Flow of Mantle Fluids Through the Ductile Lower Crust: Helium Isotope Trends

B. M. Kennedy, M. C. van Soest
2007 Science  
Heat and mass are injected into the shallow crust when mantle fluids are able to flow through the ductile lower crust. Minimum 3He/4 He ratios in surface fluids from the northern Basin and Range province, western North America increase systematically from low, crustal values in the east to high, mantle values in the west, a regional trend that correlates with the rates of active crustal deformation. The highest ratios occur where the extension and shear strain rates are greatest. The
more » ... est. The correspondence of helium isotope ratios and active trans-tensional deformation indicates a deformation enhanced permeability and that mantle fluids can penetrate the ductile lithosphere in regions even where there is no significant magmatism. Superimposed on the regional trend are local, high-3 He/ 4 He anomalies signifying hidden magmatic activity and/or deep fluid production with locally enhanced permeability, identifying zones with high resource potential, particularly for geothermal energy development. Mantle volatiles, principally water and C 2, play an important role in lithospheric rheology and the production of buoyant fluids th t can be injected into the shallow crust. Regional and local trends in the crustal occurren e of mantle volatiles provide insight into the coupling between mantle-crust tectonics (1,2), h at and mass exchange between the mantle and crust (3-5) and the occurrence and distribution 0 economic resources, such as ore minerals and oil, gas and geothermal fluids (6). Mantle-derivcl:i volatiles in the crust are traceable through helium isotopic compositions of hydrologic fluid (7). Mantle helium once injected into a crustal
doi:10.1126/science.1147537 pmid:18048684 fatcat:5wmfghimczaxvpegoea7dwfaey