Helium–strontium isotope constraints on mantle evolution beneath the Roman Comagmatic Province, Italy

A study of the He isotopic ratios of fluid inclusions in olivine and pyroxene from the Roman Comagmatic Province (RCP), Italy, is presented together with 87Sr/86Sr isotope compositions of the whole rock or pyroxene phenocrysts. A clear covariation in He and Sr isotopes is apparent, with a strong northward increase in radiogenic He and Sr being evident. He and Sr isotopes ratios range from 3He/4He=5.2 Ra and 87Sr/86Sr=0.7056 in south Campania, to 3He/4He=0.44 Ra and 87Sr/86Sr=0.715905 in the northernmost Latium. Helium isotope ratios are significantly lower than MORB values and are among the lowest yet measured in subduction zone volcanism. The 3He/4He of olivine and pyroxene phenocryst-hosted volatiles appear to be little influenced by posteruptive processes and magma–crust interaction. The 3He/4He–87Sr/86Sr covariation is consistent with binary mixing between an asthenospheric mantle similar to HIMU ocean island basalts, and an enriched (radiogenic) mantle end member generated from subduction of the Ionian/Adriatic plate. The contribution of radiogenic He from metasomatic fluids and postmetasomatism radiogenic ingrowth in the wedge is strongly dependent on the initial He concentration of the mantle. Only when asthenosphere He concentrations are substantially lower than the MORB source mantle, and metasomatism occurred at the beginning of the subduction (∼30 Ma), can ingrowth in the mantle wedge account for the 3He/4He of the most radiogenic basalts.