Phase relationships of ellenbergerite, a new high-pressure Mg-Al-Ti-silicate in pyrope-coesite-quartzite from the Western Alps

The new Mg-Al-Ti-silicate ellenbergerite occurs among abundant inclusions within decimetre-size garnets in the pyrope-quartzite layer of the Dora Maira massif, Western Alps, from which coesite relics have been reported. It is associated with pyrope (92 to 98 mole percent end-member), kyanite, talc, chlorite, rutile, zircon, and minor sodic amphibole, which all formed an apparently stable assemblage now exclusively preserved within large garnets. A petrologic analysis shows that the new mineral, which has the formula (Mg1/3, Ti1/3, □1/3)2Mg6Al6Si8O28(OH)10 with extensive Ti ⇌ Zr substitution, is a high-pressure phase with a lower pressure stability limit above 20 kbar and an upper temperature limit near 800°C or less. Its stability field extends from the Mg-carpholite field to the pyrope field and broadly overlaps that of Mg-chloritoid. The high-pressure phase relationships in the magnesian pelitic system are thus considerably modified, in particular through the reactions chlorite + talc + kyanite + rutile + H2O = ellenbergerite and chlorite + kyanite + rutile + H2O = ellenbergerite + Mg-chloritoid, which provide new upper pressure limits for two important chlorite-bearing assemblages. The different assemblages preserved within the garnets and in the matrix record variations of pressure, temperature, and water activity along a prograde metamorphic path passing near 25 kbar, 700°C, and reaching the coesite field. This finding confirms the unusual depth reached by the enclosing continental unit along a low-temperature metamorphic gradient.