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

The single-crystal X-lay technique was used to calibrate a new intracrystal line geothermometer based on equilibrium Mg-Fe* fractionation (Fe* = Fe2+ Mn2+) between M1 and M2 sites of natural P2(1)/c pigeonite, Suitable cryst als free of exsolution textures and sharp diffraction maxima were selected by careful TEM and XRD investigations from a large number of samples. Two s ingle crystals, PCA82506-3 (Wo(6)En(76)Fs(18)) from the Pecora Escarpment 8 2506 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 d egrees C. The TEM investigation, carried out on fragments of selected singl e crystals both before and after thermal treatment, shows heating-induced t exture modifications preliminary to spinodal decomposition in both crystals and a size increase in antiphase domains in BTS308-2, The two geothermomet ric 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-F e* = 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 pige onites 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).