caa a22 4500 463217101 CHVBK 20180405153200.0 cr unu---uuuuu 170326e20070801xx s 000 0 eng 10.1007/s00269-007-0158-6 doi (NATIONALLICENCE)springer-10.1007/s00269-007-0158-6 Fe-Mg diffusion in olivine II: point defect chemistry, change of diffusion mechanisms and a model for calculation of diffusion coefficients in natural olivine [Elektronische Daten] [Ralf Dohmen, Sumit Chakraborty] Analysis of existing data and models on point defects in pure (Fe,Mg)-olivine (Phys Chem Miner 10:27-37,1983; Phys Chem Miner 29:680-694, 2002) shows that it is necessary to consider thermodynamic non-ideality of mixing to adequately describe the concentration of point defects over the range of measurement. In spite of different sources of uncertainties, the concentrations of vacancies in octahedral sites in (Fe,Mg)-olivine are on the order of 10−4 per atomic formula unit at 1,000-1,200°C according to both the studies. We provide the first explicit plots of vacancy concentrations in olivine as a function of temperature and oxygen fugacity according to the two models. It is found that in contrast to absolute concentrations at ∼1,100°C and dependence on fO2, there is considerable uncertainty in our knowledge of temperature dependence of vacancy concentrations. This needs to be considered in discussing the transport properties such as diffusion coefficients. Moreover, these defect models in pure (Fe,Mg)-olivine need to be extended by considering aliovalent impurities such as Al, Cr to describe the behavior of natural olivine. We have developed such a formulation, and used it to analyze the considerable database of diffusion coefficients in olivine from Dohmen etal. (Phys Chem Miner this volume, 2007) (Part - I) and older data in the literature. The analysis documents unequivocally for the first time a change of diffusion mechanism in a silicate mineral—from the transition metal extrinsic (TaMED) to the purely extrinsic (PED) domain, at fO2 below 10−10 Pa, and consequently, temperatures below 900°C. The change of diffusion mechanism manifests itself in a change in fO2 dependence of diffusivity and a slight change in activation energy of diffusion—the activation energy increases at lower temperatures. These are consistent with the predictions of Chakraborty (J Geophys Res 102(B6):12317-12331, 1997). Defect formation enthalpies in the TaMED regime (distinct from intrinsic defect formation) lie between −66 and + 15kJ/mol and migration energies of octahedral cations in olivine are most likely ∼ 260kJ/mol, consistent with previous inferences (Phys Chem 207:147-162, 1998). Plots are shown for diffusion at various constant fO2 as well as along fO2 buffers, to highlight the difference in behavior between the two. Considering all the diffusion data and constraints from the point defect models, (Fe-Mg) diffusion in olivine along [001] is best described by the Master equations: (1) At oxygen fugacities greater than 10−10Pa: $$ \log [D_{{{{\rm FeMg}}}} (m^{2}/s)] = - 9.21 - \frac{{201000 + (P - 10^{5}) \times 7 \times 10^{{- 6}}}}{{2.303RT}} + \frac{1}{6}\log (fO_{2} /10^{{- 7}}) + 3X_{{{{\rm Fe}}}}$$ where T is in Kelvin, P and fO2 is in Pascals, X Fe is the mole fraction of the fayalite component and R is the gas constant in J/mol/K. (2) At oxygen fugacities less than 10−10 Pa: $$ \log [D_{{{{\rm FeMg}}}} (m^{2}/s)] = - 8.91 - \frac{{220000 + (P - 10^{5}) \times 7 \times 10^{{- 6}}}}{{2.303RT}} + 3X_{{{{\rm Fe}}}}$$ These equations reproduce all of the 113 experimental data points within half an order of magnitude. (3) Alternately, a global equation averaging out the change of mechanism may be used, with somewhat larger errors in reproducing the measured diffusion data. It underestimates data at higher temperatures, and overestimates them at lower temperatures on the average. Note that fO2 is not explicitly considered here, leading to additional sources of error: $$ \log [D_{{{{\rm FeMg}}}} (m^{2}/s)] = - 8.27 - \frac{{226000 + (P - 10^{5}) \times 7 \times 10^{{- 6}}}}{{2.303RT}} + 3X_{{{{\rm Fe}}}}$$ To obtain diffusion coefficients along [100] and [010], log 6 needs to be subtracted from each of the above equations. Springer-Verlag, 2007 Olivine nationallicence Point defect nationallicence Thermodynamics nationallicence Diffusion mechanism nationallicence Intrinsic nationallicence Extrinsic nationallicence Trace elements nationallicence Dohmen Ralf Institut für Geologie, Mineralogie und Geophysik, Ruhr Universität Bochum, 44780, Bochum, Germany aut Chakraborty Sumit Institut für Geologie, Mineralogie und Geophysik, Ruhr Universität Bochum, 44780, Bochum, Germany aut Physics and Chemistry of Minerals Springer-Verlag 34/6(2007-08-01), 409-430 0342-1791 34:6<409 2007 34 269 https://doi.org/10.1007/s00269-007-0158-6 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/s00269-007-0158-6 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Dohmen Ralf Institut für Geologie, Mineralogie und Geophysik, Ruhr Universität Bochum, 44780, Bochum, Germany aut NATIONALLICENCE 700 1- Chakraborty Sumit Institut für Geologie, Mineralogie und Geophysik, Ruhr Universität Bochum, 44780, Bochum, Germany aut NATIONALLICENCE 773 0- Physics and Chemistry of Minerals Springer-Verlag 34/6(2007-08-01), 409-430 0342-1791 34:6<409 2007 34 269 Metadata rights reserved Springer special CC-BY-NC licence nationallicence SWISSBIB 463217101 BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer