[SMP37-P10] FIB-TEM analysis of an ultrahigh-temperature sapphirine + quartz granulite: Implications for P-T path
Keywords:FIB-TEM analysis , sapphirine + quartz granulite, ultrahigh-temperature metamorphism, grain boundary, India
The pelitic granulite sample examined in this study is composed dominantly of garnet, mesoperthite, orthopyroxene, sillimanite, quartz, and retrograde cordierite. Garnet is poikiloblastic and contains numerous inclusions such as sapphirine, quartz, spinel, orthopyroxene, plagioclase, and K-feldspar. In this study, the boundary between sapphirine and quartz within garnet was sliced using a JEOL JIB-4000 FIB to prepare electron transparent cross-sectional samples for TEM observation. The transmission electron microscopy of the samples was obtained with a JEOL JEM-2100F TEM operated at 200 kV and equipped with energy dispersive X-ray spectroscopy (EDS) at TEM Station in NIMS. Our TEM observations and EDS analyses identified open grain boundaries of about 20 nm between sapphirine and quartz, although parts of them are filled with carbon, sillimanite, and unknown nanosized minerals that comprises Fe, Mg, Al, Si, and O with/without Ca (possibly garnet). The results of this study thus suggest that the garnet and sillimanite could have been formed from sapphirine and quartz, suggesting the progress of the following continuous reaction in FMAS system related to near-isobaric cooling:
sapphirine + quartz => garnet + sillimanite
The reaction could have taken place at low-H2O activity condition due to the occurrence of CO2, which was subsequently reduced and crystalized graphite along the grain boundary. This is the first report of nanoscale reaction textures between sapphirine and quartz obtained by FIB-TEM analysis, and we found this technique is useful for the evaluation of post-peak P-T evolution of UHT granulite terranes.
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Tateishi, K., Tsunogae, T., Santosh, M., Janardhan, A.S. (2004) First report of sapphirine + quartz assemblage from southern India: implications for ultrahigh-temperature metamorphism. Gondwana Research, 7, 899-912.