Geology http://doi.org/ks2 (2013)

Credit: © IMAGEBROKER / ALAMY

Geothermal springs in the Rocky Mountains of the western United States often occur along faults. Geochemical analyses of the spring water show the combination of faults and springs form vents that allow gases from the underlying mantle to be released at the surface.

Karl Karlstrom at the University of New Mexico, Albuquerque, USA, and colleagues analysed the carbon dioxide and helium isotopic signature of waters collected from 25 geothermal springs in the Rocky Mountains. The ratio of primordial isotope 3He to the radiogenic 4He in the waters indicates that these gases, as well as a significant fraction of the carbon dioxide dissolved in the waters, come from Earth's mantle. The researchers suggest that gases escaping from the mantle move upwards through faults and fractures in Earth's crust, then become dissolved in groundwater, and are finally vented at the surface as geothermal spring waters.

The results show that mantle degassing is surprisingly widespread and persistent, and that hot springs facilitate degassing of mantle volatile gases even in the middle of continental plates such as North America.