The negative Eu anomaly in Archean sedimentary rocks: Implications for decomposition, age and importance of their granitic sources

A negative correlation between Eu anomaly (Eu/Eu* and chemical index of weathering (CIW) is found for Archean pelites in both greenstone belts and high-grade terrains from southern Africa, western Greenland, North America, Western Australia and China. A similar negative correlation betweenEu/Eu* and chemical index of alteration (CIA) is also found for worldwide young deep-sea sediments from various tectonic settings. The increase in the indices of weathering and alteration coincide with both shifts in mineral composition and mineral decomposition from plagioclase/K-feldspar to mica/kaolinite and from amphibole to chlorite/smectite. Juvenile crustal materials from local sources are only slightly decomposed due to little or no transport. They are characterized by low CIW/CIA and negligible Eu anomalies. In contrast, crustal materials from large provenance areas have usually suffered long transport and recycling, which results in decomposition of high-temperature minerals and large CIW/CIA values that are negatively correlated with significant negative Eu anomalies. Thus, the observed trends are interpreted as mainly reflecting increasing contributions of recycled and decomposed materials from large provenances relative to juvenile crustal additions through arc magmatism at local sources. In this context, there is no obvious distinction between Archean greenstone and high-grade metasediments and young deep-sea sediments. The large provenance areas have contained a significant proportion of granitic and rhyolitic materials, which, in part, have not resisted decomposition and erosion due to their occurrence in shallow crustal layers. Granites received their distinctive negative Eu anomaly from equilibration with a feldspar-rich residuum. gative Eu anomaly of Archean metaclastic sediments apparently increased with decreasing age. Trends of minimum and averageEu/Eu* values of metaclastic sediments evolving with time are calculated for the Archean and lead us to infer that the negative Eu anormaly (Eu/Eu*< 0.9) of the upper continental crust might have appeared before 3.8 Ga. Thus, the first granite responsible for intracrustal fractionation originated very early. The negative Eu anomaly had been a common feature at least for a time span of ca. 3.2 Ga. At 2.5 Ga, the upper continental crust and clastic sediments had almost acquired their present Eu signature (solEu/Eu*∼ 0.65). The Archean-Proterozoic transition is not a boundary in the evolution of the composition of the continental crust.