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Saturday, 3 December 2011

Origin of Carbonate Rocks

Origin of Carbonate Rocks

   In geology, carbonates are a class of sedimentary rocks compose primarily of carbonate minerals. Two major types are limestone, which is composed of calcite or aragonite (different crystalline forms of CaCO3) and dolostone, which is composed of the mineral dolomite (CaMg(CO3)2).

   There are are many studies published in books, papers about carbonate rocks, mainly related to depositional environments, variations in textures, structures, facies, mineralogy, stratigraphy, diagenesis, deformation, formation of karst processes, paleontological content (fossils), isotopic studies, among others. The carbonate record is also relatively well documented by geological studies in many carbonate platforms from the Archean to recent.

   Nevertheless the problem lies in the fact that, in geology, there is little concern about the origin of this rock type.

The questions are:
  •  Where does come from carbon present in the carbonate rocks?
  •  What are the causes of the beggining of carbonate sedimentation?
  •  What are the processes responsible for the formation of dolomite and dolostone?

   The ideas based on the principles of uniformitarianism have not resolved these issues. This problem is due geology not yet provided an understanding of the process of planetary formation, the origin of natural hydrocarbons and the carbon cycle, from deep within the Earth to ocean-atmosphere-biosphere systems.

   According to the scientist Thomas Gold, surface of the Earth is very rich in carbon and deserves an explanation. Four-fifth of this carbon is oxidized, mainly in the form of carbonates. Studies of Earth's carbon budget made by the Massachusetts Institute of Technology - MIT show that the carbonates represent about 5% of global carbon and about 83% of carbon in Earth's surface or near surface.

   The carbon present in carbonate rocks can be derived from excess methane in the ocean-atmosphere by outgassing of hydrocarbons and carbon dioxide from the primordial Earth's mantle. The dissociation of methane and its reaction with oxygen is then responsible for carbon oxidation and the formation of calcium carbonate salt would be common in this paroxysm, since calcium is an abundant element on Earth.  Methane is a greenhouse gas 20 times more potent than carbon dioxide and fixation in carbonates, its precipitation in marine and lake environments would be responsible by the removal excess of carbon of the oceanic-atmospheric pool. The carbonate rocks are highly chemically reactive and are reworked by sedimentary processes in the Earth's dynamic systems. Living organisms take advantage of the calcium carbonate to build their skeletons and structures and can also be entirely reworked and re-sediment bioclastic carbonate rocks.

   Cap carbonates occur after glacial periods, mainly in Neoproterozoic (Sturtian and Marinoan glacial events). Methane released from permafrost with excess in atmosphere could  be incresead to form overlying carbonate sequences.

   The process of dolomite formation is still an enigma for geology. However it is known that dolomite mineral do not precipitate in laboratory and the features in the process of dolomitization are best explained when related hypogene hydrothermal fluids from depth through deep faults, from which the magnesium that is incorporated into calcium carbonate. Primordial hydrocarbons sometimes occur associated to dolostones (hydrothermal dolomite - HTD) and frequently remain as bituminous material after intense biodegradation in carbonate vugs. 

   It is common association of carbonate sequences with sequences of halide salt as halite (traditionally the so-called evaporites). The gypsum and barite formed as sulphates, also associated with volcanic and hydrothermal systems that brings sulphur. Hydrothermal Salt theory and abiotic hydrocarbons are maybe a clue to understanding the process of dolomite formation.

   Indeed, understand release of primordial methane maybe would be the key for understand origin of carbonate rocks in the geological record. The dolomitization process it seems related to mantle deep fluids which bring hydrocarbons (oil and gas) and halogens similar to hydrothermal process of salt formation and its interaction with carbonate rocks.

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