Magmatic Signature in Submarine Hydrothermal Fluids Vented Offshore Ventotene and Zannone Islands (Pontine Archipelago, Central Italy)

Abstract

Geochemical investigations carried out on submarine hydrothermal fluids vented offshore the Pontine Islands (Tyrrhenian Sea) revealed the existence of gas vents to the W of Zannone Island and SW of Ventotene Island. The geochemical features of the CO2-rich gas samples show a clear mantle-derived signature with3He/4He of 3.72-3.75 Ra and 1.33 Ra at Zannone and Ventotene, respectively. Gas geochemistry denotes how CO2-rich gases undergo fractionation processes due to CO2dissolution to a variable extent favoring enrichment in the less soluble gas species, i.e., CH4, N2, and He. The carbon isotope composition of CO2, expressed asδ13C vs. V-PDB, ranges from -0.71 and -6.16‰ at Zannone to 1.93‰ at Ventotene. Preliminary geothermometric and geobarometric estimations indicate equilibrium temperatures in the range of 150-200°C at Zannone and >200°C at Ventotene besides H2O pressures in the range of 5 bar and 20 bar at Zannone and Ventotene, respectively. Although the latest volcanic activity at the Pontine Archipelago is dated Middle Pleistocene, the combination of the new geochemical information along with geothermometric estimations indicates that cooling magmas are likely releasing enough thermal energy to form an efficient hydrothermal system.

Alteration and Mineralization Products of the Zannone Giant Pockmark (Zannone Hydrothermal Field, Central Tyrrhenian Sea)

“…Several active fluid emissions have been identified at the ZGP seabed, characterized by discharge of clear thermal waters and bubbling gases. Maximum temperatures of 60-70 • C were measured at the seabed [19,20].…”

“…Geochemical analyses of the Zannone hydrothermal fluids indicate emission of CO 2 -rich gases with a mantle-derived signature ( 3 He/ 4 He of 3.72-3.75), with equilibrium temperatures ranging between 150 and 200 • C [17,20] and H 2 O pressures of ca. 5 bar [20].…”

“…Geochemical analyses of the Zannone hydrothermal fluids indicate emission of CO 2 -rich gases with a mantle-derived signature ( 3 He/ 4 He of 3.72-3.75), with equilibrium temperatures ranging between 150 and 200 • C [17,20] and H 2 O pressures of ca. 5 bar [20]. The degassing suggests a significant contribution of mantle volatiles, possibly originated from residual magma batches similar to the Pleistocene trachytes cropping out in the SE sector of Ponza Island [17].…”

Hydrothermal Vent Complexes Control Seepage and Hydrocarbon Release on the Overriding Plate of the Tyrrhenian-Ionian Subduction System (Paola Basin)

“…Other large fluid-escape depressions and seafloor mounds have been observed on the continental shelf of the Pontine Archipelago (Figure 1A), where hydrothermal sulfides collected on the seabed and sub-seabed point to the possible degassing of magma similar to the one feeding the latest volcanic activity occurred on the islands in the Middle Pleistocene (Conte et al, 2020). Hydrothermal fluids discharged at this location are CO 2 -rich and show a mantle-derived signature indicating that cooling magmas are still releasing enough thermal energy to feed an active hydrothermal system (Italiano et al, 2019). An example in water depths closer to our case study, is the fault-controlled system of deep-hydrothermal circulation discovered on the Cape Vaticano Ridge (Figure 1B).…”

Hazard Scenarios Related to Submarine Volcanic-Hydrothermal Activity and Advanced Monitoring Strategies: A Study Case from the Panarea Volcanic Group (Aeolian Islands, Italy)

“…The improvement of our knowledge about the submarine hydrothermal systems has to be framed in terms of life safety and environmental preservation. Hydrothermal fields in the Southern Tyrrhenian offshore are located over the seamounts [14][15][16] and in the coastal areas of the Aeolian Archipelago [17][18][19][20][21], in front of Capo Vaticano [22,23], along the coast of Ischia island [19], in the Bay of Naples and Pozzuoli [24,25], and off the coast of Ventotene and Zannone (Pontine Islands) [26,27]. All the systems are characterized by CO 2 -rich emissions and thermal waters commonly associated to past or recent volcanic activity.…”