Silicon isotope and trace element constraints on the origin of ∼3.5Ga cherts: Implications for Early Archaean marine environments

Abstract

Silicon (Si) isotope variability in Precambrian chert deposits is significant, but proposed explanations for the observed heterogeneity are incomplete in terms of silica provenance and fractionation mechanisms involved. To address these issues we investigated Si isotope systematics, in conjunction with geochemical and mineralogical data, in three well-characterised and approximately contemporaneous, $3.5 Ga chert units from the Pilbara greenstone terrane (Western Australia).We show that Si isotope variation in these cherts is large (À2.4& to +1.3&) and was induced by near-surface processes that were controlled by ambient conditions. Cherts that formed by chemical precipitation of silica show the largest spread in d 30 Si (À2.4& to +0.6&) and are characterised by positive Eu, La and Y anomalies and overall depletions in lithophile trace elements. Silicon isotope systematics in these orthochemical deposits are explained by (1) mixing between hydrothermal fluids and seawater, and/or (2) fractionation of hydrothermal fluids by subsurface losses of silica due to conductive cooling. Rayleigh-type fractionation of hydrothermal fluids was largely controlled by temperature differences between these fluids and seawater. Lamina-scale Si isotope heterogeneity within individual chemical chert samples up to 2.2& is considered to reflect the dynamic nature of hydrothermal activity. Silicified volcanogenic sediments lack diagnostic REE+Y anomalies, are enriched in lithophile elements, and exhibit a much more restricted range of positive d 30 Si (+0.1& to +1.1&), which points to seawater as the dominant source of silica.The proposed model for Si isotope variability in the Early Archaean implies that chemical cherts with the most negative d 30 Si formed from pristine hydrothermal fluids, whereas silicified or chemical sediments with positive d 30 Si are closest to pure seawater deposits. Taking the most positive value found in this study (+1.3&), and assuming that the Si isotope composition of seawater is governed by input of fractionated hydrothermal fluids, we infer that the temperature of $3.5 Ga seawater was below $55°C.

Deciphering formation processes of banded iron formations from the Transvaal and the Hamersley successions by combined Si and Fe isotope analysis using UV femtosecond laser ablation

“…sample 3/79 with a mean δ 30 Si values of -0.77‰, suggest an enhanced terrigeneous influence. The inferred seawater composition of about 0.7‰ corresponds to the Si isotope signature of Si released by modern continental weathering (Georg et al, 2006(Georg et al, , 2009 (Robert and Chaussidon, 2006;van den Boorn et al, 2007;2010;Abraham et al, 2009;Chakrabarti et al, 2009). Both scenarios, steady state conditions with two complementary Si sinks as well as non-steady state conditions, appear possible for a Precambrian ocean basin.…”

“…The general consensus is that light Si isotopes preferentially precipitate from solution (De La Rocha et al, 1997;Ziegler et al, 2005;Basile-Doelsch, 2006), although it still lacks experimental verification. Precambrian chert deposits appear to record the original isotope signature because small-scale Si isotope variations are preserved (André et al, 2006;van den Boorn et al, 2007;2010;Steinhoefel et al, 2009a). Negative δ 30 Si values are considered to reflect a hydrothermal signature (Jiang et al, 1993;Ding et al, 1996;André et al, 2006;Steinhoefel et al, 2009a;van den Boorn et al, 2010).…”

“…Precambrian chert deposits appear to record the original isotope signature because small-scale Si isotope variations are preserved (André et al, 2006;van den Boorn et al, 2007;2010;Steinhoefel et al, 2009a). Negative δ 30 Si values are considered to reflect a hydrothermal signature (Jiang et al, 1993;Ding et al, 1996;André et al, 2006;Steinhoefel et al, 2009a;van den Boorn et al, 2010). In contrast, positive  30 Si values may reflect elevated temperature of the seawater, the influence of a continental source or precipitation from isotopically heavy seawater (Robert and Chaussidon, 2006;van den Boorn et al, 2007;2010).…”

“…Negative δ 30 Si values are considered to reflect a hydrothermal signature (Jiang et al, 1993;Ding et al, 1996;André et al, 2006;Steinhoefel et al, 2009a;van den Boorn et al, 2010). In contrast, positive  30 Si values may reflect elevated temperature of the seawater, the influence of a continental source or precipitation from isotopically heavy seawater (Robert and Chaussidon, 2006;van den Boorn et al, 2007;2010). In this study, we present high-spatial resolution Fe and Si isotope data on BIFs of the Transvaal and Hamersley successions determined by femtosecond laser ablation coupled to a multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS).…”

Effects of growth and dissolution on the fractionation of silicon isotopes by estuarine diatoms

“…These findings have been used to gain insights into oceanic Si cycling (De la Rocha et al, 1998;Reynolds et al, 2006;Beucher et al, 2008;van den Boorn et al, 2010), Si cycling in coastal areas (Cao et al, 2012 and Si cycling in lakes (Street-Perrott et al, 2008, Opfergelt et al, 2011. Although estuarine areas account for less than 10% of total ocean area but are responsible for 30-50% of global marine primary production (Field et al, 1998;Dunne et al, 2007), no previous estuarine studies have investigated Si isotope fractionation during diatom growth and dissolution.…”