• journal_title：American Mineralogist
• Contributor：Daria Pasqual ; Gianmario Molin ; Mario Tribaudino
• Publisher：Mineralogical Society of America
• Date：2000-
• Format：text/html
• Language：en
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：85
• issue：7-8
• firstpage：953
• section：Articles

The single-crystal X-ray 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 P2₁/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 (Wo₆En₇₆Fs₁₈) from the Pecora Escarpment 82506 Antarctic ureilite, and BTS308-2 (Wo₁₀En₄₇Fs₄₃) from the BTS308 Paranà rhyodacite, were annealed at temperatures ranging from 600 to 1000 °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 ln K_D* vs. 1/T are:

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 Wo₁₀.