Helium Isotropic Constraints on Terrestrial Degassing and the Transcrustal Transport of Mantle Methane. First Annual Report July 1, 1987-June 30, 1988

摘要

The research project is directed towards an evaluation of the crustal degassing process, its length- and time-scale, and its geologic and tectonic controls. The improved understanding of the crustal porefluid transport processes uncovered in the project will provide new, insight with regard to the traditional gas resources and the techniques employed to assess their potential sources and their accumulation in geologic traps. Previous measurements of the crustal degassing flux of He have been substantiated on a global scale with an atmospheric budget. Continental crustal degassing dominates the Helium 4 input to the atmosphere and a steady-state atmosphere requires a degassing flux equivalent to the whole crustal production. Measurements of the 40Ar degassing rate suggest an equivalent flux but indicate that the degassing fluxes of Helium 4 and 40 Ar are strikingly decoupled. Thus, the crustal degassing and transport processes are likely to be both tectonically and lithologically controlled. A model for continuous whole crustal degassing suggests that such a process can only be achieved by advective/convective transport in the crust in agreement with recent views on the extent and dynamics of metamorphic processes. A discontinuous process must occur even more rapidly. Large-scale fluid transport in the continental crust that is both geologically rapid and ubiquitous is strongly suggested. Further examination of the dynamics of Helium 4 release and transport in the continental crust may provide the insight to hydrocarbon accumulation and continental fluid processes that Helium 3 has provided for oceanic hydrothermal processes.