George B. Skippen

Vapour loss (" Boiling") as a mechanism for fluid evolution in metamorphic rocks

The calculation of fluid evolution paths during reaction progress is considered for multicomponent systems and the results applied to the ternary system, CO2-H2O-NaCl. Fluid evolution paths are considered for systems in which a CO2-rich phase of lesser density (vapour) is preferentially removed from the system leaving behind a saline aqueous phase (liquid). Such “boiling” leads to enrichment of the residual aqueous phase in dissolved components and, for certain reaction stoichiometries, to eventual saturation of the fluids in salt components. Distinctive textures, particularly radiating growths of prismatic minerals such as tremolite or diopside, are associated with saline fluid inclusions and solid syngenetic salt inclusions at a number of field localities. The most thoroughly studied of these localities is Campolungo, Switzerland, where metasomatic rocks have developed in association with fractures and veins at 500° C and 2,000 bars of pressure. The petrography of these rocks suggests that fluid phase separation into liquid and vapour has been an important process during metasomatism. Fracture systems with fluids at pressure less than lithostatic may facilitate the loss of the less dense vapour phase to conditions of the amphibolite facies.

Halite and sylvite as solid inclusions in high-grade metamorphic rocks

Solid inclusions of halite and sylvite, formed during regional and contact metamorphism have been identified by microscopy and by electron microprobe analysis in rocks from Campolungo, Switzerland and Cornone di Blumone, Italy. The solid inclusions occur in several of the major minerals crystallized during metamorphism and have been observed as idiomorphic crystals and dendrites. The compositions measured in 100 analyses from Campolungo, Switzerland and 40 analyses from Cornone di Blumone, Italy extend across the two-phase region in the system, KCl-NaCl, indicating that the salt inclusions are high temperature precipitates. In both localities compositionally zoned and unzoned crystals have been found. Measured compositions on the temperature maximum of the two-phase region indicate at least 500° C which can be compared with 500°±20° C determined by Mercolli (1982) and Walther (1983) from the Mg content of calcites from Campolungo. The solid inclusions have been trapped apart from CO2-rich and saline, H2O-rich fluid inclusions which have been described by Mercolli (1982) as the earliest preserved fluid inclusions in the rocks. The early precipitation of salt minerals at Campolungo indicates that fluids were saturated with NaCl and KCl at 500° C and pressures of 2,000 bars or higher. Similar relationships exist between solid and fluid inclusions in the rocks of Cornone di Blumone which formed at temperatures as high as 800° C and pressures between 0.5 and 1 kilobar (Ulmer 1983). The entrapment of halite and sylvite as solid inclusions preserves the composition of the minerals which may therefore be useful as geothermometers.

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.

Synthetic and natural tremolite in equilibrium with forsterite, enstatite, diopside and fluid

The following equilibrium among tremolite forsterite, diopside, and orthorhombic enstatite has been investigated using either synthetic tremolite or natural amphibole in the starting materials: Ca2Mg5Si8O22(OH)2+Mg2SiO4 =2 CaMgSi2O6+5MgSiO3+H2O A significant increase in the stability of the reactants was observed with natural rather than synthetic tremolite. For example, in nearly pure H2O with the H2 content of the fluid buffered by nickel-bunsenite at one kilobar (108 pascals), the breakdown of the assemblage with synthetic amphibole occurs at 708±20° C. The breakdown of the assemblage with natural amphibole, Ca2.16Mg4.94Fe0.03Si7.92 Al0.01O22(OH)2F0.03 occurs at 841±47° C. The shift in the breakdown curve is attributed to variation in the properties of the amphiboles since all other factors were common in the experiments. The reactions have also been investigated with hydrogen fugacity defined by the methane buffer and the NB, OH (XG, COH) buffer. Analysis of the experimental data by linear programming indicates that the enthalpy of reaction is tightly constrained when the calorimetrically determined entropy of 160.92 joules/degree is used. The resulting enthalpy of reaction is 113.96±1.82 kilojoules with the natural amphibole and 104.83±0.12 kilojoules with synthetic tremolite. Deviation of the natural amphibole from the ideal tremolite formula as well as a greater number of defects and dislocations in the synthetic amphibole may have contributed to the change in stability.

F-OH substitution in natural tremolite, talc, and phlogopite

AbstractThe distribution of F between tremolite and talc has been determined in metamorphosed siliceous carbonates from the Grenville Province, Ontario. Wavelength dispersive electron microprobe analyses of contiguous, texturally compatible tremolite-talc pairs indicate that the substitution of F for OH is the most significant deviation from end-member stoichiometry in the samples studied. Mixing of F and OH components has been represented by an ideal solution model for F in tremolite and an asymmetric model for F in talc. Both linear and nonlinear regression techniques have been used to derive activity coefficients for the exchange of one equivalent of OH and F components in talc. The following expressions are the result of nonlinear regression of 32 analyses from coexisting mineral pairs:

Metasomatism Involving Fluids in CO2-H2O-NaCl

\begin{gathered} \ln \gamma _{TC(OH)} = X_{TC(F)}^2 [2.447 - 2.845X_{TC(OH)} ] \hfill \\ \ln \gamma _{TC(F)} = X_{TC(OH)}^2 [1.024 + 2.845X_{TC(F} ] \hfill \\ \end{gathered}

The influence of NaCl and KCl on phase relations in metamorphosed carbonate rocks

Isobaric