| Popis: |
The radiogenic noble gas isotope composition (e.g., 4 He*, 21 Ne*, 40 Ar*, where * denotes the radiogenic component) of crustal mineral grains was investigated to evaluate their utility as a geochemical tracer for characterizing subsurface processes and conditions of crustal fluid migration, specifically within black shales. Because the production of radiogenic noble gases can be predicted by measuring the relative abundance of parent isotopes (e.g., U, Th, K), the distribution of radiogenic noble gases that are retained in crustal minerals can provide important insight into the behavior of fluids in the crust. We hypothesized that the 4 He/ 21 Ne* may be particularly useful for these purposes because the production of each isotope is fixed at a nearly constant rate of 4 He/ 21 Ne*=22x10 6 in the crust. Nonetheless, following radiogenic/nucleogenic production, the rate of diffusional release of each radiogenic isotope (e.g., 4 He*, 21 Ne*) from mineral grains in the rock matrix can vary as a function of temperature, mineral phase, and potentially the rate and extent of fluid migration. As a result, measuring the 4 He/ 21 Ne* in fluids or rocks provides information on effective rock porosity, temperature, rate, and volume of fluid flow. Here, we examine the 4 He and 21 Ne* concentrations, and their isotopic ratio, in quartz grains from the Marcellus Shale (Appalachian Basin, USA). Samples include outcrop samples, sub-crop samples collected at differing deformation regimes (e.g., along faults), and from samples collected from the lateral legs of producing wells. Each sample type experienced differing rates and maximum thermal maturities associated with their geological burial histories. Our results reflect fractionation of 4 He and 21 Ne* according to the thermal maturity of each sample set as well as with respect to the proximity of deformational features. |
