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

Dipartimento di Scienze della Terra, Università di Siena Via Laterina 8, I-53100, Siena, Italy

^* E-mail: vitic{at}unisi.it

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°C range, lizardite has to be considered as the high-temperature serpentine phase in retrograde serpentinites.

Key-words: serpentinites, olivine, lizardite, reaction, TEM.

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