Seawater-like trace element signatures (REE + Y) of Eoarchaean chemical sedimentary rocks from southern West Greenland, and their corruption during high-grade metamorphism
Modern chemical sediments display a distinctive rare earth element + yttrium (REE + Y) pattern involving depleted LREE, positive La/La*(SN), Eu/Eu*(SN), and Y(SN) anomalies (SN = shale normalised) that is related to precipitation from circumneutral to high pH waters with solution complexation of the REEs dominated by carbonate ions. This is often interpreted as reflecting precipitation from surface waters (usually marine). The oldest broadly accepted chemical sediments are c. 3,700 Ma amphibolite facies banded iron-formation (BIF) units in the Isua supracrustal belt, Greenland. Isua BIFs, including the BIF international reference material IF-G are generally considered to be seawater precipitates, and display these REE + Y patterns (Bolhar et al. in Earth Planet Sci Lett 222:43-60, 2004). Greenland Eoarchaean BIF metamorphosed up to granulite facies from several localities in the vicinity of Akilia (island), display REE + Y patterns identical to Isua BIF, consistent with an origin by chemical sedimentation from seawater and a paucity of clastic input. Furthermore, the much-debated magnetite-bearing siliceous unit of "earliest life" rocks (sample G91/26) from Akilia has the same REE + Y pattern. This suggests that sample G91/26 is also a chemical sediment, contrary to previous assertions (Bolhar et al. in Earth Planet Sci Lett 222:43-60, 2004), and including suggestions that the Akilia unit containing G91/26 consists entirely of silica-penetrated, metasomatised, mafic rock (Fedo and Whitehouse 2002a). Integration of our trace element data with those of Bolhar et al. (Earth Planet Sci Lett 222:43-60, 2004) demonstrates that Eoarchaean siliceous rocks in Greenland, with ages from 3.6 to 3.85 Ga, have diverse trace element signatures. There are now geographically-dispersed, widespread examples with Isua BIF-like REE + Y signatures, that are interpreted as chemically unaltered, albeit metamorphosed, chemical sediments. Other samples retain remnants of LREE depletion but are beginning to lose the distinct La, Eu and Y positive anomalies and are interpreted as metasomatised chemical sediments. Finally there are some siliceous samples with completely different trace element patterns that are interpreted as rocks of non-sedimentary origin, and include metasomatised mafic rocks. The positive La/La*(SN), Eu/Eu*(SN) and Y(SN) anomalies found in Isua BIFs and other Eoarchaean Greenland samples, such as G91/26 from Akilia, suggests that the processes of carbonate ion complexation controlling the REE - Y patterns were already established in the hydrosphere at the start of the sedimentary record 3,600-3,850 Ma ago. This is in accord with the presence of Eoarchaean siderite-bearing marbles of sedimentary origin, and suggests that CO(2) may have been a significant greenhouse gas at that time.
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