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Self diffusion of Mg, Ca, Ba, Nd, Yb, Ti, Zr, and U in haplobasaltic melt

LaTourrette, T. and Wasserburg, G. J. and Fahey, A. J. (1996) Self diffusion of Mg, Ca, Ba, Nd, Yb, Ti, Zr, and U in haplobasaltic melt. Geochimica et Cosmochimica Acta, 60 (8). pp. 1329-1340. ISSN 0016-7037. http://resolver.caltech.edu/CaltechAUTHORS:20131115-111016293

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Abstract

We have measured self diffusion coefficients (D) for a wide variety of elements in a Fo_(15)Di_(40)An_(45) melt at 1 bar from 1350–1500 °C. These measurements, including the first reported data for Yb, Ti, Zr, U, and preliminary values for Th and Pb, provide an internally consistent set of self diffusivities for elements covering a wide range in ionic radius and charge. Diffusion couples were formed by juxtaposing isotopically enriched and isotopically normal melts of the same chemical composition, and isotopic concentration profiles were measured with an ion probe. Well-defined diffusion coefficients were determined to within a reproducibility of ±15%. The data show good Arrhenian behavior with activation energies ranging from 170 kJ/m for Mg^(2+) to 216 kJ/m for Zr^(4+). The diffusion coefficient for U is interpreted as the weighted sum of variable proportions of U^(5+) and U^(6+), with D_(U5+) > D_(U6+). Activation energies for most elements are similar and agree well with the activation energy for viscous flow, suggesting a structural relationship between diffusion and viscosity. In contrast, diffusion coefficients display a systematic decrease with increasing ionic radius and increasing charge at any given temperature, and at 1400 °C range from 8.1 × 10^(−7) cm^2/s for Mg^(2+) to 0.9 × 10^(−7) cm^2/s for U^(5−6+). These observations suggest that diffusion of network modifying cations in low viscosity melts reflects a combination of the intrinsic mobilities of the individual cations and viscous flow of the melt network. While there is some indication that self diffusion coefficients are influenced by a combination of mechanical and electrostatic effects, a complete physical explanation for the variation of diffusivity with ionic radius and charge does not exist. The width of a boundary layer around a growing crystal is proportional to √D, and hence the small range in diffusion coefficients measured in this study implies that trace element fractionation during crystal growth in basaltic compositions will be minimal.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1016/0016-7037(96)00015-4DOIArticle
http://www.sciencedirect.com/science/article/pii/0016703796000154PublisherArticle
Additional Information:© 1996 Elsevier Science Ltd. Received July 14, 1995; accepted in revised form January 3, 1996. Hofmann for helpful discussions and advice, M. Heinrich for technical assistance with the ion probe, and A. Jurewicz for her participation in the early stages of the experiments. We also thank F. Spera, F. Ryerson, and an anonymous reviewer for insightful and constructive reviews that led to notable improvements in the manuscript. This work was supported by DOE grant DE-FG-03-88ER-13851. Division Contribution 5547(901). Editorial handling: F. J. Ryerson
Funders:
Funding AgencyGrant Number
Department of Energy (DOE)DE-FG-03-88ER-13851
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Other Numbering System NameOther Numbering System ID
Caltech Division of Geological and Planetary Sciences 5547
Lunatic Asylum Lab901
Record Number:CaltechAUTHORS:20131115-111016293
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20131115-111016293
Official Citation:T. LaTourrette, G.J. Wasserburg, A.J. Fahey, Self diffusion of Mg, Ca, Ba, Nd, Yb, Ti, Zr, and U in haplobasaltic melt, Geochimica et Cosmochimica Acta, Volume 60, Issue 8, April 1996, Pages 1329-1340, ISSN 0016-7037, http://dx.doi.org/10.1016/0016-7037(96)00015-4. (http://www.sciencedirect.com/science/article/pii/0016703796000154)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42495
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:15 Nov 2013 19:40
Last Modified:15 Nov 2013 19:40

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