Fluid phase carbonate-rich

Clark, J., Demonstration of presence and size of a C02-rich fluid phase after HCL injection in carbonate rock, in Underground Injection Science and Technology, Tsang, C.F. and Apps, J.A., Eds., Elsevier, New York, February 2007. 

Although shallow-mantle xenoliths, hosted in alkali basalts, commonly contain C02-rich fluid inclusions (see below), there have been no reports, to the author s knowledge, of H20-rich fluid inclusions in these samples. The C02-rich fluid inclusions are commonly attributed to late, possibly magma-derived, metasomatism of the samples. If such metasomatism was produced by silicate- or carbonate-rich melts, ascent of such a melt could produce saturation in a C02-rich vapor, but H2O would partition strongly into either residual melt or hydrous phases such as phlogopite or amphibole. Thus, the absence of H2O in the fluid inclusions in these samples cannot be taken as evidence that the metasomatic agent was anhydrous. 

The dense fluid that exists above the critical temperature and pressure of a substance is called a supercritical fluid. It may be so dense that, although it is formally a gas, it is as dense as a liquid phase and can act as a solvent for liquids and solids. Supercritical carbon dioxide, for instance, can dissolve organic compounds. It is used to remove caffeine from coffee beans, to separate drugs from biological fluids for later analysis, and to extract perfumes from flowers and phytochemicals from herbs. The use of supercritical carbon dioxide avoids contamination with potentially harmful solvents and allows rapid extraction on account of the high mobility of the molecules through the fluid. Supercritical hydrocarbons are used to dissolve coal and separate it from ash, and they have been proposed for extracting oil from oil-rich tar sands. 

There have been many investigations of the water content of C02-rich fluids. In general, there is reasonable agreement among the various sets of data in the low and moderate pressure regions. The benchmark investigation of the phase behavior in the system carbon dioxide + water was that of Wiebe and Gaddy (1941). 

The presence of volatile-bearing phases such as phlogopite, apatite, and carbonates in kimberhtes testify to the volatile-rich nature of the parental magma (e.g., Mitchell, 1986). The ubiquitous serpentization present in kimberlites cannot be used as evidence of magmatic water, with the exception of groundmass serpentine that is interpreted to be primary in nature. As discussed by Mitchell (1986), there are hmited stable isotopic data consistent with a meteoric origin for some of the water in the serpentine. However, it is unclear if these results could be attributed to postemplacement exchange of deuteric serpentine with meteoric fluids. 

Potential insight into the fate of a chlorinebearing fluid came from the study of Andersen et al. (1984) of xenoliths from Bullenmerri and Gnotuk maars in southwestern Australia that contained abundant C02-rich fluid inclusions and vugs up to 1.5 cm in diameter. They found the trapped fluids had reacted with the host minerals to produce secondary carbonates and amphiboles, such that the original composition of the fluid was inferred to be a chlorine- and sulfurbearing CO2-H2O fluid. The evidence for chlorine was the presence of a chlorine peak in the energy-dispersive spectmm of the amphibole unfortunately, no quantitative analyses were possible on these amphiboles. This does pose the possibility that this sort of reaction is common, and that the normal host for chlorine in the mantle is a mineral phase, such as apatite, amphibole, and mica. 

Calculations that attempt to reproduce the observed vertical successions of zeolite minerals seen in various sedimentary piles (e.g., Miyashiro and Shido, 1970) typically assume that hydrostatic and lithostatic pressures are equal and that the pore fluid is pure water. The broad features of the successions can be predicted, but there is a bewildering overlap of zeolite subfacies (Coombs, 1971). In addition to the complexities already noted, it should be recalled that zeolites occur in an unusually rich variety of crystal structures, with several different major cations, extensive cation solid solution, significant Si/Al ratio differences in different structures, and frequent growth and persistence of metastable phases. The bulk chemical composition of the parent-rock is important in determining which zeolite mineral forms, and the Pco of the diagenetic environment may determine whether a non-zeolitic clay—carbonate assemblage persists (Zen, 1961). 

The intergranular carbonate cement is always iron-rich dolomite (or ankerite). Electron probe microanalysis gives between 7 and 18 mol% FeCOj (and 0.2-0.8 mol% MnCOj) (Fig. 11). The calcium content of the dolomites is relatively stoichiometric, with an average of 51 mol% CaCOj (range 50.0-53.3 mol%). The cement is generally dull to very dull red-brown in CL, with subtle concentric zonation. It is relatively homogeneous when examined in BSEM, suggesting that intracrystalline chemistry is reasonably consistent within and between individual fractures. The dolomite crystals have a variable fluid inclusion density, with a vast preponderance of monophase aqueous inclusions. Any two-phase inclusions identified in thin section... 

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