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

Abstract

We 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) N ⩾ 1 and (La/Yb) 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 the REE patterns and ϵ Nd data of Jacobsen and Pimentel-Klose (1988a,b) from coeval BIFs and clastic sediments indicates that the source of the REEs in BIFs was distinct from that of the shales. The positive σ Nd values and positive Eu anomalies indicate that submarine hydrothermal sources were important or even dominant for the REEs, and, by extension, the Fe in BIF. We also report new measurements of REEs in hydrothermal waters from the East Pacific Rise 21 °N site. These have very large Eu anomalies (Eu/Eu∗ ∼-70), higher than previously reported. 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 mixing 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, mixed with oxidizing surface water (with some continentally derived REEs), and deposited on the continental margins.