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.     

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 entrapment and reequilibration during subduction and exhumation: A case study from the high-grade Nestos shear zone, Central Rhodope, Greece

A fluid inclusion study on metamorphic minerals of successive growth stages was performed on highly deformed paragneisses from the Nestos Shear Zone at Xanthi (Central Rhodope), in which microdiamonds provide unequivocal evidence for ultrahigh-pressure (UHP) metamorphism. The correlation of fluid inclusion density isochores and fluid inclusion reequilibration textures with geothermobarometric data and the relative chronology of micro- and macro-scale deformation stages allow a better understanding of both the fluid and metamorphic evolution along the P–T–d path. Textural evidence for subduction towards the NE is recorded by the orientation of intragranular NE-oriented fluid inclusion planes and the presence of single, annular fluid inclusion decrepitation textures. These textures occur within quartz “foam” structures enclosed in an earlier generation of garnets with prolate geometries and rarely within recrystallized matrix quartz, and reequilibrated both in composition and density during later stages of exhumation. No fluid inclusions pertaining to the postulated ultrahigh-pressure stage for microdiamond-bearing garnet–kyanite–gneisses have yet been found. The prolate shape of garnets developed during the earliest stages of exhumation that is recorded structurally by (L  S) tectonites, which subsequently accommodated progressive ductile SW shearing and folding up to shallow crustal levels. The majority of matrix kyanite and a later generation of garnet were formed during SW-directed shear under plane-strain conditions. Fluid inclusions entrapped in quartz during this stage of deformation underwent density loss and transformed to almost pure CO₂ inclusions by preferential loss of H₂O. Those inclusions armoured within garnet retained their primary 3-phase H₂O–CO₂ compositions. Reequilibration of fluid inclusions in quartz aggregates is most likely the result of recrystallization along with stress-induced, preferential H₂O leakage along dislocations and planar lattice defects which results in the predominance of CO₂ inclusions with supercritical densities. Carbonic fluid inclusions from adjacent kyanite–corundum-bearing pegmatoids and, the presence of shear-plane-parallel fluid inclusion planes within late quartz boudin structures consisting of pure CO₂-fluid inclusions with negative crystal shapes, bear witness of the latest stage of deformation by NE-directed extensional shear.This study shows that the textures of early fluid inclusions that formed already during the prograde metamorphic path can be preserved and used to derive information about the kinematics of subduction that is difficult to obtain from other sources. The textures of early inclusions, together with later generations of unaltered primary and secondary inclusions in metamorphic index minerals that can be linked to specific deformation stages and even P–T conditions, are a welcome supplement for the reconstruction of a rather detailed P–T–d path.     

Re-equilibration of natural H2O–CO2–salt-rich fluid inclusions in quartz—Part 1: experiments in pure water at constant pressures and differential pressures at 600 °C

Re-equilibration processes of natural H₂O–CO₂–NaCl-rich fluid inclusions quartz are experimentally studied by exposing the samples to a pure H₂O external fluid at 600 °C. Experimental conditions are selected at nearly constant pressure conditions (309 MPa) between fluid inclusions and pore fluid, with only fugacity gradients in H₂O and CO₂, and at differential pressure conditions (394–398 MPa, corresponding to an internal under-pressure) in addition to similar CO₂ fugacity gradients and larger H₂O fugacity gradients. Modifications of fluid inclusion composition and density are monitored with changes in ice dissolution temperature, clathrate dissolution temperature and volume fraction of the vapour phase at room temperature. Specific modification of these parameters can be assigned to specific processes, such as preferential loss/gain of H₂O and CO₂, or changes in total volume. A combination of these parameters can clearly distinguish between modifications according to bulk diffusion or deformation processes. Bulk diffusion of CO₂ according to fugacity gradients is demonstrated at constant pressure conditions. The estimated preferential loss of H₂O is not in accordance with those gradients in both constant pressure and differential pressure experiments. The development of deformation halos in quartz around fluid inclusions that are either under-pressurized or over-pressurized promotes absorption of H₂O from the inclusions and inhibits bulk diffusion according to the applied fugacity gradients.     

Fluid inclusion modification by H2O and D2O diffusion: the influence of inclusion depth, size, and shape in re-equilibration experiments

The mobility of H₂O and D₂O by diffusion through quartz is illustrated with H₂O-rich fluid inclusions synthesized at 600 °C and 337 MPa, within the α-quartz stability field. Inclusions are re-equilibrated at the same experimental conditions within a pure D₂O fluid environment. Consequently, a gradient in volatile fugacities is the only driving force for diffusion, in the absence of pressure gradients and deformation processes. Up to 100 individual inclusions are analyzed in each experiment before and after re-equilibration by microscopic investigation, microthermometry, and Raman spectroscopy. Changes in fluid inclusion composition are obtained from the ice-melting temperatures, and density changes are obtained from total homogenization temperatures. After 1-day re-equilibration, inclusions already contain up to 11 mol % D₂O. A maximum concentration of 63 mol % D₂O is obtained after 40-day re-equilibration. D₂O concentration profiles in quartz are determined from the concentration in inclusions as a function of their distance to the quartz surface. These profiles illustrate that deep inclusions contain less D₂O than shallow inclusions. At equal depths, a variety of D₂O concentration is observed as a function of fluid inclusion size: Small inclusions are stronger effected compared with large inclusions. A series of 19-day re-equilibration experiments are performed at 300, 400, 500, and 600 °C (at 337 MPa), at the same conditions as the original synthesis. The threshold temperature of diffusion is estimated around 450 °C at 337 MPa, because D₂O is not detected in inclusions from re-equilibration experiments at 300 and 400 °C, whereas maximally 26 mol % D₂O is detected at 500 °C. Our study indicates that the isotopic composition of natural fluid inclusions may be easily modified by re-equilibration processes, according to the experimental conditions at 600 °C and 337 MPa.     

Ore Genesis and Tectonic Significance of the Xiaojiashan Tungsten Deposit,Eastern Tianshan,Xinjiang, China: Evidence from Geology,Fluid Inclusions,and Stable Isotope Geochemistry

The Xiaojiashan tungsten deposit is located about 200 km northwest of Hami City, the Eastern Tianshan orogenic belt, Xinjiang, northwestern China, and is a quartz vein‐type tungsten deposit. Combined fluid inclusion microthermometry, host rock geochemistry, and H–O isotopic compositions are used to constrain the ore genesis and tectonic setting of the Xiaojiashan tungsten deposit. The orebodies occur in granite intrusions adjacent to the metamorphic crystal tuff, which consists of the second lithological section of the first Sub‐Formation of the Dananhu Formation (D₂d ₁²). Biotite granite is the most widely distributed intrusive bodies in the Xiaojiashan tungsten deposit. Altered diorite and metamorphic crystal tuff are the main surrounding rocks. The granite belongs to peraluminous A‐type granite with high potassic calc‐alkaline series, and all rocks show light Rare Earth Element (REE)‐enriched patterns. The trace element characters suggest that crystallization differentiation might even occur in the diagenetic process. The granite belongs to postcollisional extension granite, and the rocks formed in an extensional tectonic environment, which might result from magma activity in such an extensional tectonic environment. Tungsten‐bearing quartz veins are divided into gray quartz vein and white quartz veins. Based on petrography observation, fluid inclusions in both kinds of vein quartz are mainly aqueous inclusions. Microthermometry shows that gray quartz veins have 143–354°C of T_h, and white quartz veins have 154–312°C of T_h. The laser‐Raman test shows that CO₂ is found in fluid inclusions of the tungsten‐bearing quartz veins. Quadrupole mass spectrometry reveals that fluid inclusions contain major vapor‐phase contents of CO₂, H₂O. Meanwhile, fluid inclusions contain major liquid‐phase contents of Cl^?, Na⁺. It can be speculated that the ore‐forming fluid of the Xiaojiashan tungsten deposit is characterized by an H₂O–CO₂, low salinity, and H₂O–CO₂–NaCl system. The range of hydrogen and oxygen isotope compositions indicated that the ore‐forming fluids of the tungsten deposit were mainly magmatic water. The ore‐forming age of the Xiaojiashan deposit should to be ~227 Ma. During the ore‐forming process, the magmatic water had separated from magmatic intrusions, and the ore‐bearing complex was taken to a portion where tungsten‐bearing ores could be mineralized. The magmatic fluid was mixed by meteoric water in the late stage.     

Fluid inclusion and stable isotope evidence for the genesis of quartz-scheelite veins, Metaggitsi area, central Chalkidiki Peninsula, N. Greece

Scheelite mineralization accompanied by muscovite and albite, and traces of Mo-stolzite and stolzite occurs in epigenetic quartz vein systems hosted by two-mica gneissic schists, and locally amphibolites, of the Paleozoic or older Vertiskos Formation, in the Metaggitsi area, central Chalkidiki, N Greece. Three types of primary fluid inclusions coexist in quartz and scheelite: type 1, the most abundant, consists of mixed H₂O-CO₂ inclusions with highly variable (20–90 vol.%) CO₂ contents and salinities between 0.2 and 8.3 equivalent weight % NaCl. Densities range from 0.79 to 0.99 g/cc; type 1 inclusions contain also traces (<2 mol%) of CH₄. Type 2 inclusions are nearly 100 vol.% liquid CO_2, with traces of CH₄, and densities between 0.75 and 0.88 g/cc. Type 3 inclusions, the least abundant, contain an aqueous liquid of low salinity (0.5 to 8.5 equivalent weight% NaCl) with 10–30 vol.% H₂O gas infrequently containing also small amounts of CO₂ (<2 mol%); densities range from 0.72 to 0.99 g/cc. The wide range of coexisting fluid inclusion compositions is interpreted as a result of fluid immiscibility during entrapment. Immiscibility is documented by the partitioning of CH₄ and CO₂, into gas-rich (CO₂-rich) type 1 inclusions, and the conformity of end-member compositions trapped in type 1 inclusions to chemical equilibrium fractionation at the minimum measured homogenization temperatures, and calculated homogenization pressures. Minimum measured homogenization temperatures of aqueous and gas-rich type 1 inclusions of 220°–250 °C, either to the H₂O, or to the CO₂ phase, is considered the best estimate of temperature of formation of the veins, and temperature of scheelite deposition. Corresponding fluid pressures were between 1.2 and 2.6 kbar. Oxygen fugacities during mineralization varied from 10⁻³⁵ to 10⁻³¹ bar and were slightly above the synthetic Ni-NiO buffer values. The fluid inclusion data combined with δ¹⁸O water values of 3 to 6 per mil (SMOW) and δ¹³C CO₂₋ fluid of −1.2 to +4.3 per mil (PDB), together with geologic data, indicate generation of mineralizing fluids primarily by late- to post-metamorphic devolatilization reactions. Received: 8 April 1997 / Accepted: 8 July 1997     

Fluid inclusion constraints on the uplift history of the metamorphosed massive sulphide deposits at Ducktown, Tennessee

Abstract Standard petrographic, microthermometric and Raman spectroscopic analyses of fluid inclusions from the metamorphosed massive sulphide deposits at Ducktown, Tennessee, indicate that fluids with a wide range of compositions in the C–O–H–N–S–salt system were involved in the syn- to post-metamorphic history of these deposits. Primary fluid inclusions from peak metamorphic clinopyroxene contain low-salinity, H₂O–CH₄ fluids and calcite, quartz and pyrrhotite daughter crystals. Many of these inclusions exhibit morphologies resembling those produced in laboratory experiments in which confining pressures significantly exceed the internal pressures of the inclusions. Secondary inclusions in metamorphic quartz from veins, pods, and host matrix record a complex uplift history involving a variety of fluids in the C–O–H–N–salt system. Early fluids were generated by local devolatilization reactions while later fluids were derived externally. Isochores calculated for secondary inclusions in addition to the chronology of trapping and morphological features of primary and secondary fluid inclusions suggest an uplift path which was concave toward the temperature axis over the P–T range 6–3 kbar and 550–225° C. Immiscible H₂O–CH₄–N₂–NaCl fluids were trapped under lithostatic to hydrostatic pressure conditions at 3–0.5 kbar and 215 ± 20° C. Entrapment occurred during Alleghanian thrusting, and the fluids may have been derived by tectonically driven expulsion of pore fluids and thermal maturation of organic material in lower-plate sedimentary rocks which are thought to underlie the deposits. Episodic fracturing and concomitant pressure decreases in upper-plate rocks, which host the ore bodies, would have allowed these fluids to move upward and become immiscible. Post-Alleghanian uplift appears to have been temperature-convex. Uplift rates of 0.10–0.05 mm year^?1 from middle Ordovician to middle Silurian – late Devonian, and 0.07–0.12 mm year^?1 from middle Silurian – late Devonian to late Permian are suggested by our uplift path and available geochronological data.     

Synthetic fluid inclusions - VII. Re-equilibration of fluid inclusions in quartz during laboratory-simulated metamorphic burial and uplift

ABSTRACT P-T conditions inferred from fluid inclusions in metamorphic rocks often disagree with the values predicted from mineral equilibria calculations. These observations suggest that inclusions formed during early stages of regional metamorphism continue to re-equilibrate during burial and subsequent uplift in response to differential pressure. P-T conditions accompanying burial and uplift were experimentally simulated by initially forming pure H₂O inclusions in quartz at elevated temperatures and pressures, and then re-equilibrating the inclusions in the presence of a 20 wt% NaCl solution such that final confining pressures ranged from 5 kbar above to 4 kbar below the initial internal pressure of the inclusions at the temperature of re-equilibration. In all samples re-equilibrated at confining pressures below the internal pressure, some inclusions were formed that had compositions of 20 wt% NaCl and densities in accord with the final P-T conditions. Additionally, some inclusions were observed to contain fluids of intermediate salinities (between 0 and 20 wt% NaCl). Densities of these inclusions were also consistent with formation at the re-equilibration P-T conditions. The remainder of the fluid inclusions observed in these samples contained pure H₂O and their homogenization temperatures corresponded to densities intermediate between the initial and final P-T conditions. In short-term experiments (7 days) where the initial internal overpressure exceeded 1 kbar, no inclusions were found that contained the original density and none were found to have totally re-equilibrated. Instead, most H₂O inclusions re-equilibrated until their internal pressures were between ∼750 and 1500 bars above the confining pressure, regardless of the initial pressure differential. In a long-term experiment (52 days), inclusions re-equilibrated at a lower confining pressure than the initial internal pressure displayed homogenization temperatures corresponding to a range in final internal pressures between 0 kbar (i.e. total re-equilibration) and 1.2 kbar above the confining pressure. In experiments where the confining pressure during re-equilibration exceeded the initial internal pressure, densities of pure H₂O inclusions increased to values intermediate between the initial and final P-T conditions. Additionally, these inclusions were generally surrounded by a three-dimensional halo of smaller inclusions, also of intermediate density, resulting in a texture similar to that previously ascribed to decrepitation from internal overpressure. In extreme cases where confining pressures were 4–5 kbar above the initial pressure, the parent inclusion almost completely closed leaving only the three-dimensional array of small (5 μm) inclusions, the outline of which may be several times the volume of the original inclusion. Groups of such inclusions closely resemble textures commonly observed in medium- to high-grade metamorphic rocks. Inclusions containing 10 and 42 wt% NaCl solutions trapped at 600 °c and 3 kbar were re-equilibrated at 600 °c and 1 kbar for 5 days in dry argon to evaluate the importance of H₂O diffusion as a mechanism of lowering the inclusion bulk density. Salinities of re-equilibrated inclusions obtained from freezing point depressions and halite dissolution temperatures indicate that original compositions were preserved. Density changes similar to those previously described were noted in these experiments, in inclusions showing no visible microfractures. Therefore, density variations observed in inclusions in this study, re-equilibrated under rapid deformation conditions, are considered to result from a change in the inclusion volume, without significant loss of contents by diffusion or leakage.     

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.     

Fluid inclusions in rocks from the amphibolite-facies gneiss to charnockite progression in southern Karnataka, India: direct evidence concerning the fluids of granulite metamorphism

Abstract Fluid inclusion studies of rocks from the late Archaean amphibolite-facies to granulite-facies transition zone of southern India provide support for the hypothesis that CO₂,-rich H₂O-poor fluids were a major factor in the origin of the high-grade terrain. Charnockites, closely associated leucogranites and quartzo-feldspathic veins contain vast numbers of large CO₂-rich inclusions in planar arrays in quartz and feldspar, whereas amphibole-bearing gray gneisses of essentially the same compositions as adjacent charnockites in mixed-facies quarries contain no large fluid inclusions. Inclusions in the northernmost incipient charnockites, as at Kabbal, Karnataka, occasionally contain about 25 mol. % of immiscible H₂O lining cavity walls, whereas inclusions from the charnockite massif terrane farther south do not have visibile H₂O Microthermometry of CO₂ inclusions shows that miscible CH₄ and N₂ must be small, probably less than 10mol.%combined. Densities of CO₂ increase steadily from north to south across the transitional terrane. Entrapment pressures calculated from the CO₂ equation of state range from 5 kbar in the north to 7.5 kbar in the south at the mineralogically inferred average metamorphic temperature of 750°C, in quantitative agreement with mineralogic geobarometry. This agreement leads to the inference that the fluid inclusions were trapped at or near peak metamorphic conditions. Calculations on the stability of the charnockite assemblage biotite-orthopyroxene-K-feldspar-quartz show that an associated fluid phase must have less than 0.35 H₂O activity at the inferred P and T conditions, which agrees with the petrographic observations. High TiO₂ content of biotite stabilizes it to lower H₂O activities, and the steady increase of biotite TiO₂ southward in the area suggests progressive decrease of aH₂O with increasing grade. Oxygen fugacities calculated from orthopyroxene-magnetite-quartz are considerably higher than the graphite CO₂-O₂ buffer, which explains the absence of graphite in the charnockites. The present study quantifies the nature of the vapours in the southern India granulite metamorphism. It remains to be determined whether CO₂-flushing of the crust can, by itself, create large terranes of largeion lithophile-depleted granulites, or whether removal of H₂O-bearing anatectic melts is essential.     

Experimental modeling of the transformation of H2O-CO2-CH4 inclusions during isobaric cooling and isothermal compression

The transformations (density, composition, or shape alterations) of fluid inclusions under isobaric cooling and isothermal compression have been experimentally modeled. The H₂O-CO₂-CH₄ inclusions used in the model experiments have been synthesized in quartz at 1 or 3 kbar and 700°C. The parameters of the model experiments were as follows: for isobaric cooling, 700–400°C at 1 or 3 kbar; for isothermal compression, 1–3 and 3–5 kbar at 400 and 700°C, respectively. The shape of the inclusion walls changed as a result of the experiments because of the dissolution and reprecipitation of the host phase. The intensity of these changes was directly affected by the pressure and temperature differences and the amount of the aqueous phase in the inclusions. Fluid densities increased in many inclusions in the run products: the increase ranged from insignificant (0.06 g/cm³) to fairly noticeable (0.15 g/cm³). The composition of the fluid inclusions also changed. The results of the experiments, in particular of those carried out with a labeled fluid, show that the penetration of a denser external fluid into inclusions with a lower fluid density is the basic reason for the increase in the density and change in the composition of fluid inclusions. In several experiments, the methane concentration changed with an attendant change in the melting temperature of carbon dioxide but without noticeable changes in the carbon dioxide homogenization temperature. Original Russian Text ? O.V. Vasyukova, V.I. Fonarev, 2006, published in Geokhimiya, 2006, No. 12, pp. 1266–1278.     

Genesis of massive sulfide deposits in the Verkhneural’sk ore district, the South Urals, Russia: Evidence for magmatic contribution of metals and fluids

Melt inclusions and aqueous fluid inclusions in quartz phenocrysts from host felsic volcanics, as well as fluid inclusions in minerals of ores and wall rocks were studied at the Cu-Zn massive sulfide deposits in the Verkhneural’sk ore district, the South Urals. The high-temperature (850–1210°C) magmatic melts of volcanic rocks are normal in alkalinity and correspond to rhyolites of the tholeiitic series. The groups of predominant K-Na-type (K₂O/Na₂O = 0.3–1.0), less abundant Na-type (K₂O/Na₂O = 0.15–0.3), and K-type (K₂O/Na₂O = 1.9–9.3) rhyolites are distinguished. The average concentrations (wt %) of volatile components in the melts are as follows: 2.9 H₂O (up to 6.5), 0.13 Cl (up to 0.28), and 0.09 F (up to 0.42). When quartz was crystallizing, the melt was heterogeneous, contained magnetite crystals and sulfide globules (pyrrhotite, pentlandite, chalcopyrite, bornite). High-density aqueous fluid inclusions, which were identified for the first time in quartz phenocrysts from felsic volcanics of the South Urals, provide evidence for real participation of magmatic water in hydrothermal ore formation. The fluids were homogenized at 124–245°C in the liquid phase; the salinity of the aqueous solution is 1.2–6.2 wt % NaCl equiv. The calculated fluid pressure is very high: 7.0–8.7 kbar at 850°C and 5.1–6.8 kbar at 700°C. The LA-ICP-MS analysis of melt and aqueous fluid inclusions in quartz phenocrysts shows a high saturation of primary magmatic fluid and melt with metals. This indicates ore potential of island-arc volcanic complexes spatially associated with massive sulfide deposits. The systematic study of fluid inclusions in minerals of ores and wall rocks at five massive sulfide deposits of the Verkhneural’sk district furnished evidence that ore-forming fluids had temperature of 375–115°C, pressure up to 1.0–0.5 kbar, chloride composition, and salinity of 0.8–11.2 (occasionally up to 22.8) wt % NaCl equiv. The H and O isotopic compositions of sericite from host metasomatic rocks suggest a substantial contribution of seawater to the composition of mineral-forming fluids. The role of magmatic water increases in the central zones of the feeding conduit and with depth. The dual nature of fluids with the prevalence of their magmatic source is supported by S, C, O, and Sr isotopic compositions. The TC parameters of the formation of massive sulfide deposits are consistent with the data on fluid inclusions from contemporary sulfide mounds on the oceanic bottom.     

Ore-forming fluids associated with granite-hosted gold mineralization at the Sanshandao deposit, Jiaodong gold province, China

The Sanshandao gold deposit, with total resources of more than 60 t of gold, is located in the Jiaodong gold province, the most important gold province of China. The deposit is a typical highly fractured and altered, disseminated gold system, with high-grade, quartz-sulphide vein/veinlet stockworks that cut Mesozoic granodiorite. There are four stages of veins that developed in the following sequence: (1) quartz-K-feldspar-sericite; (2) quartz-pyrite±arsenopyrite; (3) quartz-base metal sulfide; and (4) quartz-carbonate. Fluid inclusions in quartz and calcite in vein/veinlet stockworks contain C-O-H fluids of three main types. The first type consists of dilute CO₂–H₂O fluids coeval with the early vein stage. Molar volumes of these CO₂–H₂O fluid inclusions, ranging from 50–60 cm³/mol, yield estimated minimum trapping pressures of 3 kbar. Homogenization temperatures, obtained mainly from CO₂–H₂O inclusions with lower CO₂ concentration, range from 267–375 °C. The second inclusion type, with a CO₂–H₂O±CH₄ composition, was trapped during the main mineralizing stages. These fluids may reflect the CO₂–H₂O fluids that were modified by fluid/rock reactions with altered wallrocks. Isochores for CO₂-H₂O±CH₄ inclusions, with homogenization temperatures ranging from 204–325 °C and molar volumes from 55 to 70 cm³/mol, provide an estimated minimum trapping pressure of 1.2 kbar. The third inclusion type, aqueous inclusions, trapped in cross-cutting microfractures in quartz and randomly in calcite, are post-mineralization, and have homogenization temperatures between 143–228 °C and salinities from 0.71–7.86 wt% NaCl equiv. Stable isotope data show that the metamorphic fluid contribution is minimal and that ore fluids are of magmatic origin, most likely sourced from 120–126 Ma mafic to intermediate dikes. This is consistent with the carbonic nature of the fluid, and the cross-cutting nature of those deposits relative to the host Mesozoic granitoid.Editorial handling: R.J. Goldfarb     

A mechanism for preferential H2O leakage from fluid inclusions in quartz,based on TEM observations

Preferential leakage of H₂O from fluid inclusions containing multiple gas components has been suspected in natural metamorphic rocks and has been demonstrated experimentally for synthetic H₂O-CO₂-rich inclusions in natural quartz. Knowledge of the physical and chemical characteristics of the leakage mechanism, which may be very complex, increases the value of natural fluid inclusions to metamorphic geology. It is proposed that crystal defects play a major role in nondecrepitative preferential H₂O leakage through quartz, and remain effective during metamorphism. Inclusions with either an internal overpressure or underpressure produce strain in the adjacent quartz crystal via the nucleation of many dislocations and planar defects (like Dauphiné twin boundaries). These defects allow preferential loss of H₂O from H₂O-CO₂-rich inclusions at supercritical conditions. The transport capacity of this leakage mechanism is enhanced by nucleation of small bubbles on defect structures. The nucleation of these bubbles seems to be a recovery process in strained crystals. Solubility gradients of quartz in water in a crystal with internally underpressurized inclusions may result in optical visible implosion halos in a three dimensional spatial arrangement, caused by the growth of small bubbles at the expense of the larger original fluid inclusion. Natural fluid inclusions from Naxos (Greece) are always associated with numerous interlinked dislocations. These dislocations may have been produced by plastic derormation or by crystal growth related processes (e.g. crack healing). The presence of small bubbles on these dislocations indicates that a similar leakage mechanism for H₂O must have occurred in these rocks.     

Evidence of magmatic CO2-rich fluids in peraluminous graphite-bearing leucogranites from Deep Freeze Range (northern Victoria Land,Antarctica)

Fine-grained peraluminous synkinematic leuco-monzogranites (SKG), of Cambro-Ordovician age, occur as veins and sills (up to 20–30 m thick) in the Deep Freeze Range, within the medium to high-grade metamorphics of the Wilson Terrane. Secondary fibrolite + graphite intergrowths occur in feldspars and subordinately in quartz. Four main solid and fluid inclusion populations are observed: primary mixed CO₂+H₂O inclusions + Al₂SiO₅ ± brines in garnet (type 1); early CO₂-rich inclusions (± brines) in quartz (type 2); early CO₂+CH₄ (up to 4 mol%)±H₂O inclusions + graphite + fibrolite in quartz (type 3); late CH₄+CO₂+N₂ inclusions and H₂O inclusions in quartz (type 4). Densities of type 1 inclusions are consistent with the crystallization conditions of SKG (750°C and 3 kbar). The other types are post-magmatic: densities of type 2 and 3 inclusions suggest isobaric cooling at high temperature (700–550°C). Type 4 inclusions were trapped below 500°C. The SKG crystallized from a magma that was at some stage vapour-saturated; fluids were CO₂-rich, possibly with immiscible brines. CO₂-rich fluids (±brines) characterize the transition from magmatic to post-magmatic stages; progressive isobaric cooling (T<670°C) led to a continuous decrease off _{O 2} can entering in the graphite stability field; at the same time, the feldspars reacted with CO₂-rich fluids to give secondary fibrolite + graphite. Decrease ofT andf _{O 2} can explain the progressive variation in the fluid composition from CO₂-rich to CH₄ and water dominated in a closed system (in situ evolution). The presence of N₂ the late stages indicates interaction with external metamorphic fluids.Contribution within the network Hydrothermal/metamorphic water-rock interactions in crystalline rocks: a multidisciplinary approach on paleofluid analysis. CEC program: Human Capital and Mobility     

The P-T-fO 2 stability of deerite,Fe 12 2+ Fe 6 3+ [Si12O40](OH)10

New equilibrium experiments have been performed in the 20–27 kbar range to determine the upper thermal stability limit of endmember deerite, Fe ₁₂ ²⁺ Fe ₆ ³⁺ [Si₁₂O₄₀](OH)₁₀. In this pressure range, the maximum thermal stability limit is represented by the oxygen-conserving reaction: deerite(De)=9 ferrosilite(Fs)+3 magnetite(Mag)+3 quartz(Qtz)+5 H₂O(W) (1). Under the oxygen fugacities of the Ni-NiO buffer the breakdown-reduction reaction: De=12 Fs+2 Mag+5 W+1/2 O₂ (10) takes place at lower temperatures (e.g. T=63° at 27 kbar). The experimental brackets can be fitted using thermodynamic data for ferrosilite, magnetite and quartz from Berman (1988) and the following 1 bar, 298 K data for deerite (per gfw): V^o=55.74 J.bar^-1, S^o=1670 J.K^-1, H _f ^o =-18334 kJ, =2.5x10^-5K^-1, =-0.18x10^-5 bar^-1. Using these data in conjunction with literature data on coesite, grunerite, minnesotaite, and greenalite, the P-T stability field of endmember deerite has been calculated for P _s=P _{H 2}O. This field is limited by 6 univariant oxygenconserving dehydration curves, from which three have positive dP/dT slopes, the other three negative slopes. The lower pressure end of the stability field of endmember deerite is thus located at an invariant point at 250±70°C and 10+-1.5 kbar. Deerite rich in the endmember can thus appear only in environments with geothermal gradients lower than 10°C/km and at pressures higher than about 10 kbar, which is in agreement with 4 out of 5 independent P-T estimates for known occurrences. The presence of such deerite places good constraints on minimum pressure and maximum temperature conditions. From log f _{O 2}-T diagrams constructed with the same data base at different pressures, it appears that endmember deerite is, at temperatures near those of its upper stability limit, stable only over a narrow range of oxygen fugacities within the magnetite field. With decreasing temperatures, deerite becomes stable towards slightly higher oxygen fugacities but reaches the hematite field only at temperatures more than 200°C lower than the upper stability limit. This practically precludes the coexistence deerite-hematite with near-endmember deerite in natural environments.     

Fluid geochemistry in the Ivigtut cryolite deposit, South Greenland

The 1.27 Ga old Ivigtut (Ivittuut) intrusion in South Greenland is world-famous for its hydrothermal cryolite deposit [Na₃AlF₆] situated within a strongly metasomatised A-type granite stock. This detailed fluid inclusion study characterises the fluid present during the formation of the cryolite deposit and thermodynamic modelling allows to constrain its formation conditions.Microthermometry revealed three different types of inclusions: (1) pure CO₂, (2) aqueous-carbonic and (3) saline-aqueous inclusions. Melting temperatures range between − 23 and − 15 °C for type 2 and from − 15 to − 10 °C for type 3 inclusions. Most inclusions homogenise between 110 and 150 °C into the liquid.Stable isotope compositions of CO₂ and H₂O were measured from crushed inclusions in quartz, cryolite, fluorite and siderite. The δ¹³C values of about − 5‰ PDB are typical of mantle-derived magmas. The differences between δ¹⁸O of CO₂ (+ 21 to + 42‰ VSMOW) and δ¹⁸O of H₂O (− 1 to − 21.7‰ VSMOW) suggest low-temperature isotope exchange. δD (H₂O) ranges from − 19 to − 144‰ VSMOW. The isotopic composition of inclusion water closely follows the meteoric water line and is comparable to Canadian Shield brines. Ion chromatography revealed the fluid's predominance in Na, Cl and F. Cl/Br ratios range between 56 and 110 and may imply intensive fluid–rock interaction with the host granite.Isochores deduced from microthermometry in conjunction with estimates for the solidification of the Ivigtut granite suggest a formation pressure of approximately 1–1.5 kbar for the fluid inclusions. Formation temperatures of different types of fluid inclusions vary between 100 and 400 °C. Thermodynamic modelling of phase assemblages and the extraordinary high concentration in F (and Na) may indicate that the cryolite body and its associated fluid inclusions could have formed during the continuous transition from a volatile-rich melt to a solute-rich fluid.     

Calculated fluid evolution path versus fluid inclusion data in the COHN system as exemplified by metamorphic rocks from Rogaland, south-west Norway

Abstract Fluid evolution paths in the COHN system can be calculated for metamorphic rocks if there are relevant data regarding the mineral assemblages present, and regarding the oxidation and nitrodation states throughout the entire P-T loop. The compositions of fluid inclusions observed in granulitic rocks from Rogaland (south-west Norway) are compared with theoretical fluid compositions and molar volumes. The fluid parameters are calculated using a P-T path based on mineral assemblages, which are represented by rocks within the pigeonite-in isograd and by rocks near the orthopyroxene-in isograd surrounding an intrusive anorthosite massif. The oxygen and nitrogen fugacities are assumed to be buffered by the coexisting Fe-Ti oxides and Cr-carlsbergite, respectively. Many features of the natural fluid inclusions, including (1) the occurrence of CO₂-N₂-rich graphite-absent fluid inclusions near peak M2 metamorphic conditions (927° C and 400 MPa), (2) the non-existence of intermediate ternary CO₂-CH₄-N₂ compositions and (3) the low-molar-volume CO₂-rich fluid inclusions (36–42 cm³ mol^?1), are reproduced in the calculated fluid system. The observed CO₂-CH₄-rich inclusions with minor N₂ (5 mol%) should also include a large proportion of H₂O according to the calculations. The absence of H₂O from these natural high-molar-volume CO₂-CH₄-rich inclusions and the occurrence of natural CH₄-N₂-rich inclusions are both assumed to result from preferential leakage of H₂O. This has been previously experimentally demonstrated for H₂O-CO₂-rich fluid inclusions, and has also been theoretically predicted. Fluid-deficient conditions may explain the relatively high molar volumes, but cannot be used to explain the occurrence of CH₄-N₂-rich inclusions and the absence of H₂O.     

Metamorphic fluid flow in the northeastern part of the 3.8-3.7 Ga Isua Greenstone Belt (SW Greenland): A re-evalution of fluid inclusion evidence for early Archean seafloor-hydrothermal systems

Fluid inclusions in quartz globules and quartz veins of a 3.8-3.7 Ga old, well-preserved pillow lava breccia in the northeastern Isua Greenstone Belt (IGB) were studied using microthermometry, Raman spectrometry and SEM Cathodoluminescence Imaging. Petrographic study of the different quartz segregations showed that they were affected by variable recrystallization which controlled their fluid inclusion content. The oldest unaltered fluid inclusions found are present in vein crystals that survived dynamic and static recrystallization. These crystals contain a cogenetic, immiscible assemblage of CO₂-rich (+H₂O, +graphite) and brine-rich (+CO₂, +halite, +carbonate) inclusions. The gas-rich inclusions have molar volumes between 44.8 and 47.5 cm³/mol, while the brine inclusions have a salinity of ∼33 eq. wt% NaCl. Modeling equilibrium immiscibility using volumetric and compositional properties of the endmember fluids indicates that fluid unmixing occurred at or near peak-metamorphic conditions of ∼460 °C and ∼4 kbar. Carbonate and graphite were precipitated cogenetically from the physically separated endmember fluids and were trapped in fluid inclusions.In most quartz crystals, however, recrystallization obliterated such early fluid inclusion assemblages and left graphite and carbonate as solid inclusions in recrystallized grains. Intragranular fluid inclusion trails in the recrystallized grains of breccia cementing and crosscutting quartz veins have CO₂-rich assemblages, with distinctly different molar volumes (either between 43.7 and 47.5 cm³/mol or between 53.5 and 74.1 cm³/mol), and immiscible, halite-saturated H₂O-CO₂-NaCl(-other salt) inclusions. Later intergranular trails have CH₄-H₂ (X_H2 up to ∼0.3) inclusions of variable density (ranging from 48.0 to >105.3 cm³/mol) and metastable H₂O-NaCl(-other salt?) brines (∼28 eq. wt% NaCl). Finally, the youngest fluid inclusion assemblages are found in non-luminescent secondary quartz and contain low-density CH₄ (molar volume > 105.33 cm³/mol) and low-salinity H₂O-NaCl (0.2-3.7 eq. wt% NaCl). These successive fluid inclusion assemblages record a retrograde P-T evolution close to a geothermal gradient of ∼30 °C/km, but also indicate fluid pressure variations and the introduction of highly reducing fluids at ∼200-300 °C and 0.5-2 kbar. The quartz globules in the pillow fragments only contain sporadic CH₄(+H₂) and brine inclusions, corresponding with the late generations present in the cementing and crosscutting veins. We argue that due to the large extent of static recrystallization in quartz globules in the pillow breccia fragments, only these relatively late fluid inclusions have been preserved, and that they do not represent remnants of an early, seafloor-hydrothermal system as was previously proposed.Modeling the oxidation state of the fluids indicates a rock buffered system at peak-metamorphic conditions, but suggests a change towards fluid-graphite disequilibrium and a logf_H2/f_H2O above the Quartz-Fayalite-Magnetite buffer during retrograde evolution. Most likely, this indicates a control on redox conditions and on fluid speciation by ultramafic rocks in the IGB.Finally, this study shows that microscopic solid graphite in recrystallized metamorphic rocks from Isua can be deposited inorganically from a fluid phase, adding to the complexity of processes that formed reduced carbon in the oldest, well-preserved supracrustal rocks on Earth.