Neodymium isotopic studies of Precambrian banded iron formations

Thesis/Dissertation · Wed Jan 01 04:00:00 EST 1986

OSTI ID:6149171

Pimentel-Klose, M R

The isotopic composition of Nd is reported for 11 different Precambrian BIFs and suggests a trend of increasingly positive epsilon/sub Nd/(T) values with age. This trend is interpreted to reflect isotopic variations in precambrian sea water. The Urucum and the Gunflint BIFs, both younger than 1.8 AE, yield negative epsilon/sub Nd/(T) values, between -4.6 and -1.1. The remaining BIFs, with ages between 1.85 and 3.4 AE, have predominantly positive values, between -0.7 and +4.0. The low UXSm/ UUNd ratio in BIFs is inconsistent with their REE being derived from rivers draining large proportions of greenstones. The positive, mantle-like values of BIFs older than 1.8 AE contrasts strongly with the negative, continental-like values of Phanerozoic sea water. Therefore, the REE budget of the oceans during most of the Precambrian was probably dominated by the hydrothermal circulation of sea water through MORBs and not by river waters, as today. A one order of magnitude higher hydrothermal contribution of Nd is suggested by the data for the Early Precambrian. This is most likely due to a higher hydrothermal water to river water flux ratio and/or a higher Nd concentration in hotter hydrothermal waters (>375 C) during this period. The large hydrothermal contribution of REE during the Early Precambrian can be explained best if the temperature of sea water fluxing through MORBs was higher than today. Experiments investigating the interaction between sea water and basalt have shown that the concentration of iron might have been about 20 times larger if the temperature of interaction was about 425 C. Such hot hydrothermal solutions could have been the most important source of iron in Precambrian BIFs.

• Research Organization: Harvard Univ., Boston, MA (USA)
• OSTI ID: 6149171
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580100* -- Geology & Hydrology-- (-1989)
AGE ESTIMATION
BASALT
CHEMISTRY
ELEMENTS
ENERGY SYSTEMS
GEOLOGIC AGES
GEOLOGIC DEPOSITS
GEOLOGIC HISTORY
GEOTHERMAL SYSTEMS
HYDROGEN COMPOUNDS
HYDROTHERMAL SYSTEMS
IGNEOUS ROCKS
IRON
ISOTOPE RATIO
METALS
NEODYMIUM
OCEANOGRAPHY
OXYGEN COMPOUNDS
PALEOTEMPERATURE
PRECAMBRIAN ERA
QUANTITY RATIO
RARE EARTHS
ROCKS
SEAWATER
TRANSITION ELEMENTS
VOLCANIC ROCKS
WATER
WATER CHEMISTRY