Abstract
Fluid inclusions and geological relationships indicate that rodingite formation in the Asbestos ophiolite, Québec, occurred in two, or possibly three, separate episodes during thrusting of the ophiolite onto the Laurentian margin, and that it involved three fluids. The first episode of rodingitization, which affected diorite, occurred at temperatures of between 290 and 360°C and pressures of 2.5 to 4.5 kbar, and the second episode, which affected granite and slate, occurred at temperatures of between 325 and 400°C and pressures less than 3 kbar. The fluids responsible for these episodes of alteration were moderately to strongly saline (~1.5 to 6.3 m eq. NaCl), rich in divalent cations and contained appreciable methane. A possible third episode of alteration is suggested by primary fluid inclusions in vesuvianite-rich bodies and secondary inclusions in other types of rodingite, with significantly lower trapping temperatures, salinity and methane content. The association of the aqueous fluids with hydrocarbon-rich fluids containing CH4 and higher order alkanes, but no CO2, suggests strongly that the former originated from the serpentinites. The similarities in the composition of the fluids in all rock types indicate that the ophiolite had already been thrust onto the slates when rodingitization occurred.
Highlights
Calc-silicate rocks associated with serpentinites are called rodingites, a name which was given originally by Bell et al [1] to lime-rich, coarse- to fine-grained, gabbro-like rocks composed primarily of grossular or prehnite in the vicinity of the Roding River in the Dun Mountain ultramafic complex, New Zealand
We present results from a detailed study of fluid inclusions in rodingitized slate, diorite and granite, and vesuvianite-rich veins from the JM Asbestos mine, Asbestos, Québec
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
Summary
Calc-silicate rocks associated with serpentinites are called rodingites, a name which was given originally by Bell et al [1] to lime-rich, coarse- to fine-grained, gabbro-like rocks composed primarily of grossular or prehnite in the vicinity of the Roding River in the Dun Mountain ultramafic complex, New Zealand. As is http://www.geochemicaltransactions.com/content/8/1/11 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. The observed distribution is most consistent with mixing accompanied by a minor loss of CH4 (Figure 8)
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