The geological CO2 degassing history of a long-lived caldera

The majority of the ~100 Holocene calderas on Earth host vigorously active hydrothermal systems, the heat and volatile budgets of which are sustained by degassing of deeply stored magma. Calderas may thus contribute a nontrivial, although poorly quantified, fraction of the global budget of magmatic volatiles such as CO2. Here we use original isotopic a d petrological results from Campi Flegrei volcano, Italy, to propose that hydrothermal calcites are natural mineral archives for the magmatic CO2 that reacted with reservoir rocks during the geological history of a caldera. We show that Campi Flegrei calcites, identified in core samples extracted from 3-km-deep geothermal wells, formed at isotopic equilibrium with magmatic fluids having δ18OH2O of +8.7‰ to +12.7‰, and δ13CCO2 of ~ -1.5‰. This inferred fossil fluid composition is virtually identical to that of present-day fumaroles, demonstratinga stable carbon source duringthe caldera's (<40 k.y.) history. We use the mass of calcites stored in the hydrothermal system to estimate that 12 Gt of magmatic CO2 reacted with the Campi Flegrei rocks during the caldera history; this corresponds to a time-averaged CO2 flux of ~ 800 t d-1. This long-term CO2 flux, the first of its kind in the geological literature, is similar to the present-day soil CO2 degassing flux (1100 ± 200 t d-1). We conclude that the actual magmatic CO2 degassing flux from calderas may be severely underestimated if subsurface calcite precipitation is not taken into account.