Hf isotope compositions of Mediterranean lamproites: Mixing of melts from asthenosphere and crustally contaminated mantle lithosphere

Mediterranean lamproites from Spain, Italy, Serbia and Macedonia are mantle-derived ultrapotassic volcanic rocks that occur exclusively in postcollisional, extension-related geodynamic settings within the Alpine-Himalaya orogenic belt. Previous studies inferred them to be multi-component melts, originating by mixing of several mantle end-members: (1) provenance-controlled crust-contaminated mantle component(s), (2) an ultra-depleted mantle component, and (3) a component ultimately derived from the convecting mantle. Hf isotope ratios of Mediterranean lamproites reported here cover a large range of εHf values from 0 to -15, for less variable εNd -2 to -13, providing further evidence for a lithospheric origin. The Hf isotope data help to distinguish two regionally distinct crustal components both derived from zircon-bearing protoliths similar to turbiditic sediments typical for continental margins. The Hf isotopes provide unique insights into mantle mixing processes by constraining the geochemistry of the component derived from the convective mantle. This component is rich in Sr and Nb, has low Ti and HFSE⁴⁺/LREE, and isotopic compositions similar to OIB. In Hf-Nd isotope space, this component is responsible for the decoupling of Hf and Nd isotopic ratios, already recognized in lamproites worldwide. This deviation results from a strongly curved hyperbolic mixing between a proto-lamproitic melt and a small, but significant contribution from carbonatitic melts ultimately derived from the convective mantle.