Single-crystal thermometric calibration of Fe-Mg order-disorder in pigeonites

The single-crystal X-lay technique was used to calibrate a new intracrystalline geothermometer based on equilibrium Mg-Fe* fractionation (Fe* = Fe2+ + Mn2+) between M1 and M2 sites of natural P2(1)/c pigeonite, Suitable crystals free of exsolution textures and sharp diffraction maxima were selected by careful TEM and XRD investigations from a large number of samples. Two single crystals, PCA82506-3 (Wo(6)En(76)Fs(18)) from the Pecora Escarpment 82506 Antarctic ureilite, and BTS308-2 (Wo(10)En(47)Fs(43)) from the BTS308 Parana rhyodacite, were annealed at temperatures ranging from 600 to 1000 degrees C. The TEM investigation, carried out on fragments of selected single crystals both before and after thermal treatment, shows heating-induced texture modifications preliminary to spinodal decomposition in both crystals and a size increase in antiphase domains in BTS308-2, The two geothermometric equations calculated by linear regression of In K-D* vs. 1/T are: In K-D* = -3291(+/- 269)/T(K) + 0.971(+/-0.253); (r(2) = 0.974) ln K-D* = -2816(+/- 83)/T(K) + 0.542(+/-0.083); (r(2) = 0.995) respectively for PCA82506-3 (X-Fe* = 0.20) and BTS308-2 (X-Fe* = 0.49) [X-Fe* = Fe*/(Fe* + Mg)]. These results imply negligible compositional effects on Mg-Fe* site partitioning within the range of compositions encompassed by the samples. Comparison of the intracrystalline fractionation data of pigeonites with those of Pbca orthopyroxenes shows a similar degree of ordering for both at a given temperature. This result suggests only a small effect of Ca on Fe*-Mg ordering in pyroxene with Ca content up to Wo(10).