Other
Scientific paper
May 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993gecoa..57.2239b&link_type=abstract
Geochimica et Cosmochimica Acta (ISSN 0016-7037), vol. 57, no. 10, p. 2239-2249.
Other
14
Earth Atmosphere, Planetary Evolution, Precambrian Period, Rare Earth Elements, Sediments, Chemical Composition, Marine Environments, Sea Water
Scientific paper
Regardless of age and metamorphic grade, the chemically precipitated component in Early Precambrian (greater than 2.3 Ga) iron formations (IFs) shows (Sm/Yb)CN less than 1 and (Eu/Sm)SN greater than 1, reflecting the corresponding ratios of contemporaneous sea water. This REE signature reveals that the REE distribution in Early Precambrian IFs must be explained by mixing between a marine bottom and a surface water component, and that the REEs (and by analogy the Fe) cannot be derived from weathering of a continental source. (Sm/Yb)N ratios of detritus-free IFs are controlled by the marine surface water component, whose REE distribution is affected by the same processes which operate today in the entire ocean. Mixing calculations reveal that (Sm/Yb)CN in Early Precambrian marine surface waters was significantly lower than today. To explain this difference, two mechanisms are discussed on the basis of higher PCO2 and lower PO2 levels of the Precambrian atmosphere. The REE distribution in Precambrian IFs is described as a result of mixing in a multicomponent system, where high-T and low-T hydrothermal fluids contributed to the marine bottom water, and REE input from the dissolved REE pool in river waters, after some modification in estuaries, dominated the REE distribution in marine surface waters.
Bau Michael
Moeller Peter
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