Caldera di Manziana, Manziana, Metropolitan City of Rome Capital, Lazio, Italyi

Regional Level Types
Caldera di Manziana	Caldera
Manziana	Municipality
Metropolitan City of Rome Capital	Metropolitan City
Lazio	Administrative Region
Italy	Country

Caldara di Manziana, presently characterised by CO₂ and H₂S gas emissions, is a subcircular structure of 0.25 km², possibly generated by a hydrothermal explosion during the alkali-potassic volcanism affecting central Italy from 0.8–0.6 Ma to 90 ka.

The rocks present in this area correspond to the Monti Sabatini Volcanic District, and they are mainly explosive, pyroclastic deposits, and some lava flows. The rocks consist mainly of undersaturated ultrapotassic rocks ranging from tephrite to phonolite, with some trachytes and latites erupted locally. The rock texture is porphyritic with 5–40% volume of phenocrysts. Abundant phenocryst phases are clinopyroxene, olivine, plagioclase, and Fe-Ti oxides. Phlogopite phenocrysts are rare. The groundmass contains leucite, clinopyroxene, K-feldspar, plagioclase, glass, sporadic phlogopite, and amphibole. Apatite is also found in the groundmass. These rocks overlay acid volcanics of the Tolfa-Cerite (Cerveteri)-Manziana volcanic complex of the Tuscany magmatic province, which correspond to older volcanic episodes 3.7 to 1.8 Ma old.

The caldera floor sustains a constant, focused, and diffuse CO₂ flow that does not allow the growth of vegetation (Rogie et al., 2000). The CO₂ originates from a deep (~2000 m) geothermal reservoir within Mesozoic carbonates with an estimated temperature of ~200 °C, and interacts with superficial aquifers before reaching the surface, where fluids are at 9–15 °C. In the approximate centre of the area of diffuse CO₂ venting there is a permanent pool of water (~10 m diametre, ~0.5 m deep) caused by the focused rising and expulsion of CO₂. The focused venting of CO₂ generates a permanent geyser ~1 m wide and ~75 cm high. The water from this pool and from several other smaller sources within the caldera form little streams that join together and run in the south-west direction. Again, all of these water sources have temperatures in the range of 9–15 °C. In the winter, the centre of the caldera becomes flooded and water is seen to bubble. Moreover, the caldera floor is an H₂S source with a diffuse, spatially variable flux (from 0.037 to > 132 g m−2 d−1). When the caldera is flooded in the winter, much of the H₂S venting occurs through the sheet of water and part of the gas dissolves in the water. The pH in water sources, pools, and streams ranges between slightly and strongly acidic (just below 7 to 1). Other gasses detected, in minor or trace concentrations, are N₂, Ar, CH₄, H₂, He, and CO.

The H₂S emissions have caused the precipitation of sulphur as well as the acidic alteration of the local volcanics with the generation of halloysite and amorphous silica (Lombardi & Mattias, 1979; Pinzari & Cuadros, 2021). Unusual perforations in phlogopite crystals, caused by sulphuric acid generated by microbial oxidation of H₂S emanations, are observed. Phlogopite alteration products include Fe-bearing beidellite or Fe-bearing halloysite, silica, Fe oxides, and gypsum.

Other Regions, Features and Areas containing this locality

References