Late metamorphic veins with dominant PS-15 polygonal serpentine in the Monte Avic ultramafite

The ultramafic body of Monte Avic (Aosta Valley, Western Alps, Italy) consists of antigorite serpentinite and Ti-clinohumite metadunite. They host late metamorphic veins, up to a couple of centimeters thick, compact, and homogeneous, with a “porcelain” appearance. Vein colors range from yellowish to light greenish, light yellowish fading to white, or rare orange. The veins consist of 15-sector PS-15 polygonal serpentine, with chemical composition Mginline-formula_2.85 Feinline-formula_0.08 Siinline-formula_2.05 Oinline-formula_7.05 [OH]inline-formula_3.95. Recognition of this unusual phase is supported by diagnostic satellite reflections in the X-ray powder diffraction pattern (e.g., at inline-formulad_obs of 2.502, 2.336, 2.151, and 1.966 Å) TEM images (showing 15-sector polygonal fibers, mostly 200 nm in diameter and a few inline-formulaµm in length, forming a randomly oriented felt) and a inline-formulaµ-Raman wavenumber, matching previous data. This different evidence affords the successful distinction of PS-15 and PS-30, alternatively using TEM images, X-ray powder diffraction, or the low- and high-wavenumber inline-formulaµ-Raman spectra. At Monte Avic, the vein emplacement was accompanied by significant fluid pressure, as suggested by deformation and dismembering of the host rock, with PS-15 grown within isotropic stress microenvironments characterized by fluid-filled voids. Random growth of the mass-fiber polygonal serpentine was favored by low-strain conditions. PS-15 veins formed at the end of the long polyphase Alpine orogenic evolution, with hydrous fluids possibly deriving from serpentinite dehydration in the depth.