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Rb-Sr and K-Ar ages on minerals at temperatures of 300°–400° C from deep wells in the Larderello geothermal field (Italy)

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Abstract

Rb-Sr and K-Ar ages have been obtained on six biotites, two muscovites and one hornblende from samples of micaschist, gneiss and amphibolite of Lower Paleozoic to Precambrian age at a depth exceeding 2,000 m in basement rocks of the Larderello-Travale geothermal region. Most of the data cluster in the range 2.5–3.7 Ma, revealing the existence of a Pliocene thermal event to which the origin of the field may be attributed. The resulting duration of the Larderello geothermal field is unexpectedly long. In the basement levels of the two wells examined, unstabilized minimum temperatures of 290° and 380° C were measured. All the biotites show almost complete 40Ar and 87Sr retention at the measured well temperatures. Petrologic evidence (stilpnomelane stability) and experimental data (activation energies and diffusion coefficients) also favour a closure temperature above 400° C for Rb-Sr and K-Ar in biotites, in agreement with recent direct experimental determinations.

For the last 3 Ma mean geothermal gradients of 120°–150° C/km have been evaluated in the first 2–3 km, and 60°–65° C/km in the underlying 2 km. A rough estimate of total cooling in the last 3 Ma gives a value of 120° C at 2,500 m depth and 50° C at 4,000 m depth in Sasso 22 well. A mean uplift rate of about 0.2 mm/year is calculated independently.

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Authors and Affiliations

  1. Istituto di Geocronologia e Geochimica Isotopica, Consiglio Nazionale delle Ricerche (CNR), Via Cardinale Maffi 36, 56100, Pisa, Italy

    A. Del Moro, F. Radicati di Brozolo & I. M. Villa

  2. Istituto Internazionale per le Ricerche Geotermiche (IIRG), CNR, Via del Buongusto 1, 56100, Pisa, Italy

    M. Puxeddu

  3. Centro di Studi sui Problemi dell'Orogeno delle Alpi Occidentali, CNR, Via S. Massimo 24, 10123, Torino, Italy

    F. Radicati di Brozolo

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  1. A. Del Moro
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  2. M. Puxeddu
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  4. I. M. Villa
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Research conducted under a collaboration agreement between the Italian National Research Council (CNR) and the Italian National Electricity Board (ENEL)

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Del Moro, A., Puxeddu, M., di Brozolo, F.R. et al. Rb-Sr and K-Ar ages on minerals at temperatures of 300°–400° C from deep wells in the Larderello geothermal field (Italy). Contr. Mineral. and Petrol. 81, 340–349 (1982). https://doi.org/10.1007/BF00371688

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