TY - JOUR
T1 - Cation diffusion in aluminosilicate garnets
T2 - Experimental determination in pyrope-almandine diffusion couples
AU - Ganguly, Jibamitra
AU - Cheng, Weiji
AU - Chakraborty, Sumit
N1 - Funding Information:
Acknowledgements We are greatly indebted to Drs. Tim Loomis and Steve Elphick for their collaboration in the early phase of this work more than a decade ago, which greatly influenced the course of our research program on diffusion kinetics in minerals, especially garnet. Thanks are due to Dr. Craig Schwandt and an anonymous reviewer for constructive reviews. The work has been supported by US National Science Foundation grants No. EAR-911797 and EAR-9418941.
PY - 1998/4
Y1 - 1998/4
N2 - Diffusion couples made from homogeneous gem quality natural pyrope and almandine garnets were annealed within graphite capsules under anhydrous conditions at 22-40 kbar, 1057-1400 °C in a piston-cylinder apparatus. The concentration profiles that developed in each couple were modeled to retrieve the self diffusion coefficients [D(I)] of the divalent cations Fe, Mg, Mn and Ca. Because of their usually low concentions and lack of sufficient compositional change across the interface of the diffusion couples, only a few reliable data can be obtained for D(Ca) and D(Mn) from these experiments. However, nine sets of D(Fe) and D(Mg) data were retrieved in the above P-T range, and cast in the form of Arrhenian relation, D = D0 exp{-[Q(1 bar) + PΔV+]/RT}. The values of the activation energy (Q) and activation volume (ΔV+) depend on whether fO2 is constrained by graphite in the system C-O or held constant. For the first case, we have for Fe: Q(1 bar) = 65,532 ± 10,111 cal/mol, D0 = 3.50 (±2.30) × 10-5 cm2/s, ΔV+ = 5.6(±2.9) cm3/mol, and for Mg: Q(1 bar) = 60,760 ± 8,257 cal/mol, D0 = 4.66 (±2.48) × 10-5 cm2/s, ΔV+ = 5.3(±3.0) cm3/mol. Here the ΔV+ values have been taken from Chakraborty and Ganguly (1992). For the condition of constant fO2, the Q values are ∼9 kcal lower and ΔV+ values are ∼4.9 cm3/mol larger than the above values. Lower temperature extrapolation of the Arrhenian relation for D(Mg) is in good agreement with the Mg tracer diffusion dat (D*Mg) of Chakraborty and Rubie (1996) and Cygan and Lasaga (1985) at 1 bar, 750-900 °C, when all data are normalized to the same pressure and to fO2 defined by graphite in the system C-O. The D*Mg data of Schwandt et al. (1995), on the other hand, are lower by more than an order of magnitude than the low temperature extrapolation of the present data, when all data are normalized to the same pressure and to fO2 defined by the graphite buffer. Comparison of the D(Fe), D(Mg) and D(Mn) data in the pyrope-almandine diffusion couple with those in the spessartine-almandine diffusion couple of Chakraborty and Ganguly (1992) shows that the self diffusion of Fe and Mn are significantly enhanced with the increase in Mn/Mg ratio; the enhancement effect on D(Mg) is, however, relatively small. Proper application of the self diffusion data to calculate interdiffusion coefficient or D matrix elements for the purpose of modeling of diffusion processes in natural garnets must take into account these compositional effects on D(I) along with the effects of thermodynamic nonideality, fO2, and pressure.
AB - Diffusion couples made from homogeneous gem quality natural pyrope and almandine garnets were annealed within graphite capsules under anhydrous conditions at 22-40 kbar, 1057-1400 °C in a piston-cylinder apparatus. The concentration profiles that developed in each couple were modeled to retrieve the self diffusion coefficients [D(I)] of the divalent cations Fe, Mg, Mn and Ca. Because of their usually low concentions and lack of sufficient compositional change across the interface of the diffusion couples, only a few reliable data can be obtained for D(Ca) and D(Mn) from these experiments. However, nine sets of D(Fe) and D(Mg) data were retrieved in the above P-T range, and cast in the form of Arrhenian relation, D = D0 exp{-[Q(1 bar) + PΔV+]/RT}. The values of the activation energy (Q) and activation volume (ΔV+) depend on whether fO2 is constrained by graphite in the system C-O or held constant. For the first case, we have for Fe: Q(1 bar) = 65,532 ± 10,111 cal/mol, D0 = 3.50 (±2.30) × 10-5 cm2/s, ΔV+ = 5.6(±2.9) cm3/mol, and for Mg: Q(1 bar) = 60,760 ± 8,257 cal/mol, D0 = 4.66 (±2.48) × 10-5 cm2/s, ΔV+ = 5.3(±3.0) cm3/mol. Here the ΔV+ values have been taken from Chakraborty and Ganguly (1992). For the condition of constant fO2, the Q values are ∼9 kcal lower and ΔV+ values are ∼4.9 cm3/mol larger than the above values. Lower temperature extrapolation of the Arrhenian relation for D(Mg) is in good agreement with the Mg tracer diffusion dat (D*Mg) of Chakraborty and Rubie (1996) and Cygan and Lasaga (1985) at 1 bar, 750-900 °C, when all data are normalized to the same pressure and to fO2 defined by graphite in the system C-O. The D*Mg data of Schwandt et al. (1995), on the other hand, are lower by more than an order of magnitude than the low temperature extrapolation of the present data, when all data are normalized to the same pressure and to fO2 defined by the graphite buffer. Comparison of the D(Fe), D(Mg) and D(Mn) data in the pyrope-almandine diffusion couple with those in the spessartine-almandine diffusion couple of Chakraborty and Ganguly (1992) shows that the self diffusion of Fe and Mn are significantly enhanced with the increase in Mn/Mg ratio; the enhancement effect on D(Mg) is, however, relatively small. Proper application of the self diffusion data to calculate interdiffusion coefficient or D matrix elements for the purpose of modeling of diffusion processes in natural garnets must take into account these compositional effects on D(I) along with the effects of thermodynamic nonideality, fO2, and pressure.
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U2 - 10.1007/s004100050386
DO - 10.1007/s004100050386
M3 - Article
AN - SCOPUS:0000460473
SN - 0010-7999
VL - 131
SP - 171
EP - 180
JO - Contributions to Mineralogy and Petrology
JF - Contributions to Mineralogy and Petrology
IS - 2
ER -