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Ion microprobe evidence for the mechanisms of stable isotope retrogression in high-grade metamorphic rocks

Eiler, J. M. and Valley, J. W. and Graham, C. M. and Baumgartner, L. P. (1995) Ion microprobe evidence for the mechanisms of stable isotope retrogression in high-grade metamorphic rocks. Contributions to Mineralogy and Petrology, 118 (4). pp. 365-378. ISSN 0010-7999. http://resolver.caltech.edu/CaltechAUTHORS:20121025-112508404

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Abstract

Retrograde interdiffusion is widely proposed as the dominant factor in producing the stable isotopic fractionation among minerals in slowly cooled igneous and metamorphic rocks. Mineral zonation consistent with interdiffusion of stable isotopes has never been directly observed, however, leaving doubt as to the mechanism responsible for the bulk-mineral isotopic compositions commonly measured. Ion microprobe analyses of oxygen isotope ratios in magnetite were combined with conventional bulk mineral analyses and diffusion modeling to document the relationship between mineral zonation and the mechanism of retrogression inferred from bulk mineral data. Two samples of magnetitebearing, quartzo-feldspathic Lyon Mountain gneiss from the Adirondack mountains, N.Y. were studied in detail. Conventional stable isotope analysis of both samples indicates that isotope thermometers are discordant and were reset by as much as 200°C from the estimated peak temperature of 750°C. The relative order of apparent temperatures recorded by various thermometers differs between the two samples, however, with T_(qtz-fsp) ≫ T_(mt-qtz) and T_(mt-fsp) in one sample and T_(qtz-fsp) < T_(mt-qtz) and T_(mt-fsp) in the other. Diffusion modeling using the Fast Grain Boundary model shows that the former pattern of apparent temperatures is consistent with closed system interdiffusion during cooling, whereas the latter is not. The modeling predicts that 0.5 mm diameter magnetite grains common to this rock type will contain isotopic zonation of 1‰ (rims lower in δ^(18)O than cores), and that the cores of smaller (0.1 mm) grains will be similarly lower than to the cores of large (0.5 mm) grains. Ion microprobe analysis reveals that the zoning patterns of magnetite grains from the first sample contain clear core to rim zonation in multiple grains (Δcore-rim=1.1±0.4‰) and predicted grain-size vs core composition variations, consistent with diffusion-controlled resetting of bulk mineral fractionations. In contrast, the second sample shows irregular inter-and intra-granular variations over an 8‰ range, consistent with open system alteration. These results provide direct documentation of the importance of interdiffusion in affecting stable isotope distributions in slowly cooled rocks. The correlations of bulk-mineral resetting with zonation show that bulk mineral data, when interpreted with detailed modeling, can be used to determinate what processes controlling retrogression.


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Additional Information:© 1995 Springer-Verlag. Received: 24 December 1993; Accepted: 4 June 1994. We gratefully acknowledge the financial support of a National Science Foundation doctoral fellowship and a Wisconsin Alumni Research Foundation fellowship (J.M.E.), NSF grants EAR-9105709 (J.W.V.) EAR-9206090 (L.P.B.) DOE grant 93ER 14389 (J.W.V.), and research grants to J.M.E. from the Geological Society of America, Sigma Xi and the University of Wisconsin. Thanks are due to Phil Whitney for supplying samples of Lyon Mountain Gneiss, to Mike Spicuzza for assistance with conventional stable isotope analysis, to Jill Banfield for enlightening discussions on oxide defects, and to an anonymous reviewer for suggestions that improved this manuscript. We especially thank John Craven for assistance, training and advice in the use of the ion microprobe. Thanks also are due to Stuart Kearns for technical assistance with electron microprobe analysis at the University of Edinburgh, and to Rick Noll for assistance with the Scanning Electron Microscope at the University of Wisconsin. Editorial responsibility: I. S. E. Carmichael.
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NSFUNSPECIFIED
Wisconsin Alumni Research FoundationUNSPECIFIED
NSFEAR-9105709
NSFEAR-9206090
Department of Energy (DOE)93ER14389
Geological Society of AmericaUNSPECIFIED
Sigma XiUNSPECIFIED
University of WisconsinUNSPECIFIED
Record Number:CaltechAUTHORS:20121025-112508404
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20121025-112508404
Official Citation:Eiler, J., J. Valley, et al. (1995). "Ion microprobe evidence for the mechanisms of stable isotope retrogression in high-grade metamorphic rocks." Contributions to Mineralogy and Petrology 118(4): 365-378.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35097
Collection:CaltechAUTHORS
Deposited By: Aucoeur Ngo
Deposited On:25 Oct 2012 20:42
Last Modified:25 Oct 2012 20:42

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