Zeolite facies

Hydrothermal alteration minerals from midoceanic basalt are analcite, stilbite, heulandite, natrolite-mesolite-scolecite series, chlorite and smectite for zeolite facies, prehnite, chlorite, calcite and epidote for prehnite-pumpellyite facies, albite, actinolite, chlorite, epidote, quartz, sphene, hornblende, tremolite, talc, magnetite, and nontronite for green schist facies, hornblende, plagioclase, actinolite, leucoxene, quartz, chlorite, apatite, biotite, epidote, magnetite and sphene for amphibolite facies (Humphris and Thompson, 1978). 

The rocks in the study have undergone zeolite facies regional metamorphism but locally intense penetrative fabrics are recognized in areas adjacent major faults. 

Because the compositions are basic, the expanding minerals are trioctahedral and they are apparently associated in all facies with chlorite. The occurrence of a regularly interstratified montmorillonite (saponite) -chlorite mineral, corrensite, is typified by an association with calcic zeolites and albite. Temperature measurement in the "hydrothermal" sequences at several hundred meters depth indicate that the ordered, mixed layered mineral succeeds a fully expandable phase between 150-200 C and this ordered phase remains present to about 280°C. In this interval calcium zeolites disappear, being apparently replaced by prehnite. The higher temperature assemblage above corrensite stability typically contains chlorite and epidote. 

The stability conditions of corrensite then cover the low grade clay mineral facies (near 100°C) and extend well into the calcium zeolite-prehnite, muscovite-chlorite facies. In pelitic rocks the upper limit will be somewhat lower near the illite-chlorite zone. It is evident that composition of a rock governs the occurrence of corrensite. It can be... 

Temperature is also of great importance, apparently decreasing the Si-content of the zeolites as it increases. Temperature and time, or approach to equilibrium, appear to determine the type of zeolite assemblages found and thus an understanding of these factors permits the tentative establishment of two alkali zeolite "facies" subdivisions the diagenetic and analcite type (Hay, 1966 Moiola, 1970 Iijima and Utada, 1970 Iijima, 1970 Studer, 1967 Coombs, 1970 Seki, 1969 Miyashiro and Shido, 1970). 

Figure 34. Alkali zeolites projected into a portion of the Na-K-Si coordinates. Anal = analcite Ph = phillipsite solid solution Ze = alkali zeolites undifferentiated Alb = albite KF = potassium feldspar Q = quartz Si = amorphous silica, a) low, b) medium, and c) high temperature facies. Shaded areas are two-phase fields.

Due to the conflicting experimental results, we will rely upon natural occurrence and we will place an 80°C limit on the initial zeolite paragenesis where solid solution is maximum. An intermediate stage exists up to temperatures near 100°C at high water pressure where analcite, potassium feldspar and alkali zeolite can coexist as can analcite and albite. Analcite persists up to 180°C where the upper limit of the alkali zeolite facies is reached. 

The first two parageneses described above, where alkali zeolites are stable, correspond to the "diagenetic" zeolite facies of Coombs (1970). 

The latter, according to Coombs, is an analcite-heulandite facies which contains other calcic zeolites in more basic rocks. The disappearance of analcite would occur near the heulandite-laumontite transition for calcic zeolites. Thus, calcic zeolites can continue to be stable at higher grades of diagenesis or epimetamorphism than alkali zeolites. 

The second division of the zeolite facies is based upon the appearance of albite as a diagenetic mineral, usually coexisting with analcite in the initial stages of its development, and also with the widespread development of montmorillonite-illite mixed layered mineral (30 to 90% expandable layers) coexisting with illite. The phase relations of this facies are indicated by Figure 35b. Assemblages can contain natrolite as above. They are ... 

In each of the different parageneses outlined here, the instability of a mineral can be denoted by its replacement with one or usually several minerals. The rocks in these facies are typified by multi-phase assemblages which can be placed in the K-Na-Al-Si system. This is typical of systems where the major chemical components are inert and where their masses determine the phases formed. The assumptions made in the analysis up to this point have been that all phases are stable under the variation of intensive variables of the system. This means that at constant P-T the minerals are stable over the range of pH s encountered in the various environments. This is probably true for most sedimentary basins, deep-sea deposits and buried sedimentary sequences. The assemblage albite-potassium feldspar-mixed layered-illite montmorillonite and albite-mixed layered illite montmorillonite-kaolinite represent the end of zeolite facies as found in carbonates and sedimentary rocks (Bates and Strahl,... 

The bulk composition of the sediments must be normally near the potassium-rich side of the zeolite facies since analcite is not reported in these sediments and potassium feldspar apparently coexists with or replaces the alkali zeolites. 

A consideration of natural occurrence and chemical composition of alkali zeolites allows a certain refinement of the zeolite facies concept previously proposed. The key factor is the grouping of the alkali zeolites into a continuous solid solution series. Other possible coexisting phases of similar composition are sodium and potassium feldspar, natrolite and analcite. The extent of solid solution decreases with temperature, possibly also with pressure. This effect allows the sequential series zeolite-K feldspar, zeolite-analcite-K feldspar, analcite-K feldspar-albite and eventually two feldspars to the exclusion of analcite, the alkali zeolite with the highest stability limits. 

The second facies is marked by the instability of the fully expanding dioctahedral phases and the existence of a kaolinite-illite tie-line (Figure 48b). In this facies the siliceous alkali zeolites (other than analcite) become unstable, the compositional range of the trioctahedral expanding phases is reduced and aluminous 14 8 chlorite-"allevardite"... 

COOMBS (D.S.), 1970. Present status of the zeolite facies. Ame. Chem. 

No significant differences have been established between heulandite and laumontite in sedimentary formations in the zones of deep epigenesis (initial metamorphism), on the one hand, and the same minerals in hydro-thermal deposits, on the other. Apparently, this will require more factual data. Nevertheless, the distribution of these zeolites and the associations of clay minerals permit a distinction between the zeolite facies of regional epigenesis-metamorphism and the zeolite mineralization in geothermal areas (recent hydrothermal systems). 

The above discussion suggests that true sedimentary formations are generally poor in types of zeolites (containing only one or two species) although the origin of the zeolites is clearly controlled by the facies conditions. An indispensible condition is the presence of fresh aluminosilicate... 

B 5 — zeolite facies and regional epigenesis. Temperatures from 100 to 300-350°C, pressures not more than 3-5 kbar. 

Coombs D. S., EUis A. J., Fyfe W. S., and Taylor A. M. (1959) The zeolite facies, with comments on the interpretation of hydrothermal synthesis. Geochim. Cosmochim. Acta 17, 53-107. 

On the basis of observations in southern New Zealand (9), Turner (18) defined a zeolitic facies to include only regionally developed assemblages, including laumontite-albite—quartz, that largely replace the preexisting rocks and conform to the mineralogical and chemical require-... 

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