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 from the H₂O apex, and mixing of two aqueous fluids would produce a line parallel to the H₂O-NaCl tie-line provided that both fluids contained the same proportion of methane. The observed distribution is most consistent with mixing accompanied by a minor loss of CH₄ (Figure 8).

The most probable conditions of trapping of the inclusion fluids is the P-T region between the homogenization pressures, isochores limiting maximum and minimum pressures, and, for rodingitized diorite, granite and slate, the upper thermal stability of prehnite, which provides a maximum estimate of the temperature. The isochores for type 2a inclusions in samples Gran-1b and Slate-1a, and for type 2b inclusions in sample Dior-2 were calculated using the program GEOFLUIDS Model 1 and bulk molar volumes were estimated as described above. Although the P-T conditions associated with the earlier phase of rodingitization of the slates could not be determined, the lower homogenization temperatures of type 1 inclusions in sample Slate-2 suggest that they were trapped at pressures comparable to those trapped in diorite.

None of the mineral assemblages in the samples studied define invariant or univariant reactions in P-T space, for which the position can be estimated by correcting activities of end-member components for the effects of solid solution. There are also no suitable mineralogical geothermometers for these rocks. However, the upper boundary of the field of stability of prehnite can be used to constrain maximum trapping temperatures, as expressed by the decomposition/dehydration reaction

5Prehnite = 3Quartz + 2Grossular + 2Zoisite (or Clinozoisite) + 4H₂O

which has a negative slope in P-T space. The position of this univariant reaction was determined with the program package THERMOCALC v2.3 4243 assuming pure phases. Because grossular and zoisite do not have end-member compositions, and quartz is not present in the rodingites, the above phase boundary provides a maximum P-T limit for the projection of the isochores.

The position of the isochores for type 2 fluid inclusions in samples Dior-2, Gran-1b and Slate-1a, and of reaction (1) are shown on the P-T diagrams in Figure 9A and 9B. Also shown on this figure are the positions of the isochores for type 1 inclusions in zoisite and grossular from sample Dior-2 and in clinopyroxene from sample Slate-1a. The position of the isochores for type 1 inclusions was calculated using program ISOC 3031 and the equation of state for pure CH₄ of Duan et al. 3233. Volume corrections were performed for the compressibility and expansion of the clinopyroxene hosting type 1 fluid inclusions in sample Slate-1a.

As noted in the section discussing the gas chromatographic analyses, bulk fluid samples from rodingitized diorite, granite and slate contain an unidentified species that may form up to 9 mol % of the total volatiles excluding H₂O if this species is N₂. To test the potential error in estimation of trapping pressures introduced by assuming that only CH₄ is present in type 1 inclusions, we used the program BULK and the equation of state of Bakker 44 to calculate the molar volume of various mixtures of CH₄ with N₂ and H₂, and compared them with results obtained for pure CH₄. A value of -118°C was selected for the homogenization temperature, which represents the average for type 1 inclusions in sample Dior-2. The molar volumes were then used to calculate isochores using program ISOC. Our calculations suggest that trapping pressures would be overestimated by about 80–90 bar between 300 and 350°C for a fluid mixture with molecular proportions of 0.9 CH₄ and 0.1 N₂.

Molecular hydrogen is a common constituent of aqueous and gas phases in the serpentinization environment, and is produced during oxidation of iron 4546474849. Assuming the molecular proportions of CH₄ and H₂ in the fluid mixture trapped by type 1 inclusions were 0.9 and 0.1, respectively, the results of our calculations suggest that the trapping pressures would be underestimated by 85–110 bar between 300 and 350°C.

Potential errors involved in the calculation of the isochores in type 2a and type 2b fluid inclusions due to the presence of a gas other than CH₄ were also evaluated. Using data presented in Table 4, new fluid compositions were calculated using program CURVES and the equation of state of Duan et al. 37. For this purpose, a mixture in the non aqueous phase with molecular proportions of 0.9 CH₄ and 0.1 N₂ was used. For each measurement, the volume fraction of aqueous solution after final clathrate melting was kept the same as that determined for fluids for which CH₄ was the only dissolved gas species. The results indicate that the pressures may be underestimated by a maximum of approximately 80 bar for the mixture considered. Unfortunately, the method could not be applied to evaluate the effect of mixing H₂ with CH₄ because suitable equations of state for modeling clathrate involving a mixture of these volatiles in saline aqueous inclusions are not available. However, by analogy to the calculations involving 0.9 CH₄-0.1 N₂ and 0.9 CH₄-0.1 H₂ mixtures for type 1 inclusions, and 0.9 CH₄-0.1 N₂ for type 2 inclusions, the change in bulk molar volume of type 2 inclusions produced by a progressive increase in the proportion of H₂ is probably small. We consider that an error in the order of ± 100 bar is a reasonable approximation on the position of the isochores for type 1 and type 2 fluid inclusions.

Despite the compositional uncertainties considered above, it is reasonable to assume, based on the petrographic observations and the estimation of the position of the isochores, that type 1 and type 2b inclusions in Dior-2 were trapped in the same environment. For these two types of inclusions to have been trapped simultaneously, a process involving immiscibility must be considered. To verify this, we calculated the composition of the low density, CH₄-rich fluid phase that should coexist with type 2a inclusions at saturated vapour pressure (P_h in Table 4) using GEOFLUIDS. The calculated compositions were then used to calculate the volumes of water and methane in the low density phase at room temperature using BULK. The results indicate that between 4 and 8 vol. % of aqueous solution should be present in type 1 inclusions for the investigated total homogenization temperature interval of between 301 and 333°C for type 2a inclusions. Such small proportions of aqueous fluid would form a film that may appear invisible in the dark coloured type 1 inclusions. The homogenization temperature calculated for the methane in the CH₄-rich, low density fluid coexisting with the water-rich fluid having a composition corresponding to that calculated for the lower salinity type 2a fluid inclusion at 301°C, as reported in Table 4, is -117.5°C. This value agrees well with those measured for type 1 fluid inclusions in sample Dior-2. For this to be the case, the trapping pressure of type 1 and type 2a inclusions in this sample would be equal to the total homogenization pressure of type 2a inclusions, i.e., between ~2.5 and 3 kbar. In sample Dior-1, which does not contain type 1 fluid inclusions, the trapping pressure of type 2a inclusions would be above that of the total homogenization.

Isochores for type 1 inclusions in Slate-1a cross the calculated P-T field of type 2a inclusions in sample Gran-1b and the calculated isochore for type 2a inclusions in sample Slate-1a. These inclusions are considered to have been trapped in a different environment to that in which type 1 inclusions in rodingitized diorite were trapped.

Based on the data analysis presented above, we estimate that pressure conditions accompanying rodingitization of diorite (samples Dior-1 and Dior-2) were between 2.5 and 4.5 kbar, and temperatures were between the minimum total homogenization temperature (286°C) and 360°C, the temperature at which the isochore projected from the highest homogenization temperature crosses reaction (1). These conditions are similar to those estimated by Laird et al. 50 from a study of calc-silicate layers in serpentinized ultramafic north of Richmond, Québec. Based on thermobarometric calculations using TWEEQU 51 on the assemblage pumpellyite + zoisite + prehnite + grossular + chlorite (quartz absent), they proposed maximum pressures between 3 and 5 kbar and maximum temperatures between 300 and 400°C. Pressures accompanying the formation of veins cutting pervasively rodingitized slate (sample Slate-1a) and rodingitization of the granite (sample Gran-1b) are interpreted to have been between 0.6 and 3 kbar; the temperature was potentially as high as 410°C.

The relatively high temperature of rodingitization of the granite raises the possibility that deserpentinization may have occurred, albeit locally, by reaction of serpentine with brucite to form forsteritic olivine by the reaction

Chrysotile + Brucite = 2Forsterite + 3H₂O

or that temperature was buffered by this reaction. This is suggested by the fact that this reaction boundary is crossed by the isochore for sample Slate-1a and the lower temperature isochore of the P-T field for sample Gran-1b, whereas the higher temperature isochore for sample Gran-1b lies entirely in the forsterite field (Figure 9B; note that for natural forsteritic olivine from ultramafic rocks, temperatures for reaction (2) are shifted by 10°C to the left; Moody 52).

Because the fluid inclusions in the vesuvianite samples contain significant proportions of an unknown species, no attempt was made to estimate the entrapment pressure of type 2c inclusions. However, if the two type 1 inclusions observed in vesuvianite were trapped at the same time as the type 2c inclusions, their homogenization temperature of -90°C would suggest a minimum pressure of entrapment of ~1 kbar.

The fluid inclusion and petrographic data indicate that there were probably three episodes of rodingitization. The earliest episode corresponded to rodingitization of diorite and probably of slate at high pressure, and was followed by a second episode of rodingitization during which there was rodingitization of granite and further rodingitization of slate. The vesuvianite-rich "veins" formed during the latest stages of ophiolite emplacement. The data thus define an early high pressure, and a later, lower pressure regime of alteration. The pressure and temperature conditions estimated from type 2b (and coexisting type 1) fluid inclusions are consistent with stability fields calculated using the GEO-CALC 535455 and THERMOCALC v2.3 4243 software packages for the assemblages with which they occur in the system CMASH. The interpreted trapping temperatures of fluid inclusions in sample Gran-1b (340 – 400°C; supported by occurrence of the assemblage hydrogrossular + clinopyroxene + wollastonite + vesuvianite in sample Gran-2c, which has a lower thermal stability of ~400°C at 1 to 2 kbar 565758) and the total homogenization temperatures of "vein" sample Ves-2 (T_h 180–203°C) suggest that vesuvianite formed over a very wide range of temperatures.

Because the granites must have intruded the serpentinites before rodingitization of the slates, the lower temperatures of the vesuvianite-rich "veins" must indicate a very late stage of rodingitization, recrystallization and intense veining. If the unidentified gas species was N₂, its presence in the vesuvianite-rich "veins" in proportions up to twice those in the other rodingites might indicate a progressive increase in the contribution of a fluid derived from sedimentary rocks of the Laurentian continental margin or of meteoric waters.

The emplacement history of the Québec Appalachian ophiolites is complex and a clearer picture of this history has emerged from recent geological and geochronological studies that are consistent with the obduction history of other ophiolites that evolved in similar tectonic settings throughout the world. It is now well-established, based on U-Pb zircon age dating of oceanic plagiogranites and granitic rocks of continental affinity intruded in the ophiolites, on Ar-Ar cooling age determinations of amphibolite soles at the basal thrust of the ophiolites, and on continental-margin metasediments in tectonic contact with the ophiolites, that the latter were obducted very soon after their formation and that the granites were emplaced during obduction on the Laurentian margin during the Ordovician 596061.

At the suggested time of intrusion of the felsic intrusive rocks through the base of the ophiolite sequences, between 475 and 465 Ma 59, the only rock unit that existed above the ophiolite was the Saint-Daniel Formation, which comprises, among other lithologies, pelagic sediments deposited on the seafloor 2162. Southeast of Asbestos, this formation is < 6 km thick. The combined thickness of this sedimentary unit (assuming it represents the original thickness) and the underlying igneous crust (2 km) indicates that the minimum depth of emplacement of the felsic intrusive rocks in the Asbestos ophiolite was 8 km, corresponding to a lithostatic pressure of > 2.2–2.4 kbar, i.e., just below the range of pressures estimated from the fluid inclusion data in rodingitized diorite.

Our calculated P-T conditions for the trapping of fluids in rodingitized diorite are intersected by the hotter P-T-t paths estimated by Whitehead et al. 59 during obduction of the Thetford-Mines ophiolite and generation of granitic rocks by melting of rock units underlying the young and hot mantle rocks at the base of the thrust. We suggest that P-T conditions similar to those for the Thetford-Mines ophiolite developed during obduction of the Asbestos ophiolite and that the high density fluids in type 1 and type 2a inclusions were responsible for rodingitization of diorite during this time (Figure 10).

The similar trapping temperatures but lower trapping pressures determined for the fluids in the rodingitized granite and slate samples imply that rodingitization also occurred near the final stages of emplacement and deformation of the Asbestos ophiolite. The relatively low, greenschist grade metamorphism that affected the slates at the base of the Asbestos ophiolite, the fact that these slates are rodingitized, and the evidence for thrusting followed by normal movement along the basal contact (Alain Tremblay, pers. comm. 2005), all suggest that the saline and Ca-rich fluids in type 2b inclusions interacted with the host rocks later than was the case for the less saline fluids represented by type 2a inclusions. The kinematic indicators for both thrust and normal faulting (striations) are superimposed on the blackwall replacing the slates between the rodingite and the serpentinized ultramafic rocks, and on the serpentinites at the contact.

A precise age for this event can not be given. However, based on the geological evidence presented above and ⁴⁰Ar/³⁹Ar dating of muscovite in the Caldwell sediments at the base of the Asbestos ophiolite 61, we can state confidently that type 2b fluids were trapped after granite intrusion at 465–475 Ma., and between Taconic metamorphism at 461–468 Ma. and extensional deformation which followed during the Silurian at 430–410 Ma. 6263.