THERMAL CONSTRAINTS ON CRUSTAL RARE-GAS RELEASE AND MIGRATION - EVIDENCE FROM ALPINE FLUID INCLUSIONS

The Palfris marl of the Helvetic Alpine Nappes contains four distinct vein fill generations. CH4-rich gas is found in abundant fluid inclusi ons within these carbonate veins, while free CH4 gas has also been pro duced from exploratory boreholes through this formation. The stable is otope and helium, neon, and argon isotopic composition of these fluids has been determined. A constant radiogenic Ar-40 concentration of 1.2 5 +/- 0.13 (1 sigma) ppm in these differently sited fluids requires an intimate association between the Ar-40rad source and the hydrocarbon phase. This can only be reasonably explained if the Ar-40rad was input into the hydrocarbon phase during hydrocarbon generation, migration, or storage prior to entrapment in the fluid inclusions. Stable isotope results constrain the maturity of hydrocarbon production, while fluid inclusion formation pressures and temperatures record values of up to 2.5 kbars and 250-degrees-C. These values place limits on the range o f thermal conditions in which the hydrocarbon/Ar-40rad relationship wa s established. All fluids within inclusions also contain radiogenic He -4/Ar-40 values at predicted crustal production ratios. These observat ions provide the first evidence that both He-4rad and Ar-40rad can be quantitatively released on a regional scale bounded by the thermal con ditions required to produce the hydrocarbon phase and the conditions u nder which the fluid inclusions were formed (T = 190-250-degrees-C). T hese results require that negligible quantities of excess Ar-40rad, de coupled from He-4rad, have been released into this system. Given the w ide array of mechanisms which can potentially cause decoupling of thes e two species, this result provides an important constraint on the rol e of these processes within the sedimentary fluid regime. In contrast, the free borehole gas contains excess radiogenic He-4 and Ne-21, rela tive to Ar-40rad, in proportions which can be accounted for by local p roduction and subsequent diffusion from the surrounding marl. The latt er pattern is consistent with rare gas migration in lower temperature environments. A conceptual model which considers both diffusional and metamorphic release of helium and argon, and the ability of the surrou nding fluid regime to transport the rare gases from their respective m ineral production sites, is consistent with both these results and dat a from regional rare gas studies.