The role of apatite in the melting of pargasite-bearing metasomatized mantle. Impliacations in the the generation of primary vaugneritic liquids

Resum

A set of high pressure experiments have been performed using as the starting material pargasite-bearing pyroxenites appearing as xenoliths in Holocene alkaline basalts from Olot (Girona). The aim to test the role of pargasite (Pg), and accompanying apatite, in the generation of liquids of monzodioritic (vaugneritic) composition. Our study show that Ap-doped runs are much more fertile compared with the Ap-free runs at the same conditions of pressure and temperature. In the absence of Ap, a Pg-bearing pyroxenite produces traces (<1 wt%) of melt, whereas in the Ap-bearing runs, the obtained melt fractions are 6.8 wt% at 1.5 GPa and 20.6 wt% at 1.0 GPa, at 1100 °C. The melts formed at 1.5 GPa are very similar to common vaugneritic rocks. These results confirm the inference that vaugnerites are (1) primary melts derived from a metasomatized mantle source and (2) their origin is related with stages of lithosphere decompression, as pargasite is the main mineral involved at the source region. We also demonstrate that Ap has a strong control by increasing the melt fraction in the source. It is envisaged that apatite is also involved in the supply of incompatible elements that characterize vaugneritic rocks