Strontium isotope signature of the PGI lemons Limone Costa d'Amalfi and Limone di Sorrento, and of the orchard soils from Sorrento peninsula.

The Limone Costa d'Amalfi and Limone di Sorrento lemons from the Sorrento Peninsula have Protected Geographical Indication (PGI) and are subject to origin fraud. The 87Sr/86Sr ratio (SrIR) signature of lemons and soils was investigated to verify its reliability to trace the PGI lemons and to highlight environmental factors responsible of the lemon SrIRs. The SrIR ranges of each PGI lemon were not discriminating as they overlapped with each other and some non-PGI lemon SrIRs fell within these ranges. The lemon SrIRs were generally not correlated with bulk and bioavailable soil SrIRs, rather, they were the result of plant Sr uptake with different SrIRs depending on interaction between water supplied to soil and soil with different chemical and physical characteristics. The study of lemon SrIRs and the causes of their origin and variability provides a reliable forecast reference for the other PGI agri-food products in the study area.

A heterogeneous subcontinental mantle under the African-Arabian Plate boundary revealed by boron and radiogenic isotopes.

The northern and northwestern margins of the Arabian Plate are a locus of a diffuse and long-lasting (early Miocene to Pleistocene) Na-alkali basaltic volcanism, sourced in the asthenosphere mantle. The upwelling asthenosphere at the Africa-Arabia margin produces very limited magma volumes in the axial zone. Therefore, portions of hot, fertile mantle continue their eastward migration and are stored at shallower depths under the 100-km thick Arabian lithosphere, which is much thinner than the African one (≈175 km): this causes the occurrence and 20-Ma persistence of magma supply under the study area. Erupted basalts sampled a continuous variation of the mantle source, with a striking correlation among temperature, pressure and isotopic composition shifting between two end members: a 100 km-deep, more depleted source, and a 60 km-deep, more enriched one. In particular, we observed an unusual variation in boron isotopes, which in the oceanic domain does not vary between more depleted and more enriched mantle sources. This study shows that, at least in the considered region, subcontinental mantle is more heterogeneous than the suboceanic one, and able to record for very long times recycling of shallow material.

No significant boron in the hydrated mantle of most subducting slabs.

Boron has become the principle proxy for the release of seawater-derived fluids into arc volcanics, linked to cross-arc variations in boron content and isotopic ratio. Because all ocean floor serpentinites so far analysed are strongly enriched in boron, it is generally assumed that if the uppermost slab mantle is hydrated, it will also be enriched in boron. Here we present the first measurements of boron and boron isotopes in fast-spread oceanic gabbros in the Pacific, showing strong take-up of seawater-derived boron during alteration. We show that in one-pass hydration of the upper mantle, as proposed for bend fault serpentinisation, boron will not reach the hydrated slab mantle. Only prolonged hydrothermal circulation, for example in a long-lived transform fault, can add significant boron to the slab mantle. We conclude that hydrated mantle in subducting slabs will only rarely contribute to boron enrichment in arc volcanics, or to deep mantle recycling.