IS THERE A SECULAR CHANGE IN OXYGEN ISOTOPE COMPOSITION OF CRUSTAL FLUIDS TRACKING THAT OF SEAWATER?

The δ¹⁸O of seawater directly or indirectly sets the starting composition of almost all crustal fluids, with the possible exception of magmatic fluids. If δ¹⁸O of seawater changed over geological time, so all crustal fluids and their subsequent rock or brine products would exhibit a secular change in δ¹⁸O. Most fossil brachiopods and sediments do show a secular trend of decreasing δ¹⁸O, which some explain as reflecting an ever lower δ¹⁸O ocean. However, the δ¹⁸O ophiolites and ore deposits do not change indicating a constant δ¹⁸O of seawater. The secular trend in sediments can be explained if the temperature of the oceans were higher. A 40 year controversy has continued on this point but has recently reignited as a result of the recent suggestion that the Archean oceans were very hot, a conclusion incompatible with some models of atmosphere and seawater compositions. Critics of a hot Archean invoke moderate temperatures but a very low δ¹⁸O ocean (-13 ppt SMOW) as the explanation for the low δ¹⁸O Archean cherts. A -13 ‰ ocean would impose a clear signal on any product of rock alteration by the hydrosphere, with much lower values in higher alteration temperature products. The recently discovered sheeted dike complex at Isua, Greenland provides insights into the 3.8 Ga ocean and may finally resolve the long standing controversy the δ¹⁸O of ancient oceans. The Isua dikes range in δ¹⁸O from 5.7 to 6.9 ppt giving no evidence of a low, but rather a slightly enriched ¹⁸O seawater at 3.8 Ga. The data from Isua as well as previous studies on ¹⁸O-enriched pillows from Pilbara (3.5 Ga.), Barberton (3.5 Ga) and Kid Creek (2.7 Ga) show that the suggestion of a very low δ¹⁸O, early ocean is untenable. We can conclude from the oxygen isotope analyses of the pillows and dykes that the earliest ocean had already reacted with the sea floor. This has great implications to the composition of the oceans themselves because seawater/seafloor rock reactions are known to massively affect if not control the chemical make up of seawater.