Late-Alpine rodingitization in the Bellecombe meta-ophiolites (Aosta Valley, Italian Western Alps): evidence from mineral assemblages and serpentinization-derived H2-bearing brine.

We report on Alpine metamorphic and fluid inclusion evolution of a polyphase rodingite occurrence within the Bellecombe antigorite-serpentinite, exposed in the Piemonte zone of Aosta Valley, NW Italy. Fine-grained rodingitic rocks, derived from a protolith of basaltic dike(s), are cross-cut by a network of at least six vein generations, consisting of chlorite, diopside, and grossular garnet (Type I), andradite-grossular garnet + diopside (Type II), andradite-rich garnet + chlorite (Type III), grossular-rich garnet (Type IV), vesuvianite (Type V), and chlorite (Type VI). The fine-grained rodingite and associated veins reveal a tectono-metamorphic history similar to that of the hosting serpentinite and characterized by an earlier high pressure metamorphism, followed by decompression/re-equilibration under greenschist facies conditions and by final cooling. The fluid inclusion study, performed on primary fluid inclusions in vesuvianite from Type V veins and on secondary fluid inclusions in andradite-rich garnet from Type III veins, revealed that at P = 0.22 GPa and T = 400°C, an H2-bearing (XH2 = 0.010) brine (6 wt.% CaCl2 + 6 wt.% NaCl) with traces of CH4 (XCH4 = 0.002) was introduced into the rock. This fluid had a composition compatible with the Ca-rich H2-bearing, reducing aqueous solutions reported from the serpentinization front. These data point to an important event of rodingitization, probably triggered by hydration of metamorphic olivine, during the late greenschist facies Alpine evolution that was probably triggered by hydration of metamorphic olivine.