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

Abstract

Serpentine minerals in natural samples are dominated by lizardite and antigorite. In spite of numerous petrological experiments, the stability fields of these species remain poorly constrained. This paper presents the petrological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from the Alpine paleo-accretionary wedge. Serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub-greenschist (P < 4 kbar, T ~ 200-300°C) to eclogite facies conditions (P > 20 kbar, T > 460°C) along a subduction geothermal gradient. We used the observed mineral assemblage in natural serpentinite along with the T max estimated by Raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the lizardite to antigorite transition at high pressures. We show that below 300°C, lizardite and locally chrysotile are the dominant species in the mesh texture. Between 320 and 390°C, lizardite is progressively replaced by antigorite at the grain boundaries through dissolutionprecipitation processes in the presence of SiO 2 enriched fluids and through a solid-state transition in the cores of the lizardite mesh. Above 390°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°C.

A large mantle water source for the northern San Andreas fault system: a ghost of subduction past

“…Based on thermodynamic modeling and experimental observations, antigorite is considered the high-temperature/ high-pressure form of serpentine that results from a reaction of peridotites with water at mantle depths (Evans et al 1976;Evans 2004;Schwartz et al 2013). Chrysotile is a low-temperature, fibrous form of serpentine and often occurs in 'veins' or systems of veins that bound antigorite blocks (Evans 2004).…”

Composition and alteration of Cr-spinels from Milia and Pefki serpentinized mantle peridotites (Pindos Ophiolite Complex, Greece)

“…During ocean-floor hydrothermal alteration, low T serpentinization did not produce any compositional or textural change in Cr-spinel, as it took place under reducing conditions (indicated by the low Fe 3+ # values), probably at T's below 300 °C where chrysotile is stable (e.g. Schwartz et al 2013). The second stage mainly has to do with the formation of ferrian chromite under higher T ( > 300 °C) hydrous fluid-saturated conditions, and involves the dissolution-precipitation reaction of primary Cr-spinel with serpentine to produce clinochlore and FeO-and Cr 2 O 3 -rich, Al 2 O 3 -poor Cr-spinel, according to the reactive mechanism proposed by Merlini et al (2009).…”

Controls on the location of compressional deformation on the NW European margin

“…Serpentinization occurs when water interacts with ultramafic rocks within the stability field of the serpentine minerals (,4608C: Schwartz et al 2013). It is possible for surface water to reach the upper mantle if the entire crust is brittle and is penetrated by faults, but if crustal temperatures are sufficiently high then a ductile lower-crustal layer may form that inhibits such penetration.…”

“…The weakening is greater where the dominant serpentinite phase is lizardite rather than antigorite, and this is likely to be the case at temperatures below 3008C. Lizardite is progressively replaced by antigorite at higher temperatures, with the latter mineral becoming ubiquitous at 3908C and secondary olivine crystallization commencing at 4608C (Schwartz et al 2013).…”