Exsolution and hydration of pyroxenes from partially serpentinized harzburgites

摘要: Ortho-and clinopyroxenes within partially-hydrated harzburgites from Elba and Val di Cecina (Italy) show lamellar exsolution textures variable replacement by biopyriboles, talc-chlorite-serpentine mixed layers serpentine. Chemical geothermometric data suggest that the pyroxenes crystallized at 1240–1051°C, followed subsolidus slightly lower T (1145–1025°C for clinopyroxene lamellae + orthopyroxene matrix pairs a 1033–982°C range pairs). Investigation transmission electron microscopy of exsolved enstatite augite reveals multistage hydration process. The first stage (highest , probably in amphibole stability field) leads to formation biopyribole augite, leaving unaffected. second (~500–300°C) corresponds topotactic layer silicates (talc chlorite serpentine, with (001)layer parallel (100)enstatite). Enstatite is also replaced randomly oriented, poorly crystalline last (<300°C) disappearence recrystallization leading completely hydrated bastites random lizardite lamellae, polygonal serpentine minor chrysotile.