Boron isotopes in Archean cherts: investigating early Earth marine conditions
Abstract
The Archean Eon was a period of intense modifications of the Earth surface environment that led to the emergence of life in the early ocean. Therefore, the chemistry of Archean seawater and its relationship to that of the solid Earth, through oceanic and continental crust alteration, remain a matter of great interest and debate. Here, we present new boron (B) isotopes data in well-characterized cherts (ca. 3.5 Ga) from the Pilbara Craton, in order to provide new constraints on their depositional environments. Boron isotopes offer a potentially powerful tool for investigating seawater/rock interactions. The modern differences between B isotopes in seawater, continental crust and oceanic crust greatly facilitate the identification of B sources, while mineral precipitation processes are also accompanied by large isotopic fractionations. We analyzed B concentrations and δ11B in 12 subsamples from two types of stratiform chert; one type (C-chert) is interpreted as resulting from direct precipitation of a seawater-hydrothermal fluid mixture, the second type (S-chert) as resulting from silicification of detritic sediment precursors (Van den Boorn et al., 2007). The C-cherts subsamples show relative large ranges of δ11B values (from -3 to +16 ‰) and B concentrations (0.4-20 ppm). The cm scale variations argue against significant post depositional alteration and thus support the hypothesis that the primary signature is preserved. The similarity of our results with B isotopes in modern cherts suggests that Archean seawater conditions during chert deposition were rather close to those in the modern ocean, at least with respect to B isotopes. In particular, the marine B isotopic budget was likely comparable to the present one. If this is correct, the B isotopes data imply that Archean seawater pH was neutral to slightly basic (pH ≈ 7-9). The S-chert subsamples show remarkably constant δ11B values (≈ -20 ‰) despite a wide range of B concentrations (3-120 ppm). These data indicate that either the δ11B of precipitating fluid was close to 0 ‰ (far from the expected Archean seawater value), or a more complex two-steps B precipitation process occurred. The lack of correlation between B and δ11B rather favors the first hypothesis, while the constancy of δ11B values over a wide range of concentrations could reflect the control of the detritic precursors on B isotopes in S-cherts. Comparison of B and Si isotopes could further help delineating the continental or marine conditions during the alteration and post-depositional evolution of the precursors. Van den Boorn, S. H. J. M., van Bergen, M. J., Nijman, W., and Vroon, P. Z., 2007. Dual role of seawater and hydrothermal fluids in Early Archean chert formation: Evidence from silicon isotopes. Geology 35, 939-942
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMPP11C1335L
- Keywords:
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- 1030 GEOCHEMISTRY / Geochemical cycles;
- 4912 PALEOCEANOGRAPHY / Biogeochemical cycles;
- processes;
- and modeling