Pressure-temperature estimates of the lizardite/antigorite transition in high pressure serpentinites

Serpentine minerals in natural samples are dominated by lizardite and antigorite. In spite of numerous laboratory experiments, the stability fields of these species remain poorly constrained. This paper presents petrological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from the Alpine paleo-accretionary wedge. Serpentine varieties were identified from a range of metamorphic pressure and temperature conditions from sub-greenschist (P<4 kbar, T similar to 200-300 degrees C) to eclogite facies conditions (P>20 kbar, T>460 degrees C) along a subduction geothermal gradient. We use the observed mineral assemblage in natural serpentinite along with the T-max estimated by Raman spectroscopy of the carbonaceous matter in associated metasediments to constrain the temperature of the lizardite to antigorite transition at high pressures. We show that below 300 degrees C, lizardite and locally chrysotile are the dominant species in the mesh texture. Between 320 and 390 degrees C, lizardite is progressively replaced by antigorite at the grain boundaries through dissolution-precipitation processes in the presence of SiO2 enriched fluids and in the cores of the lizardite mesh. Above 390 degrees C, under high-grade blueschist to eclogite facies conditions, antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460 degrees C. (C) 2012 Elsevier B.V. All rights reserved.