Constraints on mantle source and interactions from helium-strontium isotope variations in Italian volcanism

Abstract

Helium isotope ratios of olivine and pyroxene from Plio-Quaternary volcanic rocks from Southern Italy (each of the seven Eolian Islands, Mt. Vulture, Etna, Ustica, Pantelleria), range from 2.3 to 7.1 Ra. A good correlation emerges with the 3He/4He of fumarolic fluids. Importantly the phenocryst 3He/4He correlate with whole rock Sr isotopes (0.70309-0.70711) reflecting the mixing of two sources. Crustal contamination of magmatic He isotopes is recorded only occasionally (e.g., pyroxenes from Vulcano). The He isotope values of Pantelleria, Etna, Iblei, Ustica, Alicudi and Filicudi (7.0 ± 0.6 Ra) define the mantle composition least affected by subduction. That these characterise volcanoes from a variety of tectonic regimes (subduction-related, intraplate, rifting) suggest a common origin of geochemical features and are consistent with a HIMU-type mantle that is either younger than the Cook-Austral island end-member, or one with lower 238U/204Pb. When merged with data from the Roman Comagmatic Province (Latium and Campania), a remarkably strong He-Sr isotope correlation is apparent. The general northward decrease in 3He/4He corresponds to an increase in 87Sr/86Sr and decrease in 143Nd/144Nd and 206Pb/204Pb that is due to increasing metasomatic enrichment of the mantle wedge via subduction of the Ionian Adriatic plate since 30 Ma. Calculations based on the ingrowth of 4He in the wedge and on the 4He content of the subducting crust show that mechanisms of enrichment in radiogenic He are effective only if the wedge is strongly depleted in He relative to best estimates of the depleted mantle.