Ronald J. Bakker

Package FLUIDS 1. Computer programs for analysis of fluid inclusion data and for modelling bulk fluid properties

The computer package FLUIDS contains five sets of computer programs written in C++ for the calculation of fluid properties. Both model fluids and data from fluid inclusions can be analysed in the programs. Bulk density, molar volume and composition of fluid inclusions are calculated with the program BULK, which uses microthermometric data, micro-Raman spectroscopic data, cation ratios and volume-fraction estimations as input. The program offers a broad variety of equations of state, which are selected according to the fluid system or the temperature and pressure conditions of interest. The isochores that correspond to the output or test V−x properties of fluid inclusions are calculated by the program ISOC. In calculating the isochores, this program takes account of the compressibility and expansion of selected host minerals. The program TEST allows different equations of state to be compared directly with experimental data, thereby facilitating selection of the most adequate model in the programs BULK and ISOC. Loner is a group of programs that handle individual equations of state (EoS) for all fluid systems presented in the programs BULK and ISOC. Aqso is a group of programs that allows purely empirical modelling of electrolyte-bearing aqueous solutions, Henrys law for dilute gas-bearing aqueous solutions, and osmotic coefficients for dilute electrolyte-bearing aqueous solutions. The programs are available on the University of Leoben web site at http://www.unileoben.ac.at/~buero62/minpet/ronald.

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.

Experimental post-entrapment water loss from synthetic CO2-H2O inclusions in natural quartz

Artificial fluid inclusions were hydrothermally synthesized by crack healing in natural Brazilian quartz. Two original experiments E421 and E679 with a H2O-CO2 fluid were carried out at 835 K, 200 MPa, over 38 days, and at 856 K, 211 MPa, over 35 days, respectively. In both experiments homogeneous three-phase (a vapor and two liquids) fluid inclusions were synthesized with 22 and 20 mol% CO2, respectively. The CO2 phases homogenize to the vapor phase at 302.2 ± 0. l K (E679, cores 1 and 2), and to the liquid phase at 303.6 ± 0.3 K (E421, core 3), 303.9 ± 0.2 K (E421, core 4). The CO2-H2O phases homogenize to the vapor phase at 573.6 ± 0.4 K (E679, core 1 and 2), 575.6 ± 1.5 K (E421, core 3), and 576.1 ± 0.8 K (E421, core 4). Micro cracks and the new hydrothermally precipitated quartz, which directly surrounds the inclusions, were studied with TEM and SEM. The healed cracks have numerous growth imperfections that provide many possible routes for fluid transport. Dislocation arrays and small channels were observed and are often connected to the inclusions. Quartz cores 3 and 4 were subsequently re-equilibrated for 21 and 27 days respectively under hydrothermal conditions with pure H2O, at both a lower pressure of 100 MPa (E463) and a higher pressure of 365 MPa (E490) than that of the original experiment E421. The temperatures of the re-equilibration experiments were equal to the original (835 K). In E463, the internally overpressured re-equilibration, only traces of solution and precipitation of quartz were evident with minor transformation of the angular walls to more rounded forms. Volume increase for some inclusions resulted from decrepitation. The homogenization of the CO2-H2O phases to the gas phase occurred at higher temperatures, up to 604 K. In E490, the internally underpressured re-equilibration, major solution and precipitation resulted in the transformation of irregular shaped inclusion walls and formation of secondary inclusions halos. The homogenization of the CO2-H2O phases to the gas phase occurred at lower temperatures, down to 565 K. The fluids in inclusions from both re-equilibration experiments were found to have lower densities than the original fluids synthesized. This is quantified by the increased volumetric proportions of CO2 vapor. The CO2 fraction in inclusions was found to have increased, by up to 54 mol%. The change in homogenization temperatures and the decrease in the proportions of H2O in the original inclusions favours a model in which preferential transport of H2O occurs along mobile dislocation lines, small intercrystalline nanocracks, and/or channels. Results from experiment V1 and V4, using cores 1 and 2, indicate that the changes observed in the re-equilibration experiments are not artifacts of the experimental method.

ADAPTATION OF THE BOWERS AND HELGESON (1983) EQUATION OF STATE TO THE H2O-CO2-CH4-N2-NACL SYSTEM

Abstract The equation of state developed by Bowers and Helgeson ([Bowers, T.S., Helgeson, H.C., 1983. Calculation of the thermodynamic and geochemical consequences of nonideal mixing in the system H 2 O–CO 2 –NaCl on phase relations in geological systems: equation of state for H 2 O–CO 2 –NaCl fluids at high pressures and temperatures. Geochim. Cosmochim. Acta, 47, 1247–1275.] and [Bowers, T.S., Helgeson, H.C., 1985. Fortran programs for generating fluid inclusion isochores and fugacity coefficients for the system H 2 O–CO 2 –NaCl at high pressures and temperatures. Computers and Geosciences, 11, 203–213.]), which was originally designed for H 2 O–CO 2 –NaCl fluids, has been extended to CH 4 and N 2 bearing fluids. Available experimental P – V – T – X data in the H 2 O–CO 2 –CH 4 –N 2 –NaCl fluid system are accurately reproduced by this equation of state, and, therefore, isochores and fugacity coefficients can be accurately calculated up to 1000 MPa and 1300 K. This equation of state cannot be used in and near immiscibility regions and critical points, like any modified Redlich–Kwong equation of state. The empirical modifications allow isochore construction for realistic geological fluid systems which are often found in fluid inclusions, and which involve several gases and salts.

Improvements in clathrate modelling: I. The H2O-CO2 system with various salts

Abstract The formation of clathrates in fluid inclusions during microthermometric measurements is typical for most natural fluid systems which include a mixture of H 2 O, gases, and electrolytes. A general model is proposed which gives a complete description of the CO 2 clathrate stability field between 253–293 K and 0–200 MPa, and which can be applied to NaCl, KCl, and CaCl 2 bearing systems. The basic concept of the model is the equality of the chemical potential of H 2 O in coexisting phases, after classical clathrate modelling. None of the original clathrate models had used a complete set of the most accurate values for the many parameters involved. The lack of well-defined standard conditions and of a thorough error analysis resulted in inaccurate estimation of clathrate stability conditions. According to our modifications which include the use of the most accurate parameters available, the semi-empirical model for the binary H 2 O-CO 2 system is improved by the estimation of numerically optimised Kihara parameters σ = 365.9 pm and ɛ/k = 174.44 K at low pressures, and σ = 363.92 pm and e/k = 174.46 K at high pressures. Including the error indications of individual parameters involved in clathrate modelling, a range of 365.08–366.52 pm and 171.3–177.8 K allows a 2% accuracy in the modelled CO 2 clathrate formation pressure at selected temperatures below Q 2 conditions. A combination of the osmotic coefficient for binary salt-H 2 O systems and Henrys constant for gas-H 2 O systems is sufficiently accurate to estimate the activity of H 2 O in aqueous solutions and the stability conditions of clathrate in electrolyte-bearing systems. The available data on salt-bearing systems is inconsistent, but our improved clathrate stability model is able to reproduce average values. The proposed modifications in clathrate modelling can be used to perform more accurate estimations of bulk density and composition of individual fluid inclusions from clathrate melting temperatures. Our model is included in several computer programs which can be applied to fluid inclusion studies.

Estimation of volume fractions of liquid and vapor phases in fluid inclusions, and definition of inclusion shapes

Abstract The molar volume (Vm) and chemical composition (x) of saline aqueous inclusions and gas inclusions in minerals can be calculated satisfactorily from microthermometric and other analytical data. For complex gas-bearing aqueous inclusions, however, calculation of Vm-x properties requires additional input of the volume-fractions of the inclusion phases (φ). Traditional estimation of φ in non-fluorescing inclusions involves measuring area-fractions of the phases projected in the microscope and then making rough corrections for the third dimension. The uncertainties in the results are unknown and therefore the accuracies of the calculated Vm-x properties are also unknown. To alleviate this problem we present a new, routine method to estimate φ using the petrographic microscope in conjunction with a spindle-stage. Inclusions in normal thick-sections are rotated stepwise and their projected areas and area-fractions are plotted against rotation angle. The resulting data arrays are systematically related to inclusion orientation, to inclusion shape, and to φ. The dependency on orientation is minimized when area fractions are measured at the position where the inclusions project their largest total areas. The shape dependency is accounted for using a new objective classification of inclusion projections, based on parameters from digital image processing. The method has been verified with synthetic fluid inclusions of known φ. For individual liquid + vapor inclusions with regular (not .negative-crystal.) shapes, the new procedure yields φ with a relative accuracy of ±4%. This degree of accuracy permits Vm . x properties of gas-bearing, aqueous fluid inclusions to be calculated with sufficient certainty for many geochemical applications. Even better accuracy (e.g., down to ±0.6%) can be obtained by combining results from several inclusions in the same homogeneously trapped petrographic assemblage.

Determination of the composition and molar volume of H2O-CO2 fluid inclusions by microthermometry

Abstract H 2 O-CO 2 fluid inclusions are common in nature but their accurate analysis by microthermometry has been hindered by insufficient characterisation of the fluid properties along the solvus (high temperature miscibility boundary) of the system. Thus some of the previous methods of calculating fluid inclusion composition ( x ) and molar volume ( V ) have relied on notoriously unreliable optical estimates of phase volume fractions. Although the T-V-x properties of the solvus can be modelled thermodynamically from equations of state, a review shows that none of the published equations are accurate enough to be useful in fluid inclusion microthermometry. Therefore, the available experimental data have been used to construct a new V-x diagram, contoured for phase transition temperatures and phase volume fractions. This diagram allows the bulk composition and molar volume of individual fluid inclusions to be determined accurately from two microthermometric measurements: the temperature of partial homogenisation of the carbonic phases (in the presence of excess aqueous liquid), and the temperature of total homogenisation.

Hot dolomites in a Variscan foreland belt: hydrothermal flow in the Cantabrian Zone (NW Spain)

Abstract The Cantabrian Zone (CZ) in NW Spain represents the foreland belt of the Variscan Iberian Massif. It consists of aPrecambrian basement covered by Palaeozoic sediments. These underwent intense thin-skinned tectonics, diagenetic to epizonal thermal events, and several episodes of fluid flow causing large-scale hydrothermal dolomitization. Aim of this research is to trace the carbonate diagenesis in the Carboniferous Barcaliente and Valdeteja Formations in the Bodon Unit, and to define type and origin of the dolomitizing fluids. Employed methods include petrography, cathodoluminescence (CL), XRD, stable isotopes and fluid inclusion (FI) microthermometry/Raman spectrometry. The dolomitizing fluid was possibly hot (100 to 150 °C), saline, Mg-rich modified seawater, operating in a burial environment. It is assumed that the dolomitization occurred during late- to post-Variscan extensional phases. Main pathways for the fluidswere the Variscan thrust and fault planes, as well as stratification/lamination joints of the host limestones. One of the main tectonic lineaments, the Leon Line, played an effective role for fluid circulation, as reflected by the highest temperatures and often almost complete dolomitization close to this fault. Extensional tectonics may have promoted a gravity driven flow of fluids, which circulated deeply down, underwent heating and depletion in 18 O and dolomitized the primary carbonates.

Fluid inclusions as metamorphic process indicators in the Southern Aravalli Mountain Belt (India)

Abstract Fluid inclusions from a biotite-garnet schist in the Southern Aravalli Mountain Belt (India) give information on both peak metamorphic conditions and post-peak metamorphic processes during uplift. A combination of careful petrography, microthermometry and Raman spectroscopy reveals the presence of at least five generations of enclosed fluids. Lower amphibolite-facies pressure-temperature conditions of the growth of garnet rims are reproduced by the highest fluid density of the relatively oldest inclusion type of CO2 (±N2)-rich compositions. A calculated fluid composition in the COH system, in equilibrium with the graphite buffer corresponds to a CO2-rich fluid at metamorphic conditions. However, the results of these calculations are very sensitive to small fluctuations in oxygen fugacity and the accuracy of thermodynamic properties of mineral equilibria. Re-equilibration, conceived by specific size-density distribution and the absence of an aqueous phase in inclusions that contain nahcolite crystals, is monitored in these inclusions as post-peak metamorphic processes, like partial decrepitation and preferential leakage. The other fluid types represent heterogeneous fluid trapping of coexisting aqueous NaCl-bearing solutions with CO2-CH4-rich vapour bubbles in healed cracks, and probably the introduction of external fluids containing high salinity aqueous CaCl2-rich solutions in nearly pure N2 vapour bubbles, at lower P-T conditions. This study illustrates that fluid inclusions remain a valuable database of peak metamorphic conditions, moreover, alterations of the entrapped fluids and surrounding crystals are illustrative for specific exhumation evolutions.

Can the vapour phase be neglected to estimate bulk salinity of halite bearing aqueous fluid inclusions

The bulk salinity cannot be directly obtained from the dissolution temperatures of halite in highly saline fluid inclusions that contain solid, liquid, and vapour at room temperature. At least two of the following independent parameters must be determined to estimate the bulk composition and density of these inclusions: 1. dissolution temperature of halite in the presence of vapour; 2. total homogenization temperature of liquid and vapour; and 3. volume fraction of the vapour phase. A new Vm-x diagram for phase stabilities in the H2O-NaCl system has been constructed to obtain these bulk fluid properties from inclusions that homogenize liquid and vapour phase at higher temperatures than dissolution of halite.