Geochemistry of submarine warm springs in the limestone cavern of Grotta Azzurra, Capo Palinuro, Italy: evidence for mixing-zone dolomitisation

Subtidal springs in and around the submarine limestone cavern of Grotta Azzurra, at Capo Palinuro, Italy, discharge fluids which are warm (< 25°C), mildly acidic, depleted in Cl−, Na+ and Mg2+, and enriched in Si, alkalinity, Ca2+, Sr2+, Mn, NH4+, PO43− and H2S, relative to surrounding seawater. The compositions of the warm fluid samples collected in and around the cave define mixing lines which suggest dilution of a single thermal fluid (T ≥ 23°C) by cool overlying seawater (T= 17–17.6°C). The chemical data suggest that the proposed thermal fluid contains two components, one derived from seawater (< 90%) and the other from low-salinity groundwater (> 10%). Excess Si, alkalinity, Ca2+, Sr2+ and Mn relative to seawater are likely derived from the groundwater component or dissolution/hydrothermal alteration of the host rocks. Magnesium has been removed from the seawater component in exchange for Ca2+, due to dolomitisation of the limestone and/or hydrothermal alteration reactions. Saturation-state calculations suggest that the vented fluids are near saturation with respect to calcite and supersaturated with respect to dolomite. This and the presence of dolomite in the host rocks and cave-floor sediments suggest that “mixing-zone” dolomitisation of the limestones is occurring, perhaps kinetically assisted by elevated temperature and/or bacterial mediation in the reducing subseafloor zone. One possible “end-member” condition is considered for the thermal fluid — zero-Mg — which suggests an end-member temperature of 50.5°C and a fluid composition derived from ∼ 38% seawater and ∼ 62% groundwater. The heat source for the circulating fluids is uncertain, but may involve warm underlying igneous rocks or heating via the geothermal gradient. A continuous in-situ record of vent-fluid temperature, salinity, pH and O2 concentration collected within the cavern is consistent with our interpretation of the fluid origin, and suggests that tidal forcing affects circulation and venting of the warm fluids.