Geochemistry of Precambrian carbonate intercalated in pillows and its host basalt: implications for the REE composition of circa 3.4 Ga seawater

Abstract

The REE composition of early Archean seawater has been estimated through analyses of dolomite intercalated in pillow basalts from the Pilbara Block, Western Australia. We have critically evaluated the possible disturbances that have affected the chemical composition of the carbonate through geologic time. Dolomite samples from the Pilbara show a chondrite-normalized LREE-enriched pattern with profound downward convex tetrad effects, suggesting that these REE signatures are related to ancient seawater from which the dolomite precipitated. Contribution of a hydrothermal component to the seawater signature appears negligible based on the small Eu anomalies observed in dolomite. Therefore, dolomite samples from the Pilbara Block appear to preserve REEs intrinsic to ancient seawater, suggesting that the REE composition of seawater could be estimated given that carbonate–seawater partition coefficients of REEs are known. We used REE partition coefficients based on the analyses of a dolomite reference sample (JDo-1 from the Geological Survey of Japan) and show that REE concentrations of ∼3.4 Ga seawater could have been one or two orders of magnitude higher than at present. Moreover, it appears that 3.4 Ga seawater was significantly enriched in HREE and had less pronounced W-type tetrad effects relative to that of present seawater. These may reflect paleo-environmental conditions particular to seawater in the Archean, such as high-salinity and [CO 3 2−].