Geochemistry of apatite-rich layers in the Finero phlogopite–peridotite massif (Italian Western Alps) and ion microprobe dating of apatite

Highly metasomatized apatite-rich peridotite layers (AP-layer) occur in the Finero phlogopite–peridotite massif (western Italian Alps). The AP-layer and the host peridotite are characterized by higher concentration of REE (total REE > 80ppm), especially LILE, and lower HFSE, than those of the other rocks in the Finero massif. The AP-layers and their host peridotites have bulk silicate Earth-like Sr and Nd-isotope compositions except for one sample containing rare carbonate aggregates. The carbonate aggregates occur in a late serpentine-talc veinlet and show relatively high d13C and d18O, high 87Sr/86Sr and lower ɛNd than the other samples. These results, combined with the previous data, indicate that the carbonates formed late at relatively low temperatures. The petrochemical results along with the literature suggest that the AP-layer was locally formed from a metasomatic agent geochemically distinct from that formerly producing phlogopite-bearing harzburgite widely distributed throughout the massif. We propose that the metasomatic agent for the AP-layer was an orthopyroxene-saturated, CO2-bearing hydrous fluid/melt that evolved through the crystallization of metasomatic minerals upon reactions with peridotites. Sensitive high-resolution ion microprobe (SHRIMP) analyses of apatite grains yielded a Tera-Wasserburg concordia three-dimensional isochron age of 215 ± 35Ma in the 238U/206Pb–207Pb/206Pb–204Pb/207Pb diagram. The age is similar to intrusions in the Finero area, suggesting that the carbonate- and apatite-metasomatism was synchronous with Triassic magmatic activity.