Mesh textures and bastites in the Elba retrograde serpentinites

Retrograde serpentinized peridotites, belonging to the Internal Ligurid terrains, occur in eastern Elba island, Italy The mesh texture (with pyroxene bastites) is the typical textural arrangement. The mesh rims are apparently fibrous, with negative optical elongation; cores range from isotropic to microgranular. Optical and chemical determinations indicate harzburgite (75 % olivine and 25 % pyroxene) as the protolith for Elba serpentinites, with characteristics similar to mantle peridotites from ocean basins. Microstructural analysis, led by transmission electron microscopy, indicates that the mesh rims, rather than being actually fibrous, consist of elongated lizardite crystals, having sharp triangular sections with apices pointing toward the mesh center; poorly crystallized material occurs in between adjacent lizardite sectors. The mesh cores consist of lizardite, chrysotile and polygonal serpentine, intermixed in variable amount and random mutual orientation. Bastites are generally poorly crystalline, formed by tiny lizardite lamellae with minor chrysotile. Mesh rims and cores have chemical compositions variable from specimen to specimen, but almost constant within each specimen; cores are always aluminium-enriched with respect to the rims; when present, nickel (after olivine) occurs in the meshes. Bastites are chemically simpler than meshes, and, independently from their optical appearance, always have an aluminium-rich composition; chromium (after pyroxene) always characterizes bastites. Chemical data indicate absence of long range chemical fluxes during serpentinization. The formation of the mesh texture (with bastites) is explained as a two-stage process. In stage I, the peridotite undergoes thermal fracturing and lizardite flakes start to decorate the pseudocubic fracture pattern (thus producing the mesh rim). Massive water arrival into the weakened peridotite induces stage III where serpentinization is completed forming the mesh cores and bastites. In stage I the reaction is thermodynamically controlled, in stage II the reaction is kinetically controlled.