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The oxygen isotope anatomy of a slowly cooled metamorphic rock

Eiler, John M. and Valley, John W. and Graham, Colin M. and Baumgartner, Lukas P. (1995) The oxygen isotope anatomy of a slowly cooled metamorphic rock. American Mineralogist, 80 (7-8). pp. 757-764. ISSN 0003-004X.

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The distribution of O isotope ratios in a slowly cooled metamorphic rock is investigated by a combination of ion microprobe and laser probe techniques. The sample has unusually simple geometry: centimeter-size domains of feldspar and garnet separated by a one-millimeter to one-centimeter thick layer of millimeter-size magnetite grains. Isotopic zonation at two different scales is documented. A sharp decrease in δ^(18)O in the outer 10-100 μm of individual magnetite grains is in agreement with modeling of isotopic reequilibration during cooling by interdiffusion between feldspar and magnetite. The presence of this zonation in the outer portions of magnetite grains that are adjacent to garnet indicates extensive transport along grain boundaries during diffusive exchange. Millimeter-scale zonation in the polygranular layer of magnetite is caused by the interaction of volume diffusion and grain boundary transport, the effects of which depend upon local textures. Grain boundaries enhance bulk diffusion in polygranular aggregates and allow exchange between nontouching grains. The results document a complex textural control to millimeter-scale O isotope zonation and geothermometry in slowly cooled rocks and document the importance of fast grain boundary transport in controlling diffusive exchange.

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Additional Information:© 1995 Mineralogical Society of America. Manuscript received September 2, 1994; manuscript accepted March 13, 1995. We wish to acknowledge the financial support of the National Science Foundation, the Department of Energy, the Wisconsin Alumni Research Foundation, the Geological Society of America, and Sigma Xi. Thorough and helpful reviews by Ray Joesten and Jean Morrison substantially improved this paper. We also thank Stuart Keams of the University of Edinburgh for his assistance with electron microprobe analysis, Rick Noll at the University of Wisconsin for his assistance with the SEM, Eugene Cameron for providing the sample used in this study, and Mike Spicuzza and Matt Kohn for help and training in laser probe analysis. Particular thanks are due as well to John Craven for assistance, training, and advice in the use of the Edinburgh ion microprobe, which is supported by NERC.
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Department of Energy (DOE)UNSPECIFIED
Wisconsin Alumni Research FoundationUNSPECIFIED
Geological Society of AmericaUNSPECIFIED
Issue or Number:7-8
Record Number:CaltechAUTHORS:20121023-142633236
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35044
Deposited On:23 Oct 2012 22:48
Last Modified:03 Oct 2019 04:24

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