Oriented, non-topotactic olivine -> serpentine replacement in mesh-textured, serpentinized peridotites

摘要

Partially serpentinized harzburgites from Southern Tuscany, Italy, show serpentine replacing the peridotitic minerals, as rims around olivine and thin lamellae parallel to pyroxene cleavage. Exempt from post-serpentinization tectonometamorphic overprints, these mesh-textured serpentinites offer a favourable setting for the study of seafloor serpentinization.Studied by HRTEM and AEM, the olivine --> serpentine replacement reveals a complex sequence of reaction steps. Initially, olivine dissolves forming a silicon-enriched amorphous domain, where early serpentine nuclei are formed. These nuclei recrystallize producing oriented columnar lizardite. The lizardite in the rim shows silicon excess, due to intermixed amorphous or talc-like layers. No chrysotile fiber occurs at the reaction front. Although the olivine-to-lizardite reaction is clearly not-topotactic, recrystallization of early formed serpentine leads to large lizardite sectors, oriented with (001) almost parallel to the reaction front.As the olivine-to-lizardite reaction is estimated to occur in the upper 300-500 degreesC range, lizardite has to be considered as the high-temperature serpentine phase in retrograde serpentinites.