Ion microprobe analysis of oxygen isotope ratios in granulite facies magnetites; diffusive exchange as a guide to cooling history

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Authors:Valley, John W.; Graham, Colin M.
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, 109(1), p.38-52. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1991
Note:In English. 60 refs.; illus. incl. 3 tables
Summary:Ion microprobe analysis of magnetites from the Adirondack Mts, NY, yields oxygen isotope ratios with spatial resolution of 2-8 mu m and precision in the range of 1 per mille (1 sigma). These analyses represent 11 orders of magnitude reduction in sample size compared to conventional analyses on this material and they are the first report of routinely reproducible precision in the 1 per mille range for analysis of delta18O at this scale. The analysed magnetites form nearly spherical grains in a calcite matrix with diopside and monticellite. Textures are characteristic of granulite-facies marbles and show no evidence for retrograde recrystallization of magnetites. Magnetites are near to Fe3O4 in composition, optically and chemically homogeneous. A combination of ion probe plus conventional BrF5 analysis shows that individual grains are homogeneous with delta18OSMOW = 8.9 +- 1 per mille from the core to near the rim of 0.1-1.2 mm diameter grains. Depth profiling into crystal growth faces of magnetites shows that the rims are 9 per mille depleted in delta18O. These low delta18O values increase in smooth gradients across the outer 10 mu m of magnetite rims in contact with calcite. These are the sharpest intracrystalline gradients measured to date in geological materials. This discovery is confirmed by bulk analysis of 150-350 mu m diameter magnetites which average 1.2 per mille lower in delta18O than coarse magnetites, due to low delta18O rims. Conventional analysis of coexisting calcite yields delta18O = 18.19, suggesting that bulk delta18O(Cc-Mt) = 9.3 per mille and yielding an apparent equilibration 'T' of 525 degrees C, over 200 degrees C below the T of regional metamorphism. Consideration of experimental diffusion data and grain size distribution for magnetite and calcite suggests two contrasting cooling histories. The data for O in calcite under hydrothermal conditions at high PH2O indicate that diffusion is faster in magnetite and modelling of the low delta18O rims on magnetite suggest that the Adirondacks experienced slow cooling, followed by a brief period of rapid cooling, possibly related to uplift. [P.Br.]
Subjects:Granulites; High temperature; Ion probe data; Isotope ratios; Isotopes; Magnetite; Mass spectra; Metamorphic rocks; O-18/O-16; Oxides; Oxygen; Phase equilibria; Spectra; Stable isotopes; Temperature; Adirondack Mountains; New York; United States
Abstract Numbers:92M/1698
Record ID:1993026640
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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