The effect of feldspar on quartz-H2O−CO2 dihedral angles at 4 kbar,with consequences for the behaviour of aqueous fluids in migmatites

An investigation was made of the effect of trace amounts of feldspar (Na and/or K) on dihedral angles in the quartz-H₂O-CO₂ system at 4 kbar and 450–1050°C. Quartz-quartz-H₂O dihedral angles in feldspar-bearing quartz aggregates are observed to be the same as those in pure quartz aggregates at temperatures below 500°C. Above this temperature, they decrease with increasing temperature until the solidus. The final angle at the inception of melting is about 65° for microcline-quartz-H₂O and microcline-albite-quartz-H₂O, and much less than 60° (the critical value for formation of grain-edge fluid channels in an isotropic system) for the albite-quartz-H₂O system. CO₂ was observed to produce a constant quartz-quartz-fluid dihedral angle of 97° in feldspar-bearing quartz aggregates at all temperatures studied. Also examined were the dihedral angles for the two co-existing supersolidus fluids in quartz aggregates. In all systems the quartz-volatile fluid angle is greater than 60°, whereas the quartz-melt angle is lower than 60°. Both super-solidus angles decrease with increasing temperature. The transition from nonconnected to connected poro- sity with increasing temperature observed in the quartz-albite-H₂O system some tens of degrees below the solidus (termed a permeability transition), if a common feature of rocks near their melting points, will play an important role in controlling the permeability of high-grade rocks to aqueous fluids. Received: 27 October 1993 / Accepted: 11 July 1994     

The influence of the fluid phase composition on the solidus temperatures in the haplogranite system NaAlSi3O8-KAlSi3O8-SiO2-H2O-CO2

The solidus temperatures in the haplogranite-system NaAlSi₃O₈-KAlSi₃O₈-SiO₂-H₂O-CO₂ have been determined up to 15 kbar for a constant molar ratio of sodium to potassium of 11 and for fluid compositions ranging from pure water to pure carbon dioxide. The data for the water-saturated solidus are virtually identical with those of previous studies. At constant pressure, the solidus curve as a function of the fluid phase composition exhibits a point of inflection in the range of the water-rich compositions. This phenomenon is attributed to chemical interactions between the CO₂ and the H₂O in the silicate melt. The point of inflection disappears if the CO₂ in the gas phase is replaced by molecular nitrogen. The CO₂-saturated solidi have been measured at 2 and 5 kbars. The data at 5 kbar indicate a melting point depression in the order of 40° C compared to the dry solidus of Huang and Wyllie (1975). The experimental data can be used to estimate the melting temperatures of common quartz and feldspar bearing crustal rocks under the conditions of granulite facies metamorphism. Since for most fluid phase compositions, the solidus curves are very steep in the P, T-diagram, the beginning of melting is nearly exclusively determined by the fluid composition and almost independent of pressure between about 2 and more than 10 kbar. Therefore, the onset of partial melting in quartz and feldspar containing rocks under granulite facies conditions can be used to estimate the composition of a coexisting H₂O-CO₂ fluid phase if geothermometric data are available. The temperature range between the beginning of granulite facies metamorphism and the initiation of melting expands with increasing carbon dioxide content in the H₂O-CO₂ fluid phase. At a CO₂ molar fraction of 0.9, this range extends from about 600° C to 900° C and is almost independent of pressure.     

The role of fluids in the formation of talc deposits of Rema area,Kumaun Lesser Himalaya

Talc deposits of Rema area in the Kumaun Inner Lesser Himalaya are hosted within high magnesium carbonates of the Proterozoic Deoban Formation. These deposits occur as irregular patches or pockets mainly within magnesite bodies, along with impurities of magnesite, dolomite and clinochlore. Textures represent different phases of reactions between magnesite and silica to produce talc. Petrography, XRD and geochemistry reveal that the talc has primarily developed at the expense of magnesite and silica, leaving dolomite largely un-reacted. Early fluid inclusions in magnesite and dolomite associated with talc are filled with H₂O+NaCl+KCl ± MgCl₂ ± CaCl₂ fluids, which represent basin fluid system during diagenesis of carbonates. Their varied degree of re-equilibration was although not pervasive but points to increased burial, and hence requires careful interpretation. H₂O-CO₂ fluid with XCO₂ between 0.06 and 0.12 was equilibrated with talc formation. The reaction dolomite+quartz → talc was not extensive because T-X_CO2 was not favourable, and talc was developed principally after magnesite+quartz.     

Equilibrium dihedral angles in the system H2O−CO2−NaCl-calcite,and implications for fluid flow during metamorphism

Fluid-calcite-calcite dihedral angles have been measured for fluids in the system H₂O−CO₂−NaCl, between 1 and 2 kbar, and 550–750° C. It is found that the calcite-calcite-H₂O dihedral angle decreases steadily with addition of NaCl from a value of about 80° (pure water) to 44° (60 wt% NaCl). The CO₂−H₂O system displays a well-defined minimum at , with a dihedral angle of 50°, in contrast to those of pure CO₂ and H₂O which are 90° and 80° respectively. Experiments containing fluids which are immiscible at run conditions showed a bimodal distribution of dihedral angles in the CO₂−H₂O−NaCl system, which can be approximately correlated with the compositions of the two fluid phases. Such bimodality was only observed for immiscible fluids in the H₂O−NaCl system if the quench rate exceeded about 200°C per min. This is probably due to the extremely rapid establishment of the single phase dihedral angle on quenching. The fluid phase topology in devolatilising marbles will only be a connected network for very saline brines and fluids with close to 0.5. Fluids trapped in fluid inclusions in calcite grains in marbles may be predominantly H₂O-rich or CO₂-rich, and of low salinity. All other fluid compositions in the H₂O−CO₂−NaCl-calcite system will occupy isolated pores, the largest of which will grow at the expense of the smallest. Escape of fluid produced during devolatilisation reactions under such conditions will occur by fluid overpressuring and hydrofracture. In contrast, previous experimental studies of quartz-fluid dihedral angles between 950° and 1100° C (Watson and Brenan 1987) predict that quartz-dominated lithologies will permit pervasive flow of H₂O−NaCl fluids, but not of H₂O−CO₂ fluids. Documented geological examples of differences in permeability and fluid flow mechanism between metamorphic argillites, psammites and limestones which support the results of the experimental studies are discussed.     

Petrogenesis of rare-element pegmatites in the Olary Block, South Australia, part 2. Fluid inclusion study

Summary Fluid inclusions were investigated in quartz, beryl, apatite and triplite from the border and intermediate zones and core of pegmatites within the Proterozoic Olary Block, South Australia. Three compositionally distinct types of inclusions were recognized including pure CO₂ inclusions, mixed H₂O-CO₂ inclusions, and aqueous inclusions with some of them containing a solid phase. Three fluid events occurred during pegmatite formation and subsolidus alteration. Initial fluids are characterised by a low to intermediate salinity (4.1 to 23.4wt% NaCl equivalent), and a composition of about 10 mole% CO₂, 4.2 mole% NaCl equivalent, and 85.6 mole% H2O. Fluids were trapped as homogeneous H₂O-CO₂ phases. The second pulse of fluids was of intermediate to high salinity at 11 to 33 wt% NaCl equivalent. These fluids were most likely trapped as separated CO₂ and H₂O phases. Finally, intermediate to high salinity fluids of post-pegmatite origin with approximately 15 to 30 wt % NaCl equivalent were introduced. The P-T regime for the three fluid events has been estimated at 520° to > 650 °C and 2 to 5 kbars, 400° to 650 °C and 1.8 to 3.3 kbars, and 380° to 480°C and 2.0 to 2.6 kbars, respectively. These conditions indicate a declining pressure path implying a tectonic uplift of the Olary Block during successive fluid emplacements.

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Petrogenese von Seltenelementpegmatiten im Olary Block, Südaustralien, Teil 2. Untersuchung der Flüssigkeitseinschlüsse

Zusammenfassung Flüssigkeitseinschlüsse wurden in Quarz, Beryll, Apatit und Triplit von Rand-, Zwischen- und Kernzonen in Pegmatiten des proterozoischen Olary Blocks, Südaustralien, untersucht. Drei Typen von Flüssigkeitseinschlüssen mit verschiedenen Zusammensetzungen wurden erkannt: reine CO₂ Einschlüsse, gemischte H₂O-CO₂ Einschlüsse und wässerige Einschlüsse, wobei einige von diesen feste Einschlüsse aufweisen. Drei Fluid Ereignisse sind den Stadien der Pegmatitbildung und Subsolidus-Alteration zuzuordnen. Die erste Fluidgeneration ist durch geringe bis intermediäre Salinität(4.1 bis 23.4 Gewichts% NaCI Äquivalent) und eine Zusammensetzung von ungefähr 10 Mol % CO₂, 4.2 Mol% NaCl Äquivalent und 85.6 Mol% H₂O charakterisiert. Diese Fluide wurden als homogene H₂O-CO₂ Phasen eingeschlossen. Der zweite Puls von Fluiden war von intermediärer bis hoher Salinität (11 bis 33 Gewichts.% NaCI Äquivalent). Diese Fluide wurden wahrscheinlich als entmischte H₂O und CO₂ Phasen eingeschlossen. Zum Schluß wurden Fluide postpegmatitischen Ursprungs mit intermediärer bis hoher Salinität zugeführt (15 bis 30 Gewichts% NaCI Äquivalent). Der P-T Bereich für die drei Fluid-Ereignisse ist mit 520° bis > 650 °C und 2 bis 5 kbar, 400° bis 650 °C und 1.8 bis 3.3 kbar, und 380° bis 480°C und 2.0 bis 2.6 kbar abgeschätzt worden. Dies weist auf abnehmenden Druck hin und deutet damit eine tektonische Hebung des Olary Blocks während sukkzessiver Fluid-Platznahmen an.

     

The partitioning of Fe and Mg between olivine and carbonate and the stability of carbonate under mantle conditions

We have investigated the effect of Fe on the stabilities of carbonate (carb) in lherzolite assemblages by determining the partitioning of Fe and Mg between silicate (olivine; ol) and carbonates (magnesite, dolomite, magnesian calcite) at high pressures and temperatures. Fe enters olivine preferentially relative to magnesite and ordered dolomite, but Fe and Mg partition almost equally between disordered calcic carbonate and olivine. Measurement of K _d (X _Fe ^carb X _Mg ^ol /X _Fe ^ol X _Mg ^carb ) as a function of Fe/ Mg ratio indicates that Fe–Mg carbonates deviate only slightly from ideality. Using the regular solution parameter for olivine W _FeMg ^ol of 3.7±0.8 kJ/mol (Wiser and Wood 1991) we obtain for (FeMg)CO₃ a W _FeMg ^carb of 3.05±1.50 kJ/mol. The effect of Ca–Mg–Fe disordering is to raise K _d substantially enabling us to calculate W _CaMg ^carb -W _CaFe ^carb of 5.3±2.2 kJ/mol. The activity-composition relationships and partitioning data have been used to calculate the effect of Fe/Mg ratio on mantle decarbonation and exchange reactions. We find that carbonate (dolomite and magnesian calcite) is stable to slightly lower pressures (by 1 kbar) in mantle lherzolitic assemblages than in the CaO–MgO–SiO₂(CMS)–CO₂ system. The high pressure breakdown of dolomite + orthopyroxene to magnesite + clinopyroxene is displaced to higher pressures (by 2 kbar) in natural compositions relative to CMS. CO₂. We also find a stability field of magnesian calcite in lherzolite at 15–25 kbar and 750–1000°C.     

Fluid variability in 2 GPa eclogites as an indicator of fluid behavior during subduction

Fluid activity ratios calculated between millimeter- to centimeter-scale layers in banded mafic eclogites from the Tauern Window, Austria, indicate that variations in a _{H 2 O} existed between layers during equilibration at P approximately equal to 2GPa and T approximately equal to 625°C, whereas a _{CO 2} was nearly constant between the same layers. Model calculations in the system H₂O–CO₂–NaCl show that these results are consistent with the existence of different saturated saline brines, carbonic fluids, or immiscible pairs of both in different layers. The data cannot be explained by the exisience of water-rich fluids in all layers. The model fluid compositions agree with fluid inclusion compositions from eclogite-stage veins and segregations that contain (1) saline brines (up to 39 equivalent wt. % NaCl) with up to six silicate, oxide, and carbonate daughter phases, and (2) carbonic fluids. The formation of crystalline segregations from fluid-filled pockets or hydrofractures indicates high fluid pressures at 2 GPa; the record of fluid variability in the banded eclogite host rocks, however, implies that fluid transport was limited to local flow along individual layers and that there was no large-scale mixing of fluids during devolatilization at depths of 60–70 km. The lack of evidence for fluid mixing may, in part, reflect variations in wetting behavior of fluids of different composition; nonwetting fluids (water-rich or carbonic) would be confined to intergranular pore spaces and would be essentially immobile, whereas wetting fluids (saline brines) could migrate more easily along an interconnected fluid network. The heterogeneous distribution of chemically distinct fluids may influence chemical transport processes during subduction by affecting mineral-fluid element partitioning and by altering the migration properties of the fluid phase(s) in the downgoing slab.     

Melting of albite and dehydration of brucite in H2O–NaCl fluids to 9 kbars and 700–900°C: implications for partial melting and water activities during high pressure metamorphism

 The melting reaction: albite_(solid)+ H₂O_(fluid) =albite-H₂O_(melt) has been determined in the presence of H₂O–NaCl fluids at 5 and 9.2 kbar, and results compared with those obtained in presence of H₂O–CO₂ fluids. To a good approximation, albite melts congruently at 9 kbar, indicating that the melting temperature at constant pressure is principally determined by water activity. At 5 kbar, the temperature (T)- mole fraction (X _(H2O) ) melting relations in the two systems are almost coincident. By contrast, H₂O–NaCl mixing at 9 kbar is quite non-ideal; albite melts ∼70 ^°C higher in H₂O–NaCl brines than in H₂O–CO₂ fluids for X _(H2O) =0.8 and ∼100 ^°C higher for X _(H2O) =0.5. The melting temperature of albite in H₂O–NaCl fluids of X _(H2O)=0.8 is ∼100 ^°C higher than in pure water. The P–T curves for albite melting at constant H₂O–NaCl show a temperature minimum at about 5 kbar. Water activities in H₂O–NaCl fluids calculated from these results, from new experimental data on the dehydration of brucite in presence of H₂O–NaCl fluid at 9 kbar, and from previously published experimental data, indicate a large decrease with increasing fluid pressure at pressures up to 10 kbar. Aqueous brines with dissolved chloride salt contents comparable to those of real crustal fluids provide a mechanism for reducing water activities, buffering and limiting crustal melting, and generating anhydrous mineral assemblages during deep crustal metamorphism in the granulite facies and in subduction-related metamorphism. Low water activity in high pressure-temperature metamorphic mineral assemblages is not necessarily a criterion of fluid absence or melting, but may be due to the presence of low a _(H2O) brines. Received: 17 March 1995/Accepted: 9 April 1996     

Phase relations in the CH4-H2O-NaCl system at 2 kbar, 300 to 600°C as determined using synthetic fluid inclusions

Synthesis of fluid inclusions in the CH₄-H₂O-NaCl system was accomplished by subjecting fractured quartz or fluorite, along with known quantities of CH₄, H₂O, and NaCl, to a pressure of 2 kbar and temperatures of 300, 400, 500, or 600°C, in sealed Au capsules. Under the elevated P-T conditions, some of the fractures healed, trapping fluids as inclusions. Microthermometric measurements conducted on the fluid inclusions show that at 2 kbar and 400 to 600°C, there are very broad regions of fluid unmixing in the CH₄-H₂O-NaCl system. For those bulk fluid compositions that lie in the two-phase (i.e., immiscible fluids) field, the high-density phase is enriched in NaCl, whereas the low-density phase is enriched in CH₄. For any given bulk composition, the degree of NaCl enrichment in the high-density phase increases, whereas the degree of CH₄ enrichment in the low-density phase decreases, as temperature increases from 400 to 600°C. Our experimental constraints on the size of the two-phase field are generally consistent with results generated using the equation-of-state GEOFLUIDS (available at http://geotherm.ucsd.edu/geofluids/). However, when comparing the compositions of coexisting immiscible fluids, as determined experimentally vs. calculated using GEOFLUIDS, we find that some relatively small but probably significant differences exist between our experiments and this equation of state.     

Constraints on phase diagram topology for the system CaO−MgO−SiO2−CO2−H2O

Published phase diagrams for the siliceous carbonate system CaO–MgO–SiO₂–CO₂–H₂O are contradictory because of different estimates of the relative stability of magnesite. Experimental data on magnesite are too ambiguous to determine the validity of these estimates. Therefore, field evidence is used to select the correct phase diagram topology for siliceous carbonate and carbonate ultramafic rocks at pressures of about 2–5 kbar. The primary selection criterion is provided by the existence of the stable assemblage talc+dolomite+forsterite+tremolite+antigorite, which occurs in the Bergell contact aureole and Swiss Central Alps. Field evidence also is used to argue that the reaction magnesite+quartz=enstatite must occur at lower temperature than the reaction dolomite+quartz=diopside. T-X _{CO 2} and P _{CO 2}-T phase diagrams consistent with these observations are calculated from experimental and thermo-dynamic data. For antigorite ophicarbonate rocks, remarkable agreement is obtained between the spatial distribution of low variance mineral assemblages and the calculated diagrams.     

CO2-Brine immiscibility at high temperatures,evidence from calcareous metasedimentary rocks

Study of fluid inclusions in quartz segregations and in the rock matrix of a calcareous psammite and a carbonate schist suggests that brines containing 23–24 weight percent salt (NaCl equivalent) are immiscible with CO₂ at the metamorphic conditions of approximately 600° and 6.5 Kb. The presence of a high temperature solvus between saline brine and CO₂ is supported by other fluid inclusion studies as well as experimental measurements from the literature. As saline brines are common in metamorphic and hydrothermal systems, CO₂-brine immiscibility should play an important role in petrogenesis. The fluid inclusions preserved in the quartz segregations probably represent the fluids generated by prograde metamorphic reactions, whereas the compositions of the fluids trapped in the rock matrix quartz suggest they have reequilibrated with the matrix minerals during incipient retrograde reactions. The isochores from the densest inclusions observed in this study pass close to the inferred peak metamorphic conditions; other isochores suggest an episode of deformation and recrystallization at 275° C and 1.4 Kb. Using the density information preserved in all the inclusions, a convex-downward uplift path on a P-T diagram is inferred for these rocks.     

Partitioning of F between H2O and CO2 fluids and topaz rhyolite melt

Fluid/melt distribution coefficients for F have been determined in experiments conducted with peraluminous topaz rhyolite melts and fluids consisting of H₂O and H₂O+CO₂ at pressures of 0.5 to 5 kbar, temperatures of 775°–1000°C, and concentrations of F in the melt ranging from 0.5 to 6.9 wt%. The major element, F, and Cl concentrations of the starting material and run product glasses were determined by electron microprobe, and the concentration of F in the fluid was calculated by mass balance. The H₂O concentrations of some run product glasses were determined by ion microprobe (SIMS). The solubility of melt in the fluid phase increases with increasing F in the system; the solubility of H₂O in the melt is independent of the F concentration of the system with up to 6.3 wt% F in the melt. No evidence of immiscible silica- and fluoriderich liquids was detected in the hydrous but water-undersaturated starting material glasses (8.5 wt% F in melt) or in the water-saturated run product glasses. F concentrates in topaz rhyolite melts relative to coexisting fluids at most conditions studied; however, D_F (wt% F in fluid/wt% F in melt) increases strongly with increasing F in the system. Maximum values of D_F in this study are significantly larger than those previously reported in the literature. Linear extrapolation of the data suggests that D_F is greater than one for water-saturated, peraluminous granitic melts containing 8 wt% F at 800° C and 2 kbar. D_F increases as temperature and as (H₂O/H₂O+CO₂) of the fluid increase. For topaz rhyolite melts containing 1 wt% F and with H₂O-rich fluids, D_F is independent of changes in pressure from 2 to 5 kbar at 800° C; for melts containing 1 wt% F and in equilibrium with CO₂-bearing fluids the concentrations of F in fluid increases with increasing pressure. F-and lithophile element-enriched granites may evolve to compositions containing extreme concentrations of F during the final stages of crystallization. If F in the melt exceeds 8 wt%, D_F is greater than one and the associated magmatic-hydrothermal fluid contains >4 molal F. Such F-enriched fluids may be important in the mass transport of ore constituents, i.e., F, Mo, W, Sn, Li, Be, Rb, Cs, U, Th, Nb, Ta, and B, from the magma.     

The influence of H2O-H2 fluids and redox conditions on melting temperatures in the haplogranite system

Solidus temperatures of quartz–alkali feldspar assemblages in the haplogranite system (Qz-Ab-Or) and subsystems in the presence of H₂O-H₂ fluids have been determined at 1, 2, 5 and 8 kbar vapour pressure to constrain the effects of redox conditions on phase relations in quartzofeldspathic assemblages. The hydrogen fugacity (f _H2) in the fluid phase has been controlled using the Shaw membrane technique for moderately reducing conditions (f _H2 < 60 bars) at 1 and 2 kbar total pressure. Solid oxygen buffer assemblages in double capsule experiments have been used to obtain more reducing conditions at 1 and 2 kbar and for all investigations at 5 and 8 kbar. The systems Qz-Or-H₂O-H₂ and Qz-Ab-H₂O-H₂ have only been investigated at moderately reducing conditions (1 and 5 kbar) and the system Qz-Ab-Or-H₂O-H₂ has been investigated at redox conditions down to IW (1 to 8 kbar). The results obtained for the water saturated solidi are in good agreement with those of previous studies. At a given pressure, the solidus temperature is found to be constant (within the experimental precision of ± 5°C) in the f _H2 range of 0–75 bars. At higher f _H2, generated by the oxygen buffers FeO-Fe₃O₄ (WM) and Fe-FeO (IW), the solidus temperatures increase with increasing H₂ content in the vapour phase. The solidus curves obtained at 2 and 5 kbar have similar shapes to those determined for the same quartz - alkali feldspar assemblages with H₂O-CO₂- or H₂O-N₂-bearing systems. This suggests that H₂ has the behaviour of an inert diluent of the fluid phase and that H₂ solubility in aluminosilicate melts is very low. The application of the results to geological relevant conditions [HM (hematite-magnetite) > f _O2 > WM] shows that increasing f _H2 produces a slight increase of the solidus temperatures (up to 30 °C) of quartz–alkali feldspar assemblages in the presence of H₂O-H₂ fluids between 1 and 5 kbar total pressure. Received: 4 March 1996 / Accepted: 22 August 1996     

Geochemistry and origin of the ore-forming fluids in hydrothermal-magmatic systems in tectonically active zones

Data on fluid inclusions and stable isotope compositions (O, H, C, and S) in minerals have been summarized for large-and middle-scale mesothermal gold deposits (Nezhdaninsk, Berezovsk, Kochkar’, Svetlinsk, Darasun, and Maisk), cassiterite-silicate-sulfide deposits of Sikhote Alin (Solnechnoe, Arsen’evsk, and Vysokogorsk), vein silver-base metal deposits in the Southern Verkhoyansk region (Prognoz and Kupol’noe), and epithermal copper-bismuth-silver-base metal deposits of the Karamazar district in Tajikistan (Kanimansur, Tary Ekan, and Zambarak). It is shown that ores precipitated from fluids with salinity varying from brines (up to 60 wt % NaCl equiv) to dilute fluids (1–3 wt % NaCl equiv). As a rule, fluids of different compositions entered the hydrothermal-magmatic system. A fluid mixture of H₂O-CO₂-NaCl±CH₄±N₂ predominated in the orogenic (mesothermal) gold-bearing hydrothermal systems, with deposition of the final-stage gold-bearing sulfosalts from aqueous-salt fluid. Brines played a significant role in the formation of cassiterite-silicate-sulfide and vein silver-base metal deposits. The brines often coexisted with a low-density vapor-rich fluid at the ore deposition site. The obtained data suggest a predominant magmatic component in the hydrothermal-magmatic systems, with a significant contribution of meteoric waters.     

P-T evolution and fluid inclusion characteristics of retrograded eclogites,Münchberg Gneiss Complex,Germany

Kyanite eclogites occur as part of the Münchberger nappe pile in NE-Bavaria, West Germany. Eclogites are overprinted by subsequent amphibolite facies metamorphism. The preservation of primary eclogitic textures as well as symplectitic textures are indicative of rapid decompression. Eclogite formation is estimated to have occurred under conditions of high H₂O-activities at pressures between 20 and 26 kbar and temperatures ranging between 590 and 660° C, as is shown by the coexistence of omphacite (Jd 50), kyanite, zoisite and quartz. Minimum pressure estimates, independent of the water activity, range between 9 and 16 kbar at the relevant temperatures. Detailed studies of fluid inclusion reveal two predominant groups of aqueous-brine inclusions: high salinity (14–17 wt% NaCl equiv.) and low salinity (0–8 wt% NaCl equiv.) inclusions. Fluid compositions of both groups of inclusions yield isochores passing close to the estimated amphibolite facies PT-field. The compositions of these fluids are in good agreement with fluid compositions considered from mineral equilibria. None of the fluid inclusions has densities appropriate for eclogite facies metamorphism, but probably reflect later amphibolite facies metamorphism.     

Experimental and petrological constraints on local-scale interaction of biotite-amphibole gneiss with H2O-CO2-（K, Na）Cl fluids at middle-crustal conditions： Example from the Limpopo Complex, South Africa

Reaction textures and fluid inclusions in the～2.0 Ga pyroxene-bearing dehydration zones within the Sand River biotite-hornblende orthogneisses（Central Zone of the Limpopo Complex） suggest that the formation of these zones is a result of close interplay between dehydration process along ductile shear zones triggered by H₂O-CO₂-salt fluids at 750—800℃and 5.5—6.2 kbar.partial melting,and later exsolution of residual brine and H₂O-CO₂ fluids during melt crystallization at 650—700℃.These processes caused local variations of water and alkali activity in the fluids,resulting in various mineral assemblages within the dehydration zone.The petrological observations are substantiated by experiments on the interaction of the Sand River gneiss with the H₂O-CO-2-（K,Na）Cl fluids at 750 and 800℃and 5.5 kbar.It follows that the interaction of biotite-amphibole gneiss with H₂O-CO₂-（K.Na）Cl fluids is accompanied by partial melting at 750—800℃.Orthopyroxene-bearing assemblages are characteristic for temperature 800℃and are stable in equilibrium with fluids with low salt concentrations,while salt-rich fluids produce clinopyroxene-bearing assemblages.These observations arc in good agreement with the petrological data on the dehydration zones within the Sand River orthoeneisses.     

Experimental phase relations of hydrous peridotites modelled in the system H2O-CaO-MgO-Al2O3,-SiO2

The hydration of peridotites modelled by the system H₂O-CaO-MgO-Al₂O₃-SiO₂ has been treated theoretically after the method of Schreinemakers, and has been investigated experimentally in the temperature range 700°–900° C and in the pressure range of 8–14 kbar. In the presence of excess forsterite and water, the garnet- to spinel-peridotite transition boundary intersects the chlorite dehydration boundary at an invariant point situated at 865±5° C and 15.2±0.3 kbar. At lower pressures, a model spinel lherzolite hydrates to both chlorite- and amphibole-bearing assemblages at an invariant point located at 825±10° C and 9.3±0.5 kbar. At even lower pressures the spinel-to plagioclase-peridotite transition boundary intersects the dehydration curve for amphibole+forsterite at an invariant point estimated to lie at 855±10° C and 6.5±0.5 kbar.Both chlorite and amphibole were characterized along their respective dehydration curves. Chlorite was found to shift continuously from clinochlore, with increasing temperature, to more aluminous compositions. Amphibole was found to be tremolitic with a maximmum of 6 wt.% Al₂O₃.The experimentally determined curves in this study were combined with the determined or estimated stability curves for hydrous melting, plagioclase, talc, anthophyllite, and antigorite to obtain a petrogenetic grid applicable to peridotites, modelled by the system H₂O-CaO-MgO-Al₂O₃-SiO₂, that covers a wide range of geological conditions. Direct applications of this grid, although quite limited, can be made for ultramafic assemblages that have been extensively re-equilibrated at greenschist to amphibolite facies metamorphism and for some highgrade ultramafic assemblages that display clear signs of retrogressive metamorphism.     

A calculated petrogenetic grid for ultramafic rocks in the system CaO-FeO-MgO-Al2O3-SiO2-CO2-H2O at low pressures

Calculated phase equilibria among the minerals amphibole, chlorite, clinopyroxene, orthopyroxene, olivine, dolomite, magnesite, serpentine, brucite, calcite, quartz and fluid are presented for the system CaO–FeO–MgO–Al₂O₃–SiO₂–CO₂–H₂O (CaF-MASCH), with chlorite and H₂O–CO₂ fluid in excess and for a temperature range of 440°C–600°C and low pressures. The minerals chosen in CaFMASCH represent the great majority of phases encountered in metamorphosed ultramafic rocks. The changes in mineral compositions in terms of FeMg_-1 and (Mg, Fe)SiAl_-1Al_-1 are related to variations in the intensive parameters. For example, equilibria at high in the presence of chlorite involve minerals which are relatively aluminous compared with those at low . The calculated invariant, univariant and divariant equilibria are compared with naturally-occurring greenschist and amphibolite facies ultramafic mineral assemblages. The correspondence of sequences of mineral assemblages and the compositions of the minerals in the assemblages is very good.     

Controls on the mechanisms of fluid infiltration and front advection during regional metamorphism: a stable isotope and textural study of retrograde Dalradian rocks of the SW Scottish Highlands

Vein-controlled retrograde infiltration of H₂O-CO₂ fluids into Dalradian epidote amphibolite facies rocks of the SW Scottish Highlands under greenschist facies conditions resulted in alteration of calcite-rich marble bands to dolomite and spatially associated ¹⁸O enrichment of about 10%. on a scale of metres. Fluid inclusion data indicate that the retrograde fluid was an H₂O-salt mixture with a low CO₂ content, and that the temperature of the fluid was about 400d? C. Detailed petrographic and textural (backscattered electron imaging) studies at one garnet-grade locality show that advection of fluid into marbles proceeded by a calcite-calcite grain edge flow mechanism, while alteration of non-carbonate wall-rock is associated with veinlets and microcracks. Stable isotopic analysis of carbonates from marble bands provides evidence for advection of isotopic fronts through carbonate wall-rocks perpendicular to dolomite veins, and fluid fluxes in the range 2.4–28.6 m³/m² have been computed from measured advection distances. Coincidence of isotope and reaction fronts is considered to result from reaction-enhanced kinetics of isotope exchange at the reaction front. Front advection distances are related to the proportion of calcite to quartz in each marble band, with the largest advection distance occurring in nearly pure calcite matrix. This relationship indicates that fluid flow in carbonates is only possible along fluid-calcite-calcite grain edges. However, experimental constraints on dihedral angles in calcite-fluid systems require that pervasive infiltration occurred in response to calcite dissolution initiated at calcite-calcite grain junctions rather than to an open calcite pore geometry. The regional extent of the retrograde infiltration event has been documented from the high δ¹⁸O of dolomite-ankerite carbonates from veins and host-rocks over an area of least 50 × 50 km in the SW Scottish Highlands. Isotopically exotic ¹⁸O-rich retrograde fluids have moved rapidly upwards through the crust, inducing isotopic exchange and mineral reaction in wall-rocks only where lithology, pore geometry or mineral solubilities, pressure and temperature have been appropriate for pervasive infiltration to occur.     

Contact angles in CO2-water-coal systems at elevated pressures

Injection of carbon dioxide into coal seams is considered to be a potential method for its sequestration away from the atmosphere. However, water present in coals may retard injection: especially if carbon dioxide does not wet the coal as well as water. Thus contact angles in the coal-water-CO₂ system were measured using CO₂ bubbles in water/coal systems at 40 °C and pressures up to 15 MPa using five bituminous coals. At low pressures, in this CO₂/water/coal system, receding contact angles for the coals ranged between 80° to 100°; except for one coal that had both high ash yield and low rank, with a contact angle of 115°, indicating that it was hydrophilic. With increasing pressure, the receding contact angles for the different coals decreased, indicating that they became more CO₂-wetting. The relationship between contact angle and pressure was approximately linear. For low ash or high rank coals, at high pressure the contact angle was reduced to 30-50°, indicating the coals became strongly CO₂-wetting; that is CO₂ fluids will spontaneously penetrate these wet coals. In the case of the coal that was both high ash and hydrophilic, the contact angle did not drop to 90° even at the highest pressures used. These results suggest that CO₂ will not be efficiently adsorbed by all wet coals equally well, even at high pressure. It was found that at high pressures (> 2 MPa) the rate of penetration of carbon dioxide into the coals increased rapidly with decreasing contact angle, independently of pressure. Injecting CO₂ into wet coals that have both low rank and high ash will not trap CO₂ as well as injecting it into high rank or low ash coals.