Noble gases in the Finero phlogopite-peridotites, western Italian Alps

Here we report results from an investigation of noble gases in a specimen with layers of very fine-grained apatite and orthopyroxene from the Finero ultramafic complex, western Italian alps. We have also examined fresh olivine grains separated from phlogopite-harzburgites without an apatite layer. Fluid inclusions of these olivine separates appeared rich in radiogenic components such as ⁴He, ²¹Ne and ⁴⁰Ar, suggesting the derivation of fluids from a crust-like reservoir. Such a crust-like component also appeared to be contained in the samples with the apatite-opx layer, however, an additional isotopically distinct component is necessary to explain progressive increases in ³He concentration and decreases in ⁴⁰Ar/³⁶Ar ratios observed approaching the apatite-rich layer. High ³He contents and low ⁴⁰Ar/³⁶Ar ratios are consistent with the signature expected for slab-derived metasomatic fluids. There is a clear mixing trend defined in a ³He/³⁶Ar vs. ⁴⁰Ar/³⁶Ar diagram with crustal and slab-derived fluids as end-member compositions, suggesting that the complex was metasomatised by fluids derived from geochemically distinct regions and distinct geotectonic settings. The parent melt or fluids responsible for the formation of the apatite-rich layer should be derived from the slab-derived component, which is consistent with the notion that the Finero mantle was once in the mantle wedge. An in situ component produced in the previously metasomatised portion of lithospheric mantle, or fluid released from surrounding metamorphic rocks during exhumation are possible sources for the crust-like component.