Oxygen isotope variations in granulite-grade iron formations: constraints on oxygen diffusion and retrograde isotopic exchange

摘要

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 CO 2 -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 ( Δ qz − mt =10.0‰) within 2–3 meters of the orthogneiss contact to 600° C ( Δ 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.