WATER-ROCK INTERACTION IN THE LARDERELLO GEOTHERMAL-FIELD (SOUTHERN TUSCANY, ITALY) - AN O-18 O-16 AND D/H ISOTOPE STUDY/

Citation
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
Citations number
42
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Geology
ISSN journal
0377-0273
Volume
59
Issue
1-2
Year of publication
1993
Pages
145 - 160
Database
ISI
SICI code
0377-0273(1993)59:1-2<145:WIITLG>2.0.ZU;2-O
Abstract
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.