TY - JOUR
T1 - Neodymium diffusion in orthopyroxene
T2 - Experimental studies and applications to geological and planetary problems
AU - Sano, Jennifer
AU - Ganguly, Jibamitra
AU - Hervig, Richard
AU - Dohmen, Ralf
AU - Zhang, Xiaoyu
N1 - Funding Information:
We are grateful to Dr. Hexiong Yang for his help with the use of AMCS data base, visualization of the orthopyroxene crystal structure, and single crystal X-ray diffraction study of enstatite annealed at 1200 °C to check for conversion to high-temperature polymorph. We are also thankful to Eli Bloch for determining the SIMS sputtering rates in Au and garnet, and to Professor Rabi Bhattacharya for making available the pre-publication manuscript of his forthcoming book on statistics (Mathematical Statistics and Large Scale Sampling) that we have used for the statistical analysis of the data and also for helpful discussions. Thanks are due to Daniele Cherniak, Yang Liang and an anonymous reviewer for their formal reviews of the paper and helpful comments, and Rick Ryerson for thoughtful editorial handling. This research was supported by NASA grants NNX07AJ74G and NNX10A179G to JG. The SIMS facility at the Arizona State University is supported by an NSF grant EAR-0948878.
PY - 2011/8/15
Y1 - 2011/8/15
N2 - We have determined the Nd3+ diffusion kinetics in natural enstatite crystals as a function of temperature, f(O2) and crystallographic direction at 1bar pressure and applied these data to several terrestrial and planetary problems. The diffusion is found to be anisotropic with the diffusion parallel to the c-axial direction being significantly greater than that parallel to a- and b-axis. Also, D(//a) is likely to be somewhat greater than D(//b). Diffusion experiments parallel to the b-axial direction as a function of f(O2) do not show a significant dependence of D(Nd3+) on f(O2) within the range defined by the IW buffer and 1.5log unit above the WM buffer. The observed diffusion anisotropy and weak f(O2) effect on D(Nd3+) may be understood by considering the crystal structure of enstatite and the likely diffusion pathways. Using the experimental data for D(Nd3+), we calculated the closure temperature of the Sm-Nd geochronological system in enstatite during cooling as a function of cooling rate, grain size and geometry, initial (peak) temperature and diffusion direction. We have also evaluated the approximate domain of validity of closure temperatures calculated on the basis of an infinite plane sheet model for finite plane sheets showing anisotropic diffusion. These results provide a quantitative framework for the interpretation of Sm-Nd mineral ages of orthopyroxene in planetary samples. We discuss the implications of our experimental data to the problems of melting and subsolidus cooling of mantle rocks, and the resetting of Sm-Nd mineral ages in mesosiderites. It is found that a cooling model proposed earlier [Ganguly J., Yang H., Ghose S., 1994. Thermal history of mesosiderites: Quantitative constraints from compositional zoning and Fe-Mg ordering in orthopyroxene. Geochim. Cosmochim. Acta 58, 2711-2723] could lead to the observed ∼90Ma difference between the U-Pb age and Sm-Nd mineral age for mesosiderites, thus obviating the need for a model of resetting of the Sm-Nd mineral age by an "impulsive disturbance" [Prinzhoffer A, Papanastassiou D.A, Wasserburg G.J., 1992. Samarium-neodymium evolution of meteorites. Geochim. Cosmochim. Acta 56, 797-815].
AB - We have determined the Nd3+ diffusion kinetics in natural enstatite crystals as a function of temperature, f(O2) and crystallographic direction at 1bar pressure and applied these data to several terrestrial and planetary problems. The diffusion is found to be anisotropic with the diffusion parallel to the c-axial direction being significantly greater than that parallel to a- and b-axis. Also, D(//a) is likely to be somewhat greater than D(//b). Diffusion experiments parallel to the b-axial direction as a function of f(O2) do not show a significant dependence of D(Nd3+) on f(O2) within the range defined by the IW buffer and 1.5log unit above the WM buffer. The observed diffusion anisotropy and weak f(O2) effect on D(Nd3+) may be understood by considering the crystal structure of enstatite and the likely diffusion pathways. Using the experimental data for D(Nd3+), we calculated the closure temperature of the Sm-Nd geochronological system in enstatite during cooling as a function of cooling rate, grain size and geometry, initial (peak) temperature and diffusion direction. We have also evaluated the approximate domain of validity of closure temperatures calculated on the basis of an infinite plane sheet model for finite plane sheets showing anisotropic diffusion. These results provide a quantitative framework for the interpretation of Sm-Nd mineral ages of orthopyroxene in planetary samples. We discuss the implications of our experimental data to the problems of melting and subsolidus cooling of mantle rocks, and the resetting of Sm-Nd mineral ages in mesosiderites. It is found that a cooling model proposed earlier [Ganguly J., Yang H., Ghose S., 1994. Thermal history of mesosiderites: Quantitative constraints from compositional zoning and Fe-Mg ordering in orthopyroxene. Geochim. Cosmochim. Acta 58, 2711-2723] could lead to the observed ∼90Ma difference between the U-Pb age and Sm-Nd mineral age for mesosiderites, thus obviating the need for a model of resetting of the Sm-Nd mineral age by an "impulsive disturbance" [Prinzhoffer A, Papanastassiou D.A, Wasserburg G.J., 1992. Samarium-neodymium evolution of meteorites. Geochim. Cosmochim. Acta 56, 797-815].
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U2 - 10.1016/j.gca.2011.05.040
DO - 10.1016/j.gca.2011.05.040
M3 - Article
AN - SCOPUS:79960319401
SN - 0016-7037
VL - 75
SP - 4684
EP - 4698
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 16
ER -