Serpentinite dehydration at low pressures

Abstract Petrographic observations combined with mineral compositional analyses constrain the phase relations of prograde metamorphosed serpentinites in Bergell contact aureole (Italy). In a 1500 m profile perpendicular to north-eastern edge intrusion, seven dehydration reactions ran completion. Three previously undocumented have been identified within 70 intrusive contact: olivine + anthophyllite = orthopyroxene H 2 O, tremolite Cr–Al-spinel Mg-hornblende O and chlorite Cr-Al-spinel O. Petrological analysis indicates that these occur over narrow range pressure temperature, 300 ± 30 MPa 720 10 °C respectively. Computed diagram sections reproduce observed parageneses one notable exception. Due underestimation aluminium sodium contents Ca-amphibole models, plagioclase is predicted above 700 instead Mg-hornblende. comparison natural grains, content computed compositions overestimated for low grade while it underestimated near upper thermal stability limit chlorite. sections, Fe partitioning relative Mg between other silicates, suggests clear preference olivine, therefore shows lower Mg#s. contrast, microprobe pairs indicate orthopyroxene, Mg#s than equilibrium olivine. The inferred metamorphic not consistent simple heat conduction models temperature ~ 800 °C, which 120–230 higher estimated. Petrography also reveals extensive retrograde overprint 200 contact. Retrogression related fluids were released by during metamorphism magmatic expelled from tonalite intrusion. gradient intrusion country rocks induced hydrothermal circulation throughout aureole, beyond peak conditions caused parageneses. proposed high pressures, particular, transition Mg-hornblende, provides complete representation hydration processes subduction zones, along deep oceanic transform faults, at passive continental margins. latter has new implications, specifically initiation.