RARE-EARTH ELEMENT SYSTEMATICS OF THE CHEMICALLY PRECIPITATED COMPONENT IN EARLY PRECAMBRIAN IRON FORMATIONS AND THE EVOLUTION OF THE TERRESTRIAL ATMOSPHERE-HYDROSPHERE-LITHOSPHERE SYSTEM

Irrespective of provenance, age, and metamorphic grade, the chemically precipitated component in Early Precambrian (>2.3 Ga) iron formations (IFs) displays (Sm/Yb)CN < 1 and (Eu/Sm)SN > 1 which reflects the corresponding ratios of contemporaneous seawater. In conjunction with epsilon(Nd-IF) > epsilon(Nd-shale) (JACOBSEN and PIMENTEL-KLOSE, 1988a,b) this rare earth element (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. (Eu/Sm)N ratios of detritus-free IFs are controlled by the marine bottom water component, where (Eu/SM)CN > 1 results from REE input from high-temperature (high-T) hydrothermal fluids which received their REE signature during high-T alteration of oceanic crust at spreading centres. Decreasing (Eu/Sm)CN of IFs with decreasing depositional age reflect the vaning importance of high-T compared to low-T (<200-degrees-C) hydrothermal fluids, hence oceanic crust alteration, for the REE budget of marine bottom waters, and are related to decreasing upper mantle temperatures. (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 it is today. To explain this difference, two mechanisms are discussed on the basis of higher P(CO2) and lower P(O2) levels of the Precambrian atmosphere: (1) higher [CO32-] in Precambrian river and coastal waters and (2) redox cycling of the REEs at an oceanwide chemocline which separated anoxic and oxic water masses. However, these processes are synergetic and could have operated in unison. 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. After mixing of bottom and surface waters, their combined REE load was quantitatively scavenged onto precipitating iron oxihydroxides or iron carbonates and deposited with the chemically precipitated component. After addition of variable amounts of clastic detritus, this eventually resulted in the REE distribution observed in Precambrian IFs.