HR: 09:00h
AN: V51E-05    [Abstracts]
TI: UHP-Metamorphic Pyrope Quartzites From Dora Maira: Cathodoluminescence of Silica and Twinning of Coesite
AU: * Schertl, H
EM: hans-peter.schertl@rub.de
AF: Institute of Geology, Mineralogy and Geophysics, Ruhr-University Bochum, Bochum, 44780 Germany
AU: Medenbach, O
EM:
AF: Institute of Geology, Mineralogy and Geophysics, Ruhr-University Bochum, Bochum, 44780 Germany
AU: Neuser, R D
EM:
AF: Institute of Geology, Mineralogy and Geophysics, Ruhr-University Bochum, Bochum, 44780 Germany
AB: Since the first discovery of metamorphic coesite in ultrahigh-pressure (UHP) rocks from the Dora Maira Massif/Western Alps, much attention was drawn on its characteristics: the paragenesis, influence of OH on the kinetics of the coesite-quartz transition, present day overpressure in coesite inclusions, features like palisade-quartz as typical breakdown product, experimental studies on the rheology of polycrystalline coesite, oxygen isotope signatures, etc. Here we would like to focus on the cathodoluminescence (CL) of coesite and its breakdown products. Since luminescence is triggered even by minor differences in composition or structure of a mineral, in this study the CL microscope is employed not only as a powerful tool to distinguish between different mineral phases but also to characterize different generations of a coesite breakdown product. A second topic concerns the twinning of coesite which is very rarely observed in nature. The investigations were made on pyrope quartzite previously representing a pyrope coesitite at UHP metamorphic conditions (Chopin, 1984; Schertl et al., 1991). Main constituent phases are pyrope, quartz, phengite, talc, and kyanite with minor amounts of coesite and jadeite. The rock can be subdivided in a fine-grained type containing pyropes up to about 1.5 cm and a coarse-grained type with pyrope crystals up to 25 cm. The boundaries between both types are irregular, but they exhibit significant differences concerning their mineral inclusions: inclusions of coesite/quartz (in paragenesis with kyanite and phengite) are only observed in small pyropes whereas in big pyropes no silica phase occurs. Typical mineral inclusions in big pyropes essentially are kyanite, talc, and chlorite with minor amounts of ellenbergerite, Mg-dumortierite and sodic amphibole. Coesite typically shows bluish-green luminescence colours, whereas palisade-like quartz as breakdown product (interpreted to be formed at high temperatures) surrounding coesite is brownish-violet. Most interestingly these luminescence colours of quartz differ from those of quartz filling cracks (brown in colour, late generation of quartz) within the relic coesite crystals. The matrix quartz shows dark blue luminescence colours. In rare cases within the coesite/quartz inclusions in pyrope a mineral was observed exhibiting a bright brownish-yellow luminescence colour. Later studies revealed this mineral as chalcedony (Schertl et al., 2004, Fig. 1e), difficult to distinguish from quartz under the polarizing microscope because of their similar refractive indices and birefringences. Chalcedony as a low temperature phase was formed after cracking of the host pyrope. Twinning of coesite is reminiscent of polysynthethic plagioclase consisting of albite- and pericline-law individuals. Twinning of synthetic coesite was already reported by Ramsdell in 1955. He described (100)- and (021)-twins, that however, mainly derived from X-ray investigations because of the imperfect development of the run products. Further studies by Sclar et al. (1962) confirmed the occurrence of (021)-twins, Naka et al. (1975) provided evidence for (-121)- and (-233)-twins. Two respective types of lamellae in metamorphic coesites from Dora Maira were identified; their twin planes are intersecting each other with an angle of ca. 90°. The lamellae are twinned after (021) and (-101), the latter observed in natural coesite for the first time.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3625 Petrography, microstructures, and textures
DE: 3654 Ultra-high pressure metamorphism
DE: 3694 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005