Oxygen isotope variations in granulite-grade iron formations: constraints on oxygen diffusion and retrograde isotopic exchange |
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| Authors: | Z D Sharp J R O'Neil E J Essene |
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| Institution: | (1) Department of Geological Sciences, University of Michigan, 48109 Ann Arbor, MI;(2) United States Geological Survey, MS 937, 345 Middlefield Rd., 94025 Menlo Park, CA;(3) Present address: Geophysical Laboratory, Carnegie Institution of Washington, 2801 Upton St, 20008 N.W. Washington, DC, USA;(4) Present address: Department of Geological Sciences, University of Michigan, 48109 Ann Arbor, MI, USA |
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| Abstract: | The oxygen isotope ratios of various minerals were measured in a granulite-grade iron formation in the Wind River Range, Wyoming. Estimates of temperature and pressure for the terrane using well calibrated geothermometers and geobarometers are 730±50° C and 5.5±0.5 kbar. The mineral constraints on fluid compositions in the iron formation during retrogression require either very CO2-rich fluids or no fluid at all. In the iron formation, isotopic temperature estimates from quartz-magnetite fractionations are controlled by the proximity to the enclosing granitic gneiss, and range from 500° C (
qz – mt=10.0 ) within 2–3 meters of the orthogneiss contact to 600° C (
qz – mt=8.0) farther from the contact. Temperature estimates from other isotopic thermometers are in good agreement with those derived from the quartz-magnetite fractionations.During prograde metamorphism, the isotopic composition of the iron formation was lowered by the infiltration of an external fluid. Equilibrium was achieved over tens of meters. Closed-system retrograde exchange is consistent with the nearly constant whole-rock 
18Owr value of 8.0±0.6. The greater
qz-mt values in the iron formation near the orthogneiss contact are most likely due to a lower oxygen blocking temperature related to greater exchange-ability of deformed minerals at the contact. Cooling rates required to preserve the quartz-magnetite fractionations in the central portion of the iron formation are unreasonably high ( 800° C/Ma). In order to preserve the 600° C isotopic temperature, the diffusion coefficient D (for  -quartz) should be two orders of magnitude lower than the experimentally determined value of 2.5×10–16 cm2/s at 833 K. There are no values for the activation energy (Q) and pre-exponential diffusion coefficient (D
0), consistent with the experimentally determined values, that will result in reasonable cooling rates for the Wind River iron formation. The discrepancy between the diffusion coefficient inferred from the Wind River terrane and that measured experimentally is almost certainly due to the enhancement of exchange by the presence of water in the laboratory experiments. Cooling rate estimates were also determined for iron formation retrograded under water-rich conditions. Application of the experimentally determined data to these rocks results in a reasonable cooling rate estimate, supporting the conclusion that the presence of water greatly enhances oxygen diffusion.Contribution 441 from the Mineralogical Laboratory, University of Michigan |
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