Texture-specific Si isotope variations in Barberton Greenstone Belt cherts record low temperature fractionations in early Archean seawater

Sedimentary cherts are unusually abundant in early Archean (pre-3.0 Ga) sequences, suggesting a silica cycle that was profoundly different than the modern system. Previously applied for the purpose of paleothermometry, Si isotopes in ancient cherts can offer broader insight into mass fluxes and mechanisms associated with silica concentration, precipitation, diagenesis, and metamorphism. Early Archean cherts contain a rich suite of sedimentological and petrographic textures that document a history of silica deposition, cementation, silicification, and recrystallization. To add a new layer of insight into the chemistry of early cherts, we have used wavelength-dispersive spectroscopy and then secondary ion mass spectrometry (SIMS) to produce elemental and Si and O isotope ratio data from banded black-and-white cherts from the Onverwacht Group of the Barberton Greenstone Belt, South Africa. This geochemical data is then interpreted in the framework of depositional and diagenetic timing of silica precipitation provided by geological observations. SIMS allows the comparison of Si and O isotope ratios of distinct silica phases, including black carbonaceous chert beds and bands (many including well-defined sedimentary grains), white relatively pure chert bands including primary silica granules, early cavity-filling cements, and later quartz-filled veins. Including all chert types and textures analyzed, the δ^(30)Si dataset spans a range from −4.78‰ to +3.74‰, with overall mean 0.20‰, median 0.51‰, and standard deviation 1.30‰ (n = 1087). Most samples have broadly similar δ^(30)Si distributions, but systematic texture-specific δ^(30)Si differences are observed between white chert bands (mean +0.60‰, n = 750), which contain textures that represent primary and earliest diagenetic silica phases, and later cavity-filling cements (mean −1.41‰, n = 198). We observed variations at a ∼100 μm scale indicating a lack of Si isotope homogenization at this scale during diagenesis and metamorphism, although fractionations during diagenetic phase transformations may have affected certain textures. We interpret these systematic variations to reflect fractionation during silica precipitation as well as isotopically distinct fluids from which later phases originated. SIMS δ^(18)O values fall in a range from 16.39‰ to 23.39‰ (n = 381), similar to previously published data from bulk gas source mass spectrometry of Onverwacht cherts. We observed only limited examples of texture-related variation in δ^(18)O and did not observe correlation of δ^(18)O with δ^(30)Si trends. This is consistent with hypotheses that Si isotope ratios are more resistant to alteration under conditions of rock-buffered diagenesis (Marin-Carbonne et al., 2011). Our results indicate that low temperature processes fractionated silicon isotopes in early Archean marine basins, a behavior that probably precludes the application of chert δ^(30)Si as a robust paleothermometer. The values we observe for facies that sedimentological and petrographic observations indicate formed as primary and earliest diagenetic silica precipitates from seawater are more ^(30)Si-rich than that expected for bulk silicate Earth. This is consistent with the hypothesis that the silicon isotope budget is balanced by the coeval deposition of ^(30)Si-enriched cherts and ^(30)Si-depleted iron formation lithologies. Precipitation of authigenic clay minerals in both terrestrial and marine settings may have also comprised a large ^(30)Si-depleted sink, with the corollary of an important non-carbonate alkalinity sink consuming cations released by silicate weathering

Primary silica granules—A new mode of Paleoarchean sedimentation

In the modern silica cycle, dissolved silica is removed from seawater by the synthesis and sedimentation of silica biominerals, with additional sinks as authigenic phyllosilicates and silica cements. Fundamental questions remain, however, about the nature of the ancient silica cycle prior to the appearance of biologically mediated silica removal in Neoproterozoic time. The abundance of siliceous sedimentary rocks in Archean sequences, mainly in the form of chert, strongly indicates that abiotic silica precipitation played a significant role during Archean time. It was previously hypothesized that these cherts formed as primary marine precipitates, but substantive evidence supporting a specific mode of sedimentation was not provided. We present sedimentologic, petrographic, and geochemical evidence that some and perhaps many Archean cherts were deposited predominately as primary silica grains, here termed silica granules, that precipitated within marine waters. This mode of silica deposition appears to be unique to Archean time and provides evidence that primary silica precipitation was an important process in Archean oceans. Understanding this mechanism promises new insights into the Archean silica cycle, including chert petrogenesis, microfossil preservation potential, and Archean alkalinity budgets and silicate weathering feedback processes

Sedimentology and geochemistry of Archean silica granules

The production of biogenic silica has dominated the marine silica cycle since early Paleozoic time, drawing down the concentration of dissolved silica in modern seawater to a few parts per million (ppm). Prior to the biological innovation of the first silica biomineralizing organisms in late Proterozoic time, inputs of silica into Precambrian seawater were balanced by strictly chemical silica and silicate precipitation processes, although the mechanics of this abiotic marine silica cycle remain poorly understood. Cherty sedimentary rocks are abundant in Archean sequences, and many previous authors have suggested that primary precipitation of amorphous silica could have occurred in Archean seawater. The recent discovery that many pure chert layers in early Archean rocks formed as sedimentary beds of sand-sized, subspherical silica granules has provided direct evidence for primary silica deposition. Here, we provide further sedimentological and geochemical analyses of early Archean silica granules in order to gain a better understanding of the mechanisms of granule formation. Silica granules are common components of sedimentary cherts from a variety of depositional settings and water depths. The abundance and widespread distribution of silica granules in Archean rocks suggest that they represented a significant primary silica depositional mode and that most formed by precipitation in the upper part of the water column. The regular occurrence of silica granules as centimeter-scale layers within banded chert alternating with layers of black or ferruginous chert containing few granules indicates episodic granule sedimentation. Contrasting silicon isotopic compositions of granules from different depositional environments indicate that isotopic signatures were modified during early diagenesis. Looking to modern siliceous sinters for insight into silica precipitation, we suggest that silica granules may have formed via multiple stages of aggregation of silica nanospheres and microspheres. Consistent with this hypothesis, Archean ocean chemistry would have favored particle aggregation over gelling. Granule formation would have been most favorable under conditions promoting rapid silica polymerization, including high salinity and/or high concentrations of dissolved silica. Our observations suggest that granule sedimentation was often episodic, suggesting that granule formation may have also been episodic, perhaps linked to variations in these key parameters