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 cor responding ratios of contemporaneous seawater. In conjunction with eps ilon(Nd-IF) > epsilon(Nd-shale) (JACOBSEN and PIMENTEL-KLOSE, 1988a,b) this rare earth element (REE) signature reveals that the REE distribu tion in Early Precambrian IFs must be explained by mixing between a ma rine bottom and a surface water component, and that the REEs (and by a nalogy the Fe) cannot be derived from weathering of a continental sour ce. (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 fro m high-temperature (high-T) hydrothermal fluids which received their R EE signature during high-T alteration of oceanic crust at spreading ce ntres. Decreasing (Eu/Sm)CN of IFs with decreasing depositional age re flect the vaning importance of high-T compared to low-T (<200-degrees- C) hydrothermal fluids, hence oceanic crust alteration, for the REE bu dget of marine bottom waters, and are related to decreasing upper mant le temperatures. (Sm/Yb)N ratios of detritus-free IFs are controlled b y the marine surface water component, whose REE distribution is affect ed by the same processes which operate today in the entire ocean. Mixi ng calculations reveal that (Sm/Yb)CN in Early Precambrian marine surf ace waters was significantly lower than it is today. To explain this d ifference, 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 th e REEs at an oceanwide chemocline which separated anoxic and oxic wate r masses. However, these processes are synergetic and could have opera ted 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 i nput from the dissolved REE pool in river waters, after some modificat ion in estuaries, dominated the REE distribution in marine surface wat ers. After mixing of bottom and surface waters, their combined REE loa d was quantitatively scavenged onto precipitating iron oxihydroxides o r iron carbonates and deposited with the chemically precipitated compo nent. After addition of variable amounts of clastic detritus, this eve ntually resulted in the REE distribution observed in Precambrian IFs.