Testing for Magmatic CO2 Degassing Above the North Appalachian Anomaly

Abstract

The Northern 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 New York, Massachusetts, New Hampshire and Vermont. 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. We hypothesize that magmatic CO2, if present, will produce a CO2 degassing signature distinguishable from a shallow biogenic CO2 source. Samples were taken in areas with springs because the water traveling from within the Earth’s crust, and sometimes deeper, exploits the easiest pathway to reach the surface. We expect that CO2 would exploit similar paths. Data obtained thus far are unable to confidently confirm a second-high flux population that would support a possible magmatic component. The CO2 fluxes in this area are 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), which makes it hard for the two possible components to be distinguished. To address the question of whether our method is truly capable of distinguishing low concentrations of two distinct components, we generate a number of synthetic datasets representing biogenic and magmatic CO2 flux components based on data obtained in this study and compare it to data from regions of magmatic CO2 elsewhere (Natron Basin, Tanzania, and Mammoth Mountain, USA) to investigate the potential range of magmatic fluxes that could be discriminated from these high background values. The two magmatic components selected have different signatures: data from Lake Natron display moderate fluxes, mean CO2 flux of 30 g m-2 d-1 (Muirhead et al., 2019) and are treated here as a weak magmatic component; while the CO2 fluxes from Mammoth Mountain are considerably higher, mean CO2 flux of 1,991 g m-2 d-1 (Cardellini et al., 2003) and are treated as an example of a strong magmatic component. The relative proportions of the biogenic and magmatic populations modelled are 50:50, 60:40, 70:30, 80:20, 90:10, and 95:5. These data show that when investigating areas of high biogenic CO2 fluxes, if the magmatic signal is strong, or weak, the magmatic CO2 flux population is discernible even if such a population represents a small proportion of the overall dataset (e.g., 5%). Ultimately, isotopic analyses must be conducted to confidently distinguish between magmatic and biogenic CO2 signatures.