Evidence for crustal degassing of CF sub(4) and SF sub(6) in Mojave Desert groundwaters

• Published in: Geochimica et cosmochimica acta 2008-02, Vol.72 (4), p.999-1013
• Main Authors: Deeds, Daniel A, Vollmer, Martin K, Kulongoski, Justin T, Miller, Benjamin R, Muehle, Jens, Harth, Christina M, Izbicki, John A, Hilton, David R, Weiss, Ray F
• Format: Article
• Language: English
• ISSN: 0016-7037
• DOI: 10.1016/j.gca.2007.11.027

Dissolved tetrafluoromethane (CF sub(4)) and sulfur hexafluoride (SF sub(6)) concentrations were measured in groundwater samples from the Eastern Morongo Basin (EMB) and Mojave River Basin (MRB) located in the southern Mojave Desert, California. Both CF sub(4) and SF sub(6) are supersaturated with respect to equilibrium with the preindustrial atmosphere at the recharge temperatures and elevations of the Mojave Desert. These observations provide the first in situ evidence for a flux of CF sub(4) from the lithosphere. A gradual basin- wide enhancement in dissolved CF sub(4) and SF sub(6) concentrations with groundwater age is consistent with release of these gases during weathering of the surrounding granitic alluvium. Dissolved CF sub(4) and SF sub(6) concentrations in these groundwaters also contain a deeper crustal component associated with a lithospheric flux entering the EMB and MRB through the underlying basement. The crustal flux of CF sub(4), but not of SF sub(6), is enhanced in the vicinity of local active fault systems due to release of crustal fluids during episodic fracture events driven by local tectonic activity. When fluxes of CF sub(4) and SF sub(6) into Mojave Desert groundwaters are extrapolated to the global scale they are consistent, within large uncertainties, with the fluxes required to sustain the preindustrial atmospheric abundances of CF sub(4) and SF sub(6).