Metamorphism, hydrothermal

Cavaretta, G., Gianelli, G. and Puxeddu, M. (1980) Hydrothermal metamorphism in the Larderello geothermal field. Geothermics, 9, 297-314. 

It is tempting to describe the parent bodies of the future carbonaceous chondrites, once accretion was completed, as cold objects in motion around the sun. Such a description was broadly accepted until recently. The situation has now changed drastically, and the (unknown) parent bodies of Cl 1 and CM 2 carbonaceous chondrites are described as objects that have experienced hydrothermal metamorphism and which therefore were not cold and chemically dead during all their long lives. Collisions between asteroids seem not to be exceptional. Some of these collisions lead to the rupture of the asteroids which then become potential parent bodies. Other less dramatic collisions only produce heating. This heating may be an initial step towards hydrothermal metamorphism. 

The same phenomenology must be important locally on Earth, too, where thick evaporite deposits of hydrated salts and local thick beds of methane clathrate in permafrost or seafloor sediments should influence the thermal environment of the crust. The predicted control on the crust s thermal state by hydrate deposits should have consequences for the localization of hydrothermal springs around and within evaporite basins, hydrothermal metamorphism... 

It is obvious that in the case of pressure gradients determined by the maximum controlled by buffer reactions in the pile of rocks, and by the minimum in fractures (in the case of open circulation Pf(n,in) - (hydr) of the column of fluid), mechanical movement of the fluid was in one direction — from rock to fracture. For movement in the opposite direction—from fracture to rock—it was necessary to create a corresponding pressure gradient. Such phenomena, in addition to diffusion along the concentration gradient, presumably have occurred in hydrothermal metamorphism with typical reactions of hydration and carbonation. However, for normal progressive metamorphism it is hard to imagine a mechanical model in which a fluid with a strictly constant value of / h,o " h,o introduced from... 

Naboko, S.I., 1963. Hydrothermal metamorphism of rocks in volcanic regions. Izd. Akad. Nauk S.S.S.R., Moscow, 172 pp. (in Russian). 

Spooner E. T. C., Chapman H. J., and Smewing J. D. (1977) Strontium isotope contamination and oxidation during ocean floor hydrothermal metamorphism of the ophiolitic rocks of the Troodos Massif, Cyprus. Geochim. Cosmochim. Acta 41, 873-890. 

Scientists recognize several types of metamorphism regional metamorphism, contact metamorphism, dynamic metamorphism, and hydrothermal metamorphism. These occur between the low-temperature process of dia-genesis (temperature above 392°F [200°C] and pressure greater than 1,000 bars) and the high temperatures at which rocks melt and later cool to form igneous rocks (approximately 1,112-1,472°F [600-800°C] in temperature and more than 10,000 bars pressure). 

Hydrothermal metamorphism requires the presence of hot fluids derived from igneous rock nearby. The fluids react with minerals in the surrounding rock to produce... 

Talc is the major constituent of rocks known as soapstone or steatite. Its paragenesis is associated with the hydrothermal metamorphism of siliceous dolomites, and thus it might be accompanied by tremolite, which may be of concern for many potential applications. 

Fig. 6.4 Simplified summary of the geochemical carbon subcycle, showing main reservoirs and approximate sizes of annual fluxes (arrows) in kt of carbon (after Arthur 2000). dbl = deposition, burial + lithification hmv = hydrothermal metamorphism + volcanism rog = respiration + outgassing wtg = weathering.

As products of hydrothermal metamorphism in geothermal fields, such as Wairakei (10, 42), Pauzhetsk (39), and Onikobe (38). In these, a generalized downward sequence with increasing temperature is mordenite with or without analcime, rare heulandite, laumontite, wairakite, less hydrous phases. 

Schiffman P., Smith 6.M., Varga R.J. and Moores E.M., 1987, Geometry, conditions and timing of off-axis hydrothermal metamorphism and ore-depOsition m the Solea graben. Nature, 325, 423-425. 

The rare earths are very widely distributed in the earth s crust and exhibit a great diversity in the geological type of the deposits. They are found in all possible geological domains igneous-hydrothermal, metamorphic, and sedimentary. They occur as an important constituent in more than 100 different minerals and in trace quantities in many others. In fact, the study of the distribution of rare earths at trace levels in minerals is of considerable impor-... 

During the past ten years a number of papers have shown the interest of fluid inclusion studies in many different domains Metambrphism of the Westem Alps (Poty, 1969 Poty et al., 197 ), hydrothermal metamorphism of the oceanic crust (Jehl, 1975 Jehl et al., 1976), pegmatite evolution in low pressure type Hercynian metamorphism (Weisbrod and Poty, 1975) granulite facies and related rocks (Touret, 197 Hollister and Burruss, 1976). The results of these works are summarized elsewhere (Weisbrod et al., 1976). This paper will review some basic principles of the study of fluid inclusions in rocks. For reasons of space, only some aspects and techniques (notably microthermometry) will be discussed in detail and for a more general presentation the interested reader must consult the classical papers of Roedder, notably Roedder,... 

Sources. Iron ore deposits were formed by many different processes, eg, weathering, sedimentation, hydrothermal, and chemical. Iron ores occur in igneous, metamorphic, and sedimentary deposits. Normally, as-mined iron ore contains 25 to 68% iron. 

Talc of metasedimentary origin is formed by hydrothermal alteration of a dolomitic host rock by a silica-containing fluid. This type of deposit is typical of Montana and AustraUa. It is usually quite pure with talc content of 90 to 98% and often very white as well. Dolomite [17069-72-6], CaMg(C02)2, is the most common accessory mineral. The fourth type is of metamorphic origin, where a siUcaceous dolostone is first converted to tremolite [14567-73-8] or actinohte [13768-00-8] and then partially converted to talc. The Balmat, New York, and Death Valley, California, deposits are of this type. Tremolite, dolomite, and serpentine are common accessory minerals. This type of talc deposit has a variable talc content (30—80%), but is usually white and often commercially exploited because of the properties of its accessory minerals rather than the talc. 

Talc and pyrophylUte are found in metamorphic rocks that are rich in Mg and Al, respectively. Talc is most common in metamorphosed ultrabasic rocks and in metamorphosed siUceous dolomite. PyrophylUte is found in metapeUtes, including metabauxites and metakaoUnites, and in rocks enriched in Al by hydrothermal processes (106). 

Besshi-type deposits in Sanbagawa metamorphic terrain occur in the Minawa Formation which is composed of basic schist. Sometimes, they are associated with quartz schists. Probably, quartz has been originally formed from hydrothermal solution like siliceous ore in Kuroko deposits. Original rocks of basic schists are basaltic lava and hyaloclastics. Detailed geochemical investigation on the basic schists in the Sanbagawa... 

Host rocks in the Hitachi area suffered regional metamorphism, contact metamorphism by Cretaceous granitic rocks and hydrothermal alteration associated with sulfide mineralization. 

In the other areas, generally, the host rocks weakly suffered regional and contact metamorphisms but suffered ocean-floor hydrothermal alteration. For example, hydrothermal alteration mineral assemblages in the Minamidani mine district in the Maizuru range from prehnite-pumpellyite facies to a transition state from green schist to amphibolite... 

Kase and Horiuchi (1996) obtained a large number of analytical data on sphalerites from sixteen Besshi-type deposits, mainly at Besshi and its vicinity, Hitachi, and Shimokawa. They revealed that (1) the Mn/Zn and Co/Zn ratios of sphalerite may have markedly increased during contact metamorphism, while the Cd/Zn ratios remained unchanged (2) the Emco/ lwzn (2/n total dissolved concentration in ore fluids) and Emco/S/wzn ratios in the initial ore solutions responsible for the mineralizations at Besshi which was calculated ba.sed on the equilibrium fractionation model between hydrothermal solution and sphalerite and analytical data on sphalerites are quite similar to the ratios of hydrothermal solutions at EPR 21 °N (3) however, these ratios for the Hitachi solutions are very low and different from those of the Besshi-subtype solution. 

Tertiary climate change in relation to CO2 flux by volcanic, hydrothermal and metamorphic activities... 

In addition to hydrothermal and volcanic activity, metamorphism may have influenced the CO2 levels of the atmosphere and caused climate changes. Based on a model of the Cenozoic extension in the North American Cordillera, Nesbitt et al. (1995) demonstrated that CO2 generation associated with crustal extension may have been a major contributor to the elevated CO2 levels of the Cenozoic atmosphere and the resulting global warming due to the CO2 greenhouse effect. 

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