Open AccessThe structure of a hydrothermal system from an integrated geochemical, geophysical, and geological approach: The Ischia Island case study
Author(s) -
Di Napoli R.,
Martorana R.,
Orsi G.,
Aiuppa A.,
Camarda M.,
De Gregorio S.,
Gagliano Candela E.,
Luzio D.,
Messi.,
Pecoraino G.,
Bitetto M.,
de Vita S.,
Valenza M.
Publication year - 2011
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2010gc003476
Subject(s) - geology , hydrothermal circulation , geophysics , geochemistry , earth science , seismology
The complexity of volcano‐hosted hydrothermal systems is such that thorough characterization requires extensive and interdisciplinary work. We use here an integrated multidisciplinary approach, combining geological investigations with hydrogeochemical and soil degassing prospecting, and resistivity surveys, to provide a comprehensive characterization of the shallow structure of the southwestern Ischia's hydrothermal system. We show that the investigated area is characterized by a structural setting that, although very complex, can be schematized in three sectors, namely, the extra caldera sector (ECS), caldera floor sector (CFS), and resurgent caldera sector (RCS). This contrasted structural setting governs fluid circulation. Geochemical prospecting shows, in fact, that the caldera floor sector, a structural and topographic low, is the area where CO 2 ‐rich (>40 cm 3 /l) hydrothermally mature (log Mg/Na ratios < −3) waters, of prevalently meteoric origin ( δ 18 O < −5.5‰), preferentially flow and accumulate. This pervasive hydrothermal circulation within the caldera floor sector, being also the source of significant CO 2 soil degassing (>150 g m −2 d −1 ), is clearly captured by electrical resistivity tomography (ERT) and transient electromagnetic (TEM) surveys as a highly conductive (resistivity < 3 Ω·m) layer from depths of ∼100 m, and therefore within the Mount Epomeo Green Tuff (MEGT) formation. Our observations indicate, instead, that less‐thermalized fluids prevail in the extra caldera and resurgent caldera sectors, where highly conductive seawater‐like (total dissolved solid, TDS > 10,000 mg/l) and poorly conductive meteoric‐derived (TDS < 4,000 mg/l) waters are observed, respectively. We finally integrate our observations to build a general model for fluid circulation in the shallowest (<0.5 km) part of Ischia's hydrothermal system.