E. Petrucci et al., WATER-ROCK INTERACTION IN THE LARDERELLO GEOTHERMAL-FIELD (SOUTHERN TUSCANY, ITALY) - AN O-18 O-16 AND D/H ISOTOPE STUDY/, Journal of volcanology and geothermal research, 59(1-2), 1993, pp. 145-160
Stable isotope and fluid inclusions investigations were carried out on
metamorphic rocks belonging to the Southern Tuscany Paleozoic basemen
t that constitutes the main deep fluid reservoir ( > 1000 m below grou
nd level) of the Larderello Geothermal Field. Whole-rock samples have
deltaO-18 and deltaD values ranging from + 2.9 to + 11.6 parts per tho
usand and from - 65 to - 76. parts per thousand, respectively; most ne
gative deltaO-18 and deltaD values have been obtained on rocks from hi
ghly fractured levels. Calculated deltaO-18 and deltaD values for the
fluids in equilibrium with the rocks are from + 3 to + 8 parts per tho
usand and - 31 to - 43 parts per thousand respectively, temperatures o
f homogenization of fluid inclusions range from 245-degrees-C to 324-d
egrees-C and salinities from 1.0 to 12.8 wt.% NaCl eq. The combined da
ta are consistent with mixing between a deep circulating (> 2500 m bel
ow 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 O-18-rich
hydrothermal fluids at T> 250-degrees-C in the deeper explored levels
contrast with the much less strongly O-18-shifted meteoric-hydrotherm
al waters which circulate in the upper 1000 m or so of the system at T
< 250-degrees-C. These new stable isotope data and other petrological
, geophysical and geochemical data are all consistent with the existen
ce of two apparently separate hydrothermal systems (and productive hor
izons) at different depths bearing fluids of different physico-chemica
l characteristics. The ''shallow'' upper meteoric system may not be co
ntinuosly connected to the lower meteoric-magmatic system because, at
least locally, fluid pressures in the lower levels are higher than hyd
rostatic pressures.