Distinguishing primary versus secondary geochemical and silicon isotope characteristics of Precambrian chert and iron formation

Precambrian chert and banded iron formation (BIF) are defined as chemical sediments precipitated directly from seawater. As such, their geochemistry, including trace elements, rare earth elements (REE), and isotopes (O, Si) may preserve Precambrian seawater conditions. However, the chemical signature of these rocks may not simply reflect seawater, because hydrothermal interactions and metasomatism may produce rocks that petrographically and chemically resemble chert and BIF, including seawater-like compositional characteristics acquired during mineral replacement. Interpreting the major-, trace-, and rare-earth element signatures of these rocks requires the identification of geochemical and isotopic fingerprints that differentiate rocks formed from chemical sedimentation from rocks formed through replacement.

Previously, REE proxies have been used to identify chemically precipitated sedimentary rocks such as chert. However, these established proxies have proven to be non-diagnostic in depositional settings where alteration has resulted in element mobility. Silicon isotopes, by contrast, have the potential to discriminate rocks formed by chemical sedimentation from those formed through replacement, because they record process-specific information concerning the mechanism of quartz precipitation. Of equal importance, silicon isotopes can provide key information that may identify marine silicon sources.

Using information concerning what isotopic signatures can be preserved in quartz precipitates from different depositional settings, a compilation of the silicon isotope composition of chert can be deciphered to understand lithology-dependent trends. Archean-aged silicified rocks formed through replacement of primary minerals by quartz have a silicon-isotope composition similar to the original rock. Chemical sedimentary chert formed through quartz precipitation proximal to hydrothermal systems has a wide range of silicon-isotope compositions, and shows no apparent trend with respect to time. Only chemical sedimentary cherts associated with iron formation formed in continentally influenced depositional settings record a systematic trend with respect to time. These rocks record a unidirectional positive shift in silicon-isotope compositions. We interpret this trend to reflect chemical precipitation from a changing ocean reservoir, where, as a result of steady growth and emergence of continental masses, the ocean reservoir evolved from one dominated by a hydrothermal silicon isotope signature to a continental one.