TY - JOUR
T1 - Ca-Mg diffusion in diopside
T2 - Tracer and chemical inter-diffusion coefficients
AU - Zhang, Xiaoyu
AU - Ganguly, Jibamitra
AU - Ito, Motoo
N1 - Funding Information:
Acknowledgments This research was supported by a grant from the NASA Cosmochemistry program NNX07AJ74G. We are grateful to Prof. N. Sugiura and Dr. Miyazaki for providing access to the SIMS facility at the University of Tokyo while M.I. has been at the same University on a JSPS fellowship, and to Prof. Richard Hervig for providing access to the SIMS facility at the Arizona State University and helpful advice on the analytical procedures. Our sincere thanks are due to Profs. Youxue Zhang and Jim Van Orman for constructive reviews of the manuscript, and to Prof. Rabi Bhattacharya for deriving the relation for constrained regression for Mg diffusion. We also thank Dr. Marilena Stimpfl for some of the initial studies that have partly laid the groundwork of the experimental investigations.
PY - 2010/2
Y1 - 2010/2
N2 - We have experimentally determined the tracer diffusion coefficients (D*) of 44Ca and 26Mg in a natural diopside (̃Di96) as function of crystallographic direction and temperature in the range of 950-1,150 °C at 1 bar and f(O2) corresponding to those of the WI buffer. The experimental data parallel to the a*, b, and c crystallographic directions show significant diffusion anisotropy in the a-c and b-c planes, with the fastest diffusion being parallel to the c axis. With the exception of logD*(26Mg) parallel to the a* axis, the experimental data conform to the empirical diffusion "compensation relation", converging to logD ̃ -19. 3 m2/s and T ̃ 1,155 °C. Our data do not show any change of diffusion mechanism within the temperature range of the experiments. Assuming that D* varies roughly linearly as a function of angle with respect to the c axis in the a-c plane, at least within a limited domain of ̃20° from the c-axis, our data do not suggest any significant difference between D*(//c) and D*(⊥(001)), the latter being the diffusion data required to model compositional zoning in the (001) augite exsolution lamellae in natural clinopyroxenes. Since the thermodynamic mixing property of Ca and Mg is highly nonideal, calculation of chemical diffusion coefficient of Ca and Mg must take into account the effect of thermodynamic factor (TF) on diffusion coefficient. We calculate the dependence of the TF and the chemical interdiffusion coefficient, D(Ca-Mg), on composition in the diopside-clinoenstatite mixture, using the available data on mixing property in this binary system. Our D*(Ca) values parallel to the c axis are about 1-1.5 log units larger than those Dimanov et al. (1996). Incorporating the effect of TF, the D(Ca-Mg) values calculated from our data at 1,100-1,200 °C is ̃0. 6-0.7 log unit greater than the experimental quasibinary D((Ca-Mg + Fe)) data of Fujino et al. (1990) at 1 bar, and ̃0.6 log unit smaller than that of Brady and McCallister (1983) at 25 kb, 1,150 °C, if our data are normalized to 25 kb using activation volume (̃4 and ̃6 cm3/mol for Mg and Ca diffusion, respectively) calculated from theoretical considerations.
AB - We have experimentally determined the tracer diffusion coefficients (D*) of 44Ca and 26Mg in a natural diopside (̃Di96) as function of crystallographic direction and temperature in the range of 950-1,150 °C at 1 bar and f(O2) corresponding to those of the WI buffer. The experimental data parallel to the a*, b, and c crystallographic directions show significant diffusion anisotropy in the a-c and b-c planes, with the fastest diffusion being parallel to the c axis. With the exception of logD*(26Mg) parallel to the a* axis, the experimental data conform to the empirical diffusion "compensation relation", converging to logD ̃ -19. 3 m2/s and T ̃ 1,155 °C. Our data do not show any change of diffusion mechanism within the temperature range of the experiments. Assuming that D* varies roughly linearly as a function of angle with respect to the c axis in the a-c plane, at least within a limited domain of ̃20° from the c-axis, our data do not suggest any significant difference between D*(//c) and D*(⊥(001)), the latter being the diffusion data required to model compositional zoning in the (001) augite exsolution lamellae in natural clinopyroxenes. Since the thermodynamic mixing property of Ca and Mg is highly nonideal, calculation of chemical diffusion coefficient of Ca and Mg must take into account the effect of thermodynamic factor (TF) on diffusion coefficient. We calculate the dependence of the TF and the chemical interdiffusion coefficient, D(Ca-Mg), on composition in the diopside-clinoenstatite mixture, using the available data on mixing property in this binary system. Our D*(Ca) values parallel to the c axis are about 1-1.5 log units larger than those Dimanov et al. (1996). Incorporating the effect of TF, the D(Ca-Mg) values calculated from our data at 1,100-1,200 °C is ̃0. 6-0.7 log unit greater than the experimental quasibinary D((Ca-Mg + Fe)) data of Fujino et al. (1990) at 1 bar, and ̃0.6 log unit smaller than that of Brady and McCallister (1983) at 25 kb, 1,150 °C, if our data are normalized to 25 kb using activation volume (̃4 and ̃6 cm3/mol for Mg and Ca diffusion, respectively) calculated from theoretical considerations.
KW - Cooling rates
KW - Diffusion
KW - Diopside
KW - Eucrites
KW - Thermodynamic factor
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U2 - 10.1007/s00410-009-0422-5
DO - 10.1007/s00410-009-0422-5
M3 - Article
AN - SCOPUS:77951204089
SN - 0010-7999
VL - 159
SP - 175
EP - 186
JO - Contributions to Mineralogy and Petrology
JF - Contributions to Mineralogy and Petrology
IS - 2
ER -