Raymond Siever

The silica cycle in the Precambrian

Abstract Whereas the modem silica cycle is heavily dependent on the biology of silica-secreting organisms such as the diatoms, the Precambrian silica cycle was dominated by inorganic reactions among dissolved silica, clay and zeolite minerals, and organic matter. There is no evidence for deposition of a layered amorphous silica but abundant evidence for diagenetic silicification in the Neoproterozoic, the time period chosen for this analysis. The influx of silica to the Neoproterozoic ocean would have been governed, as today, by the balance among tectonism, weathering, and hydrothermal input. The evidence of Neoproterozoic rocks favors tectonic and weathering regimes not very dissimilar to those of the early Phanerozoic and hydrothermal inputs significantly altered at certain periods. One flux would have been very different: the present diffusional influx of silica from interstitial waters into the oceans would have been altered to an efflux from the oceans to interstitial waters. Reactions of dissolved silica with inorganic phases would have controlled silica concentrations at a level of about 60 ppm. This figure is based on experimental determinations of solubility and silica sorption on clay minerals and zeolites. As silica in interstitial waters was lowered by opal-CT precipitation, a diffusional efflux from the ocean into the sediment was set up. Silicification would have started in early diagenesis as opal-CT precipitated from interstitial waters. Diffusion of silica from overlying seawater declined as the sediment was buried a few hundred meters and the diffusion path lengthened. During this still relatively shallow burial, silicification continued, but this time powered by hydrologie transport of dissolved silica from coastal plain sediments during transgressions and regressions. Thus the major removal of silica from the Neoproterozoic ocean took place by diagenetic reactions.

Secular Change in Chert Distribution: A Reflection of Evolving Biological Participation in the Silica Cycle

In the modern oceans, the removal of dissolved silica from sea water is principally a biological process carried out by diatoms, with lesser contributions from radiolaria, silicoflagellates, and sponges. Because such silica in sediments is often redistributed locally during diagenesis to from nodular or bedded chert, stratigraphic changes in the facies distribution of early diagenetic chert provide important insights into the development of biological participation in the silica cycle. The abundance of chert in upper Proterozoic peritidal carbonates suggests that at this time silica was removed from seawater principally by abiological processes operating in part of the margins of the oceans. With the evolution of demosponges near the beginning of the Cambrian Period, subtidal biogenetic cherts became increasingly common, and with the Ordovician rise of radiolaria to ecological and biogeochemical prominence, sedimented skeletons became a principal sink for oceanic silica. Cherts of Silurian to Cretaceous age share many features of facies distribution and petrography but they differ from Cenozoic siliceous deposits. These differences are interpreted to reflect the mid-Cretaceous radiation of diatoms and their subsequent rise to domination of the silica cycle. Biogeochemical cycles provide an important framework for the paleobiological interpretation of the organisms that participate in them.

Dissolution kinetics and the weathering of mafic minerals

Abstract Fayalite, hypersthene, basalt, and obsidian were dissolved in buffered solutions (25°C; pH 4.5 and 5.5) under air, N2 or O2 atmospheres, in order to follow the kinetics of dissolution. Each dissolved more rapidly at lower pH values, dissolving most rapidly in the initial few days, followed by slower dissolution for periods up to six months. Dissolution was more rapid when air was excluded. In oxygen atmospheres an Fe(OH)3 precipitate armors mineral surfaces, thus inhibiting further dissolution, and further affects the solution by scavenging dissolved silica and cations. Dissolution reactions include initial exchange between cations and H+, incongruent dissolution of silicate structures, oxidation of Fe2+ in solution, precipitation of Fe(OH)3, and scavenging of dissolved silica and cations by Fe(OH)3. Dissolution kinetics may explain weathering of mafic rocks and minerals at the Earths surface, the formation of Fe-oxide coatings on mineral grains, weathering of submarine mafic rocks and intrastratal solution of mafic minerals in buried sandstones. Early Precambrian weathering would have been more rapid before the appearance of large amounts of oxygen in the atmosphere, and continental denudation rates may have been higher than at present because of this effect and the predominance of mafic igneous rocks at an early stage of continent formation and growth.

Sorption of silica by clay minerals

Abstract Clay minerals were reacted with silica-spiked solutions of unbuffered distilled water; water buffered at pH 5.5, 8 and 10; alkali chloride solutions; natural and artificial sea water to assess the influence of pH, silica and cation activities. The data are plotted as silica produced by dissolution or sorption of silica by clay surface as a function of initial silica concentration at a given pH and solution composition. This allows the determination of the dissolved silica value at which the clay mineral surface neither dissolves nor sorbs silica. The values of the various activities in different solutions are used to infer the phase equilibria between solution, clay mineral and the surface phase produced either by dissolution or sorption. Most intensively investigated were sorption reactions of kaolinite in sea water and other ionic solutions to form silica-rich, cation-rich surface phases in cationic solutions and silica-rich phases in cation-free solutions. Inferred equilibrium constants imply that silicate reconstitution is doubtful as a mechanism for partial control of silica and cation composition of sea water but is reasonable in silica-rich interstitial waters.

Diagenetic replacement controlled by force of crystallization

Many diagenetic replacements in sedimentary rocks occur along thin solution films between the authigenic and host phases. Rather than conventionally invoking bulk pore-water undersaturation for host-phase dissolution, we propose that many diagenetic replacements occur by a force of crystallization-controlled replacement mechanism whereby nonhydrostatic stresses resulting from authigenic crystal growth are principally responsible for host-phase dissolution. This model explains diagenetic features such as the restriction of host-phase dissolution to authigenic-host crystal contacts and euhedral authigenic crystal faces in planar contact with unreplaced host phases, as well as the relative replacement tendencies of some common authigenic minerals.

A new mechanism for pressure solution in porous quartzose sandstone

Abstract The mechanism of pressure solution, a source of controversy for years, must be understood before we can evaluate the effectiveness of pressure solution during geological processes. The water film diffusion (WFD) mechanism proposed by Weyl (1959) and Rutter (1976, 1983) is believed by many to be the primary mechanism responsible for intergranular pressure solution (IPS) in non-porous metamorphic rocks as well as porous sedimentary rocks. Tada and Siever (1986), experimenting with halite single crystals, suggested the new plastic deformation plus free-face pressure solution (PD + FFPS) mechanism. The effectiveness of PD + FFPS as an IPS mechanism is theoretically evaluated for porous quartzose sandstone and compared with WFD. The result suggests that, though the driving force of the reaction (relative activity increase) is 4 to 5 orders of magnitude larger in WFD, the ease of diffusion (diffusion path width times the diffusion coefficient) is 7 to 9 orders of magnitude larger in PD + FFPS. Consequently. PD + FFPS yields diffusion rates 2 to 5 orders of magnitude faster than WFD. In WFD, diffusion is always the rate-controlling process, whereas either dissolution at IPS contacts or precipitation on free grain surfaces may be the rate-controlling process in PD + FFPS, when temperatures are low and/or grain sizes are small. The dissolution or precipitation rate of PD + FFPS is faster than the diffusion rate of WFD except when the total free grain surface area is very small. In final stages of compaction, when the total free grain surface area has become very small, WFD replaces PD + FFPS.

Experimental knife-edge pressure solution of halite

Abstract Pressure solution experiments were carried out, using a quartz knife-edge 0.26 mm wide on halite single crystals in halite saturated solutions, to observe the detailed development of pressure solution contacts and the rates of pressure solution. A rate of about 3 μm/day was observed for initial knife-edge stresses ranging from 4.5 to 15 MPa. Close examination of the contact leads to the conclusion that the mechanism of pressure solution is a combination of plastic deformation at the contact and free surface pressure dissolution near its periphery. Free surface pressure dissolution increases the contact stress to about 18 MPa, high enough to cause plastic deformation, by changing the area of contact. This mechanism differs from a water film diffusion mechanism, previously suggested by many authors, but is similar in some ways to the undercutting hypothesis of Bathurst (1958). We infer a steady state plastic deformation instead of catastrophic grain crushing at the contact. Free surface dissolution plus the plastic deformation mechanism may be primarily responsible for pressure solution in relatively porous rocks.

Diffusion and mass balance of Mg during early dolomite formation, Monterey Formation

Abstract Diffusion from overlying seawater is capable of supplying enough Mg to account for all of the early diagenetic dolomite observed in the Monterey Formation (0.5–20 vol %). The amount formed depends on the Mg flux magnitude into the sediment, which is proportional to the rate of dolomite precipitation and inversely proportional to the sediment accumulation rate. The amount of dolomite formed is calculated for accumulation rates of 50 and 600 m/Ma, using reaction rate constants obtained from the Mg concentration profile of DSDP sites 479, 533 and 147, and proposed profiles that assume significant dolomite formation in the uppermost tens of meters of sediment. Advective pore water flow from compaction decreases the Mg flux and becomes increasingly significant as the accumulation rate increases and the reaction rate decreases. Advection alone during compaction supplies sufficient Mg to account for a realistic maximum of 2 vol % dolostone within a compacted section. An approximate evaluation of the Rayleigh number suggests that the low permeability of siliceous ooze prevents convective fluid flow at shallow burial depths. The Mg mass balance is calculated from dolomite abundance in Monterey Formation sections, Santa Maria basin area, California. Pore water and solid particle velocities are calculated as a function of sediment depth using a porosity-depth relation for average siliceous sediment based on a composite of DSDP data. The bulk sediment diffusion coefficient increases with sediment depth at a rate proportional to the geothermal gradient. At shallow burial depths seawater is probably the only significant source of Mg; dolomite, volcanic ash, and clay minerals are the only significant sinks. Clay mineral, organic matter, and silica diagenesis may provide additional Mg at deeper burial depths. The more or less regular spacing of dolomitic layers may be controlled by the residence time of the dolomite forming layer within the uppermost sediment, where the Mg flux is a maximum.

Establishment of equilibrium between clays and sea water

Kaolinite, montmorillonite and illite were reacted with silica-spiked natural and artificial sea water to demonstrate the effect of prologed grinding of the clay, pH, and dissolved silica concentration. Grinding greatly increases the reactivity to that the extent of reaction is appreciable after one week. Clays dissolve at low pH and low dissolved silica concentrations but will combine with alkalies and silica at higher pH and higher dissolved silica concentrations, presumably forming a more siliceous, more alkaline clay. Equilibrium pH and silica concentration can be bracketed from initial reactions in this way.

Morphology and Sediments of a Portion of the Mid-Atlantic Ridge.

In October 1964, a detailed geophysical and sampling survey was made of the central part of the Mid-Atlantic Ridge between 22� and 23� north latitude. The results indicate a large difference in age between the relief of the crest and that of the flanks of the Ridge and suggest that the crest portion is very young. Detailed surveys of two sediment-filled valleys on the upper western flank of the Ridge reveal different sedimentary sequences in the two valleys and indicate the probable existence of a locally controlled depositional regime and a significant local supply of sediment.