Anomalously low Al 2O 3/TiO 2 values for Archean cherts from the Pilbara Block, Western Australia—possible evidence for extensive chemical weathering on the early earth

Abstract

Geochemical, sedimentological and mineralogical features of 3.0 Ga Archean cherts and intercalated BIFs from the Pilbara Block, Western Australia, were studied. Chert layers that are well exposed at Point Samson, 20 km north-northeast of Roebourne consist of various types of chert: white-light gray chert, brownish-yellow chert, gray chert, dark gray chert, massive black chert, and light gray chert. The cherts other than brownish-yellow chert consist mostly of microcrystalline quartz with a minor amount of fine silicate minerals and carbonates. The brownish-yellow chert is characterized by a high concentration of goethite. Most cherts contain thin layers characterized by high concentrations of Ti-bearing minerals such as rutile (anatase), FeTi aggregates, and fine TiO 2 grains. Accordingly, Al 2O 3/TiO 2 values in some samples are very low (<1.0), indicating that these samples contain more TiO 2 than Al 2O 3. This geochemical feature is attributed mainly to the supply of detrital materials with high Ti-bearing minerals to the site of deposition, because the Ti-bearing minerals are generally associated with detrital quartz and zircon. Ti-enriched detrital materials are possibly formed by AlTi fractionation during: (1) extensive chemical weathering, or (2) hydrothermal alteration of the source rocks. In the first hypothesis, which we prefer, it is suggested that low pH values (<4) of meteoric waters accelerated the decomposition of rock-forming minerals and prevented precipitation of dissolved Al 3+. Such low pH values of meteoric waters could have been attributed to more violent volcanic activities issuing acid materials (e.g., H 2S and HCl) and/or a higher concentration of atmospheric CO 2 in the Archean than today.