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Sedimentology and Geochemistry of Archean Silica Granules Sedimentology and Geochemistry of Archean Silica Granules

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Sedimentology and Geochemistry of Archean Silica Granules Sedimentology and Geochemistry of Archean Silica Granules Sedimentology and geochemistry of Archean silica granules Sedimentology and geochemistry of Archean silica granules Elizabeth J.T. Stefurak1,†, Donald R. Lowe1, Danielle Zentner1, and Woodward W. Fischer2 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA 2Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91126, USA ABSTRACT via multiple stages of aggregation of silica Many pre–3.0 Ga cherty sequences include nanospheres and microspheres. Consistent <10-cm-thick layers of white-weathering or The production of biogenic silica has with this hypothesis, Archean ocean chemis- translucent chert composed of nearly pure SiO2 dominated the marine silica cycle since early try would have favored particle aggregation (commonly >99 wt%), now microcrystalline Paleozoic time, drawing down the concentra- over gelling. Granule formation would have quartz (Lowe, 1999a). These white chert beds tion of dissolved silica in modern seawater been most favorable under conditions pro- are interbedded with compositionally contrast- to a few parts per million (ppm). Prior to moting rapid silica polymerization, includ- ing beds containing primary organic sedimen- the biological innovation of the fi rst silica ing high salinity and/or high concentrations tary grains (Lowe, 1999a; Walsh and Lowe, biomineralizing organisms in late Protero- of dissolved silica. Our observations suggest 1999), mixtures of siderite and fi ne aluminosili- zoic time, inputs of silica into Precambrian that granule sedimentation was often epi- cates, or hematite. These alternating lithologies seawater were balanced by strictly chemi- sodic, suggesting that granule formation may are known as banded black-and-white chert, cal silica and silicate precipitation processes, have also been episodic, perhaps linked to banded ferruginous (iron-bearing) chert, and although the mechanics of this abiotic ma- variations in these key parameters. banded iron formation, respectively (Fig. 1). rine silica cycle remain poorly understood. Stefurak et al. (2014) reported that many of the Cherty sedimentary rocks are abundant in INTRODUCTION white chert bands in these units are distinct sedi- Archean sequences, and many previous au- mentary beds composed entirely of sand-sized thors have suggested that primary precipita- The abundance and sedimentary style of chert subspherical primary silica granules (Fig. 1) and tion of amorphous silica could have occurred in early Archean rocks highlight a fundamental that silica granules were a common component in in Archean seawater. The recent discovery distinction between the modern and Archean other sedimentary layers as well, suggesting that that many pure chert layers in early Archean silica cycles. The early Archean silica cycle silica granules were a widespread grain type and rocks formed as sedimentary beds of sand- lacked the key components that dominate the represented a signifi cant mode of silica deposi- sized, subspherical silica granules has pro- modern silica cycle: continental weathering as tion in early Archean time. This observation aug- vided direct evidence for primary silica the dominant source and silica biomineraliza- mented the emerging view that the deposition of deposition. Here, we provide further sedi- tion as the dominant sink for dissolved silica in mud-, silt-, and sand-sized chemical sand grains mentological and geochemical analyses of marine waters (Maliva et al., 1989, 2005; Siever, composed of silica (Stefurak et al., 2014), iron early Archean silica granules in order to gain 1957, 1992; Treguer et al., 1995). Although the silicates (Rasmussen et al., 2013), and siderite a better understanding of the mechanisms of volume of continental crust during Archean time (Köhler et al., 2013) was involved in the deposi- granule formation. Silica granules are com- was much less than on modern Earth (Cawood tion of early Precambrian cherts and iron forma- mon components of sedimentary cherts from et al., 2012; Dhuime et al., 2012), Archean tions. This study presents additional petrographic a variety of depositional settings and water oceans had abundant silica sources in the form and geochemical analyses of silica granules with depths. The abundance and widespread dis- of weathering and alteration of mafi c and the goal of providing further insight into controls tribution of silica granules in Archean rocks ultramafi c rocks (Siever, 1992); mass balance on their formation and deposition. suggest that they represented a signifi cant requires that abundant sinks must also have been primary silica depositional mode and that present. Previous authors have suggested that GEOLOGIC BACKGROUND most formed by precipitation in the upper banded iron formations, diagenetic silicifi cation, part of the water column. The regular oc- and authigenic clay precipitation (Maliva, 2001; This study focuses on chert samples collected currence of silica granules as centimeter- Maliva et al., 1989, 2005; Siever, 1992; Stefurak in early Archean strata from both the Pilbara scale layers within banded chert alternating et al., 2015) were important sinks in the Archean block of Western Australia and the Barberton with layers of black or ferruginous chert silica cycle. It has also long been suggested that greenstone belt of South Africa. Samples from containing few granules indicates episodic primary chemical precipitation of amorphous the Pilbara block are from the Antarctic Creek granule sedimentation. Contrasting silicon silica phases also played a major role as a silica Member of the 3470 ± 1 Ma Apex Basalt (Byerly iso topic compositions of granules from dif- sink during Precambrian time (Lowe, 1999a; et al., 2002), which forms the middle part of the ferent depo sitional environments indicate Maliva et al., 2005; Siever, 1992), which is sup- Warrawoona Group (Fig. 2; Van Kranendonk that isotopic signatures were modifi ed during ported by the abundance of chert, especially in et al., 2002). The Antarctic Creek Member is a early diagenesis. Looking to modern siliceous pre–3.0 Ga Archean sequences. However, many 4–14-m-thick unit composed largely of silicifi ed sinters for insight into silica precipitation, we cherts also formed by diagenetic replacement felsic volcaniclastic sediments that also include suggest that silica granules may have formed and meta somatism of nonsiliceous primary an event bed of impact spherules and current- material (Lowe, 1999a), making primary silica deposited layers of silica granules (Stefurak †E-mail: [email protected] precipitates diffi cult to identify unambiguously. et al., 2014). The spherule bed has been corre- GSA Bulletin; Month/Month 2015; v. 1xx; no. X/X; p. 1–18; doi: 10.1130/B31181.1; 10 fi gures; 1 table; Data Repository item 2015105.; published online XX Month 2014. GeologicalFor permissionSociety ofto copy,America contact Bulletin [email protected], v. 1XX, no. XX/XX 1 © 2015 Geological Society of America Stefurak et al. ABC 2 cm 1 cm 500 μm D E F 1 cm 1 mm 1 mm GH I 2 cm 1 cm 500 μm Figure 1. Layers of silica granules at different scales. (A–C) Black-and-white banded chert: (A) outcrop photo, Buck Reef Chert (locality Buck Reef Chert), (B) polished slab of white chert band showing faintly granular texture, upper Mendon Formation (locality BH-03), and (C) thin section of silica granules from a white chert band, upper Mendon Formation (locality SAF 521). (D–F) Lenticular granular layers within ferruginous shale, lower Mapepe Formation (locality SAF 183): (D) polished slab with lenses of two distinct granule types (arrows), both shown in thin section—a lower lens of mini- mally compacted, slightly ferruginous granules (E) and an upper lens of more compacted non-ferruginous granules (F). (G–I) Banded iron formation, lower Mapepe Formation: (G) outcrop photo showing chert bands (arrows) within jasper , (H) core photo highlighting ~1-cm-thick granular layer within banded iron formation (dashed outline) (sample SAF 649– 14), and (I) thin-section image of pure silica granules within slightly ferruginous matrix from the layer shown in H. See supplementary Table DR1 for additional stratigraphic information for samples shown and supplementary Figure DR1 for map of sample localities (see text footnote 1). lated with a similar 3470 ± 2 Ma unit, spherule Mendon Formations are part of the Onverwacht ferruginous chert at the base of the Kromberg bed S1, in the Barberton greenstone belt (Byerly Group, an 8–10-km-thick sequence of mafi c Formation (Lowe and Byerly, 1999). The age of et al., 2002; Glikson et al., 2004; Hickman and and ultramafi c rocks with thin, interbedded the Buck Reef Chert is bracketed by two sets of Van Kranendonk, 2008; Lowe and Byerly, 1986). sedimentary chert units representing sediments U/Pb zircon ages of 3416 ± 5 Ma from a con- Samples from the Barberton greenstone belt deposited during periods of volcanic quiescence glomeratic unit in the lowest part of the Buck were collected from the Buck Reef Chert Mem- (Lowe and Byerly, 1999). The overlying Fig Reef Chert (Krüner et al., 1991) and 3334 ± ber of the Kromberg Formation, the upper Men- Tree Group is an ~180-m-thick sequence of 3 Ma from a felsic tuff from the uppermost don Formation, and the basal Mapepe Forma- immature terrigenous clastic rocks and felsic Kromberg Formation (Byerly et al., 1996). The tion (Fig. 2; Fig. DR11). Both the Kromberg and volcaniclastic units (Condie et al., 1970; Lowe basal unit of the Buck Reef Chert was deposited and Byerly, 1999). The Mapepe Formation is in a shallow, restricted setting, indicated by the the basal unit of the Fig Tree Group south of the common occurrence of wave ripples and evapo- 1GSA Data Repository item 2015105, Figures DR1-DR8 and Tables DR1 and DR2, is available at Inyoka fault (Lowe and Byerly, 1999). rite pseudomorphs after nahcolite (Lowe and http:// www .geosociety .org /pubs /ft2015 .htm or by re- The Buck Reef Chert is a 200–400-m-thick Fisher Worrell, 1999). The middle Buck Reef quest to editing@ geosociety .org. unit of banded black-and-white and banded Chert, composed of banded black-and-white 2 Geological Society of America Bulletin, v.
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