Compositional re-equilibration of fluid inclusions in quartz

Compositional modifications to salt-water fluid inclusions in quartz were observed following exposure to disequilibrium conditions in a series of laboratory experiments in which samples containing inclusions of known composition were annealed at 3 kbar and 600≤T≤825°C in the presence of fluids having different compositions for time intervals ranging from a few days to one month. Changes in inclusion compositions following re-equilibration were monitored using salt crystal dissolution temperatures and/or IR (infra red) spectroscopy. The behaviors of both synthetic and natural fluid inclusions were studied. The synthetic samples were re-equilibrated under P _int=P _conf conditions to minimize stress in the crystal surrounding the inclusions, and were subjected to both f _H2O ^int f _H2O ^conf and f _H2O ^int f _H2O ^conf . After re-equilibration for four days at T≥600°C, most inclusions displayed significant compositional changes Without decrepitation. Salinity variations as large as ≈ 25 wt% were inferred for brine inclusions exposed to f _H2O ^int ≠f _H2O ^conf for 16 days at 825°C. The majority of our observations are consistent with the net diffusion of water toward the reservior having the lowest μH₂O; i.e., synthetic NaCl−H₂O fluid inclusions exhibited increased Tm(NaCl)s (implying lower relative H₂O contents) when re-equilibrated in the presence of fluids having lower μH₂O, whereas, similar (and, in some cases, the same) inclusions exhibited decreased Tm(NaCl)s (implying higher H₂O contents) after exposure to fluids having higher μH₂O. The behavior of natural salt-water fluid inclusions during re-equilibration was generally consistent with corresponding observations on synthetic samples verifying that compositional changes are not restricted to synthetic inclusions. Our results clearly show that there was chemical communication between fluids trapped as inclusions in quartz and the external fluid reservoir. Additionally, it is evident that although applied stress can significantly enhance the re-equilibration rate, strain in the crystal host around the inclusions resulting from large pressure differentials between the internal and confining fluids is not a necessary prerequisite for compositional change. Finally, because significant compositional changes can be induced in brine inclusions in quartz during shortterm exposure to non-equilibrium conditions at 600≤T≤825°C in the laboratory, it is likely that similar changes may result at much lower temperatures during exposure of natural rocks to non-equilibrium conditions over geologic time.     

Texturally-early fluid inclusions in garnets: evidence of the prograde metamorphic path?

Well-formed, texturally-early fluid inclusions in garnets from the Archean Pikwitonei granulite domain, Manitoba, Canada, have been analyzed using microthermometric methods. The mean CO₂ homogenization temperature (to liquid) for inclusions in 12 of 13 samples from the Cauchon Lake-Nelson River area is +15.2° C (n=125, 2σ=8.2° C), corresponding to a CO₂ density of 0.82 g/cm³. Inclusions in the remaining sample have somewhat lower CO₂ homogenization temperatures (mean=+5.4° C, n=24). The studied inclusions contain an estimated 10 to 20 vol. percent H₂O, with minor amounts of other fluid species such as CH₄, N₂, and/or H₂S. The fluid inclusions were probably trapped during early garnet growth at relatively low pressures (≤5 kbar if at 750° C), and appear to have undergone only limited or possibly no subsequent re-equilibration. This interpretation is consistent with the “anti-clock-wise” P-T-t path (heating before loading) determined for the Pikwitonei region by other workers. For such a prograde path, inclusions entrapped early, at high temperatures but at relatively low pressures, would experience internal underpressures during most of the subsequent prograde and retrograde phases of metamorphism. The texturally-early fluid inclusions in garnets from the Pikwitonei region therefore cannot be used to provide direct information about the highest metamorphic temperature and pressure conditions (750° C and 7 kbar). However, the results obtained in this study suggest that texturally-early fluid inclusions in garnets may, in some cases, retain evidence of the prograde metamorphic path.     

Preferential water loss from synthetic fluid inclusions

A fundamental question in most fluid inclusion studies is whether inclusions behave as compositionally closed systems after trapping, and, thus, represent samples of the fluid phase(s) present in the system at the time of their formation. This question was addressed in high-temperature laboratory experiments with synthetic fluid inclusions in quartz and it was found that at 825°C the inclusions exhibited open-system behavior with respect to water. Synthetic salt-water fluid inclusions in quartz were reequilibrated for 12 hours to 35 days at 825° C in a dry argon atmosphere under 1.5 kbar confining pressure. These conditions created initial internal overpressures (P _int> P _conf) of 1.5–4 kbar in the inclusions and differential water fugacities in the same sense i.e., fH₂OfH₂O. After 108 hours of reequilibration, preferential water loss had resulted in salinity increases as large as 22 wt% salt (e.g., from 57 to 79 wt% NaCl, as determined from measured temperatures of salt dissolution). Also, following reequilibration, a strong inverse correlation between salinity and inclusion volume was observed, and this trend became more pronounced with increasing reequilibration time. These observations, together with a lack of evidence for selective H₂O removal via hydration reactions, suggest that water loss occurred by a diffusion-related mechanism. Fluxes of 4x10^-11 g/cm²-s and diffusion coefficients on the order of 10^-9 cm²/s are calculated for water loss from the inclusions. The calculated H₂O diffusion coefficient is consistent with the determination of Blacic (1981) derived from hydrolytic weakening experiments, but is much larger than the value obtained by Giletti and Yund (1984) for volume diffusion of oxygen in isotope exchange experiments. These observations suggest that the mechanism of water loss from our synthetic fluid inclusions may have been pipe diffusion along dislocations, subgrain boundaries or other structural defects rather than bulk volume diffusion.The results of this study are relevant to the interpretation of fluid inclusions in quartz from several natural high-temperature environments where water fugacities of included and ambient fluids are known to have evolved along separate paths over geologic time.     

Synthetic fluid inclusions - VI. Quantitative evaluation of the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure

ABSTRACT The decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure has been evaluated using pure H₂O synthetic inclusions formed by healing fractures in natural quartz. Three different modes of non-elastic deformation, referred to as stretching, leakage or partial decrepitation, and total decrepitation have been observed. The internal pressure required to initiate non-elastic deformation is inversely related to inclusion size according to the equation: internal pressure (kbar) = 4.26 D^-0.423 where D is the inclusion diameter in microns. Regularly shaped inclusions require a higher internal pressure to initiate non-elastic deformation than do irregularly shaped inclusions of similar size. Heating inclusions through the α/β quartz inversion results in mechanical instability in the quartz crystal and leads to mass decrepitation of inclusions owing to structural mismatches generated by pressure gradients in the quartz around each inclusion. Long-term heating experiments (∼2 years) suggest that the internal pressure required to initiate non-elastic deformation does not decrease significantly with time and indicates that short-lived thermal fluctuations in natural systems should not alter the inclusion density and homogenization temperature. Inclusions that do exhibit decreased density (higher homogenization temperature) are, however, always accompanied by a change in shape from irregular to that of a negative crystal. Observations of this study are consistent with elasticity theory related to fracture generation and propagation around inclusions in minerals. These results indicate that an inclusion will not be influenced by a neighbouring inclusion, or other defect in the host phase, as long as the distance between the two is >2–4 diameters of the larger of the two inclusions.     

Fluid inclusions in Adirondack granulites: Implications for the retrograde P-T path

Investigation of fluid inclusions in granitic and cale-silicate gneisses from the Adirondack Mountains, New York, has revealed the presence of various types, including: (1) CO₂-rich, (2) mixed H₂O–CO₂±salt and (3) aqueous inclusions with no visible CO₂. Many, if not all, of these inclusions were trapped or modified after the peak of granulite facies metamorphism, as shown by textural relations or by the lack of agreement between the composition of the fluids found in some inclusions and the composition of the peak-metamorphic fluid as estimated from mineral equilibria. Many fluid inclusions record conditions attained during retrograde cooling and uplift, with minimum pressures and temperatures of 2 to 3 kbar and 200 to 300°C. The temperatures and pressures derived from the investigation of these inclusions constrain the retrograde P-T path, and the results indicate that a period of cooling with little or no decompression.     

A boundary layer-induced immiscibility in naturally re-equilibrated H2O-CO2-NaCl inclusions from metamorphic quartz (Western Carpathians,Czechoslovakia)

Naturally re-equilibrated fluid inclusions have been found in quartz crystals from alpine fissures of the Western Carpathians. Re-equilibration textures, such as planar arrangement of the decrepitation clusters as well as the quartz c- and a-axis oriented fracturing indicate explosion of fluid inclusions. The extent of fracturing, which is dependent on inclusion diameters, suggests inclusion fluid overpressures between 0.6–1.9 kb. Microthermometry data are controversial with the textures because of indicating roughly fixed initial fluid composition and density during re-equilibration, although inclusion volumes have been sometimes substantially reduced by crystallization of newly-formed quartz. It is concluded that fluid loss from re-equilibrated inclusions must have been compensated for by replacing equivalent quartz volume from cracks into parent inclusion. Such a mechanism has operated in a closed system and the re-equilibration related cracks have not been connected with mineral surface. The compositional and density differences between aqueous inclusions in decrepitation clusters and CO₂-rich parent inclusions cannot be interpreted in terms of classical fluid immiscibility. Moreover, monophase liquid-filled aqueous inclusions and coexisting monophase CO₂ vapour-filled inclusions in the decrepitation clusters are thermodynamically unacceptable under equilibrium metamorphic conditions. The effect of disjoining pressure resulting from structural and electrostatic forces in very thin fractures is suspected to have caused density and compositional inconsistencies between parent and cluster inclusions, as well as the unusual appearance of cluster inclusions. In high-grade metamorphic conditions, the re-equilibration probably leads to boundary layer-induced immiscibility of homogeneous H₂O–CO₂–NaCl fluids and to formation of compositionally contrasting CO₂-rich and aqueous inclusions.     

P-T-X conditions of calc-silicate formation: evidence from fluid inclusions and phase equilibria; Llano Uplift, central Texas, USA

Abstract The Llano Uplift in central Texas is a Grenville aged (c. 1.1 Ga) metamorphic terrane consisting predominantly of amphibolite facies mineral assemblages. The formation of these assemblages has been attributed to the emplacement of relatively late granite plutons throughout the area. Two types of granitic intrusion have previously been recognized: (1) Town Mountain Granites, which occur as relatively large, circular-shaped bodies of coarse-grained granite, and (2) Younger Granites which are present as smaller and more irregular bodies of finer-grained granite. In the central part of the uplift, wollastonite-bearing calc-silicate rocks occur within the Valley Spring Gneiss. The development of these calc-silicate rocks has been linked to infiltrating fluids presumably derived from spatially associated Younger Granites. The stability of coexisting quartz, calcite, wollastonite, grossular and anorthite and coexisting quartz, calcite, wollastonite, andradite and hedenbergite shows that the calc-silicate rocks equilibrated under H₂O-rich conditions with χCO₂ <0.10. Fluid inclusions present within the calc-silicate minerals are H₂O-rich with salinities of <17 wt% equivalent NaCl. The absence of any detectable CO₂ in the fluid inclusions may indicate entrapment of the inclusions at lower pressures and more H₂O-rich conditions compared to the stability of the peak metamorphic mineral assemblage. Homogenization temperatures, measured for texturally primary inclusions, range from 360 to 368° C corresponding to a density range from 0.53 to 0.82 g/cm³. Isochores for these fluid inclusions, when combined with the stability of the solid-solid equilibria Grs + Qtz = Wo + An, yield formation conditions of 500–550° C at 1–2 kbar. This indicates that the granitic intrusions involved in the formation of the Blount Mountain calc-silicates were emplaced at a pressure of at least 1–2 kbar.     

Synthetic fluid inclusions. XV. TEM investigation of plastic flow associated with reequilibration of fluid inclusions in natural quartz

The nature and abundance of dislocations in quartz surrounding fluid inclusions were studied to obtain a better understanding of processes associated with fluid inclusion reequilibration. Synthetic fluid inclusions containing 10 wt% NaCl aqueous solution were formed in three samples at 700 °C and 5 kbar. One of the samples was quenched along an isochore to serve as a reference sample. The other two samples were quenched along a P-T path that generated internal pressures in excess of the confining pressure. The two samples were held at the final reequilibration P-T conditions of 625 °C and 2 kbar for 30 and 180 days, respectively. Following the experiments, microstructures associated with fluid inclusions were examined with the TEM. Quartz in healed fractures in the reference sample that was quenched isochorically shows a moderate dislocation activity. Quartz adjacent to reequilibrated fluid inclusions in the other two samples, however, showed a marked increase in dislocation activity compared to the un-reequilibrated sample. Deformation of the inclusion walls occurred anisotropically by expansion of mobile dislocations in their slip systems. Dislocation expansion was controlled by glide in the rhombohedral planes {1 0 1 1} that was restricted to narrow zones (≤3 μm) in the immediate vicinity of the fluid inclusion walls outside of the healed fracture plane. These plastic zones were observed after both short term (30 days) and long term (180 days) experiments and are attributed to hydrolytic weakening of quartz around fluid inclusions owing to diffusion of water into the quartz matrix during the experiment. The close spatial association of submicroscopic water bubbles with dislocations, and the rarity of water bubbles in the reference sample, show clearly that in both the 30 and 180 day experiments reequilibration involves water loss from the fluid inclusions. Our results indicate that synthetic fluid inclusions in this study recover (chemically and volumetrically), even at relatively fast experimental loading rates, such that internal stresses never reach the point of brittle failure. The driving force for fluid inclusion deformation involves two related mechanisms: plastic deformation of hydrolytically weakened wet quartz in the healed fracture, and water leakage associated with preexisting and strain-induced dislocations. Received: 5 May 1998 / Accepted: 10 February 2000     

Fluid evolution during metamorphism at increasing pressure: carbonic- and nitrogen-bearing fluid inclusions in granulites from Øksfjord,north Norwegian Caledonides

Three successive metamorphic stages M1, M2 and M3 have been distinguished in polymetamorphic granulite facies quartz-feldspathic gneisses from the Seiland Igneous Province, Caledonides of northern Norway. An early period of contact metamorphism (M1; 750–950°C, ca. 5 kbar) was followed by cooling, accompanied by strong shearing and recrystallization at intermediate-P granulite facies conditions (M2; 700–750°C, 5–6kbar). High-P granulite facies (M3; ca. 700°C, 7–8 kbar) is related to recrystallization in narrow ductile shear zones and secondary growth on M2 minerals. On the basis of composition, fluid inclusions in cordierite, quartz and garnet can be divided into three major types: (1) CO₂ inclusions; (2) mixed CO₂–N₂ inclusions; (3) N₂ inclusions. Fluid chronology and mineral assemblages suggest that the earliest inclusions consist of pure CO₂ and were trapped at the M1 contact metamorphic episode. A carbonic fluid was also present during the intermediate-P granulite facies M2 metamorphism. The CO₂-rich inclusions in M2 garnet can be divided into two generations, an early lower-density and a late higher-density, with isochores crosscutting the P-T box of M2 and M3, respectively. The nitrogen-rich fluids were introduced at a late stage in the fluid evolution during the high-P M3 event. The mixed CO₂–N₂ inclusions, with density characteristics compatible with M3 conditions, are probably produced from intersection between pre-existing pure CO₂ inclusions and N₂ fluids introduced during M3. The fluid inclusion data agree with the P-T evolution established from mineral assemblages and mineral chemistry.     

Fluid inclusion evidence for an anticlockwise metamorphic P-T path in central Massachusetts

Acadian (Late Silurian to Early Devonian) metamorphism in the Central Maine Terrane (CMT) in central Massachusetts is characterized by an early low-P, high-T (Buchan-type) metamorphism followed by thickening at high temperature (>650d? C) and then by cooling to 100-200d? C below peak recorded temperatures before eventual unroofing. Mineralogical and textural evidence for this path includes sillimanite pseudomorphs after early andalusite, abundant cordierite in pelitic lithologies, replacement of low-P cordierite-bearing assemblages by high-P garnet-bearing assemblages, and recrystallization of mylonites associated with late shear zones to form lower-T and higher-P assemblages. Peak conditions in the highest grade rocks were 685-780d? C and 5-6 kbar; the cooling path passed through 550d? C at about 6.5 kbar. The well-constrained P-T path documented from geological and mineralogical evidence for the CMT offers an unusual opportunity to examine characteristics of fluid inclusions that have experienced a long-lived metamorphic event spanning a broad range of P-T conditions. Fluid inclusion data from the CMT document a range of fluid compositions (CO₂-rich, mixed CO₂-N₂-rich, N₂-rich and H₂O-rich) and densities during metamorphism. Densities of CO₂ fluid inclusions range from 0.20 to 1.03 g cm^-3. Medium-density CO₂ fluid inclusions are contained in quartz inclusions within garnets in partial melt leucosomes, and in quartz grains within migmatites. Fluid inclusions within the quartz inclusions indicate trapping conditions of 650-700d? C at pressures below 5 kbar. Other CO₂ fluid inclusions from matrix quartz yield isochores which pass through 700d? C and 5.2 kbar. The highest density inclusions associated with rocks containing the late high-P assemblages have isochores which pass below the estimated P-T conditions for recrystallization of the mylonite. Fluid inclusion evidence suggests an early low-P heating event, followed by thickening at high temperature, and then by nearly isobaric cooling to about 500d? C with later decompression. This interpretation is also consistent with previously published petrological models and supports an anticlockwise P-T path for the CMT of south-central Massachusetts.     

Experimental diffusion of hydrogen into synthetic fluid inclusions in quartz

Abstract Quartz-hosted, synthetic CO₂-H₂O fluid inclusions behave as open systems with respect to diffusional transfer of hydrogen during laboratory-simulated metamorphic re-equilibration at 650, 750 and 825°C and 1.5 kbar total pressure with f_O2 defined by the C-CH₄ buffer. Microthermometry and Raman spectroscopy show that the initial CO₂-H₂O inclusions become CO₂-CH₄-H₂-H_2O
inclusions after diffusive influx of hydrogen from the reducing confining medium. Measurable changes are observed in inclusion compositions after only 15 days of re-equilibration, implying significant hydrogen mobility at still lower temperatures over geological time spans. Results of synthetic inclusion re-equilibrium experiments have profound implications for the interpretation of natural fluid-inclusion data; failure to account for potential hydrogen migration in inclusions from high-temperature geological environments may lead to erroneous estimates of P-T, and/or the compositions of metamorphic fluids.     

Fluid inclusions in high-pressure granulites of the Pan-African belt in Tanzania (Uluguru Mts): a record of prograde to retrograde fluid evolution

The high-pressure granulites of the Uluguru Mountains are part of the Pan-African belt of Tanzania, the metamorphic evolution of which is characterized by an anticlockwise P-T path. Mineral assemblages that represent distinct metamorphic stages are selected for fluid inclusion studies in order to deduce the fluid evolution in metapelites and pyroxene granulites from the prograde to the retrograde stage. Fluid inclusion data improve the petrologically derived P-T path and confirm the anticlockwise evolution. Fluid inclusions in quartz enclosed in garnet porphyroblasts in metapelites preserve prograde fluids of CO₂–N₂ composition and later-trapped pure CO₂. During isochoric heating at temperatures near the peak of metamorphism, deformation and recrystallization led to fluid homogenization yielding N₂-poor CO₂ composition in the metapelites. Near-peak CO₂–N₂ fluid inclusions in quartz of metapelites and CO₂ inclusions in garnet-pyroxene granulites are characterized by perfect negative crystal shape. Garnet formed in veins and as coronas around orthopyroxene represent the near-isochoric/isobaric cooling stage which is characterized by high-density CO₂-rich fluid inclusions. Up to 15 mol% N₂ in some primary CO₂ inclusions in corona garnet indicate small-scale fluid heterogeneity during the static garnet growth. The fact that high-density fluid inclusions are preserved, suggests a shallow dP/dT slope of the uplift path. Nevertheless, some fluid inclusions decrepitated or re-equilibrated and low-density CO₂ inclusions were trapped in the garnet-pyroxene granulite while N₂–CH₄ inclusions formed in the metapelites. Different fluid compositions in metapelite and metabasite argue for an internal control of the fluid composition by phase equilibria. In shear zones where the pyroxene granulite was transformed into scapolite-biotite schist, CO₂–N₂ and low-density N₂–CH₄ fluid inclusions indicate several stages of tectonic activity and suggest fluid influx from the nearby metapelites. High- and low-salinity aqueous inclusions observed beside CO₂ inclusions in garnet-pyroxene granulites, in vein quartz and shear zones could be of high-grade origin but are mainly re-equilibrated or re-trapped along healed microfractures during lower-grade stages. Received: 21 May 1997 / Accepted: 6 October 1997     

Phase equilibria in fluid inclusions from the Khtada Lake metamorphic complex

The Khtada Lake. British Columbia, metamorphic complex consists of high grade amphibolite and metasedimentary units with development of gneiss, migmatite and homogeneous autochthonous plutons. Maximum metamorphic conditions are estimated to have exceeded 5 kbar and 700°C.Fluid inclusions in matrix quartz are highly variable in density and composition, ranging from apparently pure CO₂ (gas or liquid or both at room temperature) through CO₂ + H₂O ± CH₄ mixtures to inclusions which are entirely aqueous. They occur along cracks, in groups without planar features and as isolated inclusions. The latter and some which occur in groups, are interpreted to most nearly approximate, in density and composition, the fluids present during the peak of metamorphism.The density and fluid composition data are derived from direct observations of phase changes between ? 180 and + 380°C and from the application of published experimental data in the system CH₄-CO₂-H₂O-NaCl. The most dense, pure CO₂ inclusions indicate a pressure of entrapment at 5 kbar, if a temperature of 700°C is assumed. This is in close agreement with the minimum P-T estimates from the mineral assemblages. Methane was positively identified in inclusions in graphite-bearing specimens. Salt content is concluded to be about 5–6 wt% NaCl equivalent in the aqueous phase in both aqueous and CO₂ + H₂O inclusions. There is evidence of immiscible separation of CO₂-rich and H₂O-rich fluids at temperatures at least as high as 375°C.     

Re-equilibration of CO2 fluid inclusions at controlled hydrogen fugacities

Abstract Natural, pure CO₂ inclusions in quartz and olivine (c. Fo₉₀) were exposed to controlled f_H2 conditions at T= 718–728°C and P_total= 2 kbar; their compositions were monitored (before and after exposures) by microsampling Raman spectroscopy (MRS) and microthermometry. In both minerals exposed at the graphite–methane buffer (f_H2= 73 bar), fluid speciations record the diffusion of hydrogen into the inclusions. In quartz, room-temperature products in euhedral isolated (EI type) inclusions are carbonic phases with molar compositions of c. CO₂(60) + CH₄(40) plus graphite (Gr) and H₂O, whereas anhedral inclusions along secondary fractures (AS type) are Gr-free and contain H₂O plus carbonic phases with compositions in the range c. CO₂(60) + CH₄(40) to CO₂(10) + CH₄(90). EI type inclusions in olivine evolved to c. CO₂(90–95) + CH₄(5–10) without Gr, whereas AS type inclusions have a range of compositions from CO₂(90) + CH₄(10) ± Gr to CH₄(50) + H₂(50) ± Gr; neither H₂O nor any hydrous species was detected by optical microscopy or MRS in the olivine-hosted products. Differences in composition between and among the texturally distinct populations of inclusions in both minerals probably arise from variations in initial fluid densities, as all inclusions apparently equilibrated with the ambient f_H2. These relations suggest that compositional variability among inclusions in a given natural sample does not require the entrapment of multiple generations of fluids. In addition, the absence of H₂O in the olivine-hosted inclusions would require the extraction of oxygen from the fluids, in which case re-equilibration mechanisms may be dependent on the composition and structure of the host mineral. Many of the same samples were re-exposed to identical P–T conditions using Ar as the pressure medium, yielding ambient f_H2= 0.06 bar. In most inclusions, the carbonic fluids returned to pure CO₂ and graphite persisted in the products. Reversal of the mechanisms from the prior exposure at f_H2= 73 bar did not occur in any inclusions but the AS types in olivine, in which minor CO₂ was produced at the expense of CH₄ and/or graphite. The observed non-reversibility of previous mechanisms may be attributed to: (1) slower fluid–solid reactions compared to reactions in the homogeneous fluid phase; (2) depressed activities of graphite due to poor ordering; and/or (3) low ambient f_O2 at the conditions of the second run.     

High-density nitrogen inclusions in barite from a giant siderite vein: implications for Alpine evolution of the Variscan basement of Western Carpathians, Slovakia

Contrasting compositions and densities of fluid inclusions were revealed in siderite–barite intergrowths of the Dro?diak polymetallic vein hosted in Variscan basement of the Gemeric unit (Central European Carpathians). Primary two‐phase aqueous inclusions in siderite homogenized between 101 and 165 °C, total salinity ranged between 18 and 27 wt%, and CaCl₂/(NaCl + CaCl₂) weight ratios were fixed at 0.1–0.3. By contrast, mono‐ and two‐phase aqueous inclusions in barite exhibited total salinities between 2 and 22 wt%, and the CaCl₂/NaCl ratios ranged from NaCl‐ to CaCl₂‐dominated compositions. The aqueous inclusions in barite were closely associated with very high‐density (0.55–0.745 g cm^?3) nitrogen inclusions, in some cases containing up to 16 mol.% CO₂. Crystallization P–T conditions of siderite (175–210 °C, 1.2–1.7 kbar) constrained by the vertical oxygen isotope gradient along the studied vein, isochores of fluid inclusions and the K/Na exchange thermometer corresponded to minimal palaeodepths between 4.3 and 6.3 km, assuming lithostatic load and average crust density of 2.75 g cm^?3. Maximum fluid pressure during barite crystallization attained 3.6–4.4 kbar at 200–300 °C, and the most dense nitrogen inclusions maintained without decrepitation the residual internal pressure of 2.2 kbar at 25 °C. Contrasting fluid compositions, increasing depths of burial (～4–14 km) and decreasing thermal gradients (～40–15 °C km^?1) during initial mineralization stages of the Dro?diak vein reflect Alpine orogenic processes, rather than an incipient Permian rifting suggested in previous metallogenetic models. Siderite crystallized at rising P–T in a closed, rock‐buffered hydrothermal system developed in the Variscan basement during the north‐vergent Cretaceous thrusting and thickening of the Gemeric crustal wedge. Variable salinities of the barite‐hosted inclusions reflect a fluid mixing in open hydrothermal system, and re‐equilibration textures (lengths of decrepitation cracks proportional to fluid inclusion sizes) correspond to retrograde crystallization trajectory coincidental with transpression or unroofing. Maximum recorded fluid pressures indicate ～12‐km‐thick pile of imbricated nappe units accumulated over the Gemeric basement during the Cretaceous collision.     

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.     

Reaction textures and fluid behaviour in very high pressure calc-silicate rocks of the Münchberg gneiss complex, Bavaria, Germany

Calc-silicate rocks occur as elliptical bands and boudins intimately interlayered with eclogites and high-pressure gneisses in the Münchberg gneiss complex of NE Bavaria. Core assemblages of the boudins consist of grossular-rich garnet, diopside, quartz, zoisite, clinozoisite, calcite, rutile and titanite. The polygonal granoblastic texture commonly displays mineral relics and reaction textures such as post kinematic grossular-rich garnet coronas. Reactions between these mineral phases have been modelled in the CaO-Al₂O₃-SiO₂-CO₂-H₂O system with an internally consistent thermodynamic data base. High-pressure metamorphism in the calc-silicate rocks has been estimated at a minimum pressure of 31 kbar at a temperature of 630d? C with X_H2, O ≥ 0.03. Small volumes of a CO₂-N₂-rich fluid whose composition was buffered on a local scale were present at peak-metamorphic conditions. The P-T conditions for the onset of the amphibolite facies overprint are about 10 kbar at the same temperature. X_Co2 of the H₂O-rich fluid phase is regarded to have been <0.03 during amphibolite facies conditions. These P-T estimates are interpreted as representing different stages of recrystallization during isothermal decompression. The presence of multiple generations of mineral phases and the preservation of very high-pressure relics in single thin sections preclude pervasive post-peak metamorphic fluid flow as a cause of a re-equilibration within the calc-silicates. The preservation of eclogite facies, very high-pressure relics as well as amphibolite facies reactions textures in the presence of a fluid phase is in agreement with fast, tectonically driven unroofing of these rocks.     

Microthermometrical and chemical studies of fluid inclusions in minerals from Alpine veins from the penninic rocks of the central and western Tauern Window (Austria/Italy)

The fluid inclusions in samples of quartz, apatite, epidote, diopside, beryl and phenakite from Alpine veins in gneisses, amphibolites and mica schists from the western Tauern Window were analysed by microthermometrical, chemical and neutron activation methods. The inclusions of the eclogites contain a high density CO₂ phase without optically detectable H₂. In the Greiner Schieferserie the fluid inclusions show high CO₂/H₂O ratios and low salt contents. In the Zentralgneis area inclusions with low CO₂/H₂O ratios and high salt contents are typical. In the calcareous mica schists of the lower Schieferhülle, in the eastern part of the investigated area, generally no CO₂ could be detected in the inclusions. These inclusions contain aqueous solutions showing a low salt content. The only CO₂ bearing inclusions observed here were in the graphite-rich rocks of the so-called Habachzungen and in the eclogites from south of the Großvenediger. Trapping pressures estimated from the fluid inclusions are up to 7.5 kbar in the eclogites, but in general the pressures are between 2 and 4 kbar. These pressure data are in good agreement with the pressure data of mineral equilibria. The chemically analysed elements in the fluid inclusions are Na, K, Cs, Mg, Ca, Mn, As, Cl and Br. From the K/Na ratios temperatures between 435 and 490°C can be deduced. The very low Cl/Br ratios (<110) suggest that the dissolved elements came from the country rocks. The alkali/chlorine ratios (～1) indicate that the positive loadings of the cations are balanced by Cl.     

Combined microthermometric and Raman spectroscopic technique to determine the salinity of H2O-CO2-NaCl fluid inclusions based on clathrate melting

Fluid inclusions approximated by the system H₂O-CO₂-NaCl are common in many geologic environments. In order to apply microthermometric data from these inclusions to infer P-T (pressure-temperature) trapping conditions, the composition of the inclusions, including the salinity, must be known. Normally, salinities of aqueous inclusions are determined from ice-melting temperatures obtained during microthermometry. However, when CO₂-bearing aqueous fluid inclusions are cooled they often form a hydrate that incorporates H₂O into the structure, and salinities estimated from ice-melting temperatures are therefore higher than the actual salinity. A technique that combines data from Raman spectroscopic and microthermometric analyses of individual inclusions was developed to determine the salinity of CO₂-bearing aqueous inclusions based on measured clathrate melting temperatures and CO₂ pressures obtained from Raman analyses. In this study, the pressure within inclusions was determined using Raman spectroscopy based on the splitting of the Fermi diad of CO₂, measured at the clathrate melting temperature. The CO₂ densities (and pressures) predicted by the equation developed in this study are in relatively good agreement with previously published equations, except for very low densities and correspondingly low pressures. The combined Raman spectroscopy - microthermometry technique thus provides both the temperature and the pressure in the inclusion at clathrate melting. For inclusions in which the clathrate melts in the presence of CO₂ liquid, the salinity can be determined with a precision of a few tenths of a wt% NaCl, whereas for inclusions in which clathrate melts in the presence of CO₂ vapor the salinity error may be a few wt% NaCl. Applying the method to synthetic fluid inclusions with known salinity suggests that the technique is valid for determining salinity of H₂O-CO₂-NaCl fluid inclusions in which clathrate melts in the presence of liquid CO₂ only or vapor CO₂ only.     

Textural evolution of synthetic fluid inclusions in quartz during reequilibration,with applications to tectonic reconstruction

Experimental reequilibration of synthetic 10 wt% NaCl-H₂O inclusions in natural quartz reveals that reequilibration textures show distinct differences depending upon the P-T path followed by the inclusion after formation. These differences combined with other geological information may be used to determine whether the sample (rock) followed a dominantly isothermal or isobaricP-T path following entrapment. The intensity and style of inclusion reequilibration features is related to the direction and magnitude of the departure of theP-T path from the original isochore for the inclusion. Thus, fluid inclusion reequilibration textures not only permit inclusionists to determine whether the rocks followed an isothermal or isobaric retrogradeP-T path, but also the magnitude of departure of this path from one that is isochoric. Editorial responsibility: J. Touret