• Influence of bulk chemistry on phase assemblages
  • HP in acid rocks
  • high and low strain domains
  • Fe oxidation state
  • Low grade HP metasediments
  • Mineralogy, new minerals
  • Variscan Magmatism

  • Influence of bulk composition on phase assemblages

    During my thesis on blueschists from Mesozoic metabasites in the Central Alps, I documented the effect of bulk composition and oxidation state of iron on the occurrence and stability of glaucophane (Oberhänsli, 1977).

    The eclogitic pillow lava of the Rimpfischhorn near Zermatt (Oberhänsli, 1981, 82) rest in a greenschist to lower amphibolite matrix and exhibt blueschist rims. This was interpreted as effect of fluid forced pre- metamorphic changes in the bulk composition and oxidation state of the primary mafic volcanics.

    Similarly we interpret the variolitic rims of UHP pillow lava (Oberhänsli et al., 2002) and isotopic signatures of UHP marbles (Romer et al., 2003) from the Dabie Shan as effect of bulk composition on the reactivity and changes in mineral assemblages.

    High pressure record in acidic and granitoid compositions

    G. Martinotti and H. Hunziker introduced me to the Sesia-Lanzo geology and the Monte Muccrone intrusive complex. There we showed the influence of fluid and deformation enabling the evolution of high-pressure phase assemblages in granitoid compositions (MM1), quantified pressure temperature conditions (MM2) and provided one of the first attempts to date HP-phases (MM3) in the Alps (Oberhänsli et al., 1985).

    High and low strain domains

    Prograde solid inclusion changing their specific volume during exhumation can provoke internal overpressure leading to crack in the host mineral as obvious in coesite bearing pyrope from Dora Maira. Similarly cracks can form at the a-b quartz transition. A crack barometer was developed for such general cases (Wendt et al., 1993)
    Relics of a prograde evolution can survive in low strain islands if the local bulk chemical composition is not reactive along the exhumation P-T trajectory. This was demonstrated on variolitic pillow lava from the UHP egion in the Dabie Shan (Oberhänsli et al., 2002)

    Fe determination for petrology

    TI failed with my first attempt to determine the oxidation state of iron in glaucophane with the Mössbauer method due to minuscule epidote inclusions that could not be separated by hand picking during my doctorate thesis.
    Later approaches with the electron microprobe in Mainz (Höfer et al., 199y) were partly successful although only for spinels and garnets.
    Finally with XANES we succeeded to determine the oxidation state fro garnet, omphacite and phengite in UHP rocks of the Dabie Shan
    Together with Max Wilke and Robert Schmid (Schmid et al., 2003 ), we showed the significant difference between measurement and stoechoimetric calculation of Fe3+/Fe2+ in OH-free phases and were able to quantify the influence of the of the oxidation state for temperature estimates

    Low grade HP metasediments

    Metapelites may host blueschist facies minerals such as carpholite which can be associated with kaolinite, sudoite, pyrophyllite, white mica, chlorite, chloritoid or kyanite. The reaction carpholite-> chloritoid + quartz is dependant on XFe and can be used as a geothermometer. A petrogenetic grid for the stability of carpholite and these phases demonstrates the dependance of Si component in phengite as a geobarometer on the phengite producing reactants (Oberhänsli et al., 1995). Reinvestgation in the Grisons allowed to establish the a blueschist belt , using Al-rich metasediments (Bousquet et al., 2002). Thus, to assess pressure a multi equilibrium approach has been suggested by Vidal et al., (2000) and tested in Western Turkey (Rimmele et al., 2005). Up to now so far unconstrained metasedimentary “blueschist belts” could be established in Oman, the Central Alps, Western and Eastern Turkey and the Himalaya.

    Mineralogy, new minerals

    Work on mineralogical topics stated with a collaboration on kosmochlore together with Irmgard Abs (Abs et al., 1985). Thomas Armbruster motivated me to collaborate on the crystal chemistry of double ring silicates. Among other minerals such as millarite an dsugilithe we investigated ossumilithe from volcanic and hightemperatur e metamorphic rocks (Armbruster et al., 1988a,b, 1989). Later work on lawsonite like structures from the Wessel mine in South Africa. In the course of this cooperation we described two new minerals hennomartinite and kornite (Armbruster et al., 1992, 1993a,b). Lately I fount a new locality of blue jadeite in the SE Iranian blueschist belt near Shorkhan.. The jadeitites are of very high purity and the color is sky blue.. Contrarily to other “blue jade” this jadeite is not highly enriched in Titanium (Oberhänsli et al., 2007)

    Variscan Magmatism

    During my time at the University in Bern I investigated the metalamprophyres from the Aar- and Gotthard massiv. This investigations were motivated by the idea that mafic to intermediate compositions should react more sensitive to the low grade metamorphism than the acid granitic host rocks of these external massifs (Oberhänsli, 1986; 1987, 1991; Stille et al., 1989).
    Shortly before the unification of the two Germanys Thomas Wenzel was invited to Berne with a Swiss stipend. During his stay in Berne we initiated a cooperation on the evolution of magmatic rocks along the Elbe lineament, especially the Meissen Massif (Wenzel et al., 1991,1997, 2000)
    During his post doctoral stay in Potsdam Sakae Sano from Matsuyama and I had the unique chance to work on the samples form a prospecting-well into the Harzburg intrusion and to get geochemical and isotopic insights into fresh samples of the type locality. This allowed to reflect on the origin of the harzburgites from the Harz Mountains (Sano et al., 2002)