Ancient recycled lower crust in the mantle source of recent Italian magmatism.

Recycling of Earth's crust through subduction and delamination contributes to mantle heterogeneity. Melt inclusions in early crystallised magmatic minerals record greater geochemical variability than host lavas and more fully reflect the heterogeneity of magma sources. To date, use of multiple isotope systems on small (< 300 µm) melt inclusions was hampered by analytical limitations. Here we report the first coupled Sr-Nd-Pb isotope data on individual melt inclusions from potassium-rich lavas from neighbouring Quaternary volcanoes in central Italy and infer the presence of a previously unidentified ancient lower crustal component in the mantle. We suggest derivation from Variscan or older basement included in the upper mantle by either delamination, sediment recycling, subduction erosion and/or slab detachment processes during Cenozoic subduction and collision of the western Mediterranean. The capability to determine isotope ratios in individual melt inclusions permits the detection of distinctive mantle contaminants and can provide insights into how geodynamic processes affect subduction recycling.

The amount of recycled crust in sources of mantle-derived melts.

Plate tectonic processes introduce basaltic crust (as eclogite) into the peridotitic mantle. The proportions of these two sources in mantle melts are poorly understood. Silica-rich melts formed from eclogite react with peridotite, converting it to olivine-free pyroxenite. Partial melts of this hybrid pyroxenite are higher in nickel and silicon but poorer in manganese, calcium, and magnesium than melts of peridotite. Olivine phenocrysts' compositions record these differences and were used to quantify the contributions of pyroxenite-derived melts in mid-ocean ridge basalts (10 to 30%), ocean island and continental basalts (many >60%), and komatiites (20 to 30%). These results imply involvement of 2 to 20% (up to 28%) of recycled crust in mantle melting.

An olivine-free mantle source of Hawaiian shield basalts.

More than 50 per cent of the Earth's upper mantle consists of olivine and it is generally thought that mantle-derived melts are generated in equilibrium with this mineral. Here, however, we show that the unusually high nickel and silicon contents of most parental Hawaiian magmas are inconsistent with a deep olivine-bearing source, because this mineral together with pyroxene buffers both nickel and silicon at lower levels. This can be resolved if the olivine of the mantle peridotite is consumed by reaction with melts derived from recycled oceanic crust, to form a secondary pyroxenitic source. Our modelling shows that more than half of Hawaiian magmas formed during the past 1 Myr came from this source. In addition, we estimate that the proportion of recycled (oceanic) crust varies from 30 per cent near the plume centre to insignificant levels at the plume edge. These results are also consistent with volcano volumes, magma volume flux and seismological observations.