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

Abstract

Partially serpentinized harzburgites from Southern Tuscany, Italy, show serpentine replacing the peridotitic minerals, as
\nrims 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.
\nStudied by HRTEM and AEM, the olivine serpentine replacement reveals a complex sequence of reaction steps. Initially,
\nolivine 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 talclike layers. No chrysotile fiber occurs at the reaction front. Although the olivine-to-lizardite reaction is clearly not-topotactic,
\nrecrystallization of early formed serpentine leads to large lizardite sectors, oriented with (001) almost parallel to the reaction front.
\nAs the olivine-to-lizardite reaction is estimated to occur in the upper 300-500°C range, lizardite has to be considered as the
\nhigh-temperature serpentine phase in retrograde serpentinites.