No need for involvement of a hidden mantle reservoir in the origin of lamproites from Mediterranean

Recent discussion about Hf-Nd isotope geochemistry focused on the composition of lamproites and kimberlites as possible evidence for the existence of a hitherto unrecognized ‘hidden’ terrestrial reservoir. This was triggered by the discovery of an apparent discrepancy between terrestrial Hf- Nd isotopes and the BSE chondritic reference value, which could be resolved by the existence of a new geochemical reservoir with Hf-Nd isotopic values below the terrestrial array. Most lamproites do not follow the Hf-Nd array defined by other terrestrial samples, but have variable Nd at almost constant Hf isotopic values. This triggered an attractive hypothesis about their role in the evolution of the Earth and their potential to sample the ‘hidden’ reservoir. Here, we present Hf, Sr, Nd and Pb isotope results on Mediterranean lamproites that can be used to further test this possibility. Mediterranean lamproites are derived from multicomponent melts, which combine depleted and enriched endmembers( s). The extremely variable radiogenic isotope composition of lamproites points to the importance of mixing relations between three contrasting geochemical components which appear in 206Pb/204Pb, 87Sr/86Sr and 143Nd/144Nd space: a crustal component, an ultra-depleted mantle component derived from the lithospheric mantle and a convecting mantleoriginating component. It is the third component which is the most relevant for the deflection of lampoites from the terrestrial Hf-Nd array. Our modelling shows that the convective-mantle derived component has high Sr and Nb contents, low HFSE4+ relative to LREE, significantly higher Nb/Ta and Zr/Hf ratios, and lower Zr/Nb and Zr/Ta ratios relative to OIB and chondrite, whereas isotopic compositions are similar to OIB. This geochemical signature is considered as a “hallmark” of mantle carbonatite. In Hf-Nd isotopic space, this component is responsible for the shift of the lamproite values from terrestrial array. We interpret this shift as a mixing hyperbola between carbonatitic melts derived from sublithospheric mantle and lamproitic melts. The deviation from the array is due to the large range in Hf/Nd ratios that vary from up to 0.20 in lamproitic melts to <0.01 in asthenospheric melts, caused by extremely high Zr and Hf concentrations in lamproites (up to 1000 and 30 ppm, respectively), and very low Hf contents in carbonatitic melts. We discuss a geodynamic scenario which provides a suitable environment for the interaction of coeval asthenosphere-derived and lamproitic melts throughout the Mediterranean region.