1 and M2) in orthopyroxenes show that the equilibrium Fe-Mg distribution coefficient (K-D) is roughly constant at intermediate ferrosilite content (X-Fs of 0.2 to 0.6) but at low X-Fs (< 0.2) is either anomalously high (Mossbauer data) or low (XRD data) compared to K-D at intermediate X-Fs. We report an experimental study on the equilibrium K-D values for two natural single crystals of orthopyroxene (with X-Fs of 0.011 and 0.160) using a high-sensitivity Mossbauer spectrometer. The K-D values were determined at 1 bar and temperatures between 600 and 1000 degrees C, and reversed at 600 to 900 degrees C. The values are found to be independent of orthopyroxene composition at a given temperature within analytical uncertainties, and in agreement with K-D values at intermediate X-Fs. A least-squares fit to the K-D data yields: InKD = 0.391(+/- 0.131) - 2205(+/- 141)/T, where T is temperature in K-D and uncertainties are at the 2 sigma level. This equation is valid for X-Fs values up to at least 0.6. We suggest that previous Mossbauer and XRD data at low X-Fs were compromised by analytical difficulties. The new result that K-D is roughly independent of X-Fs greatly simplifies treatment of equilibrium and speedometry based on Fe-Mg order-disorder in orthopyroxene. Copyright (c) 2005 Elsevier Ltd.