TY - JOUR

T1 - Kinetics of Fe2+-Mg order-disorder in orthopyroxene

T2 - Experimental studies and applications to cooling rates of rocks

AU - Stimpfl, M.

AU - Ganguly, J.

AU - Molin, Gianmario

N1 - Funding Information:
Acknowledgements This research was supported by NASA grants NAG5-10486 and NNG04GG26G. The single crystal X-ray diffraction studies were carried out in the X-ray laboratory of the Department of Chemistry, University of Arizona. The CCD equipped Area detector was purchased with an NSF grant CHE-9610374. We are grateful to Dr. M. Carducci for his help in using the X-ray equipments and the software for data reduction. Thanks are due to Prof. Chiara Domeneghetti and an anonymous reviewer for careful reviews constructive suggestions that led to significant improvement of the analysis of the data and clarity of presentation, to Prof. Hans Annersten for the Mössbauer analysis of orthopyroxene, and Prof. Linc Hollister for the KLMC samples. The senior author acknowledges helpful discussions with Dr. Robert P. Ilchik.

PY - 2005/10

Y1 - 2005/10

N2 - We determined the forward rate constant (K+) for the Fe2+-Mg order-disorder between the M2 and M1 sites of orthopyroxene (OPx), which is described by the homogeneous reaction Fe2+ (M2) + Mg(M1) ↔ Mg(M2) + Fe2+ (M1), by both ordering and disordering experiments at isothermal condition and also by continuous cooling experiments. The rate constant was determined as a function of temperature in the range of 550-750°C, oxygen fugacity between quartz-fayalite-iron and Ni-NiO buffers, and at compositions of 16 and 50 mol% ferrosilite component. The K+ value derived from disordering experiment was found to be larger than that derived from ordering experiment at 550°C, while at T >580°C, these two values are essentially the same. The fO2 dependence of the rate constant can be described by the relation K+ α (fO2)n with n=5.5-6.5, which is compatible with the theoretically expected relation. The Arrhenius relation at the WI buffer condition is given by ln (CoK+) = -41511 - 12600XFe/T(K) + 28:26 + 5:27 XFe, min-1 where Co represents the total number of M2 + M1 sites occupied by Fe2+ and Mg per unit volume of the crystal. The above relation can be used to calculate the cooling rates of natural OPx crystals around the closure temperature (Tc) of Fe-Mg ordering, which are usually below 300°C for slowly cooled rocks. We determined the Fe-Mg ordering states of several OPx crystals (∼ Fs50) from the Central Gneissic Complex (Khtada Lake), British Columbia, which yields Tc ∼290°C. Numerical simulation of the change of Fe2+-Mg ordering in OPx as a function of temperature using the above expression of rate constant and a non-linear cooling model yields quenched values of ordering states that are in agreement with the observed values for cooling rates of 11-17°C/Myr below 300°C. The inferred cooling rate is in agreement with the available geochronological constraints.

AB - We determined the forward rate constant (K+) for the Fe2+-Mg order-disorder between the M2 and M1 sites of orthopyroxene (OPx), which is described by the homogeneous reaction Fe2+ (M2) + Mg(M1) ↔ Mg(M2) + Fe2+ (M1), by both ordering and disordering experiments at isothermal condition and also by continuous cooling experiments. The rate constant was determined as a function of temperature in the range of 550-750°C, oxygen fugacity between quartz-fayalite-iron and Ni-NiO buffers, and at compositions of 16 and 50 mol% ferrosilite component. The K+ value derived from disordering experiment was found to be larger than that derived from ordering experiment at 550°C, while at T >580°C, these two values are essentially the same. The fO2 dependence of the rate constant can be described by the relation K+ α (fO2)n with n=5.5-6.5, which is compatible with the theoretically expected relation. The Arrhenius relation at the WI buffer condition is given by ln (CoK+) = -41511 - 12600XFe/T(K) + 28:26 + 5:27 XFe, min-1 where Co represents the total number of M2 + M1 sites occupied by Fe2+ and Mg per unit volume of the crystal. The above relation can be used to calculate the cooling rates of natural OPx crystals around the closure temperature (Tc) of Fe-Mg ordering, which are usually below 300°C for slowly cooled rocks. We determined the Fe-Mg ordering states of several OPx crystals (∼ Fs50) from the Central Gneissic Complex (Khtada Lake), British Columbia, which yields Tc ∼290°C. Numerical simulation of the change of Fe2+-Mg ordering in OPx as a function of temperature using the above expression of rate constant and a non-linear cooling model yields quenched values of ordering states that are in agreement with the observed values for cooling rates of 11-17°C/Myr below 300°C. The inferred cooling rate is in agreement with the available geochronological constraints.

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U2 - 10.1007/s00410-005-0016-9

DO - 10.1007/s00410-005-0016-9

M3 - Article

AN - SCOPUS:27744517449

SN - 0010-7999

VL - 150

SP - 319

EP - 334

JO - Contributions of Mineralogy and Petrology

JF - Contributions of Mineralogy and Petrology

IS - 3

ER -