Cherts over geological time

Fine grained chemical sediments, composed mostly or entirely of silica (SiO2), are called cherts.

The origin of the term “chert” dates back to the seventeenth century and was probably derived from a local English term – "Chert, perhaps originally chirt, is believed to be a local English term that was taken into geological use. It may be of onomatopoeic origin. The name chert may be of more recent origin than flint, and unlike flint, is not found in literary usage. It was well

established in meaning in 1679" (Frondel, 1962).

Folk (1974) gave the scientific definition of a chert as “a chemically precipitated sedimentary rock, essentially monominerallic and composed chiefly of microcrystalline and/or chalcedonic quartz, with subordinate megaquartz and minor amounts of impurities”. Common impurities present within cherts are clay minerals, silt, carbonate, pyrite and organic matter. They are compact rocks with low porosity (Folk et al 1952; Smith 1960), highly insoluble, highly impermeable and very resistant to alteration (Knauth, 1973).

4

Cherts or flints were first used by prehistoric man as tools and later as weapons, employing the nature of the rock to naturally break to produce conchoidal fractures and sharp edges. The rock was frequently fashioned into knife blades, spear points, arrowheads, scrapers, axes, drills and other sharp tools (Fig 1).

Figure 1Two principle types of chert and prehistoric usage of the rock. (A) Bedded chert from Governors beach, Cyprus. Image source: sandatlas.org (B) Nodular chert from Cyprus. Image source:

sandatlas.org (C) Chert or flint tools from about 14,000 years ago. Image source: Historic Scotland, historicagraphica.com

Naturally occurring authigenic silica exists in a wide variety of forms – from X-ray amorphous to well ordered α-quartz. Amorphous silica or “opal” can be of three types – opal-A, opal-CT and opal-C (Jones and Segnit, 1971). Of these, opal-A, precipitated in abundance by modern

5

organisms like diatoms and sponges etc., is the most common form. It is isotropic, its diffuse X-ray peak lies at 4.1 Å and contains up to 12 weight % H2O within its structure (Knauth, 1992).

The crystalline varieties of sedimentary silica are microcrystalline quartz, chalcedony or fibrous silica, mesocrystalline quartz and megaquartz (e.g., Knauth, 1994; Maliva et al., 2005).

Petrographical studies have demonstrated most cherts to be mostly composed of microquartz (Knauth, 1994). Descriptions of the types of silica can be found in Chapter 3.

Figure 2 Schematic diagram of major authigenic silica phases and their possible diagenetic

transformations. Vertical dimension represents qualitative burial depth with associated increase in temperature and loss of permeability. Horizontal scale represents qualitative depth of initial depositional environment. In general, deep sea oozes lie to the left of the diagram, while

epicontinental deposits lie toward the right. Diagenetic path (A) represents silica initially deposited as opal-A (diatoms, radiolarians) which then transforms to opal-CT and then microquartz via solution – reprecipitation steps. Path (C) represents early diagenetic cherts, in which microquartz forms during shallow burial. Megaquartz forms by metamorphic re-crystallization of microquartz or

6

by direct growth into voids at any stage of burial. Fibrous silica can grow in vugs and fractures at all burial depths. Modified after Knauth (1992).

The origin of authigenic quartz is highly variable and complicated. Broadly speaking, cherts may form as pure orthochemical precipitates from silica rich fluids (C-type cherts) or via replacement of a precursory volcanic or sedimentary rock (S-type cherts) (e.g., Van den Boorn et al., 2007).

Figure 2 shows the common forms of authigenic silica, hydrologic activity dissolving and

precipitating the silica, and different diagenesis pathways. At room temperature (25°C) all forms of silica are soluble at pH >9. Fluids containing >6 μg/g dissolved silica are potentially silicifying fluids. At high silica concentrations of >80,000 μg/g, however, it is more likely that opal-A (>120,000 μg/g) or opal-CT (>80,000 μg/g) will precipitate instead of quartz (Knauth, 1992). It is also possible for quartz to precipitate directly from water, but only with high silica concentration,

>4 μg/g, as shown by Mackenzie and Gees (1971) in laboratory experiments.

Modern concentration of silica in seawater is very low, < 1 μg/g in surface water and ~ 15 μg/g in some bottom waters (Hesse 1988) – in stark contrast to a 60 μg/g concentration suggested by Siever (1992) in Precambrian ocean water.

The Phanerozoic silica cycle is controlled by an immense number of silica-secreting organisms like diatoms (70% of the silica secreting population - Lisitzin, 1972) (oldest accepted fossil evidence from Lower Jurassic - Barron, 1993), followed in importance by radiolaria (fossils discovered in strata as old as Middle Cambrian - Won and Below, 1999) and silica sponges (oldest fossil records in Proterozoic - Li, Chen and Hua, T., 1998). Higher organisms like silicoflagellates (oldest fossils from Early Cretaceous – McCartney, 1993) also contribute to the Phanerozoic silica budget but on a much smaller scale. Rivers, pore water reflux, submarine weathering and submarine volcanism/hydrothermal activity provide silica input into the oceans.

The silica output flux is controlled by biogenic precipitation of silica and the total present day silica production by marine organisms is about 25 times the input of silica to the oceans (e.g., Heath, 1974; Edmond, 1979; Hesse, 1989). The imbalance in the silica budget is not a real one because 90-99% of silica extracted from surface seawater by silica secreting organisms redissolves before burial and is returned to the ocean (e.g., Hurd, 1973).

7

The silica secreting organisms produce shells or “tests” or “frustules” that are made of the amorphous opal-A. This amorphous polymorph of silica contains a large amount of water, up to 12 weight percent (wt %), taking into account both hydroxyl and molecular water (Knauth and Epstein, 1982). Opal-A is unstable in seawater and dissolves easily because of the very low silica concentrations of Phanerozoic oceans and only up to ~15 ppm in some bottom waters. This is way smaller than the equilibrium solubility of amorphous silica, which is between 70-150 ppm at 25°C and pH < 9 (Iler, 1979). The silica secreting microorganisms, while producing silica, raise the otherwise highly undersaturated seawater to silica saturation levels (on a local micro-scale) with the help of the catalyzing influence of enzyme controlled bio-reactions (Hesse, 1989). As the organisms die, the enzyme aided reactions cease and the micro-environment of

silica-supersaturation is lost. As a result, the unstable opal-A begins to dissolve back into seawater.

Thus, silica concentration of ocean surface waters is lowest due to the biogenic silica extraction, and the value gradually increases downwards with increasing water depth due to post-mortem settling and dissolution of the siliceous microorganism tests. The silica concentration of seawater attains a “mid-depth maximum” from which point downwards seawater loses its silica

concentration as it mixes with silica poor surface water masses from the Poles, especially in the North Atlantic (Hesse, 1989).

Extensive chert formations are amongst some of the oldest rocks found on earth, e.g., 3.5 Ga Onverwacht Group in South Africa represents the least metamorphosed Archean cherts.

Precambrian sedimentary rocks contain cherts in abundance, in the form of distinct beds / stratiform bedded deposits, or as lenses or nodules within other sediments like carbonate. Cherts are found as replacement nodules or silicified laminae in stromatolitic carbonate rocks, silica rich layers or cements in iron formations, beds and veins in greenstone belts and volcanic sequences, and as beds within argillites. Their occurrence almost throughout earth’s history and in a variety of different geological settings renders them important candidates for looking into the ancient geological past.

Principle questions involved in studying cherts are the origin of silica, the depositional environment of the silica or siliceous sediments and their subsequent diagenesis (Hesse, 1988). The method of silica deposition has varied over time, “present is key to the past” not

8

holding true in this respect. The modern silica cycle is governed by opal secreting diatoms but they have been active only for the past 50 Ma (Siever, 1991; Knauth, 1992) and have formed thick bedded chert sequences since then. In fact, biogenic silica is common throughout the Phanerozoic – until the diatom explosion ~50 Ma ago, the primary sources of silica were radiolaria, sponges and vascular plants. All silica secreting organisms deposit silica in its

amorphous opal-A form which then undergoes digenetic maturation and transformation to stable microquartz via opal-CT (Calvert, 1971). Oxygen isotope studies (¹⁸O/¹⁶O fractionations) suggest these changes are associated with a rise in burial temperature. Temperatures of 45°C for

the opal-A to opal-CT conversion and 80°C for the final transformation to quartz have been suggested by oxygen isotopic studies on deep sea drilled cores (Knauth and Epstein, 1975;

Kolodny and Epstein, 1976; Murata et al., 1977; Pisciotto 1981) and by studying the geothermal gradient therein (Kastner, 1981).

Precambrian cherts could not have been deposited the same way as the Phanerozoic counterparts, simply because of the absence of diatom, radiolarian opal-A and lack of firm evidence suggesting that such microorganisms may have existed during that time. Thus, inorganic pathways for the deposition and origin of Precambrian chert are required (Hesse, 1989; Siever, 1991). Within the Precambrian sedimentary rocks chert is found in abundance and in different forms, e.g. chertified stromatolitic carbonates, iron formations, beds within argillites etc. Both bedded and vein type cherts are seen, for e.g. within the Dresser Formation, Pilbara Group, Western Australia. Deposition from big hydrothermal plumes spreading out on the ocean floor may have formed some of these thick chert beds (e.g., van den Boorn et al., 2007), which are often found associated with volcanics, but it is still controversial. Moreover, some chert types like the chert layers within thick iron formations e.g., within the Gunflint Range, Canada have no modern analogues. It is possible that bacterial precipitation of silica gel (opal-A) played a major role in the Precambrian (Konhauser and Ferris, 1996; Konhauser et al., 2002) or that the ancient oceans were supersaturated in silica, leading to direct precipitation of microquartz but none of these mechanisms have been confirmed so far (Knauth, 1992; Knauth, 1994).

Despite the origin and diagenesis of cherts being complicated, advances in isotope geology have made cherts, along with other marine sediments, important for information regarding ancient

9

ocean water chemistry (e.g., Perry 1967), temperature (e.g., Knauth and Epstein, 1976) and paleoclimatic conditions. An oxygen isotope record of past earth surface might be partially preserved in cherts, even though it is controversial. However, the possibility of finding information on paleoenvironments of the early Earth from cherts important because oxygen fractionation between silica and water is a function of temperature.

In his classic paper Urey (1947) first calculated stable isotope fractionation factors between species of geochemical interest. The silica-water oxygen isotope exchange - T relationship is

expressed in the form , where is the

fractionation factor between the two phases (silica – water) and a,b, c are constants. The first experimentally determined quartz-water thermometer was given by O'Neil and Clayton (1964). So, the oxygen isotope composition of cherts can potentially tell us about the oyxgen isotope composition of ancient hydrosphere as well as its temperature.

This study focuses on the oxygen isotope study of cherts and the information regarding ancient seawater that may be obtained from that. In this thesis and additional parameter, the δ¹⁷O compositions of cherts, shall be introduced to resolve this enigmatic topic. This parameter is described in Section 1.4 of this chapter.