Istituto di Geoscienze e Georisorse     
Valori A., Cathelineau M., Marignac C. Early fluid migration in a deep part of the Larderello geothermal field: a fluid inclusion study of the granite sill from well Monteverdi 7. In: Journal of Volcanology and Geothermal Research, vol. 51 (1-2) pp. 115 - 131. Elsevier, 1992.
Metamorphic/magmatic to hydrothermal fluids trapped in fluid inclusion planes (FIP) were studied in a microfractured granite sill from a deep level (3483-3486 m b.g.l.) of the well Monteverdi 7 (Larderello geothermal field). This granite has recorded a complex succession of fluid migration through a dense set of healed microcracks. Nine generations of fluids trapped within specific microcrack networks are distinguished and include vapors and liquids of the C-H-O-N-S system, aqueous liquids and brines with distinctive sets of cation chlorides: LiCl+NaCl, NaCl+CaCl2 and NaCl. Detailed studies of the geometrical and chronological relationships between the fluid inclusion planes allow the establishment of a clear relative chronology, with the C-H-O-N-S and LiCl fluids being earlier than the NaCl+CaCl2 and NaCl aqueous liquids and brines. In particular, it can be shown that the C-H-O-N-S and LiCl fluids are strongly associated in space and time, since the corresponding fluid inclusion planes display evidence of mutual contamination of inclusions within the planes. Significant changes in the physical-chemical parameters occurred in the conditions of fluid trapping during the evolution of the Larderello field. All observed paleofluids have compositions differing from that encountered in the present-day fluids and record a complex P-T-t-X evolution, from an early stage dominated by magmatic- and metamorphic-derived fluids towards a later stage dominated by more surficial waters: - At the early stage, hydrothermal circulation occurred under lithostatic pressure, at a depth of ca. 4.5 km, at temperatures as high as 450-500C. Such conditions are very reminiscent of contact metamorphism and were probably created by the deep intrusive body detected by geophysical studies (Batini et al., 1983, 1985). The involved fluids are C-H-O-N-S vapors and liquids of metamorphic derivation and aqueous Li-bearing brines of probable magmatic derivation. They are consistent with the hypothesis of the intrusive being a granite. - At the later stage, physical conditions had evolved towards hydrostatic pressure at a depth close to the present 3.5 km one and temperatures of 400-350C, still higher than the present-day ones. Early fluids were no longer present. The observed fluids record the involvement of more surficial waters, both meteoric and connate. Transient boiling events were probably occurring, heralding the present-day conditions (vapor-dominated geothermal system). Nevertheless, no trapped fluids display similar composition and density with the present-day geothermal fluids at Larderello. In "old" geothermal fields such as Larderello, it must be emphasized that the use of fluid inclusions in order to reconstruct past hydrothermal evolution is strongly dependent onto pressure estimation, since significant temperature corrections may be needed. In the Larderello case, failing to take in account the pressure effects would have led to seriously misleading conclusions.
URL: http://https://www.journals.elsevier.com/journal-of-volcanology-and-geothermal-research/
Subject Larderello
petrography and mineralogy
thermodynamic characteristics

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