Anomalously Low ³ He/ ⁴ He and Elevated Heat Flow in the Northern Apennines Explained by Lithospheric Delamination: A Modeling Approach

The 3 He/ 4 He ratios measured in 12 CO 2 ‐dominated gas seeps from the Tuscany‐Roman Volcanic Provinces in Italy range between 0.07 and ∼1.6 R a . Helium and δ 13 CO 2 isotope systematics indicate a dominant crustal source with a variable but significantly lower contribution from the mantle. Low He ( 3 He/ 4 He = 0.6–1.6 R a ) and high Sr ( 87 Sr/ 86 Sr = 0.709514–0.710595) isotope systematics of olivine and pyroxene mantle phenocrysts have been previously interpreted as related to the addition of a crustal signature into a HIMU‐type magma (6.7 ± 0.4 R a ) in the mantle by subduction of continental crust fluids and rocks, thus explaining the much lower 3 He/ 4 He ratio compared to typical oceanic subduction zones. This is consistent with the interpretation that CO 2 is derived from the decarbonation of limestones at mantle depth, as corroborated by the presence of crustal carbonate melts within the low‐velocity mantle wedge. Here, we explore a model in which crustal contamination expresses the delamination of the continental lithosphere of the (previously subducted) Adria microplate, as previously inferred from geophysics and mechanical modeling. We present coupled thermal and helium isotopic modeling that accounts for radiogenic heat and 4 He production in the U, Th, and K‐rich delaminated continental crust and in the variably aged (depending on SCLM age and He residence time) 4 He‐rich subcontinental lithospheric mantle. Our models support a delamination tectonic scenario in which the unusually low 3 He/ 4 He values and high heat flow data are reproduced by introducing a 15–20 km—thick continental crust into the mantle.