The Oxygen Isotope Composition of 3,800 M.Y. Old Metamorphosed Chert and Iron Formation from Isukasia, West Greenland

Abstract

$$\delta^{18}O$$ of metamorphosed chert from a thick sequence of chert and iron formation at Isukasia, west Greenland ranges from +12.9 to +20.4‰. Rocks at this locality satisfy three criteria suggesting that isotope exchange occurred only in a limited volume of material. It is our conclusion that the maximum observed $$\delta^{18}O$$ of metamorphosed chert of 20.4‰ may be close to the sedimentary or diagenetic oxygen isotope composition of this rock. Magnetite $$\delta^{18}O$$ ranges from +1.1 to +7.6‰ and the mean quartz-magnetite oxygen isotope fractionation ($$\Delta_{QM}$$) is 11.6 indicating a mean temperature of metamorphism of about 390°C. Extrapolation of data assuming closed system metamorphic conditions suggests an initial $$\delta^{18}O$$ for magnetite or its precursor oxide of -8.7‰ or lower. If this is so, $$\Delta_{QM}$$ during diagenesis or sedimentation was 28.9 or more, and the corresponding temperature was about 100°C or less. A review of published isotope data on the Proterozoic Biwabik/Gunflint and Hamersley Iron Formations indicates probable primary $$\delta^{18}O_{Q}$$ values of 24‰ for these rocks, and a primary quartz-magnetite oxygen isotope fractionation of about 39. The temperature of the oceans in which these 2,000 m.y. old iron formations were deposited thus is estimated to have been about 22°C. Evidence is strong that ancient cherts are depleted in 18O compared to modern cherts and that this phenomenon is not entirely the result of metamorphism. Whether caused by a surface temperature difference or differing $$\delta^{18}O$$ of the ocean, the $${}^{18}O$$ content of chert seems to have begun to increase significantly about 2,500-2,700 m.y. ago, a time at which $$^{87}Sr/^{86}Sr$$ ratios in sedimentary rocks and $$Na_{2}O/K_{2}O$$ ratios in crustal igneous rocks also changed. We suggest that $$\delta^{18}O$$ of the early ocean was controlled by weathering and/or low temperature hydrothermal alteration of volcanic rocks and that recycling of these volcanic rocks as tonalitic gneisses may be primarily responsible for the increase in $$\delta^{18}O$$ of crustal igneous rocks from about +6 to about +8.5‰.