Lithosphere temperature

Needless to say, very much remains to be learned about the chemistry of the inner lithosphere. It is a high temperature and high pressure laboratory whose door has not yet been opened. 

There are several environmentally significant mercury species. In the lithosphere, mercury is present primarily in the +II oxidation state as the very insoluble mineral cirmabar (HgS), as a minor constituent in other sulfide ores, bound to the surfaces of other minerals such as oxides, or bound to organic matter. In soil, biological reduction apparently is primarily responsible for the formation of mercury metal, which can then be volatilized. Metallic mercury is also thought to be the primary form emitted in high-temperature industrial processes. The insolubility of cinnabar probably limits the direct mobilization of mercury where this mineral occurs, but oxidation of the sulfide in oxygenated water can allow mercury to become available and participate in other reactions, including bacterial transformations. 

The presence of a lithosphere with a thickness up to 100 km above the plume head obscures observations that could be made in terms of heat flow, gravity field or seismic structure. Establishing the temperature and flow fields beneath a hotspot thus becomes a difficult exercise. Several key parameters (Fig. 2) are poorly constrained and mostly result from theoretical fluid dynamics model, which underlines their large uncertainty. The temperature anomaly within the hotspot region is generally estimated to be approximately 200 100°C with large uncertainties (Shilling 1991 Sleep 1990). These temperature anomalies will induce smaller densities in the plume and the flux of the density anomalies is called buoyancy flux as defined in (Sleep 1990) ... 

The course taken by any particular fossilization process is, therefore, determined by the physical and chemical factors prevalent in the environment of the dead remains. The physical factors include temperature, degree of aeration, and rate of flow of groundwater. The nature of minerals and rocks, and of the groundwater at the site of burial, are the most important chemical factors. Reconstructing and explaining the processes undergone by dead remains, from the time of death to when they are fully fossilized, is the concern of taphonomy, the study of the processes taking place when dead remains pass from the biosphere to the lithosphere (see Textbox 69). 

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

Goetze C, Evans B (1979) Stress and temperature in the bending lithosphere as constrained by experimental rock mechanics. Geophys J Royal Astr Soc 59(3) 463-478... 

A chemical substance has its chemical energy in terms of the chemical potential and has its chemical exergy as well. Let us consider a chemical substance present at unit activity in the normal environment at temperature T0 and pressure p0 and examine its chemical exergy in relation with the exergy reference species in the atmospheric air, in seawater, and in lithospheric solids (Refs. 9 and 11). 

We next consider metallic iron whose exergy reference species are oxygen molecules in the atmospheric air and solid iron oxide Fe203, which is the most stable existence of iron in the top layer of the lithosphere. In the atmospheric air metallic iron reacts with oxygen gas to form iron oxide (corrosion of metallic iron). The reaction at the standard state (unit activity, standard pressure 101.3 kJ, and standard temperature 298 K) is expressed in Eq. 10.30 ... 

In the case of solid substances the reference species is often set at the most stable solid compounds in lithospheric rocks. For example, metallic iron is most stable in the form of its oxides. The standard chemical exergy of metallic iron can then be obtained from the standard affinity Aaf of the formation of iron oxide, Fe +0.75O2 = 0.5Fe2O3 A° = e e + 0.75s 2 - 0.5 pe2Oj and = 0 hence e°c = A° -0.75e° . Table 10.3 shows the standard molar chemical exergy of a few substances relative to the solid reference species in the lithosphere at the standard temperature and pressure. 

There were times on our planet when the barren dryness of uninhabited continents sharply contrasted with the densely populated sea. The continental lithosphere was then essentially represented by rock surfaces of different types. Sedimentary rocks were rare, if not absent. As rock materials became exposed to the subaerial environment at the Earth s surface, they encountered a whole range of environmental challenges such as temperature fluctuations, water, unbuffered cosmic and solar irradiation and atmospheric gases and solids instead of dissolved species. These influences resulted in rocks undergoing alterations in material properties leading to erosion and breakdown into ever-smaller particles and constituent minerals, formation of sandy sediments, and mineral soils (Ehrlich, 1996). Primordial terrestrial environments can therefore be visualized as a freshly exposed and only slightly physically pre-weathered rock surface. 

The strain-rate estimates suggest that the microstructures were formed by flow in the mantle related to large-scale tectonic processes. In the case of the Voltri peridotites, Drury et al. (1990) have related this deformation to extension and rifting of a continental lithospheric plate. The high-temperature deformation occurred under asthenospheric conditions, whereas the low-temperature deformation occurred within the lithosphere. The estimates of upper mantle viscosity obtained from the study of microstructures are lower than those for average viscosity obtained independently from other methods, such as analysis of glacial rebound of land surfaces (Nakada and Lambeck 1987), which are also based on a number of assumptions. All the samples studied are derived from the uppermost part of the mantle, so the viscosity estimates obtained from the microstructural study are consistent with the presence of a low viscosity zone between the... 

To explain the denudation of peridotites on the seafloor before the onset of oceanic accretion, Lemoine et al. (1987) and Trommsdorff et al. (1993) proposed a model involving extensional exhumation of subcontinental mantle along a major, normal detachment fault rooted in the crust-mantle boundary (Wernicke, 1981,1985). In most cases, however, the peridotites record a higher-temperature evolution than would be expected from stable lithospheric mantle. This suggests that ... 

The formation of replacive pyroxenites can be explained by melt-consuming reactions at pressure and temperature conditions close to the peridotite solidus. Ronda represents a situation where the reaction was associated with relaxation of thinned and thermally eroded subcontinental lithosphere (Garrido and Bodinier, 1999). Upon cooling of the melting domain developed during the thermal erosion (Lenoir et al., 2001), the interstitial melt became saturated in pyroxene ( aluminous phases) and reacted with olivine to produce secondary cpx, opx, and spinel... 

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