Exsolution and hydration of pyroxenes from partially-serpentinized harzburgites

Ortho- and clinopyroxenes within partially-hydrated harzburgites from Elba and Val di Cecina (Italy) show lamellar exsolution textures and variable replacement by biopyriboles, talc-chlorite-serpentine mixed layers and serpentine. Chemical and geothermometric data suggest that the pyroxenes crystallized at 1240-1051°C, followed by subsolidus exsolution at slightly lower T (1145-1025°C for clinopyroxene lamellae + orthopyroxene matrix pairs and a 1033-982°C range for orthopyroxene lamellae + clinopyroxene matrix pairs). Investigation by transmission e lectron microscopy of exsolved enstatite and augite reveals a multistage hydration process. The first stage (highest T, probably in the amphibole stability field) leads to the formation of biopyribole lamellae within exsolved augite, leaving the enstatite matrix unaffected. The second stage (∼500-300°C) corresponds to the topotactic replacement of enstatite by layer silicates (talc + chlorite + serpentine, with (001)layer silicates parallel to (100)enstatite). Enstatite is also replaced by randomly oriented, poorly crystalline serpentine. The last hydration stage (<300°C) corresponds to the disappearence of augite and recrystallization of serpentine, leading to completely hydrated bastites with random lizardite lamellae, polygonal serpentine and minor chrysotile. © 2005 The Mineralogical Society.