TESTING FOR MAGMATIC CO2 DEGASSING ABOVE THE NORTH APPALACHIAN ANOMALY

Abstract

The North Appalachian Anomaly is a thermal anomaly inferred from shear wave velocities beneath New England and interpreted as a mantle plume. Possible evidence of magmatism is investigated here by analyzing soil CO2 flux emissions in areas surrounding regional springs in NY, MA, NH and VT. To test the presence of magmatism at depth, we measured the flux of diffusely degassing soil CO2 using an EGM-5 portable CO2 gas analyzer. The working hypothesis is that magmatic CO2, if present, will produce a CO2 degassing signature distinguishable from a shallow biogenic CO2 source. Areas where springs are present were sampled, because the water traveling from within the Earth’s crust, and sometimes deeper, exploit the easiest pathway to reach the surface, and we expect that CO2 would exploit similar paths. Data obtained thus far are unable to confirm a second high flux population that would support a possible magmatic component. The biogenic CO2 fluxes in this area are nonetheless quite high (mean CO2 flux of 26 g m-2 d-1), and similar to magmatic fluxes observed in regions of moderate magmatic CO2 degassing (e.g., Natron Basin, Tanzania). To investigate the potential range of magmatic fluxes that could be discriminated from these high background values, we generate a number of synthetic datasets representing biogenic and magmatic CO2 flux components based on data obtained in this study, and regions of magmatic CO2 elsewhere (Natron Basin, Tanzania, and Mammoth Mountain, USA). The two magmatic components selected have different signatures. Data from Lake Natron (Tanzania) display moderate fluxes (mean CO2 flux of 30 g m-2 d-1) and are treated here as a “conservative” magmatic component, while the CO2 fluxes from Mammoth Mountain (USA) are considerably higher (mean CO2 flux of 1,991 g m-2 d-1) and are treated as an example of an “extreme” magmatic component. The relative proportions of the biogenic and magmatic populations were 50:50, 70:30, 90:10, and 95:5. These data show that, when investigating areas of high biogenic CO2 fluxes, if the magmatic signal is “extreme” the magmatic CO2 flux population is discernible even if such a population represents a small proportion of the overall dataset (e.g., 5%), while higher concentrations are needed with a “conservative” magmatic flux.