Buffering of pore fluids by metamorphic reactions

Greenwood, H. J.

doi:10.2475/ajs.275.5.573

Cited by 155 publications

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“…The qualitative relationships between anatexis, subsolidus dehydration and v a n m e a,,, may be i-iustrated with isobaric T-u,,, diagrams (Greenwood, 1975;Powell, 1983;Grant, 1985;Waters, 1988). Corresponding P-T and T-a,,, diagrams for selected reactions involving hornblende (Hbd), quartz (Qtz), orthopyroxene (Opx), clinopyroxene (Cpx), plagioclase (PI), silicate liquid (L) and vapour (V) in the system Ca0-Mg0-A1,03-Si02-H20 (CMASH) are illustrated schematically in Fig.…”

Section: Dehydration Melting and Dehydration-meltingmentioning

confidence: 99%

Infiltration‐driven dehydration and anatexis in granulite facies metagabbro, Grenville Province, Ontario, Canada

Pattison

1991

Journal Metamorphic Geology

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Dark hornblende + garnet-rich, quartz-absent rnetagabbro boudins from the Seguin subdomain, Ontario Grenville Province, are transected by anastomosing light-coloured veins rich in orthopyroxene, clinopyroxene, plagioclase and sometimes quartz. The veins vary in texture from fine-grained diffuse veins and patches that overprint the metagabbro, to coarse tonalitic leucosomes with sharp borders. The diffuse veins and patches are suggestive of channellized subsolidus dehydration of the metagabbro, while the tonalitic leucosomes are suggestive of local internally-derived anatexis. All vein types grade smoothly into each other, with the tonalitic leucosomes being the latest.Relative to the host metagabbro, the veins have higher Si, Na, Ba & Sr, lower Fe, Mg, Ca & Ti, and similar Al. The coarser veins are enriched in K. Plagioclase becomes steadily enriched in Na in the transition from host metagabbro (An,,) to the veins (An35), and in the coarsest veins it is antiperthitic. Differences in composition of the other minerals between host metagabbro and vein are minor. Pressure-temperature estimates are scattered, but indicate a minimum temperature during vein formation of 700" C at about 8 kbar.Mass balance constraints indicate that the veins formed from the metagabbro in an open system. The transecting veins are interpreted to represent pathways of Si + Na + Ba + Sr f K f Al-enriched, low aHZ0fluids that metasornatized the host metagabbro to form the anhydrous veins., An initial period of localized solid-state dehydration of the metagabbro, represented by the diffuse veins, was followed by a transition to localized anatexis, represented by the tonalitic leucosomes. The change to anatexis may have been due to the addition of K to the infiltrating fluid, The source and delivery mechanism of the fluids is unknown.

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Section: Dehydration Melting and Dehydration-meltingmentioning

confidence: 99%

Infiltration‐driven dehydration and anatexis in granulite facies metagabbro, Grenville Province, Ontario, Canada

Pattison

1991

Journal Metamorphic Geology

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“…There is also evidence to suggest that progressive reduction during metamorphism does not necessarily take place. Rather, fo~ can change abruptly as different divariant equilibria are encountered in P-Tfo ~ space as shown by Greenwood (1975).…”

Section: Occurrence Of Fe 3 +-Rich Zones In 9arnetmentioning

confidence: 99%

Magnetite exsolution in almandine garnet

Brearley

Champness

1986

Mineral. mag.

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A B S T R A C T. Three almandine-rich metamorphic garnets have been studied by analytical electron microscopy and electron microprobe analysis. Electron microprobe analyses with total Fe calculated as Fe 2 + show that there are no significant departures from stoichiometry due to the presence of Fe 3 § in any of the garnets studied. However, in the transmission electron microscope (TEM) all the garnets were found to contain myriad spherical, iron-rich particles up to 400/~ in diameter. Microdiffraction techniques have revealed that the particles are a cubic spinel phase, consistent with magnetite. There is no crystallographic relationship between the host garnet and the particles, a rare situation for exsolution processes. The presence of such particles is interpreted in terms of the exsolution of magnetite from almandine garnet during cooling. This can apparently occur at temperatures below 550 ~ The size of the particles is a qualitative indicator of the cooling rate of the rock, but is also dependent on the original Fe a + content of the host garnet.KEYWORDS: magnetite, almandine, garnet, exsolution, transmission electron microscopy.THE exsolution of oxide minerals is a common phenomenon in many silicate phases. Several examples of such exsolution have been successfully studied by transmission electron microscopy (TEM). Various authors have investigated exsolved oxide phases such as ilmenite and magnetite in olivines and pyroxenes, e.g. Moseley (1981Moseley ( , 1984, and recently Mongkoltip and Ashworth (1983) have discussed the exsolution of rutile and ilmenite from hornblende. In all the above examples the presence of a secondary exsolved, opaque phase had been initially observed by conventional light microscopy. The further use of TEM enabled detailed crystallographic, morphological and, in some cases, chemical information to be obtained in order to deduce a possible mechanism for the exsolution process. In contrast to the examples above, the present paper is concerned with the exsolution of magnetite particles from almandine garnet on a scale which is well beyond the resolution of the light microscope. Sample descriptionsThree samples have been used for this study. All are from pelitic metamorphic rocks with differing ~) Copyright the Mineralogical Society metamorphic histories. Sample localities and petrography are presented below.Sample RM5020 is a pelitic gneiss from the Moine inlier on the Ross of Mull, Isle of Mull, Scotland and has been studied in detail by Brearley (1984). The rock contains the assemblage garnet + biotite + muscovite + plagioclase + quartz. Garnet is abundant, occurring as small crystals less than 500/~m in diameter. The core regions are optically homogeneous ( fig. la) and are separated from the outer inclusion-free rim of the garnet by a zone full of inclusions, dominantly quartz with subordinate opaques. These distinct zones are probably attributable to periods of growth at different rates during the metamorphic evolution of the area. The calculated conditions of formation for t...

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“…The first appearance of tremolite + K-feldspar is marked by the disappearance of calcite and/or biotite and is considered to reflect the reaction: Thus, the metamorphic fluid composition associated with these stabilities was mineralogically buffered with respect to H 2 0 and CO2 (Greenwood, 1975) and became increasingly water rich with increasing grade (Fig. 8).…”

Section: Quartzmentioning

confidence: 99%

Stockwork tungsten (scheelite)-molybdenum mineralization, Lake George, southwestern New Brunswick

Seal¹,

Clark²,

Morrissy³

1987

Economic Geology

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Scheelite-molybdenite stockwork mineralization constitutes one component of the Lake George polymetallic (Sb-W-Mo-Au-base metal) deposit, a complex hydrothermal center of Late Silurian (ca. 412 m.y.) age in the Fredericton trough of the northern Appalachians. The stockwork, hosted by Silurian gra~wackes, in part calcareous, is spatially and temporally related to a postkinematic cupola of biotite monzogranite, and its formation overlapped in time with the emplacement of monzogranitic porphyry dikes. Mineralogical and textural evidence indicates that contact metamorphism associated with the cupola had ceased before the initiation of W-Mo mineralization and that it occurred, at pressures of less than 1.75 kb, in two stages: a peak stage (T > 600°C), evident only in rocks of pelitic composition; and a lower temperature reequilibration (T < 500°C), recorded in rocks of both pelitic and marly compositions.The W-Mo deposit comprises three different scheelite-and/or molybdenite-bearing veinlet types. Type 1 bodies, the earliest formed, are calc-silicate (granditic garnet, wollastonite, ~l i n o p~r o x e n e , and calcic amphibole) quartz veinlets, with ubiquitous Ca and H metasomatic alteration envelopes. Mineralogical and fluid inclusion relationships indicate that the fluids ranged in temperature from 550° to 22B°C and that temperature decreased away from the cupola. The succeeding type 2 veinlets comprise quartz and lesser amounts of perthitic alkali feldspar, muscovite, calcite, scheelite, molybdenite, and pyrite. Fluid inclusion evidence shows that mineralization dominantly occurred from 400' to 175OC, under a confining pressure of 1.3 kb. Higher grade scheelite and molybdenite deposition was focused in a lower temperature zone, to the north of the cupola, in which CO, effervescence occurred. Type 3 veinlets, the last to form, consist of prehnite, mol~bdenite, and quartz and represent a volumetrically minor mineralization type.In both type 1 and 2 systems, scheelite and molybdenite deposition appears to have been controlled by decreasing temperature and increasing pH. Temperature was a function of distance from the cupola for both veinlet types, but the controls on pH were specific to each. Thus, the pH of type 1 fluids was controlled by wall-rock interaction (H metasomatism), whereas that of type 2 fluids was controlled by CO, effervescence.The economic stibnite-quartz veins (Scratch et al., 1984) occupy fractures which transect, and therefore, postdate all stages of W-Mo mineralization.

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Buffering of pore fluids by metamorphic reactions

Cited by 155 publications

References 0 publications

Infiltration‐driven dehydration and anatexis in granulite facies metagabbro, Grenville Province, Ontario, Canada

Infiltration‐driven dehydration and anatexis in granulite facies metagabbro, Grenville Province, Ontario, Canada

Magnetite exsolution in almandine garnet

Stockwork tungsten (scheelite)-molybdenum mineralization, Lake George, southwestern New Brunswick

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