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

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Authors:Eiler, J. M.; Valley, J. W.; Graham, C. M.; Baumgartner, L. P.
Author Affiliations:Primary:
University of Wisconsin, Department of Geology and Geophysics, Madison, WI, United States
University of Edinburgh, United Kingdom
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 118(4), p.365-378. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1995
Note:In English. 56 refs.; illus., incl. 2 tables, geol. sketch map
Summary: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 O isotope ratios in magnetite were combined with conventional bulk mineral analyses and diffusion modelling to document the relationship between mineral zonation and the mechanism of retrogression inferred from bulk mineral data. Two samples of magnetite-bearing, quartzofeldspathic Lyon Mountain gneiss from the Adirondack Mts were studied in detail. Conventional stable isotope analysis of both samples indicates that isotopic thermometers are discordant and were reset by as much as 200°C from the estimated peak T of 750°C. The relative order of apparent T recorded by various thermometers differs between the two samples, with Tqtz-fsp >>>> Tmt-qtz and Tmt-fsp in one sample and Tqtz-fsp << Tmt-qtz and Tmt-fsp in the other. Diffusion modelling using the Fast Grain Boundary model shows that the former pattern of apparent T is consistent with closed-system interdiffusion during cooling, whereas the latter is not. The modelling predicts that 0.5 mm diam. magnetite grains common to this rock type will contain isotopic zonation of 10/00 (rims lower in Δ18O than cores) and that the smaller grain cores will be similarly lower than 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 (Δ core-rim = 1.1 ± 0.40/00) 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 80/00 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. [P.Br.]
Subjects:Closed systems; Electron probe; Grain size; High-grade metamorphism; Ion probe; Isotopes; Mass spectroscopy; Metamorphism; O-18/O-16; Oxygen; Retrograde metamorphism; Spectroscopy; Stable isotopes; Temperature; Adirondack Mountains; New York; United States; Fractionation; Lyon Mountain Gneiss
Abstract Numbers:95M/3164
Record ID:1995034548
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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