Larryn William Diamond

Stability of CO2 clathrate hydrate + CO2 liquid + CO2 vapour + aqueous KCl-NaCl solutions: Experimental determination and application to salinity estimates of fluid inclusions ∗☆

The dissociation temperature of CO2 clathrate hydrate (Tmcla) in the presence of CO2 liquid + CO2 vapour + aqueous KCl-NaCl solutions has been determined by microthermometry of fluid inclusions synthesized in quartz. The reliability of the experimental method as a means to examine clathrate equilibria in this four-phase (Q2) assemblage has been confirmed by reproducing independent results in the literature on the NaCl-CO2-H2O system. Regression fitting of the new experimental measurements and interpolation with published NaCl-CO2-H2O data allows the stability of CO2 clathrate in the Q2 assemblage of the KCl-NaCl-CO2-H2O quaternary system to be described as follows: wt% (KCl + NaCl) = 15.5296 + 4.2947 XKCL + 0.5061 (XKCL)2 −1.0709TmCLA − 0.4751 TmCLA − 0.048(TmCLA)2 (1) where wt% (KC1 + NaCl) denotes the total salt concentration in the aqueous liquid (with respect to the salt-H2O subsystem), TmCLA is in °C, and XKCL denotes the relative weight fraction of salts: wt% KCl/ (wt% KCl + wt% NaCl). This equation is valid over the range 0–19 wt% (KC1 + NaCl), and 0–1 XKCL When coupled with fluid inclusion KCl/NaCl ratios analyzed by methods other than microthermometry, Eqn. (1) yields compositions in terms of individual electrolyte concentrations. In natural fluid inclusions, depending on their bulk compositions and densities, clathrate may dissociate as part of the Q2 assemblage (for which ThCO2>TmCLA), or in either of two three-phase assemblages (for which ThCO2<TmCLA). The new experimental data on the Q2 assemblage, and those in the literature, may be applied to natural three-phase assemblages as well. This is enabled by the following functions which relate TmCLA, and ThCO2 measured in the metastable absence of clathrate, to equivalent wt% NaCl in the salt-H2O subsystem of the aqueous liquid. For inclusions in which CO2 homogenizes via a bubblepoint transition (ThCO2<TmCLA): eq. wt% NaCl = 15.6151 − 0.03627x + 0.00164x2 − 0.9491y − 0.00287xy − 0.02464y2 − 0.00107xy2 − 0.00222y3. (2) For inclusions in which CO2 homogenizes via a dew-point transition (ThCO2dew < TmCLA): eq. wt% NaCl = 15.5131 + 0.065705x − 0.00778x2 − 1.05135y + 0.02687xy − 0.04717y2 + 0.00138xy2 − 0.00411y3 (3) where x denotes ThCO2 between −20 and + 10°C, y denotes TmCLA, and eq. NaCl concentration is between 0 and 21 wt%. Equations (2) and (3) thus permit salinities to be estimated for the large class of natural KCl-NaCl-CO2-H2O fluid inclusions in which ThCO2is higher than the clathrate nucleation temperature.

Review of the systematics of CO2–H2O fluid inclusions

Abstract Aqueous solutions that contain volatile (gas) components are one of the most important types of fluid in the Earths crust. The record that such fluids have left in the form of fluid inclusions in minerals provides a wealth of insight into the geochemical and petrologic processes in which the fluids participated. This article reviews the systematics of CO 2 –H 2 O fluid inclusions as a starting point for interpreting the chemically more complex systems. The phase relations of the binary are described with respect to a qualitative P – T – X model, and isoplethic–isochoric paths through this model are used to explain the equilibrium and non-equilibrium behaviour of fluid inclusions during microthermometric heating and cooling. The P – T – X framework is then used to discuss the various modes of fluid inclusion entrapment, and how the resulting assemblage textures can be used to interpret the P – T conditions, phase states, and evolution paths of the parent solutions. Finally, quantitative methods are reviewed by which bulk molar volume and composition of CO 2 –H 2 O fluid inclusions can be determined from microthermometric observations of phase transitions.

Salinity of multivolatile fluid inclusions determined from clathrate hydrate stability

Abstract Measurements of the final dissociation temperature of gas-clathrate hydrates (TmCLA) are routinely used to determine the salinity of fluid inclusions which contain a volatile component in addition to water. Traditionally, experimental data are used to quantitatively relate TmCLA to the inclusion electrolyte concentration. Because of limitations in the experimental database, however, this method has hitherto not been applicable to the multivolatile fluid inclusions that are common in crustal rocks. A general solution to this problem is provided by statistical thermodynamics predictions of multivolatile clathrate stability. Published theoretical models explicitly account for the effect of aqueous NaCl in depressing the stability of clathrates composed of any mixture of CO2, N2, H2S, CH4 and higher-order hydrocarbons. Analysis of phase relations in complex clathrate systems shows that such theoretical predictions yield model salinities if the following fluid inclusion data are available: (1) the identity of the phase assemblage at TmCLA, (2) the relative concentrations of the volatile species, and (3) either the homogenization temperature of the volatile fluid fraction (bubble point or dew point, either stable or metastable), or an independent estimate of internal pressure at TmCLA. Additional data on fluid inclusion cation ratios can be incorporated in the calculations to recast equivalent weight percent aqueous NaCl in terms of effective electrolyte concentrations. New experimental data on mixed N2-CO2 clathrates, obtained from synthetic fluid inclusions, provide a test of both the model predictions and of the analytical procedure proposed for natural fluid inclusions. While the accuracy of the predictions varies between volatile compositions, the uncertainties in the salinities derived from the statistical thermodynamics method are generally of the order acceptable for geochemical applications. Applications to multivolatile, multi-electrolyte fluid inclusions from gold-quartz deposits illustrate the practical operations involved in determining salinity.

Thermodynamic description of aqueous nonelectrolytes at infinite dilution over a wide range of state parameters

Abstract A new, virial-like equation of state (EoS) for describing the thermodynamic properties of aqueous nonelectrolytes at infinite dilution is proposed. It is based on the accurate EoS for a solvent (H 2 O) given by Hill (1990) and requires only three empirical parameters to be fitted to experimental data, and these are independent of temperature and pressure. Knowledge of the thermodynamic properties of a pure gas, together with these three parameters, enables prediction of the whole set thermodynamic properties of the solute at infinite dilution (chemical potential, entropy, molar volume, and apparent molar heat capacity) over a wide range of temperatures (0 to 500°C) and pressures (1 to 2000 bars), including the near-critical region. In the cases in which experimental thermodynamic data are lacking, the empirical parameters can be estimated solely from the known standard-state properties of the solute. The new EoS is compatible with the Helgeson-Kirkham-Flowers model for aqueous electrolytes, and thus it can be applied to reactions involving minerals, gases, and aqueous ions, in addition to uncharged species.

Mesothermal gold veins and metamorphic devolatilization in the northwestern Alps: The temporal link

A long-standing debate surrounds the source of hydrothermal fluids that form gold lodes late in the metamorphic history of orogenic belts. 40 Ar/ 39 Ar dating of hydrothermal muscovite shows that gold lodes in the northwestern Alps formed from 31.6 to 10.6 Ma. The deposits young progressively along the orogen, parallel to field gradients showing decreasing age of peak metamorphism, increasing metamorphic grade, and increasing extent of exhumation of the host rocks. These correlations suggest that the ore-bearing fluid was produced by a 20 m.y. history of prograde metamorphic devolatilization of calc-schists at depth. The age trend of mineralization reflects continuous ascent of fluid and vein formation in retrograding rocks during extreme differential uplift and denudation of the Alpine orogenic wedge. The intimate correlations between collisional orogenesis, metamorphism, and gold mineralization in the Alps suggest that similar genetic scenarios may apply to late orogenic gold lodes elsewhere.

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.

Elemental analysis of individual fluid inclusions in minerals by Secondary Ion Mass Spectrometry (SIMS): Application to cation ratios of fluid inclusions in an Archaean mesothermal gold-quartz vein☆

Abstract A SIMS technique using a CAMECA-4F Ion Microprobe has been developed to quantitatively analyse element ratios of individual, preselected fluid inclusions as small as 3 μm in diameter and as dilute as 0.37 mol% salts. The method, first attempted by Nambu et al. (1977), is destructive; fluid inclusions are opened under vacuum by sputtering through the host crystal with a 12.5 KeV beam of O − ions. Emitted secondary ions are analysed in a mass spectrometer, their intensities being proportional to the concentration of parent elements in the target inclusion. Whereas Nambu et al. (1977) analysed frozen inclusions, K Na and Ca Na ratios have been obtained in this study from inclusions in the liquid state, with a relative reproducibility ( 2σ x ) of 50–65 and 80–200%, respectively. The secondary ion yields of Na + , K + , and Ca + have been calibrated empirically using fluid inclusions synthesized with bulk salinities of 0.37–3.3 mol% and K Na and Ca Na ratios of 0.005–1.0. Thus individual gold-bearing fluid inclusions in quartz from the San Antonio Archaean gold deposit, Bissett, SE Manitoba, yield K / Na = 0.036 + 0.04/−0.02 and Ca / Na = 0.034 + 0.07/−0.03. The excellent spatial control of the primary ion beam permits individual inclusions to be opened without interfering with adjacent inclusions. The most powerful application of the method is therefore in the analysis of natural samples that contain multiple generations of fluid inclusions.

RbSr isotopic analysis of fluid inclusions in quartz: Evaluation of bulk extraction procedures and geochronometer systematics using synthetic fluid inclusions

Abstract Analysis of Rb Sr isotopic and elemental signatures of fluid inclusions is technically demanding, but it offers enormous potential in elucidating the timing, sources, and geochemical reaction paths of fluids in the Crust. Fluid inclusions of known isotopic and elemental ratios have been synthesized in quartz to serve as control samples. With these standards, two previously used fluid extraction procedures for bulk quartz samples have been evaluated and improved: crushing in a mortar followed by leaching and thermal decrepitation followed by leaching. Our experiments on quartz show that 87Sr/86Sr analysis is straightforward using both methods, regardless of the leaching solution employed. However, analysis of accurate 87Rb/86Sr ratios is not trivial. Leaching with pure water is only 93% efficient and yields elevated ratios owing to failure to inhibit surface adsorption of Sr. Leaching with a La3+-doped acid solution solves this problem; it is 99.7% efficient and yields accurate ratios. Crushing is the only fluid liberation method which delivers the correct 87Rb/86Sr values. In contrast, thermal decrepitation yields 87Rb/86Sr ratios which are quite reproducible, but exclusively incorrect. If inert sample containers are used, falsely elevated ratios are obtained at all decrepitation temperatures due to three concurrent extraction artefacts. However, if silica glass tubes are used, reaction of Rb with the glass at temperatures above ∼ 500°C falsely lowers the 87Rb/86Sr signatures. We, therefore, suspect all published isotopic analyses of fluid inclusions obtained by thermal decrepitation to be in error. The trace contents of Rb and Sr in the host quartz crystals are measurable and have markedly different Rb/Sr ratios compared to the hydrothermal parent fluid. Thus analyses of leachates and host-mineral residues by the crushing method allow accurate ages to be calculated with certainties of an order which are useful for geochronology. Since fluid extraction levels vary between samples, the residues define an array on an isochron diagram, whereas the leachates define a unique point. These systematic characteristics explain hitherto poorly understood analyses in the literature. Because they represent mixing-lines, such isochrons carry only the significance of two-point ages which require independent verification of constant initial 87Sr/86Sr. Thus their ultimate relevance to geology hinges on whether the analysed fluid inclusions can be identified petrographically as being primary.

Mesothermal gold lodes in the north-western Alps A review of genetic constraints from radiogenic isotopes

Identifying the source of the hydrothermal fluid responsible for mesothermal gold lodes in orogenic belts has proven to be a formidable hurdle. As a consequence, several key aspects of the genesis of this worldwide class of deposits remain poorly understood. This article reviews a wide spectrum of published data on the Monte Rosa Gold District, a belt of mesothermal gold lodes in the Alpine orogen. The data include Sr-, Pb- and He-Ar isotopes, 40 Ar/ 39 Ar chronology, fluid-inclusion compositions, mineralogy, and the geological framework of the deposits. It is demonstrated that simultaneous consideration of several radiogenic isotope systems is highly valuable in reconstructing the source characteristics and processes of ore deposition in open hydrothermal systems. The genetic model suggested by these data involves prograde metamorphic devolatilization of Mesozoic calcschists during mid-Tertiary continental collision and orogenic uplift. The liberated fluids scavenged gold from metabasites interlayered with the calcschists, then ascended to form auriferous quartz-carbonate-sulphide veins in rocks undergoing retrograde metamorphism. Metamorphic hydrothermal systems of this type recurred along the district over a period of at least 20 Ma, their location and timing being controlled by the progress of differential uplift of the north-western Alps. The nature of this “temporal continuum” of mineralisation contrasts with that reported for Archean gold-lode deposits. As well as clarifying aspects of gold-lode genesis, the isotopic approaches discussed have great potential to constrain scenarios of large-scale fluid flow – with or without mineralising potential – in orogenic belts of all ages.