Oxygen isotopes in detrital zircons: Insight into crustal recycling during the evolution of the Greenland Shield

Abstract

Insight into the interactions between crust and hydrosphere, through the protracted evolution of the Greenland Shield, can be provided by oxygen isotopes in the mineral remnants of its denuded crust. Detrital zircons with ages of 3900 Ma to 900 Ma found within an arkosic sandstone dike of the Neoproterozoic (?Marinoan) Moraeneso Formation, North Greenland, provide a time-integrated record of the evolution of part of the Greenland Shield. These zircon grains are derived from a wide variety of sources in northeastern Laurentia, including Paleoproterozoic and older detritus from the Committee-Melville orogen, the Ellesmere-Inglefield mobile belt, and the subice continuation of the Victoria Fjord complex. Archean zircon crystals have a more restricted range of δ 18 O SMOW values (between 7.2‰ and 9.0‰ relative to standard mean ocean water [SMOW]) in comparison to Paleoproterozoic 1800–2100 Ma grains, which display significant variation in δ 18 O SMOW (6.8‰–10.4‰). These data reflect differences in crustal evolution between the Archean and Proterozoic Earth. Through time, remelting or reworking of high δ 18 O materials has become more important, consistent with the progressive emergence of buoyant, cratonized continental lithosphere. A secular increase in the rate of crustal recycling is implied across the Archean-Proterozoic boundary. This rate change may have been a response to differences in the composition of sediments and/or the stabilization of continental crust. One Eoarchean oscillatory-zoned zircon grain, free of cracks and with concordant U-Pb systematics, has an elevated δ 18 O SMOW value of 7.8‰. This is interpreted to reflect a primary magmatic signature, supporting the presence of heavy oxygen that may be compatible with a hydrosphere on early Earth, as previously determined only from Jack Hills zircons.