Soil Radon and Elemental Mercury Distribution and Relation to Magmatic Resurgence at Long Valley Caldera

Abstract

The response of a large geothermal system to magmatic resurgence was analyzed by a survey of soil gas radon and elemental mercury at 600 sites in the silicic Long Valley Caldera, California. The broad geochemical anomaly over the caldera has superimposed on it a small zone of pronounced radon enrichment and mercury depletion coincident with the surface projection of a postulated dike of rising magma. Soil gas geochemistry studies can complement traditional geophysical and geodetical methods in the evaluation of potential volcanic eruption hazards.

Historical unrest at large calderas of the world

“…On at least four occasions in 1983, hydrogen gas emissions from the soil near Casa Diablo increased within a few hours or days before small earthquake swarms (K. McGee, written commun., 1983). Mercury concentrations in soils of Long Valley were significantly higher in 1982 than in a previous survey in 1975 (Varekamp and Buseck, 1984), and they were higher still in 1983 (Williams, 1985). In 1983, anomalously high radon was also found in soil gas from the area of uplift and the area of the January 1983 seismic swarm (Williams, 1985) (fig.…”

“…Note lack of appreciable deformation prior to 1975 and progressive uplift since 1980 (Hill and others, 1985c). Williams (1985). Copyright by the American Association for the Advancement of Science.…”

Soil radon concentration along fault systems in parts of south eastern Ghana

“…The characteristic high concentrations of radon over faults reflect gas migration dominated by brittle deformation both at macro-and/or micro-scale. Therefore spatial patterns of soil-gas radon in faulted areas appear to be suitable tools for identifying and mapping active tectonic structures (Crenshaw et al, 1982;Cox, 1983;Cox and Cuff, 1981;Williams, 1985;Ciotoli et al, 1998Ciotoli et al, , 1999Papastefanou et al, 2001;Baubron et al, 2002). Burton et al (2004) used soil-gas radon concentrations to map active faults, as faults and fractures in the underlying rocks provide paths of high permeability and therefore serve as conduits for radon gas migration.…”

Precursory Subsurface 222Rn and 220Rn Degassing Signatures of the 2004 Seismic Crisis at Tenerife, Canary Islands

“…The parent isotopes of these two elements are often transported in groundwater and their concentrations depends on several factors such as oxidation/reduction potentials, water pH and chemistry (SMITH et al, 1976). They can also be brought to the surface by fluid convection caused by high geothermal gradients, and their transport therefore occurs efficiently in areas of high permeability by fracturing (WILLIAMS, 1985). 220 Rn/ 222 Rn ratio is an important parameter to monitor since provides information about the source of the radon (GIAMMANCO et al, 2007).…”