>2.7 Ga metamorphic peridotites from southeast Greenland record the oxygen isotope composition of Archean seawater

The δ¹⁸O value of seawater is presently buffered by high-temperature hydrothermal alteration and low-temperature weathering of lithospheric rocks. It is much debated whether the δ¹⁸O 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 δ¹⁸O of seawater was buffered at a higher value before continents emerged, and has been buffered at its current value (δ¹⁸O ∼ −1‰) since the emergence of continents. In this contribution, we reconstruct the δ¹⁸O of Archean seawater from triple oxygen isotope (δ¹⁷O, δ¹⁸O) 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 δ¹⁸O values for peridotites (in olivine, 1.7‰≤δ¹⁸O≤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 δ¹⁸O value than that of seawater in the Phanerozoic, but could have been derived from seawater with a value of δ¹⁸O ∼ −1‰, i.e., the modern (ice-free) seawater composition. We conclude that the δ¹⁸O of seawater had reached its current steady state value by 2.7 Ga ago.