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In geology, silicification is a petrification process in which silica-rich fluids seep into the voids of Earth materials, e.g., rocks, wood, bones, shells, and replace the original materials with silica (SiO2). Silica is a naturally existing and abundant compound found in organic and inorganic materials, including Earth's crust and mantle. There are a variety of silicification mechanisms. In silicification of wood, silica permeates into and occupies cracks and voids in wood such as vessels and cell walls. [1] The original organic matter is retained throughout the process and will gradually decay through time. [2] In the silicification of carbonates, silica replaces carbonates by the same volume. [3] Replacement is accomplished through the dissolution of original rock minerals and the precipitation of silica. This leads to a removal of original materials out of the system. [3] [4] Depending on the structures and composition of the original rock, silica might replace only specific mineral components of the rock. Silicic acid (H4SiO4) in the silica-enriched fluids forms lenticular, nodular, fibrous, or aggregated quartz, opal, or chalcedony that grows within the rock. [5] Silicification happens when rocks or organic materials are in contact with silica-rich surface water, buried under sediments and susceptible to groundwater flow, or buried under volcanic ashes. Silicification is often associated with hydrothermal processes. [1] Temperature for silicification ranges in various conditions: in burial or surface water conditions, temperature for silicification can be around 25°−50°; whereas temperatures for siliceous fluid inclusions can be up to 150°−190°. [6] [7] Silicification could occur during a syn-depositional or a post-depositional stage, commonly along layers marking changes in sedimentation such as unconformities or bedding planes. [5] [8]
The sources of silica can be divided into two categories: silica in organic and inorganic materials. The former category is also known as biogenic silica, which is a ubiquitous material in animals and plants. The latter category is the second most abundant element in Earth's crust. [9] Silicate minerals are the major components of 95% of presently identified rocks. [10]
Biogenic silica is the major source of silica for diagenesis. One of the prominent examples is the presence of silica in phytoliths in the leaves of plants, ie. grasses, and Equisetaceae. Some suggested that silica present in phytoliths can serve as a defense mechanism against the herbivores, where the presence of silica in leaves increases the difficulty in digestion, harming the fitness of herbivores. [11] However, evidence on the effects of silica on the wellbeing of animals and plants is still insufficient.
Besides, sponges are another biogenic source of naturally occurring silica in animals. They belong to the phylum Porifera in the classification system. Silicious sponges are commonly found with silicified sedimentary layers, for example in the Yanjiahe Formation in South China. [12] Some of them occur as sponge spicules and are associated with microcrystalline quartz or other carbonates after silicification. [12] It could also be the main source of precipitative beds such as cherts beds or cherts in petrified woods. [12]
Diatoms, an important group of microalgae living in marine environments, contribute significantly to the source of diagenetic silica. They have cell walls made of silica, also known as diatom frustules. [13] In some silicified sedimentary rocks, fossils of diatoms are unearthed. This suggests that diatoms frustules were sources of silica for silicification. [13] Some examples are silicified limestones of Miocene Astoria Formation in Washington, silicified ignimbrite in El Tatio Geyser Field in Chile, and Tertiary siliceous sedimentary rocks in western pacific deep sea drills. [13] [14] [15] The presence of biogenic silica in various species creates a large-scale marine silica cycle that circulates silica through the ocean. Silica content is therefore high in active silica upwelling areas in the deep-marine sediments. Besides, carbonate shells that deposited in shallow marine environments enrich silica contents at continental shelf areas. [16]
The major component of the Earth's upper mantle is silica (SiO2), which makes it the primary source of silica in hydrothermal fluids. SiO2 is a stable component. It often appears as quartz in volcanic rocks. Some quartz that is derived from pre-existing rocks, appear in the form of sand and detrital quartz that interact with seawater to produce siliceous fluids. [12] In some cases, silica in siliceous rocks are subjected to hydrothermal alteration and react with seawater at certain temperatures, forming an acidic solution for silicification of nearby materials. In the rock cycle, the chemical weathering of rocks also releases silica in the form of silicic acid as by-products. [12] Silica from weathered rocks is washed into waters and deposit into shallow-marine environments. [17]
The presence of hydrothermal fluids is essential as a medium for geochemical reactions during silicification. In the silicification of different materials, different mechanisms are involved. In the silicification of rock materials like carbonates, replacement of minerals through hydrothermal alteration is common; while the silicification of organic materials such as woods is solely a process of permeation. [17] [18]
The replacement of silica involves two processes:
1) Dissolution of rock minerals [18]
2) Precipitation of silica [18]
It could be explained through the carbonate-silica replacement. Hydrothermal fluids are undersaturated with carbonates and supersaturated with silica. When carbonate rocks get in contact with hydrothermal fluids, due to the difference in gradient, carbonates from the original rock dissolve into the fluid whereas silica precipitate out of it. [18] The carbonate that dissolved is therefore pulled out from the system while the silica precipitated recrystallizes into various silicate minerals, depending on the silica phase. [17] The solubility of silica strongly depends on the temperature and pH value of the environment [3] where pH9 is the controlling value. [18] Under a condition of pH lower than 9, silica precipitates out of the fluid; when the pH value is above 9, silica becomes highly soluble. [3]
In the silicification of woods, silica dissolves in hydrothermal fluid and seeps into lignin in cell walls. Precipitation of silica out of the fluids produces silica deposition within the voids, especially in the cell walls. [1] [19] Cell materials are broken down by the fluids, yet the structure remains stable due to the development of minerals. Cell structures are slowly replaced by silica. Continuous penetration of siliceous fluids results in different stages of silicification ie. primary and secondary. The loss of fluids over time leads to the cementation of silicified woods through late silica addition. [21]
The rate of silicification depends on a few factors:
1) Rate of breakage of original cells [21]
2) Availability of silica sources and silica content in the fluid [1] [3]
3) Temperature and pH of silicification environment [1] [3]
4) Interference of other diagenetic processes [3] [22]
These factors affect the silicification process in many ways. The rate of breakage of original cells controls the development of the mineral framework, hence the replacement of silica. [21] Availability of silica directly determines the silica content in fluids. The higher the silica content, the faster silicification could take place. [1] The same concept applies to the availability of hydrothermal fluids. The temperature and pH of the environment determine the condition for silicification to occur. [3] [22] This is closely connected to the burial depth or association with volcanic events. Interference of other diagenetic processes could sometimes create disturbance to silicification. The relative time of silicification to other geological processes could serve as a reference for further geological interpretations. [1] [19] [21] [22]
In the Conception Bay in Newfoundland, Southeastern coast of Canada, a series of Pre-Cambrian to Cambrian-linked volcanic rocks were silicified. The rocks mainly consist of rhyolitic and basaltic flows, with crystal tuffs and breccia interbedded. Regional silicification was taken place as a preliminary alteration process before other geochemical processes occurred. [23] The source of silica near the area was from hot siliceous fluids from rhyolitic flow under a static condition. [23] A significant portion of silica appeared in the form of white chalcedonic quartz, quartz veins as well as granular quartz crystal. [23] Due to the difference in rock structures, silica replaces different materials in rocks of close locations. The following table shows the replacement of silica at different localities: [23]
Location | Material Replaced | Form of silica |
---|---|---|
Manuels | Spherulites of rhyolites | Chalcedonic quartz |
Clarenville | Groundmass of rocks | Chalcedonic quartz with sericite along glassy cracks |
In the Semail Nappe of Oman in the United Arb Emirates, silicified serpentinite was found. The occurrence of such geological features is rather unusual. It is a pseudomorphic alteration where the protolith of serpentinite was already silicified. [24] Due to tectonic events, basal serpentinite was fractured and groundwater permeated along the faults, forming a large-scale circulation of groundwater within the strata. [24] Through hydrothermal dissolution, silica precipitated and crystallized around the voids of serpentinite. [25] Therefore, silicification can only be seen along groundwater paths. [25] The silicification of serpentinite was formed under the condition where groundwater flow and carbon dioxide concentration are low. [24] [25]
Silicified carbonates can appear as silicified carbonate rock layers, [3] or in the form of silicified karsts. The Paleogene Madrid Basin in Central Spain is a foreland basin resulted from the Alpine uplift, an example of silicified carbonates in rock layers. The lithology consists of carbonate and detritus units that were formed in a lacustrine environment. The rock units are silicified where cherts, quartz, and opaline minerals are found in the layers. [26] It is conformable with the underlying evaporitic beds, also dated from similar ages. It is found that there were two stages of silicification within the rock strata. [26] The earlier stage of silicification provided a better condition and site for the precipitation of silica. The source of silica is still uncertain. [26] There are no biogenic silica detected from the carbonates. However, microbial films in carbonates are found, which could suggest the presence of diatoms. [26]
Karsts are carbonate caves formed from a dissolution of carbonate rocks such as limestones and dolomites. They are usually susceptible to groundwater and are dissolved in these drainage. Silicified karsts and cave deposits are formed when siliceous fluids enter karsts through faults and cracks. [17] The Mid-Proterozoic Mescal Limestone from the Apache Group in central Arizona is classic examples of silicified karsts. A portion of the carbonates are replaced by cherts in early diagenesis and the remaining portion is completely silicified in later stages. [17] The source of silica in carbonates are usually associated with the presence of biogenetic silica; however, the source of silica in Mescal Limestone is from weathering of overlying basalts, which are extrusive igneous rocks that have high silica content. [17]
Silicification of woods usually occur in terrestrial conditions, but sometimes it could be done in aquatic environments. [19] Surface water silicification can be done through the precipitation of silica in silica-enriched hot springs. On the northern coast of central Japan, the Tateyama hot spring has a high silica content that contributes to the silicification of nearby fallen woods and organic materials. Silica precipitates rapidly out of the fluids and opal is the main form of silica. [18] With a temperature of around 70°C and a pH value of around 3, the opal deposited is composed of silica spheres of different sizes arranged randomly. [18]
Mafic magma dominated the seafloor at around 3.9 Ga during the Hadean-Archean transition. [27] Due to rapid silicification, the felsic continental crust began to form. [28] In the Archean, the continental crust was composed of tonalite–trondhjemite–granodiorite (TTG) as well as granite–monzonite–syenite suites. [28]
The Mount Goldsworthy in the Pilbara Craton located in Western Australia holds one of the earliest silicification example with an Archean clastic meta-sedimentary rock sequence, revealing the surface environment of the Earth in the early times with evidence from silicification and hydrothermal alteration. The unearthed rocks are found to be SiO2 dominant in terms of mineral composition. [8] The succession was subjected to a high degree of silicification due to hydrothermal interaction with seawater at low temperatures. [8] Lithic fragments were replaced with microcrystalline quartz and protoliths were altered during silicification. [8] The condition of silicification and the elements that were present suggested that the surface temperature and carbon dioxide contents were high during either or both syn-deposition and post-deposition. [8]
The Barberton Greenstone Belt in South Africa, specifically the Eswatini Supergroup of around 3.5–3.2 Ga, is a suite of well-preserved silicified volcanic-sedimentary rocks. With the composition ranging from ultramafic to felsic, the silicified volcanic rocks are directly beneath the bedded chert layer. Rocks are more silicified near the bedded chert contact, suggesting a relationship between chert deposition and silicification. [29] The silica altered zones reveal that hydrothermal activities, as in seawater circulation, actively circulate the rock layers through fractures and fault during the deposition of bedded chert. [30] The seawater was heated up and therefore picked up silicious materials from underneath volcanic origin. The silica enriched fluids bring about silicification of rocks through seeping into porous materials in the syn-depositional stage at a low-temperature condition. [30] [31]
Limestone is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of CaCO3. Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as the accumulation of corals and shells in the sea, have likely been more important for the last 540 million years. Limestone often contains fossils which provide scientists with information on ancient environments and on the evolution of life.
Sandstone is a clastic sedimentary rock composed mainly of sand-sized silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.
Chert is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood.
Petrified wood, also known as petrified tree, is the name given to a special type of fossilized wood, the fossilized remains of terrestrial vegetation. Petrifaction is the result of a tree or tree-like plants having been replaced by stone via a mineralization process that often includes permineralization and replacement. The organic materials making up cell walls have been replicated with minerals. In some instances, the original structure of the stem tissue may be partially retained. Unlike other plant fossils, which are typically impressions or compressions, petrified wood is a three-dimensional representation of the original organic material.
Skarns or tactites are coarse-grained metamorphic rocks that form by replacement of carbonate-bearing rocks during regional or contact metamorphism and metasomatism. Skarns may form by metamorphic recrystallization of impure carbonate protoliths, bimetasomatic reaction of different lithologies, and infiltration metasomatism by magmatic-hydrothermal fluids. Skarns tend to be rich in calcium-magnesium-iron-manganese-aluminium silicate minerals, which are also referred to as calc-silicate minerals. These minerals form as a result of alteration which occurs when hydrothermal fluids interact with a protolith of either igneous or sedimentary origin. In many cases, skarns are associated with the intrusion of a granitic pluton found in and around faults or shear zones that commonly intrude into a carbonate layer composed of either dolomite or limestone. Skarns can form by regional or contact metamorphism and therefore form in relatively high temperature environments. The hydrothermal fluids associated with the metasomatic processes can originate from a variety of sources; magmatic, metamorphic, meteoric, marine, or even a mix of these. The resulting skarn may consist of a variety of different minerals which are highly dependent on both the original composition of the hydrothermal fluid and the original composition of the protolith.
In geology, petrifaction or petrification is the process by which organic material becomes a fossil through the replacement of the original material and the filling of the original pore spaces with minerals. Petrified wood typifies this process, but all organisms, from bacteria to vertebrates, can become petrified. Petrifaction takes place through a combination of two similar processes: permineralization and replacement. These processes create replicas of the original specimen that are similar down to the microscopic level.
Magnesite is a mineral with the chemical formula MgCO
3. Iron, manganese, cobalt, and nickel may occur as admixtures, but only in small amounts.
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical composition. The minerals which compose the rocks are dissolved and new mineral formations are deposited in their place. Dissolution and deposition occur simultaneously and the rock remains solid.
Serpentinization is a hydration and metamorphic transformation of ferromagnesian minerals, such as olivine and pyroxene, in mafic and ultramafic rock to produce serpentinite. Minerals formed by serpentinization include the serpentine group minerals, brucite, talc, Ni-Fe alloys, and magnetite. The mineral alteration is particularly important at the sea floor at tectonic plate boundaries.
Volcanogenic massive sulfide ore deposits, also known as VMS ore deposits, are a type of metal sulfide ore deposit, mainly copper-zinc which are associated with and created by volcanic-associated hydrothermal events in submarine environments.
Various theories of ore genesis explain how the various types of mineral deposits form within Earth's crust. Ore-genesis theories vary depending on the mineral or commodity examined.
In geology, a vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation. The hydraulic flow involved is usually due to hydrothermal circulation.
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks, and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic to refer to sedimentary rocks and particles in sediment transport, whether in suspension or as bed load, and in sediment deposits.
Marine sediment, or ocean sediment, or seafloor sediment, are deposits of insoluble particles that have accumulated on the seafloor. These particles have their origins in soil and rocks and have been transported from the land to the sea, mainly by rivers but also by dust carried by wind and by the flow of glaciers into the sea. Additional deposits come from marine organisms and chemical precipitation in seawater, as well as from underwater volcanoes and meteorite debris.
Hydrothermal mineral deposits are accumulations of valuable minerals which formed from hot waters circulating in Earth's crust through fractures. They eventually create metallic-rich fluids concentrated in a selected volume of rock, which become supersaturated and then precipitate ore minerals. In some occurrences, minerals can be extracted for a profit by mining. Discovery of mineral deposits consumes considerable time and resources and only about one in every one thousand prospects explored by companies are eventually developed into a mine. A mineral deposit is any geologically significant concentration of an economically useful rock or mineral present in a specified area. The presence of a known but unexploited mineral deposit implies a lack of evidence for profitable extraction.
The silica cycle is the biogeochemical cycle in which biogenic silica is transported between the Earth's systems. Silicon is considered a bioessential element and is one of the most abundant elements on Earth. The silica cycle has significant overlap with the carbon cycle and plays an important role in the sequestration of carbon through continental weathering, biogenic export and burial as oozes on geologic timescales.
The Dresser Formation is a Paleoarchean geologic formation that outcrops as a generally circular ring of hills the North Pole Dome area of the East Pilbara Terrane of the Pilbara Craton of Western Australia. This formation is one of many formations that comprise the Warrawoona Group, which is the lowermost of four groups that comprise the Pilbara Supergroup. The Dresser Formation is part of the Panorama greenstone belt that surrounds and outcrops around the intrusive North Pole Monzogranite. Dresser Formation consists of metamorphosed, blue, black, and white bedded chert; pillow basalt; carbonate rocks; minor felsic volcaniclastic sandstone and conglomerate; hydrothermal barite; evaporites; and stromatolites. The lowermost of three stratigraphic units that comprise the Dresser Formation contains some of the Earth's earliest commonly accepted evidence of life such as morphologically diverse stromatolites, microbially induced sedimentary structures, putative organic microfossils, and biologically fractionated carbon and sulfur isotopic data.
Silicon isotope biogeochemistry is the study of environmental processes using the relative abundance of Si isotopes. As the relative abundance of Si stable isotopes varies among different natural materials, the differences in abundance can be used to trace the source of Si, and to study biological, geological, and chemical processes. The study of stable isotope biogeochemistry of Si aims to quantify the different Si fluxes in the global biogeochemical silicon cycle, to understand the role of biogenic silica within the global Si cycle, and to investigate the applications and limitations of the sedimentary Si record as an environmental and palaeoceanographic proxy.
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