The chemical evolution of Precambrian seawater: Evidence from REEs in banded iron formations

Journal Article · Thu Nov 01 04:00:00 EST 1990 · Geochimica et Cosmochimica Acta; (USA)

DOI:https://doi.org/10.1016/0016-7037(90)90114-Z· OSTI ID:6026651

Derry, L A; Jacobsen, S B ^[1]

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1. Harvard Univ., Cambridge, MA (USA)

The authors present REE patterns determined by isotope dilution mass spectrometry from 20 Precambrian banded iron formation (BIF) samples, ranging in age from 3.4 to 0.65 Ga. The samples studied represent a variety of depositional settings and sedimentary facies. The oxide facies BIFs typically show LREE depleted patterns relative to shales, with (La/Nd){sub N} {ge} 1 and (La/Yb){sub N} mostly 0.3-0.8. The NASC-normalized REE patterns from the oxide facies BIFs are similar to modern metalliferous sediments, except for Ce and Eu. Positive Eu anomalies were found in all BIF samples, whereas Ce anomalies were variable. Both the oxide facies BIFs and modern metalliferous sediments typically have La/Yb ratios slightly higher than seawater. The REE data suggest that the Fe and REEs in BIFs coprecipitated from seawater as Fe oxy-hydroxides. The variable Ce anomalies are probably the result of diagenetic processes and redox cycling, similar to modern reducing environments. Comparison of mass balance estimates for REEs in the oceans based on Nd isotopic composition and on Eu anomalies results in a large discrepancy, which probably is a result of non-conservative behavior of Eu during mining of hydrothermal waters with ambient seawater. The REE data are consistent with a model for BIFs in which Fe and REEs derived from submarine hydrothermal systems are upwelled from the deep ocean, mixing with oxidizing surface water (with some continentally derived REEs), and deposited on the continental margins.

• OSTI ID: 6026651
• Journal Information: Geochimica et Cosmochimica Acta; (USA), Journal Name: Geochimica et Cosmochimica Acta; (USA) Vol. 54:11; ISSN GCACA; ISSN 0016-7037
• Country of Publication: United States
• Language: English

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58 GEOSCIENCES
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CERIUM ISOTOPES
CHEMICAL COMPOSITION
CHEMICAL REACTIONS
CHEMISTRY
DEPOSITION
DIAGENESIS
ELEMENTS
ENERGY SYSTEMS
EUROPIUM ISOTOPES
GEOCHEMISTRY
GEOLOGIC AGES
GEOLOGIC HISTORY
GEOTHERMAL SYSTEMS
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IRON ORES
ISOTOPE APPLICATIONS
ISOTOPE DILUTION
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MASS BALANCE
MASS SPECTROSCOPY
METALS
NEODYMIUM ISOTOPES
ORES
OXYGEN COMPOUNDS
PACIFIC OCEAN
PETROCHEMISTRY
PRECAMBRIAN ERA
RARE EARTH ISOTOPES
RARE EARTHS
REDOX REACTIONS
ROCK-FLUID INTERACTIONS
SEAS
SEAWATER
SPECTROSCOPY
SURFACE WATERS
THERMAL WATERS
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