Relationship between Guinier-Preston zones and the kinetics of the intracrystalline Fe2+, Mg exchange reaction in Johnstown meteoritic orthopyroxene

Abstract

The Fe2+, Mg order/disorder process in orthopyroxenes from the Johnstown meteoritic diogenite is extremely slow. It is controlled by an activation energy of ≈ 100 kcal/mol, whereas in other orthopyroxenes with similar compositions the activation energy is only 60 and 65 kcal/mol. The microstructure of the orthopyroxenes is characterized by the exsolution of abundant coherently intergrown Guinier Preston (GP) zones that have been suggested to be responsible for the slow exchange kinetics. In order to test this hypothesis, orthopyroxene crystals were homogenized by dry heating at 1075 °C for 4 days at Fe/FeO buffer conditions. The variation of the distribution of Fe2+ and Mg on the M1 and M2 was followed by site refinements employing conventional X-ray intensity data. The kinetic behaviour of the homogenized crystals was studied in six controlled continuous cooling experiments in which the crystals were cooled from 850 °C to 250 °C at various rates. The quenched ordering states could be best reproduced by the Arrhenius relation \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[ln \mathit{k}_{\mathit{dis}}[d^{{-}1}] = 34.0 {-}71.1 [kcal/mol] /\mathit{RT},\] \end{document}(1) where k dis is the rate constant for the microscopic disordering step. It appears that homogenization of the GP zones has in fact lowered the activation energy from ≈ 100 kcal/mol to 71 kcal/mol. Equilibrium experiments were performed at three different temperatures. Comparing the refined Fe2+, Mg distribution coefficients, K D , with data previously obtained on non-homogenized Johnstown orthopyroxenes (Heinemann et al ., 2000) shows that the presence or absence of GP zones has no effect on the equilibrium distribution. Regression analysis of the combined ln K D data yielded \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[ln \mathit{K}_{\mathit{D}} = 0.467(77) {-}2535(87)/\mathit{T}.\] \end{document}(2) Using the derived k dis and K D relations, the cooling rate of the original Johnstown orthopyroxenes was calculated from the Mueller (1967, 1969) rate law yielding dT / dt ≈ −0.4 K/My near the temperature of apparent equilibrium T ae ≈ 325 °C. This rate is the maximum rate at which the orthopyroxenes have cooled, and T ae , albeit low, is the maximum temperature at which the GP zones have nucleated.