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Comparison of element and isotope diffusion of K and Ca in multicomponent silicate melts

van der Laan, Sieger and Zhang, Youxue and Kennedy, Allen K. and Wyllie, Peter J. (1994) Comparison of element and isotope diffusion of K and Ca in multicomponent silicate melts. Earth and Planetary Science Letters, 123 (1-3). pp. 155-166. ISSN 0012-821X. doi:10.1016/0012-821X(94)90264-X.

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Recent experimental work has shown that the homogenization of elemental concentrations can be much slower than that of isotopic ratios when there are strong concentration gradients in SiO_2 and Al_2O_3. The ramifications of this result for magma homogenization and other petrological problems related to diffusion are significant. We report here a comparison of experimental profiles of elemental concentrations and isotopic fractions of K and Ca in rhyolite-andesite (large concentration gradients) and rhyolite-rhyolite (small concentration gradients) melt couples. When the concentration profile and the isotopic profile of the same element in a single couple are compared, the former is much shorter than the latter in the rhyolite-andesite couple, consistent with other recent studies. However, the lengths of both concentration and isotopic profiles are similar in the rhyolite-rhyolite couple. Therefore, diffusion of an element or oxide may be decoupled from or coupled with isotopic ‘diffusion’, depending on whether large concentration gradients of major components are present. When the two couples are compared, the intrinsic effective binary diffusivities obtained from isotopic profiles are similar for each element in the two couples, whereas the effective binary diffusivity of K obtained from the concentration profile in the rhyolite-rhyolite couple is 37 times that in the rhyolite-andesite couple. Therefore, isotopic homogenization is roughly independent of elemental homogenization and the presence of SiO_2, Al_2O_3, and other concentration gradients, whereas elemental homogenization is strongly affected by concentration gradients. Our experimental data (isotopic and concentration profiles including uphill diffusion profiles) can be modeled quantitatively to a good approximation using a modified effective binary diffusion model in which the flux of a component is assumed to be proportional to its activity gradient instead of its concentration gradient. Therefore, the multicomponent diffusion effect in the silicate systems of our experiments seems to be largely due to contributions of non-ideal mixing to the cross-terms of the diffusivity matrix.

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Zhang, Youxue0000-0002-7439-0086
Additional Information:© 1994 Elsevier Science B.V. Received May 26, 1993; revision accepted March 11, 1994. SvdL thanks the Hawaii Institute of Geophysics/SOEST for a fellowship. We thank D.R. Baker, J.N. Christensen, E.J. Essene, H.N. Pollack, F.J. Ryerson, F.J. Spera and an anonymous reviewer for their helpful comments. This research was supported by the US National Science Foundation (EAR-89-04375, EAR-89-16707 and EAR-93-04161) and by NASA (NAGW-1472 and NAG-9-43).
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Hawaii Institute of GeophysicsUNSPECIFIED
Issue or Number:1-3
Record Number:CaltechAUTHORS:20160111-154022819
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Official Citation:Sieger van der Laan, Youxue Zhang, Allen K. Kennedy, Peter J. Wyllie, Comparison of element and isotope diffusion of K and Ca in multicomponent silicate melts, Earth and Planetary Science Letters, Volume 123, Issue 1, 1994, Pages 155-166, ISSN 0012-821X, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:63556
Deposited By: Tony Diaz
Deposited On:12 Jan 2016 23:40
Last Modified:10 Nov 2021 23:17

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