HR: 1340h
AN: T23B-0557    [Abstracts]
TI: Radon anomaly - crustal strain relationships at the Selfoss geothermal area, Iceland
AU: * Kadko, D C
EM: dkadko@rsmas.miami.edu
AF: University of Miami, RSMAS, 4600 Rickenbacker Causeway, Miami, Fl 33149 United States
AU: Gronvold, K
EM: karlgr@hi.is
AF: Nordic Volcanologic Institute, Askja - Sturlugata 7 , Reykjavik, 101 Iceland
AU: Agustsson, K
EM: kri@vedur.is
AF: Icelandic Meteorologic Office, Bustadavegur 9, Reykjavik, 150 Iceland
AU: Hovgaard, J
EM: hovgaard@sympatico.ca
AF: Exploranium/SAIC, 6108 Edwards Blvd, Mississauga, On L5T2V7 Canada
AB: Time-series measurements of radon-222 (a chemically inert, radioactive gas) within groundwater and hydrothermal fluids have previously been used to monitor tectonic and magmatic inflation/ deflation events. Typically, water samples have been collected directly from wells or bore holes and the radon extracted for measurement. This requires field visits and lab extractions that limit the frequency of sampling, leading to either missed events or ill defined event - radon emanation relationships. To overcome this difficulty, we have developed an in-situ gamma spectrometer for automatic (and remote) bore-hole water measurements of radon activity. The sensor is a sodium iodide (NaI) detector coupled to a 256-channel spectrometer and data recorder. Changes in radioactivity of low temperature fluids (< 100 deg C) are monitored by recording snapshots of the gamma spectrum over preset intervals. Laboratory tests of the gamma system whereby the detector was immersed in a water bath with varied 222Rn content (measured by standard gas extraction technique) indicate that the instrument responds linearly to changes in the radon activity over a range that has been observed for hydrothermal fluids. We deployed the system at the Selfoss geothermal site in Iceland, where for 3 years we obtained a quasi-continuous time-series of radon activity consisting of measurements taken every 30 minutes. This sampling frequency has permitted details of radon variability not previously seen, and we noted that the radon activity at Selfoss varied inversely with crustal strain changes induced by earth tides and atmospheric pressure. As strain changes accompany earthquakes derived from magmatic and tectonic events, these results yield insight into previous studies suggesting radon anomalies as precursors to earthquakes. The radon in groundwater is generated by decay of its parent radium-226, and ejected by alpha recoil into interstial fluids. Our results suggest that strain release events (by whatever mechanism) promote dilation of the rock matrix, enhancing pore space connectivity, and thereby permitting the interstitial radon to be released rapidly into the crustal fluid flow.
DE: 0450 Hydrothermal systems (1034, 3017, 3616, 4832, 8135, 8424)
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1829 Groundwater hydrology
DE: 4860 Radioactivity and radioisotopes
SC: Tectonophysics [T]
MN: Fall Meeting 2005