The thinner dykes of rodingitized diorite (< 15 cm) are generally composed of grossular-clinopyroxene-predominant assemblages, whereas the mineral assemblages in the thicker ones (> 2 m) also include zoisite and prehnite. The mineral assemblages observed in the pervasively altered diorites include grossular + clinopyroxene + chlorite, grossular + clinopyroxene, grossular + clinopyroxene + zoisite or grossular + clinopyroxene + zoisite + prehnite and zoisite + clinopyroxene. Veins cutting the pervasively rodingitized diorites contain highly variable proportions of the same phases present in the host rock (e.g., Dior-2b*; Table 1). Where they cut diorite, which has not been pervasively rodingitized, the veins measure up to 8 cm thick and are prehnite-, feldspar-(either albite or K-feldspar) or more rarely grossular-rich. A rim of prismatic to acicular clinopyroxene crystals is commonly observed on the walls of these veins, whereas the core is filled with coarse-grained Ca-Al-silicate minerals. In addition to these veins, there are also veinlets composed of dark brown vesuvianite and chlorite cutting grossular + clinopyroxene rodingites (Table 1; Dior-3).

Sample Dior-1 (Table 1) represents an example of the smaller, completely rodingitized diorite dykes. The dyke is 8 cm thick and shows blackwall margins that are in sharp contact with the host serpentinized harzburgites. This contact is not deformed. The rodingite is composed of fluid inclusion-bearing grossular and clinopyroxene, and minor interstitial proportions of chlorite-group minerals.

Sample Dior-2 (Table 1) was collected from a pervasively rodingitized margin of a 3.84 m thick diorite dyke. The serpentinites in contact with the dyke were sheared and replaced by antigorite. A thin, less than 2 cm thick, biotite-bearing blackwall zone of alteration occurs in the dyke at the contact with serpentinite. The diorite was completely rodingitized to a pinkish zoisite + diopside + garnet ± prehnite assemblage over a distance of 7 cm adjacent to the blackwall, grading sharply into strongly rodingitized diorite with relict biotite up to 18 cm from the contact with serpentinite. Fluid inclusions are common in zoisite and grossular.

Two contrasting types of mineral assemblage are present in the two dykes of rodingitized granitic rock investigated in this study. The first dyke is a pegmatitic leucogranite in which the assemblage chlorite >> grossular + prehnite locally forms a thick zone of blackwall-type alteration near its contact with serpentinite (Table 1; Gran-1a), whereas the pervasively rodingitized granite is composed of a coarse-grained assemblage of grossular + clinopyroxene (Table 1; Gran-1b) which contains abundant fluid inclusions.

The second dyke (large block; provenance in the mine unknown) is a biotite granite which was mylonitized at the margins prior to alteration. The first four cm of the dyke from the contact with serpentinite were altered to chlorite + hydrogrossular + clinopyroxene (Table 1; Gran-2a), and are followed by a six cm wide zone composed of wollastonite + vesuvianite + clinopyroxene + hydrogrossular (Table 1; Gran-2b, -2c), and a 35 cm wide zone of bleached, albite and K-feldspar-dominant rock. Fluid inclusions suitable for microthermometric measurements were not identified in the samples.

Rodingitization of slate was preceded by an episode of sodic-calcic alteration, which produced a halo of albite-tremolite-biotite rock extending up to two meters from the contact with serpentinite. Relict patches of this type of alteration are commonly found in the rodingitized slate, which forms a zone ~60 cm thick between albitized slate and a ~10 cm thick zone of blackwall-altered slate immediately adjacent to the serpentinites. The original sedimentary textures were preserved during albitization and rodingitization.

At one location, pervasive rodingitization of the slate produced a fine-grained calc-silicate mineral assemblage composed of 1) colorless hydrogrossular + zoisite + clinopyroxene ± prehnite ± K-feldspar at the contact with the blackwall (Table 1; Slate-1a) and 2) colorless hydrogrossular + prehnite + clinopyroxene ± K-feldspar further from this contact (Table 1; Slate-1b). The rodingitized slate adjacent to the blackwall is cut by numerous veins that contain fluid inclusion-rich orange grossular and clinopyroxene crystals measuring up to 5 mm in diameter, and lesser proportions of space-filling prehnite practically devoid of fluid inclusions. No fluid inclusions were observed in the fine-grained mineral assemblages that compose the pervasively altered slate.

At one other location, a 20 cm zone of slate in contact with blackwall was pervasively replaced by a porous assemblage of coarse-grained, fluid inclusion-bearing clinopyroxene and small proportions of prehnite (Table 1; Slate-2).

Vesuvianite-rich veins are concentrated at the base of the cumulate dunite-pyroxenite unit near the transition into the tectonized harzburgite. They commonly display a very high vug porosity (visually estimated at up to 25%). Many of the veins have thick haloes of diopsidized serpentinite, which are cut by vesuvianite-rich veinlets. Mineral parageneses include white grossular + green and purple vesuvianite + clinopyroxene, vesuvianite + clinopyroxene, vesuvianite + wollastonite and vesuvianite + clinopyroxene + chlorite (see Table 1).

Two types of primary fluid inclusions were recognized from microscopic examination of the samples at 25°C. Type 1 inclusions consist of a single fluid phase and type 2 inclusions of two immiscible fluids (an aqueous fluid and a carbonic phase forming a bubble). A primary origin for these inclusions was inferred based on their distributions as three dimensional groups or occurrence as comparatively large, isolated inclusions. Where fluid inclusions formed planar arrays within or cutting across crystals, they were interpreted to be of secondary origin.

Type 1 inclusions measure up to 20 μm in diameter, commonly appear very dark under the microscope and have variable shapes (irregular, tubular and rectangular). They are abundant in sample Dior-2, where they occur as three dimensional clusters in zoisite and grossular. However, they were not found in sample Dior-1. Type 1 fluid inclusions also occur as three dimensional clusters in clinopyroxene in the rodingitized slate. They are rare in sample Slate-1a (an isolated group composed of four inclusions was observed) and common in sample Slate-2. Only two type 1 fluid inclusions were detected in vesuvianite from the vesuvianite-rich veins (Ves-1), where they formed an isolated group. The occurrence of type 1 inclusions in zoisite from sample Dior-2 is illustrated in Figure 2.

The two-phase type 2 inclusions occur in all the rodingite-forming minerals. They are generally prismatic in clinopyroxene (Figure 3B and 3E) and vesuvianite (Figure 3F), and oriented parallel to the c axis (and cleavage in clinopyroxene). In garnet and zoisite, they are prismatic, equant or irregularly shaped (Figure 3A and 3D). Their diameter varies between < 1 μm and ~50 μm (one tubular inclusion in garnet in sample Gran-1b measured 150 μm in length). The volume fraction occupied by the aqueous solution at room temperature in type 2 inclusions was visually estimated to be 84 ± 6% in rodingitized diorite, 86 ± 4% in rodingitized granite and slate, and approximately 95% in vesuvianite. Type 2 inclusions were considered primary when they were large and isolated, or occurred in tight three dimensional groups away from the contacts between grains. They were considered secondary when they occurred in planar arrays.

Type 1 and type 2 inclusions coexist in the same crystals in grossular and zoisite from one of the rodingitized diorite samples (sample Dior-2; Figure 2B and 3A). In rodingitized slate, type 1 and type 2 fluid inclusions were observed in the same crystals only in sample Slate-2. The two type 1 fluid inclusions observed in vesuvianite (sample Ves-1) were in close spatial association with type 2 inclusions.

Gas chromatographic analyses of the inclusion fluids in samples Slate-1a, Gran-1b, Dior-2 and Ves-1 confirmed the presence of CH₄. The latter formed the principal carbonic species (> 90 mol % excluding water) in the volatile mixtures. Hydrocarbons detected in addition to CH₄ are C₂H₆, C₃H₈, n-C₄H₁₀ and n-C₅H₁₂. Carbon dioxide was not detected in any of the samples.

Small amounts of N₂ or CO or Ar or H₂ were also detected. However, co-elution of N₂, CO, Ar, and H₂ did not permit differentiation between these species by gas chromatographic analysis using He as the carrier gas. Results of gas chromatographic analyses for selected rodingite samples are presented in Table 3. Also included in this table are values for N₂ assuming this species represents the unknown gas. The values for H₂O reported in Table 3 should be considered qualitative as a good calibration curve for this species could not be obtained. The lower H₂O/CH₄ ratio in sample Dior-2 is consistent with the abundant occurrence of type 1 inclusions in the sample.

Assuming that the unidentified gas is N₂, XN2 MathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacH8akY=wiFfYdH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8kuc9pgc9s8qqaq=dirpe0xb9q8qiLsFr0=vr0=vr0dc8meaabaqaciaacaGaaeqabaqabeGadaaakeaacqqGybawdaWgaaWcbaGaeeOta40aaSbaaWqaaiabikdaYaqabaaaleqaaaaa@305C@ in the bulk fluid (excluding H₂O) would be ~0.05 and 0.07 for samples Dior-2 and Gran-1b, respectively, ~0.09 for sample Slate-1a, and up to ~0.35 for vesuvianite. The similarity of these values among the samples of rodingitized felsic intrusive rocks and slates suggests that the difference in the inferred internal pressure of type 2a and type 2b inclusions is not due to large variations in the proportion of the volatiles, but is real. However, the proportions of this unidentified species in the samples from vesuvianite-rich veins, which contain type 1 and type 2c inclusions, are significantly higher.

Details on the analytical procedures and results, and a discussion of the origin of the hydrocarbons will be presented elsewhere.

Microthermometric and bulk gas chromatographic data indicate that type 1 inclusions in rodingitized diorite and slate are composed predominantly of CH₄. Although a rim of aqueous solution could not be detected optically, this does not mean that it is not present in type 1 inclusions. Very low proportions of an aqueous solution in the form of a thin film on the walls of an inclusion may appear invisible. The above notwithstanding, the molar volume of type 1 inclusions was calculated from their homogenization temperature assuming, as a first order approximation, that they are composed of only one phase and that this phase is pure CH₄. The calculations were performed using the program BULK 3031 and the equation of state for pure CH₄ of Duan et al. 3233.

The mole fraction of components in type 2 fluid inclusions was estimated using the programs CURVES (version 12/02) and ICE (version 12/02), implemented in the package of programs CLATHRATES 3034. Fugacity calculations based on clathrate equilibria and molar volumes of the aqueous solutions at the temperature of final clathrate melting were estimated using equations of state in Duan et al. 3233. The program DENSITY could not be used as clathrate always formed in the fluid inclusions before a vapour bubble nucleated in the CH₄ phase 3034. The program ICE could be used only for fluid inclusions in vesuvianite containing a low density gas phase 30.

Owing to the large errors inherent in the visual estimation of the bubble volume fraction in fluid inclusions (especially if they are irregularly shaped), we used a method to estimate the volumetric properties that involved application of the programs CURVES and GEOFLUIDS model 1 (available at http://geotherm.ucsd.edu; 323334353637). The method consisted of first determining the mole fractions of the inclusion components and their molar volumes for a range of liquid proportions relative to fluid inclusion volume using CURVES. Compositional data determined using CURVES were then used in GEOFLUIDS to obtain molar volumes for the inclusions at CH₄ saturation (i.e., at the temperature of total homogenization). The correct composition of a specific inclusion corresponded to that for which the molar volumes calculated using the two programs corresponded. This method also allowed determination of the minimum trapping pressure. Lamb et al. 38 have shown that location of the solvus in the system CH₄-H₂O-NaCl calculated using GEOFLUIDS is consistent with their experimentally determined data at temperatures between 300 and 600°C, and pressures of 1 and 2 kbar 3839.

Program CURVES permits determination of inclusion composition provided the salt content is known. An assumption about the salt concentration in the aqueous phase was therefore necessary. For this, we used last ice melting data to obtain an equivalent wt.% NaCl. We realize that this method may overestimate the salinity as the presence of clathrate at final ice melting indicates that the aqueous phase is impoverished in water. In addition, first ice melting data indicate that the salts dissolved in the inclusions comprise a significant proportion of divalent cations. However, in the absence of additional compositional data, this was the best approach available to us. The only salt considered by GEOFLUIDS Model 1 is NaCl. Thus, the homogenization pressures determined using this program may be slightly overestimated.

We selected data from two representative type 2b fluid inclusions in sample Dior-2 for the calculation of fluid inclusion composition and total homogenisation pressure in rodingitized diorite. The first inclusion, with a final ice melting temperature of -14.2°C and a clathrate melting temperature of 9.1°C, is typical of a large proportion of the fluid inclusions. The volume fraction of aqueous fluid in this inclusion was visually estimated to be 86%. The second inclusion, with a final ice melting temperature of -22.4°C and a clathrate melting temperature of 3.7°C, is typical of the most saline inclusions in the sample. The volume fraction of aqueous fluid in this inclusion was also visually estimated to be 86%. Using the equation of Bodnar 40, the wt.% equivalent concentration of NaCl in the two fluid inclusions was calculated to be 18 and 24, respectively. The latter value slightly exceeds the range of salinities that can be reliably estimated with the equation (the eutectic temperature in the system NaCl-H₂O is -21.2°C) but is considered to be a close approximation. The final homogenization temperature could not be obtained for these fluid inclusions. However, as indicated above, type 2b inclusions in the sample homogenized between 286 and 333°C and at an average temperature of 312°C. Applying the method described above, the volume fraction of aqueous fluid at final clathrate melting for the first inclusion was calculated to be between 84 and 80%, corresponding to total homogenization temperatures of 301 (the lowest temperature for which the program GEOFLUID is designed) and 333°C, respectively. For the second inclusion, the volume fraction of aqueous fluid at final clathrate melting was calculated to vary between 85 and 81%. The composition and molar volumes of the fluid of the two inclusions calculated using the method are reported in Table 4. The molar fraction of methane is estimated to vary between 0.033 and 0.051.

Volumetric and compositional values for type 2a fluid inclusions in the rodingitized granite sample Gran-1b and the rodingitized slate sample Slate-1a were estimated in the same manner. A representative type 2a fluid inclusion was selected from data collected for sample Gran-1b with a final ice melting temperature of -23.8°C and a clathrate melting temperature of -6.1°C. As stated above, use of the program GEOFLUID is limited to the system H₂O-CH₄-NaCl. A final ice melting temperature of -23.8°C in the system H₂O-NaCl indicates salinities too elevated to be used in calculations using the program CURVES (where calculations are limited to fluids having salinities at or below that corresponding to the eutectic temperature of the H₂O-NaCl system), Volumetric and compositional data for the type 2a inclusion considered where thus extrapolated from a set of data calculated at salinities corresponding to the eutectic temperature and below. Again, a complete data set with final ice and clathrate melting, and total homogenization temperatures could not be compiled. The final homogenization temperatures of type 2a inclusions in sample Gran-1b, however, form a tight grouping at 344 ± 15°C. Calculations of composition and volume were therefore conducted for temperatures between 330 and 360°C. The results are reported in Table 4. A data set including the temperature of final ice and clathrate melting, and the final homogenization temperature was obtained for two type 2a includions in sample Slate-1a. The final homogenization temperatures for these two inclusions were 300 and 322.5°C. Because the data set of homogenization temperatures has a mode at 319°C in this sample, calculations were performed only for the inclusion that homogenized at 322.5°C (Table 4). As is immediately apparent from inspection of Table 4, the calculated homogenization pressures are consistent with the lower density of fluid inclusions in samples Gran-1b and Slate-1a inferred above from clathrate melting temperatures and salinity relationships.

The behaviour of type 2c fluid inclusions in vesuvianite during cryogenic experiments indicates that these inclusions are of relatively low density and that the salt dissolved in the aqueous phase is predominantly NaCl. In the absence of detailed information on the nature of the salts dissolved in the fluid inclusions, we assumed, as a first order approximation, that the aqueous phase contained only NaCl. Based on estimates of the bubble volume fraction and ice and clathrate melting temperatures, the program ICE yields molar fractions of CH₄, H₂O and NaCl of 0.01, 0.97 and 0.02, respectively, assuming methane is the only gas present. Using these data, an homogenization pressure of less than 1000 bar was obtained using the equation of state of Duan et al. 41.

As can be observed in Figure 6, type 2b fluid inclusions have a wide range of salinities from less than 10 wt.% to approximately 25 wt.% eq. NaCl. A variation in salinity could be produced by progressive separation of a methane-rich fluid during cooling, consumption of water during serpentinization, or mixing between more and less saline fluids. Fractional phase separation would produce a distribution of data points moving (approximately) away from the CH₄ apex, consumption of water due to serpentinization would produce a distribution of data points moving away fr