Water-Rock Interaction In The Larderello Geothermal Field (Southern Tuscany, Italy)- An And Isotope Study

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Journal Article: Water-Rock Interaction In The Larderello Geothermal Field (Southern Tuscany, Italy)- An And Isotope Study

Abstract
Stable isotope and fluid inclusions investigations were carried out on metamorphic rocks belonging to the Southern Tuscany Paleozoic basement that constitutes the main deep fluid reservoir (> 1000 m below ground level) of the Larderello Geothermal Field. Whole-rock samples have Δ18O and ΔD values ranging from +2.9 to +11.6‰ and from -65 to -76.‰, respectively; most negative Δ18O and ΔD values have been obtained on rocks from highly fractured levels. Calculated Δ18O and ΔD values for the fluids in equilibrium with the rocks are from +3 to +8‰ and -31 to -43‰ respectively, temperatures of homogenization of fluid inclusions range from 245°C to 324°C and salinities from 1.0 to 12.8 wt.% NaCl eq. The combined data are consistent with mixing between a deep circulating (> 2500 m below ground level) meteoric-hydrothermal water and a deep thermal water of probable magmatic or metamorphic origin. After mixing and exchange the water/rock mass ratios are small, typically <0.3. These 18O-rich hydrothermal fluids at T>250°C in the deeper explored levels contrast with the much less strongly 18O-shifted meteoric-hydrothermal waters which circulate in the upper 1000 m or so of the system at T<250°C. These new stable isotope data and other petrological, geophysical and geochemical data are all consistent with the existence of two apparently separate hydrothermal systems (and productive horizons) at different depths bearing fluids of different physico-chemical characteristics. The "shallow" upper meteoric system may not be continuosly connected to the lower meteoric-magmatic system because, at least locally, fluid pressures in the lower levels are higher than hydrostatic pressures.

Authors 
E. Petrucci, S. M. F. Sheppard and B. Turi








Published Journal 
Journal of Volcanology and Geothermal Research, 1993





DOI 
Not Provided
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Citation

E. Petrucci,S. M. F. Sheppard,B. Turi. 1993. Water-Rock Interaction In The Larderello Geothermal Field (Southern Tuscany, Italy)- An And Isotope Study. Journal of Volcanology and Geothermal Research. (!) .