Hydrothermal heat loss

While most studies of seafloor hydrothermal systems have focused on the currently active plate boundary ( 0-1 Ma crust), pooled heat-flow data from throughout the world s ocean basins (Figure 1) indicate that convective heat loss from the oceanic lithosphere actually continues in crust from 0-65 Ma in age (Stein et al, 1995). Indeed, most recent estimates would indicate that hydrothermal circulation through this older (1-65 Ma) section, termed flank fluxes, may be responsible for some 70% or more of the total hydrothermal heat loss associated with spreading-plate boundaries—either in the form of warm (20-65 °C) altered seawater, or as cooler water, which is only much more subtly chemically altered (Mottl, 2003). 

For example, assuming anhydrite-magnetite-calcite-pyrite-pyrrhotite buffers redox in sub-seafloor reaction zones and a pressure of 500 bars, dissolved H2Saq concentrations of 21 °N EPR fluid indicate a temperature of 370-385°C. However, the estimated temperatures are higher than those of the measurement. This difference could be explained by adiabatic ascension and probably conductive heat loss during ascension of hydrothermal solution from deeper parts where chemical compositions of hydrothermal solutions are buffered by these assemblages. 

Cumulative percent convective heat loss. The effect of high-temperature hydrothermal emissions is included in the 0.1-million-year-old crustal age bin. Data from Wheat, C. G. (2003). Geophysical Research Letters 30, 1895. 

Alternatively, the duration of hydrothermal convection in the oceanic crust can be estimated by mapping the distribution of nonlinear temperature profiles taken during heat-flow measurements as a function of oceanic-crustal age. Purely conductive heat loss (i.e., no hydrothermal circulation) results in linear temperature profiles in sediments, while convective heat loss results in concave or convex profiles, depending on whether the water penetrates into or comes out of the sediments. 

Ma, the oceanic cmst has lost only half of its total convective heat loss (C. A. Stein and S. Stein, 1994b). Hydrothermal fluxes from the rock record at sites 417 and 418, for example, offer a complete set of chemical changes occurring during the entire hydrothermal history of the oceanic cmst. 

If fracturing occurs in hydrothermal system, hydrothermal solution in reservoir begins to ascend very rapidly along the fractures. It seems likely as an approximation that hydrothermal solution ascends with adiabatic expansion without the heat loss from hydrothermal solution to surrounding rocks because of very rapid flow rate of hydrothermal solution (1-10 m s ) at the sea floor from which hydrothermal solution issues. Temperature of hydrothermal solution decreases due to adiabatic expansion. Hydrothermal solution sometimes boils by the changes in temperature and pressure in underground. 

Differences have been noted between species in the way in which they respond to heat treatment, but most notably between hardwoods and softwoods. Thermal, hydrothermal or hygrothermal treatment of various woods results in weight losses that are generally found to be higher for hardwood compared to softwood species (MacLean, 1951 Zaman etal., 2000 Militz, 2002). 

A titanosilicate derivative of NU-1 framework has been prepared following primary hydrothermal synthesis. The manner in which the hydrogel adjusts to accommodate Ti cation is demonstrated in the form of supplemented hydrogel systems. Evidence from IR and UV-vis supports the conclusion of Ti insertion into the NU-1 framework. The Ti located at framework undergo partial modification after the loss of organic moieties and heating above 973 K collapses the TS-NU-1 structure. 

Table 1.1 also contains data for average concentrations of the 25 selected elements in the oceanic crust. As a first approximation, we have assumed for this part of the compilation that the oceanic crust mainly consists of the so-called ocean ridge basalt (MORE). Large volumes of this basaltic ocean crust have undergone hydrothermal alteration connected with a gain of H2O, CO2, Na, Mg and S from heated sea water and losses of Si, Ca, Ee, Mn, etc. from the altered basalt to the ocean water reservoir. 

The isotopic composition of fluids in equilibrium with the smectites are expected to be close to the meteoric water line at temperatures between 30° and 90 °C. Therefore, it can be assumed, that predominantely heated meteoric water is responsible for the formation of the bentonites. The slight enrichment of 0 and D isotopes compared with meteoric water may be due to steam loss or mixing with both meteoric water and water from hydrothermal reservoir. The production of andesitic lavas, 20.2 Ma ago (Abdelkader, 1997), was accompanied by subvolcanic intrusive magmatism which could be considered as a possible heat source. At lower temperatures 6 0 values of smectites would almost exceed -i-25 7m. If heated water of 80 °C was responsible for bentonitization (Christidis et al., 1995) smectites should have 5 0 values of -fl9 °/m. Alteration by water similar to the fluids of the deep hydrothermal reservoir (Dietrich et al., 1992) is not likely as well. The isotopic composition of these fluids at temperatures above 200 °C should provide smectites with 5 0 value around +12 7m. 

XRD patterns of hydrothermally treated titania particles are shown in Figure 14-9 (a) is for as-treated particles and (b) for the particles after heat-treated at 600°C. By the hydrothermal treatment, the crystallization of amorphous titania particles occurs to form mainly anatase and a small amount ofbrookite as shown in (a) the transformation of these phases to rutile does not occur after the heat treatment at 600°C for 17 h as shown in (b). DTA-TG curves of the hydrothermally treated titania particles showed no exothermic peak or weight loss. Since the residual organics are probably eliminated during the hydrothermal treatment, the lowering of the transformation temperature does not occur. 

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