Estimation of the pressure of entrapment of the primary Pbrine + PCH4 inclusion assemblage using intersecting isochores on a pressure–temperature section for the NaCl–H2O system (position of relevant fields and phase boundaries taken from Sourirajan and Kennedy 1962; Bodnar 1994; Atkinson 2002). Note that the determination of pressure of entrapment for the Pbrine + PCH4 inclusion assemblage is an approximation only because we have applied isochore calculations for the pure NaCl–H2O system to a complex mixed salt system. Isochores for the coexisting Pbrine inclusions cannot be determined for the majority of inclusions because the inclusions homogenize by halite dissolution and vapor disappearance temperatures are considerably lower than Th; for those inclusions, isochores in the H+L field of the NaCl–H2O system are not constrained. However, Tv≈Th in several inclusions (inclusion #s ju22a07, ju22a08, ju22a10, ss02a03, ss03a04 and ss03b07; see Electronic Supplementary Material). Isochores for the listed inclusions originate near the intersection of the halite liquidi and bubble curves (for inclusions ranging from 31.1 wt% and 34.63 wt% eq. NaCl) and lie primarily in the L-only field. For the listed inclusions, isochores were calculated using Eq. 12 from Zhang and Franz (1987), the inclusion salinities, and values of Th; the isochores define the grey field labeled “brine isochores”. Most PCH4 inclusions homogenized by critical behavior near the critical T for pure CH4 (−2.6°C); the density of the CH4 phase is close to critical density (0.162 g/mL) and the isochore for the CH4 phase is relatively shallow (labeled “methane critical isochore”). The maximum pressure of entrapment (Ptrapping) for those inclusions is constrained by the intersection of the critical CH4 isochore with the brine inclusion isochores and is ∼500 bar. For comparison, the range of Ptrapping (1.1–1.5 kbar) for the primary assemblage where the assemblage contains only the brine phase (Molnar et al. 2001). This would suggest that a minimum fluid decompression of ∼600 bar resulted in brine–CH4 immiscibility
Graphically (Fig. 13 ), we estimate by the intersection of
brine and CH 4 isochores that the pressure of entrapment
for the P brine + P CH4 assemblage
could not have exceeded ∼500 bars, a pressure considerably
lower than the estimate of Molnar et al.
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