Metamorphism of sediments

Prograde metamorphism of sediments causes the liberation of volatiles, which can be described by two end-member processes (Valley 1986) ... 

Prograde metamorphism of sediments (and to a lesser degree igneous and metamorphic rocks) causes the liberation of volatile components by the reaction of lower temperature, volatile rich minerals. If no externally derived fluids infiltrate the rock, volatilization is often referred to as closed system even though it is clear that evolved fluids have left the rock. Dehydration is most common, but decarbonation also occurs in carbonate-bearing lithologies (Ferry and Burt 1982) and desulfidation can locally be important (see also Cartwright and Oliver 2000). 

HOWER (J.) ESLINGER (E.V.), HOWER (M.E.) and PERRY (E.A.), 1976. Mechanism of burial metamorphism of argillaceous sediment 1. Mineralogical and chemical evidence. Geol. Soc. Arne. Bull. (17, 725-37. 

Olivine crystallizes from magmas that are rich in magnesia and low in silica and which form such rocks as gabbros, norites, peridotites and basalts. The metamorphism of impure dolomites or other sediments in which the magnesia content is high and silica low seems to produce olivine. 

Erosion from wind, water, or glaciers picks up materials from weathering rocks and deposits them as sediments or soil. A process called lithification describes the conversion of sediments to sedimentary rocks. In contrast to the parent igneous rocks, sediments and sedimentary rocks are porous, soft, and chemically reactive. Metamorphic rock is formed by the action of heat and pressure on sedimentary, igneous, or other kinds of metamorphic rock that are not in a molten state. 

Figure 10.1. A generalized diagram for the steady-state rock cycle. Sediments, S, and continental crystalline crust, C, masses are in units of metric tons. Ss, Cs, Sc, Cc, and M are fluxes in units of 109 tons y-l due to erosion of sediments, metamorphism, erosion of crystalline rocks, recycling of crystalline rocks (resetting of ages during tectogenesis), and cycling of oceanic crust, respectively. Total sedimentation rate is 9 x 109 tons y-l. (After Gregor, 1988.)...

At the onset of diagenesis of wet sediments and metamorphism of lithified rocks there are substantial amounts of intergranular aqueous and saline solutions. As the grade of metamorphism increases, the porosity and permeability of a rock decrease and intergranular fluids become less continuous and more localized. As a result, the availability of ions for metamorphic reactions becomes increasingly dependent on the composition of the rock in situ. 

An analysis of sedimentation in the Precambrian made by Tugarinov and Voytkevich (1966) on the basis of a summary of recent geochemical data gives a clear picture of the directionality and irreversibility of the geologic evolution of the Earth s crust. Whereas in the Early Precambrian the composition of the sedimentary-metamorphic complexes is characterized by the predominant development of basic and ultrabasic effusives, altered... 

The sequence of dehydration reactions in the progressive metamorphism of silicate rocks depends on the presence of excess iron hydroxide in the original sediment, inasmuch as the lowest-temperature reaction is the transformation of the greenalite + hematite association into magnetite according to the reaction ... 

As a result, a physicochemical model for the formation of the BIF is proposed which is consistent with modern ideas on the evolution of sedimentation and volcanism and of the atmosphere, hydrosphere, and biosphere in the Precambrian. This model, which proposes a mainly volcanic source for the iron and silica and a biochemical and chemical mechanism of deposition, is the most likely but not the only possible one. Other versions, or different interpretations, are not ruled out, but it is perfectly obvious that in any genetic postulates, the specific physicochemical data must be taken into account. It is also quite understandable that in a work which is a first attempt at physicochemical analysis of the entire geological cycle— source of the material transport deposition diagenesis metamorphism—not all the problems have been worked out in sufficient detail and not all the evidence is conclusive far from it. Further investigations in this direction are needed, including not only determination of the role of the individual parameters in ore formation, but also direct experimental modeling of the process. 

It is critical to clarify the fluid flow picture during HP-LT metamorphism since subduction of sediment and hydrothermally altered oceanic crust and mantle is the primary means by which reactive volatiles including H2O and CO2 are returned to the deep Earth, ultimately giving rise to arc magma genesis at —100-150 km depth... 

The porosity and fracture content of rocks determine the maximum possible volume of gases in rocks and the gas permeability of rocks determines the speed of migration of these gases. On the basis of these parameters, typical geological structures may be divided into closed and open structures. Igneous and metamorphic rocks tend to have closed structures, whereas sediments have open structures. The porosity of igneous rocks is typically 0.5-2% and changes little with depth. Their gas permeability is typically less than 10 pm and mainly depends on fracture content. The porosity of sediments decreases from 30-35% at surface to 10-20% at a depth of 2 km. Their gas permeability varies from 10 pm to 3 pm (Fridman, 1970 Dortman, 1992). 

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