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Kinetics of Fe²⁺-Mg order-disorder in P2₁/c pigeonite

Dipartimento di Scienze della Terra, Università di Pavia, Via Ferrata 1, 27100 Pavia, Italy

Correspondence: ^* E-mail: domeneghetti{at}crystal.unipv.it

The kinetics of the Fe-Mg intracrystalline exchange reaction in P2₁/c pigeonite (Wo₁₀En₄₇Fs₄₃) free of exsolved augite, from the Paraná rhyodacite sample BTS308, was studied by single-crystal X-ray diffraction (XRD). Isothermal disordering annealing experiments, with oxygen fugacity controlled at the IW buffer, were performed on two crystals at 650, 700, 750, and 800 °C until the Fe-Mg exchange equilibrium was reached. The XRD data were collected from the two untreated crystals and after each annealing experiment. Structure refinements were carried out taking into account the recently discovered stronger preference of Mn for the M2 site compared to Fe²⁺. The linear regression of ln k_D* vs. 1/T yielded the following equation:

The T_c values calculated using this equation were 566 (±6) and 571 (±6) °C for the two crystals. Analysis of the kinetic data was performed according to Mueller’s model, which allowed retrieval of the disordering rate constants C₀K_dis⁺ for all four temperatures. The Arrhenius relation:

yielded an activation energy of 50.03 (±3) kcal/mol for the Fe-Mg exchange process. Cooling time constants, calculated at the QMF buffer conditions of the host rock were, for the two crystals, = 0.94 x 10^–1 K^–1year^–1 and = 1.10 x 10^–1 K^–1year^–1, and gave cooling rates on the order of 10 °C/h consistent with very fast lava cooling.

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