Rare earth element systematics of the chemically precipitated component in Early Precambrian iron formations and the evolution of the terrestrial atmosphere-hydrosphere-lithosphere system

Journal Article · Sat May 01 00:00:00 EDT 1993 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(93)90566-F· OSTI ID:6085184

Bau, M; Moeller, P ^[1]

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1. Hahn-Meitner-Institut, Berlin (Germany)

The chemically precipitated component in Early Precambrian (> 2.3 Ga) iron formations (IFs) displays (Sm/Yb)[sub CN] < 1 and (Eu/Sm)[sub SN] > 1 which reflects the corresponding ratios of contemporaneous seawater. In conjunction with [epsilon][sub Nd-IF] > [epsilon][sub Nd-shale] 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. Mixing calculations reveal that (Sm/Yb)[sub 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[sub CO[sub 2]] and lower P[sub O[sub 2]] levels of the Precambrian atmosphere: (1) higher [CO[sub 3][sup 2[minus]]] 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. 94 refs., 6 figs., 3 tabs.

• OSTI ID: 6085184
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Vol. 57:10; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
EARTH ATMOSPHERE
PLANETARY EVOLUTION
HYDROSPHERE
IRON ORES
CHEMICAL COMPOSITION
RARE EARTHS
PRECIPITATION
SURFACE WATERS
MIXING
CARBON DIOXIDE
EUROPIUM
OXYGEN
PALEOCLIMATOLOGY
PRECAMBRIAN ERA
SAMARIUM
YTTERBIUM
CARBON COMPOUNDS
CARBON OXIDES
CHALCOGENIDES
ELEMENTS
GEOLOGIC AGES
METALS
NONMETALS
ORES
OXIDES
OXYGEN COMPOUNDS
PALEONTOLOGY
SEPARATION PROCESSES
SOLAR SYSTEM EVOLUTION
540310* - Environment
Aquatic- Basic Studies- (1990-)
540110