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It has long been recognized that the stable oxygen isotope 18O/16O ratios of chemical sediments show a systematic decrease with increasing age. This continuous, 3.8 Ga long secular trend has been attributed to 80 °C hot early Earth oceans, a ~15‰ lower 18O/16O ratio of Archean seawater, or diagenetic obliteration of the original isotope signal. We demonstrate that high-precision triple oxygen isotope measurements including the rare third stable oxygen isotope 17O on silica rocks (cherts) can be used to shed light on this major problem in geochemistry. Our triple oxygen isotope data do not support 80 °C hot oceans in the Archean. The data also exclude that the 18O/16O of seawater was much lower in the Archean. Also, a simple freshwater-origin of cherts is not supported by the new data. We discuss the effects of diagenetic alteration on the triple oxygen isotope compositions of cherts and show that the Archean and Proterozoic cherts could have equilibrated with fluids with higher 18O/16O than today's oceans. For the Archean cherts, the high 18O/16O fluids required to explain the observations could reflect the composition of ambient seawater or of modified hydrothermal fluids. Oceans with higher 18O/16O ratio would be expected for an Archean "water world" without large continents. High 18O/16O fluids, however, could also have formed locally by reactions between seawater and the oceanic crust. Our study shows that high-precision triple oxygen isotope data provide new understandings of processes deep in time. [ABSTRACT FROM AUTHOR]

The δ 18 O value of seawater is presently buffered by high-temperature hydrothermal alteration and low-temperature weathering of lithospheric rocks. It is much debated whether the δ 18 O of seawater has been buffered to this steady-state value throughout Earth's history or, alternatively, whether it gradually increased towards the present value since the Archean. A third possibility is that the δ 18 O of seawater was buffered at a higher value before continents emerged, and has been buffered at its current value (δ 18 O ∼ −1‰) since the emergence of continents. In this contribution, we reconstruct the δ 18 O of Archean seawater from triple oxygen isotope (δ 17 O, δ 18 O) variations in >2.7 Ga ultramafic rocks that reacted with fluids that, in turn, had been derived from seawater in the Archean. The samples that were studied are peridotites from the Ivnartivaq complex, an ultramafic lens in the Archean Kuummiut terrane (Rae craton, southeast Greenland) that have unusually low δ 18 O values for peridotites (in olivine, 1.7 ‰ ≤ δ 18 O ≤ 4.6 ‰ , relative to VSMOW). Bulk rock trace element concentrations, mineral compositions and U-Pb dating of zircon grains in the peridotites indicate that these samples are metamorphic peridotites, that formed by the dehydration of serpentinites ∼2.7 Ga ago. The serpentinite protoliths, in turn, had formed by alteration of ultramafic cumulate rocks at high temperatures (250–450 °C) by fluids that were derived from seawater. Triple oxygen isotope variations in olivine from the peridotites indicate that the fluids could not have been derived from seawater with a significantly higher or lower δ 18 O value than that of seawater in the Phanerozoic, but could have been derived from seawater with a value of δ 18 O ∼ −1‰, i.e., the modern (ice-free) seawater composition. We conclude that the δ 18 O of seawater had reached its current steady state value by 2.7 Ga ago. • Archean peridotites from southeast Greenland contain olivine with 1.7‰ ≤ δ 18 O ≤ 4.6‰. • They are crystal cumulates that were altered by seawater to low- δ 18 O serpentinites. • The serpentinites became dehydrated during metamorphism ∼2.7 Ga ago. • We reconstruct the δ 18 O of >2.7 Ga seawater from triple O isotope variations in olivine. • Seawater had reached its current steady state value (δ 18 O ∼ −1‰) by 2.7 Ga ago. [ABSTRACT FROM AUTHOR]