Polyphase polygonal serpentine veins in the Rocciavrè meta-ophiolite (Western Alps, NW Italy)

Widespread polygonal serpentine (PS) veins occur in the ultramafics of the Rocciavre` meta-ophiolite, which consists of alternating metadunite and antigorite-serpentinite bands. The PS veins exhibit a complex polyphasic history that includes three PS domains (Dom 1, Dom 2 and Dom 3), porphyroblasts of aluminian-lizardite (Al-Lz) and minor brucite. Four steps of vein dilation have been recognized. The microstructure indicates that Dom 1 is constituted by a PS pseudomorph after the original olivine, the first mineral that heterogeneously nucleated on the fracture wall. Al-Lz and brucite porphyroblasts developed near the wall-rock overgrowing olivine. Dom 2 is constituted by an aggregate of PS + brucite that completely seals the vein. Dom 3 is constituted by the last generation of PS that grew as coarser-grained fiber bundles partially replacing Dom 2. Chemical analyses showed that PS of Dom 1 and Dom 2 have a similar, very low Fe-content, whereas PS of Dom 3 have systematically slightly higher Fe, which is possibly responsible for the rusty color typical of weathered surfaces. Porphyroblastic Al-Lz has high Al and minor Fe contents. Late barite sealed fractures opened in correspondence of Al-Lz cleavage planes. X-ray powder diffraction and mu-Raman revealed the presence of both PS polytypes, PS-15, with 15 sectors, and PS-30, with 30 sectors, respectively, and minor brucite. TEM images showed the presence of both PS-15 and PS-30, locally associated with brucite and rare chrysotile. mu-Raman maps confirmed that all domains contain the same submicroscopic association of PS-15, PS-30 and brucite, in highly variable ratios from point to point. Each PS vein is bounded by two reaction zones, where the host metadunite is altered to an aggregate of lizardite + magnetite + brucite + minor chlorite with accessory awaruite and heazlewoodite. Therefore, the close relationship between metadunite and PS veins supports a local origin for vein feeding elements. The vein mineralogy indicates genetic conditions characterized by CO2-free aqueous fluid, slightly silica-undersaturated bulk composition, and low fO2 compatible with magnetite, Fe=Ni alloys and Ni sulphides. The PS veins are crossed by later chrysotile veinlets, in turn cut by very thin veins of euhedral brucite included in a fine-grained carbonate aggregate, identified by mu-Raman spectroscopy as a mixture of artinite and hydromagnesite. The coeval development of PS in the vein and lizardite in the reaction zone of the host metadunite suggests that these two phases must have, at least in part, a superposed stability field and that other factors, including kinetics factors, must be responsible for the fibrous habit of serpentine-either chrysotile or PS, since the latter derives from the former-in the vein.