HR: 1340h
AN: T23B-0541    [Abstracts]
TI: Different Behaviour of Heat Flow and Helium Anomalies Through the Geothermal Areas of Central Italy
AU: * Bellani, S
EM: sbellani@igg.cnr.it
AF: Istituto di Geoscienze e Georisorse - CNR, 1, Via Moruzzi, Pisa, 56124 Italy
AU: Magro, G
EM: g.magro@igg.cnr.it
AF: Istituto di Geoscienze e Georisorse - CNR, 1, Via Moruzzi, Pisa, 56124 Italy
AB: The high enthalpy geothermal areas and more than 30% of the Italian spas are concentrated in central Italy (Tuscany and Latium regions). A wide part of this area is affected by a large heat-flow anomaly, with maxima corresponding to the geothermal fields of Larderello, Mt. Amiata and Latera. Mt.Amiata and Latera are located in areas of quaternary quiescent volcanism, while there are no outcrops of extrusive rocks in the Larderello geothermal field. Surface heat flow in a continental area has an average value of 50-60 mW/m2. The background regional value in these geothermal areas is 120-150 mW/m2, with peaks up to 1000 mW/m2 at Larderello, 600 mW/m2 at Mt. Amiata and 300 mW/m2 at Latera. Anomalous heat flow is very often related to the presence of 3He-enriched fluids. In central Italy the He isotopic ratio (3He/4He expressed as R/Ra), in gases from cold and thermal manifestations and in geothermal wells ranges from 0.2 to 3.2 R/Ra. In particular, Larderello fluids range between 0.5 and 3.2, while the interval for Mt. Amiata and Latera fluids spans from 0.2 to 0.6. The R/Ra and heat flow profiles through central Italy, NW-SE trending, show no straightforward correlation between the two parameters. A phased correlation exists between heat flow and R/Ra relative maxima at Larderello, while they are decoupled at Mt. Amiata and Latera geothermal fields. Different heat and He transport mechanisms through the crust are the most likely explanation: mantle 3He-enriched fluids move upwards mainly via fluid filled conduits, while heat diffuses faster than He throughout the entire bulk rock. At Larderello, the almost constant values of R/Ra in geothermal wells and paleo-fluids (fluid inclusions) indicate that the contribution of hot 3He-enriched fluids must have occurred through deeply rooted faults at least in the last 1 Ma. In the Mt. Amiata area the presence of several spas, marked by low R/Ra ratios down to crustal values, could indicate the addition of radiogenic 4He. Rock leaching enhanced by redistribution of heat and/or longer residence time of deep fluids in the upper crust accounts for the radiogenic 4He addition. The decoupling of the heat and He anomalies in these two areas is then related to a different permeability due to a lower degree of fracturing and/or a different depth of the faults roots.
DE: 1040 Radiogenic isotope geochemistry
DE: 8130 Heat generation and transport
DE: 8135 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8424)
DE: 8199 General or miscellaneous
SC: Tectonophysics [T]
MN: Fall Meeting 2005