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Helium isotopes are useful for tracing terrestrial water movement and calculating residence times, which have important implications for radioactive waste disposal and carbon capture and storage projects. Helium concentrations are generally measured directly in water samples; however, this is not always possible. In this scenario, the He concentration equilibrium between pore waters and He accessible volumes (e.g., fluid inclusions, hereafter HAV) in minerals (e.g., quartz) can be utilized to determine He concentrations. In this study, the He partial pressure was measured in HAVs of quartz grains collected from cores of the Kola Super Deep Borehole (KSDB) using isothermal He extraction/saturation experiments. A large fraction of He was released from these samples during the first interval of isothermal heating, indicating a destruction of the unstable HAVs, likely due to decrepitation of fluid inclusions. We present a new approach to calculate He partial pressures using only the He fraction released from the stable HAVs. This approach gives a He partial pressure pHe of ≅ 1.5 atm at depths of ~ 10 km. When combined with solubility data, this value suggests a high He concentration in the deep pore waters nearby the KSDB, ≅ 1.3 × 10−6 mol (cc H2O)−1, indicating a long residence time of He atoms in a deep water – mineral system, close to the age of the regional metamorphism (≅1700 Ma). The 3He/4He in the deep KSDB quartz samples (and the pore waters) are ~ 3 × 10−8, typical of radiogenic He generated in 10 km deep Archean rocks of the KSDB. Importantly, external fluxes are not needed to explain He isotope abundances in rocks, minerals and pore waters at these depths.

Natural waters of various origins exhibit systematic differences in their D/H and 18 O/ 16 O ratios. Several recent spectroscopic and isotopic studies indicate that in addition to molecular water from fluid inclusions isotopically shifted structurally bound water may contribute to the hydrogen isotope budget in hydrothermal minerals. To further clarify these effects, a combined FTIR and stable isotope study has been carried out on hydrothermal fluorite from fluorite–barite veins, Schwarzwald district, Germany. Water for δD analysis was extracted at two different temperatures from the same samples, at 400 °C and 650 °C. In one series of experiments, different sample aliquots were used for the runs at both temperatures, whereas in a second series a step heating technique was employed to extract water from the same sample aliquot. Compared to the results at 400 °C, the δD values obtained at 650 °C are systematically more negative for most of the samples. The isotopic shift ranges from a few permil up to as much as �55‰. FTIR spectroscopic measurements of the fluorites show that a broad absorption band at 3400 cm �1 is present, which results from O–H stretch vibrations. A band at 5200 cm �1 , which is related to a combination of bend and stretch vibrations from molecular H2O, was not found in the inclusion-free areas of the fluorites. The spectroscopic observations show that significant (but variable) amounts of structurally bound water are present in the hydrothermal fluorites. The results of our study indicate that most likely isotopically distinct fluid inclusion and structurally bound water contribute to the bulk δD signature. This calls for extreme caution in setting up appropriate analytical procedures for determination of δD values by thermal decrepitation and has considerable implications for the interpretation of paleofluid signatures. © 2007 Elsevier B.V. All rights reserved.

In the restitic crustal enclaves in the Neogene volcanics of El Hoyazo and Mazarron (SE Spain), associations of fluid and silicate melt inclusions indicative of immiscibility are frequently observed in the Bt-poor, Crd-rich graphitic metapelites. These occurrences, extremely rare for anatectic crustal rocks, have been studied by microthermometry, micro-Raman spectroscopy and EMP analysis. Both at El Hoyazo and Mazarron the immiscible FI and MI are hosted in plagioclase and cordierite, with microstructures that suggest primary trapping. The FI hosted in cordierite are monophase, whereas they may contain crystals of calcite when hosted in plagioclase, indicating fluid–host reaction during cooling. Decrepitation microstructures have not been observed. The MI contain fresh glass of peraluminous, felsic rhyolitic composition, typical of anatectic S-type melts. In all samples the fluids are CO2-dominated (N85 mol%), with minor amounts of N2 and CH4, and traces of CO and H2. The nucleation of graphite, induced by the laser beam in some inclusions, demonstrates that the fluids are graphite-saturated. Large scatter in microthermometric behaviour of FI indicates significant density variations. Only in one sample from El Hoyazo are fluid densities compatible with the estimated P–Tconditions of trapping (5–7 kbar, 850±50 °C), whereas in the remaining samples the extremely low densities, suggesting trapping pressures b1 kbar, are in contrast with the microstructural indications of primary entrapment and little evidence of decrepitation. In addition, although some measured compositions are compatible with C–O–H–N fluid speciation at the estimated P–T conditions of anatexis, the lack of H2O in all FI is inconsistent with the slightly hydrated character of coexisting glasses. The possible mechanisms accounting for departure of densities and compositions of FI from expected values are discussed, and leakage of H2O suggested as the most plausible.

Hydrogen isotope investigations on hydrothermal quartz reveal two H-reservoirs: (i) trapped fluid inclusions, and (ii) structurally bound water in homogenously distributed small clusters or bubbles. Varying mixing ratios of the two reservoirs are sampled by means of mechanical and thermal decrepitation applied to different grain size fractions. A two-component mixing calculation results in an isotopic characterisation of the two H-reservoirs, which fractionate hydrogen isotopes close to the known MOH–H 2 O system, with water being enriched in deuterium. Temperature controls both the internal fractionation as well as the abundance ratio of inclusion water to bonded water. At high temperatures, fractionation is small but bound water becomes more abundant, comprising a significant amount of water from both thermal and mechanical extraction techniques. Hence, the isotope composition of the extracted water does not reflect the original hydrogen isotope composition of the hydrothermal fluid especially at temperatures higher than 200°C. Previously reported δ D data of fluid inclusion, which were used to elucidate the origin of the hydrothermal fluid, tend to be too low.

Hydrothermal alteration and the active vapor-dominated geothermal system at The Geysers, CA are related to a composite hypabyssal granitic pluton emplaced beneath the field 1.1 to 1.2 million years ago. Deep drill holes provide a complete transect across the thermal system and samples of the modern-day steam. The hydrothermal system was liquid-dominated prior to formation of the modern vapor-dominated regime at 0.25 to 0.28 Ma. Maximum temperatures and salinities ranged from 440°C and 44 wt.% NaCl equivalent in the biotite hornfels adjacent to the pluton to 305°C and 5 wt.% NaCl equivalent at distances of 1730 m from the intrusive contact. The major, minor, and noble gas compositions of fluid inclusions in the hydrothermally altered rocks were integrated with microthermometric and mineralogic data to determine their sources and the effects of mixing and boiling. Major and minor gaseous species were released from the inclusions by crushing or thermal decrepitation; noble gases were released by crushing. The samples were analyzed by mass spectrometry. The analyses document the presence of magmatic, crustal, and meteoric components in the trapped fluids. Hydrothermal fluids present during the liquid-dominated phase of the system contained gaseous species derived mainly from crustal and magmatic sources. At The Geysers, N 2 /Ar ratios greater than 525 and 3 He/ 4 He ratios of 6–10.7 Ra are diagnostic of a magmatic component. Crustal gas has CO 2 /CH 4 ratios less than 4, N 2 /Ar ratios between 45 and 525, and low 3 He/ 4 He ratios (0.5 Ra). Meteoric fluids have CO 2 /CH 4 ratios greater than 4 and N 2 /Ar ratios between 38 (air-saturated water) and 84 (air). However, N 2 /Ar ratios between 15 and 110 can result from boiling. Ratios less than 15 reflect the incorporation of N 2 into NH 3 -bearing clay minerals. In the central Geysers, the incursion of meteoric fluids occurred during the transition from the liquid- to vapor-dominated regime. Variations in the relative CH 4 , CO 2 , and H 2 contents of the gas analyses demonstrate that boiling took place under open-system conditions. The gas data indicate that the inclusions have remained closed to the diffusion of He and H 2 since their formation.

Analysis of nano- to picomole quantities of gas released from microscopic fluid inclusions in minerals can be critical in determining details of the physical and chemical conditions under which mineral deposits formed. Such analyses can routinely be performed using quadrupole mass spectrometry of volatiles released by inclusion decrepitation as minerals are heated or crushed. However, steady-state calibration of mass spectrometric factors has proved inadequate to accurately quantify small amounts of inclusion gas mixtures analyzed under dynamic conditions. An on-line gas aliquot valve mass spectrometer calibrating system has been developed, using adaptation of an high-pressure liquid chromatography valve. By rapidly discharging controlled aliquots of standard gases or gas mixtures, dynamic calibrations simulate nearly all analytical parameters that affect volatiles released during mechanical rupture of fluid inclusions. Analytical sensitivity of the mass spectrometry system is non-linear in its dependence on partial pressure, an effect which could not be deduced using steady-state calibrations. After applying this additional correction, microsample measurements previously wildly in contrast with presumed trapping conditions now accurately reproduce fluid compositions known from geothermal well flow tests or bulk inclusion analyses.

Chemical analyses of hydrothermal quartz by means of instrumental neutron activation, coulometric, dewatering, and decrepitation analysis are combined with previously published microthermometric and isotopic data in order to unravel the processes involved in the formation of the post-Variscan hydrothermal system “Bavarian Pfahl”. The Bavarian Pfahl is interpreted as a fossil zone of tectonic activity and large-scale channelized fluid movement in the upper continental crust. The predominance of K relative to Ca and Na in quartz as well as in fluid inclusions strongly supports the hypothesis that incongruous dissolution of feldspar during wall-rock alteration and K-metasomatism during fluid ascent along the shear zone accompanied by intense silicification are characteristic features of important fluid pathways in the crust. The element contents detected in quartz along a 70 km long profile can be correlated with variable wall-rock compositions. High concentrations of Ba, As, Rb, Sr, and Cs are found in the vicinity of sedimentary basins at the western margin of the Bohemian Massif. Elevated element contents in this part of the profile probably result from intense water-rock interaction within the adjoining sedimentary basin. High Ba-contents in the vicinity of the fluorite-barite mining district of Nabburg/Wolsendorf indicate a genetic relationship between both mineralizations. Low Cl/Br-ratios can be generated either by interaction with carbonaceous sediments in these basins or by interaction with graphite in basement rocks. The calculated salinities of the fluid inclusions vary widely, but correlate well with microthermometric data, and indicate mixing of basement or sedimentary brines with high salinity and meteoric water with low salinity. The effects of polyphase cataclasis, a result of seismic pumping, on the fluid inclusion population and the mechanical properties of quartz have been studied by combined dewatering and decrepitation analyses. Despite minor regional variations in the chemical composition of quartz along the profile, the overall uniform element pattern supports homogeneity of the hydrothermal system over distances of at least 70 km and probably over the total length of about 150 km. The fluid volume required for the formation of the Bavarian Pfahl is estimated to correspond to the volume of the Baltic Sea (approx. 23,000 km3). In analogy with hydrothermal springs in the vicinity of recently active shear zones and faults the lifetime of the hydrothermal system “Bavarian Pfahl” is estimated to be about 160,000 years.

Further developments in the analysis of fluid inclusion decrepitates by inductively-coupled plasma emission spectroscopy (ICP) are outlined. Decrepitates from samples of quartz have been analysed and give useful results. As several mechanisms of transfer of material into the plasma are feasible it is important that the temperature of heating should be restricted to the temperature range of decreptiation. However, contamination by silicate phases in the quartz appears to be of little significance. Reproducibility in our analyses is usually better than 30% (relative standard deviation, RSD) and most of this appears to be due to heterogeneity in the abundance of fluid inclusions. We have been able to estimate the ore metal content of some hydrothermal fluids from SW England and have obtained values in the range one hundred to a few thousand parts per million for some elements (Cu, Pb, Zn, Sn and B). Because of the sensitivity of the method we feel that its greatest potential is in the field of routine mineral exploration, and we have had considerable success in this application.

Means of extracting and quantitatively analyzing volatiles in fluid inclusions have been studied and details of the methods developed are presented. Thermal decrepitation is the preferred method to open inclusions because experiments indicate that this method yields more accurate measurements of the sulphur-species gases and water. Two assemblages of inclusion species have repeatedly been detected in inclusions from mineral deposits. A CO 2 -dominated assemblage typically has been measured in inclusions from porphyry-type deposits. A CO 2 N 2 C n H n assemblage is typical of inclusion volatiles from sediment-hosted and epithermal deposits. The probable source of N 2 and organic compounds is biologic material which is consistent with a crustal source for the ore solutions. Inclusion volatiles from most ore deposits have 1–3 mole% gaseous species. However, levels of 5–15% have been measured in assemblages of vapor-dominant and liquid dominant inclusions from epithermal deposits. The interpretation is that vapor-dominant inclusions are rich in gaseous species. This implies that the indicated “boiling” of the ore solutions was not strictly aqueous boiling but more an unmixing of the gaseous components. Analytical data on volatiles in fluid inclusions have been used to calculate f O 2 , f S 2 , f CO 2 and pH of ore fluids, and these values agree with values calculated from other data. Data on gases in fluid inclusions have also been used to estimate trapping pressure and study paragenesis.

Fluorite and quartz samples from Oseja (Oviedo, Spain) and Sierra de Lujar (Granada, Spain) contain multiphase fluid inclusions with two immiscible liquids, a gas and a solid phase at room temperature. Studies by Raman scattering microspectrometry permitted: 1. (a) An unambiguous identification of native sulphur daughter solid. 2. (b) Determination of CO 2 H 2 S molar ratios in the gas phase and in the two liquids (aqueous solution and a mixture of liquefied CO2 and H2S). 3. (c) In H2S-rich inclusions, identification of a H2S hydrate (probably the clathrate H2S · 5.75 H2O), observed at low temperature. A check of internal consistency is given by the comparison of the theoretical ( CO 2 H 2 S ) gas ( CO 2 H 2 S ) water ratio (value = 3) and the observed ratio (mean value = 3.3). Homogenization of the gas and (CO2 + H2S) liquid occurs generally in the liquid phase. In the Lujar samples it occurs between +24° and 38°C and there is a fair correlation with H2S content. In the Oseja samples it occurs between +27° and +48°C, and the trend with composition is not clear. H2S hydrate was observed during some freezing runs in fluorite from Oseja and CO2 hydrate in samples from Sierra de Lujar. Because of decrepitation, total homogenization was observed only in small inclusions of the quartz from Lujar at above 300°C. The temperature of total homogenization corresponds to the vanishing of the sulphur droplet, which determines the paleofugacity of S2 in the fluid (10−3.6 bar). The main equilibria describing the composition of the fluid inclusions before total homogenization are the following: 1. (a) 2S /af S2 (buffering fS2) 2. (b) H 2 S + 1 2 O 2 /af H 2 O + 1 2 S 2 (buffering fO2). An example of calculation is given at +250°C. One may see that inclusions are just outside the graphite field and that CH 4 CO 2 is very low which explains that methane was not detected by Raman scattering microspectrometry. The origin of the sulphur-rich fluid at Sierra de Lujar is presently poorly understood. It could result from diagenetic processes, similar to those responsible for deposition of sulphur ores in the neighbouring Sierra de Gador.

Trace components in fluid inclusions from the Aberfoyle tin-tungsten deposit in Tasmania, Australia, were examined by thermal decrepitation-mass spectrometry. The technique involves the decrepitation of fluid inclusions by stepwise heating in the source of a mass spectrometer. In addition to water, other species identified were carbon dioxide and hydrocarbons. Detailed analysis of these hydrocarbons showed that they ranged in carbon number from C1 up to at least C33 and probably comprised, in part, biological marker compounds, previously unreported in such high-temperature environments. Their presence suggests that the fluid responsible for mineralisation was not entirely magmatic but contained a non-magmatic component evolved through fluid-rock interaction in the sedimentary country rocks.

The volatiles (CO 2 and H 2 O) from the inclusions in fluorite crystals from seven sites of the Mikhalkovo ore deposit were determined. Vacuum heating up to the decrepitation temperature (400°C) was used to extract the volatiles and they were later analysed mass-spectrometrically. The salinity of the mineral-forming solutions (⩽1%) was evaluated using freezing temperature data. On the basis of the analytical results and some geological data, and assuming an equilibrium state between hydrothermal solutions and carbonate, the pH-values of the fluorite-forming media were calculated. In the acidity of the solution vs. temperature plot, the points from the seven ore sites delineate the crystallization field of the Mikhalkovo fluorites. The different crystal habits pertain to different regions of this field.

Inductively coupled plasma atomic emission spectrometry (ICP) has been used to investigate the composition of fluid inclusions in samples of pegmatite topaz from Malaysia and carbonatite apatite from Kenya. In this preliminary study we describe a method whereby material ejected from the sample during decrepitation at elevated temperatures is transferred to the plasma by a stream of Ar. Up to 40 elements can be determined simultaneously in the resulting decrepitate. The method is rapid and permits the determination of a number of important elements which, in conventional fluid inclusion isolates, are present in a concentration so low that there are problems of detection even with sensitive methods such as ICP. A further advantage of the ICP-linked decreptiation method is the small sample size (∼ 0.5 g) normally required. Comparison of results obtained by the proposed method and the ICP analysis of leachates for the same sample shows that most elements give higher values of X Na by the decrepitation method. Possible explanations for this difference include: (1) selectivity of decrepitation as a means of nebulishing inclusion contents; (2) hydrolysis and precipitation or adsorption of metal ions during the leaching method; and (3) extraction of Na + ions from the host mineral during leaching. Regardless of these differences on-line decrepitation is a highly efficient method of nebulishing inclusion fluids for introduction into the plasma, and the method has considerable potential as a routine means of analysing inclusions.

The decrepitometric method of fluid inclusion study is based on the analysis of acoustic emissions associated with the breaking of fluid inclusions during the heating of mineral grains. The decrepitation behaviour of a mineral depends on its mechanical characteristics, the inclusion size distribution and the composition and density of the inclusion fluid. Where the size of the inclusions is greater than some critical dimension, which depends on the mineral (∼ 15 μm for quartz), there appears to be a broad correlation between the actual fluid inclusion populations and the decrepitometric populations. For such samples containing inclusion fluids with 0–25 wt.% NaCl and low gas content, corrections in the range − 75 ± 20°C to the means of Gaussians fitted to the decrepitometric data give good estimations of the mean temperatures of homogenization of the inclusion populations. For other inclusion size distributions and fluid compositions, particularly if the fluid contains a high gas content, the relationships between the temperatures of homogenization and decrepitation are less precise and estimated homogenization temperatures may not be particularly meaningful. Nevertheless samples may still be compared and classified on the basis of selected peak parameters derived from curve fitting. The decrepigram may therefore serve to “fingerprint” veins of different generations. The present procedures are compared and contrasted with those previously used by other workers.

A high-pressure heating-freezing stage constructed at our laboratory permits continuous microscopic observations of fluid inclusions during experimental runs in pressure-temperature paths (patent No. 85.025.12). Conditions currently attainable are − 180° to + 450°C and 1 to 900 bar. Runs were performed on fluid inclusions in fluorite to study the mechanism of stretching and deformation of the cavities by overheating under confining pressure. Confining pressures of 200, 410 and 620 bar were applied on aqueous (H2O, 15 eq.wt.% NaCl) cogenetic secondary inclusions and hydrocarbon primary inclusions, having volumes between 103 and 105μm3 in Tunisian fluorite from Djebel Guebli. In most cases leakage (deformation with loss of content, sometimes observed at 1 bar) was avoided under confining pressure and stretching (yielding with volume change) is the main mechanism observed. Combining our experiments with the law of stretching at 1 bar, we have tried to derive an empirical law of the pressure effect: P I s =P I s1 +KPc (mean value of K=07) Confining pressures permit to reach very high temperatures of stretching. The values of internal pressure at the stretching temperatures calculated from the isochore curves of the H2ONaCl system are between 850 and 1100 bar. The values of ΔTh increase regularly with the amount of overheating ΔToh; on the curves ΔTh = f(ΔToh) we observed two parts: in the first part, the value of Th is constant, that means that the internal pressure is not sufficient to induce a deformation of the mineral; and in the second part, the function increases in relation to plastic deformations of the mineral; the latter deformations are a function of the temperature. When an external overpressure is applied on a preliminary stretched inclusion, we observed a decrease of the new homogenization temperature (Th), that means a decrease of the inclusion volume. This reversible effect could be related to a mechanism of relaxation of the deformation. We have observed that decrepitation of fluid inclusions could also occur by rapid releasing of pressure as well as by overheating. This should be taken into account for interpretation of experiments in blind autoclaves. Those observations could be explained by a microfracturing effect connected with dislocations in a matrix with elasto-plastic deformation under change of temperature, and the possibility of strain relaxation.

The effects of feldspar and mica contamination on the analyses of fluid inclusion contents, in quartz from granitic rocks, are evaluated when the fluid inclusions are analysed by the decrepitation-inductively coupled plasma emission spectroscopy (D-ICP) method.