Helium-3 anomalies and crust-mantle interaction in Italy

The distribution of primordial ³He in the crust of Italy has been investigated through a survey of groundwaters, hot-springs, exploration and production geothermal wells. The mantle-derived component varies from 0% to > 60% and reaches a maximum at Vulcano in the Eolian Islands. ³He anomalies occur where the conductive heat flow is relatively high, but the relationships between heat flow and anomaly are uncertain.The distribution of ³He in the main reservoir and cap rock to the Larderello geothermal system shows that the mantle-derived ³He varies from ~5% to ~40% of total He and reaches a maximum in fluids extracted from a depth of ca. 3 km within the basement. Bomb-produced tritiogenic ³He appears to make a near-negligible contribution. Well 107 at Larderello has been monitored over a period of 17 months and the ${}^3\text{He}{}^4\text{He}$ ratio (R) varies from 1.4 to 1.6 times the atmospheric ratio (Ra) whilst He/Ne varies from 9 to 160. A model is proposed for the Larderello geothermal system whereby mantle-derived heat and volatiles are advected to shallow (6–8 km) depths in association with melts and released into the main geothermal reservoir at Larderello in a non-uniform manner possibly associated with fracturing. Isotopic and elemental equilibrium of gaseous species (C, O, H) appears to have taken place in the basement to the main reservoir from where fluid with R/Ra ≥ 3.2 has been extracted.