Origin of continuous hydrogen flux in gas manifestations at the Larderello geothermal field, Central Italy

Abstract

The back arc extensional area of Tuscany is characterized by anomalously high heat flow as a result of lithospheric thinning facilitated by locally intruded granite, which ultimately led to the occurrence of numerous fumaroles and other manifestations of geothermal activity. The present study builds up on molecular and isotopic geochemical analyses of fumarolic gases discharging near Larderello geothermal field in order to report on their origin and generation mechanisms. The fumarole gases are rich in CO2 (15.65–92.5 vol%), poor in CH4 (0.08–3.70 vol%) and highly-depleted in C2+ (< 0.1 vol%) whereas H2 content is rather constant (~4.5 vol%). The δ13CCH4 (from −16.7 to −29.5‰VPDB) and δDCH4 (from −155 to −185.6‰SMOW) suggest an abiotic origin of methane. The heavy δ13CCO2 values (>−4.2‰VPDB) reflect thermo-metamorphic of CO2, however magmatic-volcanic contribution cannot be ruled out.This work tests various hypotheses of the origin of H2: the intruded granite, the ophiolite and/or a deeper source. Petrographic and electron microbeam investigations performed on the Ligurian ophiolite and plutonic granite, which outcrops in the vicinity of the study area, revealed serpentinization of ophiolite (in contact with geothermal fluids at 200 °C) as the most probable source of the measured H2. Hydrogen that forms deeper, potentially near or within the granitic intrusion, at higher temperatures, is likely consumed to form abiotic methane.