Primary Silica Granules—A New Mode of Paleoarchean Sedimentation
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Primary silica granules—A new mode of Paleoarchean sedimentation 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 SAMPLES AND METHODS In the modern silica cycle, dissolved silica is removed from sea- Outcrops, polished hand samples, and polished petrographic thin water by the synthesis and sedimentation of silica biominerals, with sections were used to examine silica granules. Some samples are from the additional sinks as authigenic phyllosilicates and silica cements. BARB4 core from the 2011 International Continental Scientifi c Drilling Fundamental questions remain, however, about the nature of the Program drilling project in the Barberton Greenstone Belt (South Africa). ancient silica cycle prior to the appearance of biologically mediated Elements of interest (Ca, Mg, Fe, Al, and P or Ti) were mapped in carbon- silica removal in Neoproterozoic time. The abundance of siliceous coated (~14 nm thick) polished thin sections using a JEOL JXA-8200 sedimentary rocks in Archean sequences, mainly in the form of chert, advanced electron probe microanalyzer at the Division of Geological strongly indicates that abiotic silica precipitation played a signifi - and Planetary Sciences Analytical Facility at the California Institute of cant role during Archean time. It was previously hypothesized that Technology (Pasadena, California, USA) and using the JEOL JXA-8230 these cherts formed as primary marine precipitates, but substantive SuperProbe electron probe microanalyzer at the School of Earth Sciences evidence supporting a specifi c mode of sedimentation was not pro- Mineral Analysis Facility at Stanford University (Stanford, California). vided. We present sedimentologic, petrographic, and geochemical Qualitative intensity maps without background corrections were col- evidence that some and perhaps many Archean cherts were deposited lected, operating the electron probe in wavelength dispersive X-ray spec- predominately as primary silica grains, here termed silica granules, trometer mode at 15 kV accelerating voltage, 100 nA beam current, and that precipitated within marine waters. This mode of silica deposi- 100 ms dwell time. tion appears to be unique to Archean time and provides evidence that primary silica precipitation was an important process in Archean OBSERVATIONS oceans. Understanding this mechanism promises new insights into the Silica granules are round, internally unstructured, sand-sized silica Archean silica cycle, including chert petrogenesis, microfossil preser- particles (Fig. 1). The granules are composed of essentially pure micro- vation potential, and Archean alkalinity budgets and silicate weather- crystalline quartz, although minor Fe-bearing impurities, especially sid- ing feedback processes. erite and hematite, occur locally. The occurrence of these silica grains is not limited to white chert bands. In many cases, cherty layers as much as INTRODUCTION 50 cm thick are composed largely of silica-rich grains, and many detrital It has been suggested that primary chemical precipitation of amor- sedimentary deposits include virtually pure silica grains mixed with a vari- phous silica played a major role as a silica sink during Precambrian time ety of carbonaceous, volcaniclastic, and other sediment and particle types. (Lowe, 1999a; Maliva et al., 2005; Posth et al., 2008; Siever, 1992), al- Most granules display evidence of compaction (Fig. 2); granule cross sec- though unambiguous examples of primary silica phases were elusive. tions are elliptical in planes perpendicular to the bedding plane, with an Pre–3.0 Ga Archean sedimentary units include abundant chert litholo- average grain shape of an oblate spheroid. Unlike boudins, barrel-shaped gies formed through silica replacement and/or cementation of volcanic deformation structures formed during extension, compacted granule cross ash, detrital sediments, and a variety of other primary sediment types. sections are similar along any plane perpendicular to the bedding plane. One common element of these cherty sequences is the occurrence of lay- This compaction and the current microcrystalline nature of the granules ers or bands of white- to light gray–weathering chert, often translucent, exclude an origin as monocrystalline quartz sand, suggesting instead an generally <10 cm thick, and composed of nearly pure SiO2 (>99 wt%) initial composition as amorphous silica. (Lowe, 1999a). These layers are widely interbedded with carbonaceous Granules can be easily distinguished in hand specimen and thin sec- layers containing trace organic matter (Lowe, 1999a; Walsh and Lowe, tion when not compacted, but are more diffi cult to recognize when se- 1999), ferruginous bands, or sideritic layers of comparable thickness to verely fl attened (Fig. 2) or when the surrounding cement is also composed form black and white banded chert, banded iron formation, and banded of nearly pure and largely homogeneous microquartz (Figs. 1A and 2A). ferruginous chert, respectively. Some larger silica grains appear to be aggregates of individual granules Banded black and white cherts have been considered likely candi- (Fig. 1C), but the sand-sized granules are unstructured in thin section and dates for primary silica precipitates; early deformation features (Lowe, distinct from other types of subspherical grains within the same Archean 1999a) and oxygen isotopic data (Hren et al., 2009; Knauth and Lowe, sequences, such as accretionary lapilli (Lowe, 1999b) or impact spherules 1978, 2003) are consistent with primary or earliest diagenetic band for- (Lowe and Byerly, 1986). They lack relict textures that would indicate mation. Two hypothetical band formation mechanisms have been pro- diagenetic transformation or replacement of primary carbonate (Maliva et posed: (1) primary precipitation of silica on the seafl oor (Lowe, 1999a; al., 2005) or volcanic (DiMarco and Lowe, 1989) grains. Electron probe van den Boorn et al., 2007), and (2) earliest diagenetic segregation of maps of Al, Fe, Ca, Mg, P, and Ti do not reveal internal structuring (Fig. 3) adsorbed silica, originally deposited homogeneously with carbonaceous and support petrographic observations that granules are generally distin- matter and/or iron oxides, into distinct layers (Lowe, 1999a). While guishable from the surrounding microquartz matrix because they contain many white chert layers are massive, a surprising number display pre- few trace impurities. served internal granular textures characterized by sand-sized grains of Granules are common in sedimentary units representing a variety of nearly pure silica. This observation suggests that many, if not all, white depositional environments, from intertidal to deep-water basinal settings chert bands originated via a third novel mechanism, i.e., deposition of in the Barberton Greenstone Belt in South Africa and the Pilbara Craton primary silica grains. These pure to nearly pure silica particles are here in Western Australia. In the following discussion we describe the proper- termed silica granules. ties and distribution of granules in four specifi c occurrences that provide examples of the morphology and environmental diversity of granules in *E-mail: [email protected]. the 3.2–3.5 Ga Barberton and Pilbara sequences. GEOLOGY, April 2014; v. 42; no. 4; p. 1–4; Data Repository item 2014106 | doi:10.1130/G35187.1 | Published online XX Month 2013 GEOLOGY© 2014 Geological | April Society 2014 | ofwww.gsapubs.org America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 1 AB A C D B E S F S S Q Q C G Figure 2. Hypothetical sequence of progressive com- paction of silica granules in white chert band illustrates how matrix material can take on appearance of anas- tomosing laminations with increasing compaction. Figure 1. Silica granules. Granules are outlined (dotted lines) for clar- A: Minimally compacted. B: Moderately compacted. ity in right panels of A–C. Scale bars are 500 µm. See Table DR2 (see C: Severely compacted. Scale bars are 500 µm. See Ta- footnote 1) for more detailed stratigraphic information for samples ble DR2 (see footnote 1) for more detailed stratigraphic shown. A: Pure silica granules from within white chert band, barely and location information for samples shown. distinguishable due to presence of trace carbonate inclusions in sur- rounding material. Upper Mendon Formation, Onverwacht Group, South Africa. B: Granules (possibly including aggregates of granules, based on irregular shapes) with carbonaceous grains and matrix. Lower Mapepe Formation, Fig Tree Group, South Africa. C: Large in- traclast (dashed outline) composed of round, uncompacted granules. Upper Mendon Formation. D: Granules rimmed with diagenetic hema- A B tite (upper left) or completely fi lled with diagenetic siderite and hema- tite (lower right). Antarctic Creek Member at base of Mount Ada Basalt, Warrawoona Group, Western Australia. E, F: Pure silica granules in plane-polarized light (E) and cross-polarized light (F), some rimmed with diagenetic siderite (S) or replaced with coarse quartz cement (Q). 1 cm 1 mm Antarctic Creek Member at base of Mount Ada Basalt. G: Lens of silica granules within ferruginous shale, including micron-scale hematite grains within matrix. Lower Mapepe Formation. C 50 40 Figure 3. Layer