TY - JOUR
T1 - Diffusion kinetics of Cr in spinel
T2 - Experimental studies and implications for 53Mn-53Cr cosmochronology
AU - Posner, Esther S.
AU - Ganguly, Jibamitra
AU - Hervig, Richard
N1 - Funding Information:
We are grateful Dr. Daniele Cherniak, two anonymous reviewers, the associate editor, Dr. Qing-Zhu Yin, and the editor-in-chief, Dr. Marc Norman, for their constructive reviews and helpful suggestions for improvement. Thanks are due to Dr. Kenneth Dominik for help with microprobe analysis, David Steinke and Benjamin McElhaney for technical support, Dr. Thomas Sharp and Dr. Kurt Roggensack for providing access to the wire saw sectioning facility at ASU. We are also thankful to Drs. Dimitri Papanastassiou, Giulio Ottonello, Massimiliano Tirone and Jim Van Orman for helpful discussions and Drs. Eli Bloch and Weiji Cheng for help with the experimental studies. The experimental and analytical components of this work were supported by a NASA Grant NNX10AI79G to JG. Subsequent data and theoretical analyses, and manuscript preparation were supported by a NASA Grant 13-COS 130027 to JG, and funding from the European Research Council (ERC) Advanced Grant “ACCRETE” (contract number 290568 ) plus German Research Foundation (DFG) Grant RU 1323/9-1 to support EP. The SIMS facility at Arizona State University is supported by NSF EAR-0948878 .
Publisher Copyright:
© 2015 Elsevier Ltd.
PY - 2016/2/15
Y1 - 2016/2/15
N2 - The 53Mn-53Cr decay system, in which 53Mn decays to 53Cr (t1/2 = 3.7 Ma) has been widely used to construct 53Cr/52Cr vs. 55Mn/52Cr isochrons and thus determine relative ages of early solar system objects or events, assuming that the initial Cr isotopic ratio, (53Cr/52Cr)o, equals (53Mn/52Cr)o. With the primary objective of interpretation of these ages within a diffusion kinetic framework, we have determined the tracer diffusion coefficient of Cr in natural spinels, which are very close to the MgAl2O4 end-member composition, as a function of temperature and oxygen fugacity (f(O2)). It is found that the diffusion coefficient of Cr, D(Cr), in two stocks of spinels (referred to as cut-gems and gem-gravels) with very similar major element chemistry is consistently different, but the data in each stock yield well defined Arrhenius relations that show a difference of log D of 0.6-1.0, depending on temperature, with the D(Cr) in gem-gravel being higher than that in the cut-gem stock. The D(Cr) was found to have a positive dependence on f(O2) in the range of f(O2) of around ±2 log units relative to that of the wüstite-magnetite buffer. The difference in the D(Cr) between the two stocks and the observed D(Cr) vs. f(O2) relation has been explained in terms of a change of point defect concentration resulting from heterovalent substitution of trace elements and equilibration with the imposed f(O2) conditions, respectively. Assuming a homogeneous semi-infinite matrix, the closure temperature (Tc) of Cr diffusion in spinel has been calculated as a function of grain size, cooling rate, peak temperature (To) and f(O2). Also the dependence of D(Cr) and Tc(Cr) on the Cr# (i.e. Cr/(Cr + Al) ratio) has been accounted for using available D(Cr) vs. Cr# data in Suzuki et al. (2008). We argue, on the basis of crystal chemical considerations and available diffusion kinetic data for minerals, that the Tc for Mn should be much lower than that for Cr in spinel, olivine and orthopyroxene, and discuss the potential implications of the anticipated disparity between Tc(Cr) and Tc(Mn) for the estimation of the (53Mn/55Mn)o ratio from an internal isochron defined by these minerals. Finally, we discuss the problem of determining the Tc for an internal isochron in relation to the individual Tc(Cr) for spinel, olivine and orthopyroxene.
AB - The 53Mn-53Cr decay system, in which 53Mn decays to 53Cr (t1/2 = 3.7 Ma) has been widely used to construct 53Cr/52Cr vs. 55Mn/52Cr isochrons and thus determine relative ages of early solar system objects or events, assuming that the initial Cr isotopic ratio, (53Cr/52Cr)o, equals (53Mn/52Cr)o. With the primary objective of interpretation of these ages within a diffusion kinetic framework, we have determined the tracer diffusion coefficient of Cr in natural spinels, which are very close to the MgAl2O4 end-member composition, as a function of temperature and oxygen fugacity (f(O2)). It is found that the diffusion coefficient of Cr, D(Cr), in two stocks of spinels (referred to as cut-gems and gem-gravels) with very similar major element chemistry is consistently different, but the data in each stock yield well defined Arrhenius relations that show a difference of log D of 0.6-1.0, depending on temperature, with the D(Cr) in gem-gravel being higher than that in the cut-gem stock. The D(Cr) was found to have a positive dependence on f(O2) in the range of f(O2) of around ±2 log units relative to that of the wüstite-magnetite buffer. The difference in the D(Cr) between the two stocks and the observed D(Cr) vs. f(O2) relation has been explained in terms of a change of point defect concentration resulting from heterovalent substitution of trace elements and equilibration with the imposed f(O2) conditions, respectively. Assuming a homogeneous semi-infinite matrix, the closure temperature (Tc) of Cr diffusion in spinel has been calculated as a function of grain size, cooling rate, peak temperature (To) and f(O2). Also the dependence of D(Cr) and Tc(Cr) on the Cr# (i.e. Cr/(Cr + Al) ratio) has been accounted for using available D(Cr) vs. Cr# data in Suzuki et al. (2008). We argue, on the basis of crystal chemical considerations and available diffusion kinetic data for minerals, that the Tc for Mn should be much lower than that for Cr in spinel, olivine and orthopyroxene, and discuss the potential implications of the anticipated disparity between Tc(Cr) and Tc(Mn) for the estimation of the (53Mn/55Mn)o ratio from an internal isochron defined by these minerals. Finally, we discuss the problem of determining the Tc for an internal isochron in relation to the individual Tc(Cr) for spinel, olivine and orthopyroxene.
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U2 - 10.1016/j.gca.2015.11.018
DO - 10.1016/j.gca.2015.11.018
M3 - Article
AN - SCOPUS:84960392515
SN - 0016-7037
VL - 175
SP - 20
EP - 35
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -