17:15 〜 18:30
[SMP25-P12] Hidden intact coesite in deeply subducted rocks and its geological implications
キーワード：コース石、石英仮像、相転移、ラマン分光法、透過型電子顕微鏡法
Coesite, a high-pressure phase of SiO2, is a diagnostic indicator of ultrahigh-pressure (UHP) metamorphism, oftentimes implying subduction of a rock to a minimum depth of 80 km. Deciphering the petrology of UHP rocks will improve interpretations of not only the tectono-metamorphic evolution in ancient subduction and collision zones, but also the underlying framework of plate tectonics. Tracking the spatial extent of UHP metamorphic terranes within a convergent plate margin is the first step toward understanding the fluxes of subducted crust. Previous petrological studies indicate that metamorphic coesite rarely occurs as inclusions in rigid minerals, such as zircon and garnet (Chopin, 2003 and references therein). Most coesites are often partially or completely obscured owing to the formation of polycrystalline quartz grains surrounded by radial cracks (i.e., quartz pseudomorphs), driven by an increase in the volume during the exhumation process. In contrast, recent numerical modeling and deformation experiments show that UHP conditions may reach shallower depths in the lithosphere than previously estimated, presumably via overpressure at both microscopic and macroscopic scales (e.g., Schmalholz and Podladchikov, 2013; Richter et al., 2016). This debatable conflict between petrographic observations and theoretical notions has led to intense debate regarding the pressure-to-depth conversion of metamorphic rocks. Through integrated micro-Raman analyses and modern nano-observation techniques employing a focused ion beam system and transmission electron microscopy (FIB–TEM), this study reports the occurrence of minuscule coesite (<20 µm) as abundant inclusions in garnet-rich impure quartzites from the Lago di Cignana Unit (LCU) in the Italian Western Alps.
Coesite was identified by a strong Raman band at a wavenumber (ν) of 521 cm−1. Representative bands of coesite inclusions completely sealed inside garnet grains showed slight shifts to higher frequencies (ν = ∼522 cm−1), which is indicative of residual pressures retained by the coesite inclusion in the garnet. Raman intensity maps revealed that these SiO2 inclusions are composed entirely of coesite, with no quartz present. The results obtained via Raman intensity mapping of the LCU samples yielded residual pressure values for coesite inclusions of up to 0.5 GPa, with heterogeneous stress observed within the crystals. Bright-field TEM images show that the interface structures form notably sharp boundaries with no crystal defects, such as dislocations, stacking faults, or microcracks observed within either coesite or garnet. The high-resolution TEM image displays the lattice fringes of coesite along the [100] direction, corresponding to the lattice spacings of the (020) plane for coesite (d values = 0.62 nm). Our TEM results show only the lattice fringes of either coesite or garnet, further indicating that no quartz crystallized at the nano-interfaces.
We suggest that common coesite-derived quartz pseudomorphs are less typical structures in UHP metamorphic rocks. The discovery of such intact coesite inclusions may fill the gaps in the predicted and observed abundance of coesite worldwide. In addition, abundant intact minuscule coesite inclusions may be hidden in many established UHP localities, and overlooked because of their size. Our findings would play an essential role in reappraisal of the extent of deeply subducted rocks and their rheology.
Coesite was identified by a strong Raman band at a wavenumber (ν) of 521 cm−1. Representative bands of coesite inclusions completely sealed inside garnet grains showed slight shifts to higher frequencies (ν = ∼522 cm−1), which is indicative of residual pressures retained by the coesite inclusion in the garnet. Raman intensity maps revealed that these SiO2 inclusions are composed entirely of coesite, with no quartz present. The results obtained via Raman intensity mapping of the LCU samples yielded residual pressure values for coesite inclusions of up to 0.5 GPa, with heterogeneous stress observed within the crystals. Bright-field TEM images show that the interface structures form notably sharp boundaries with no crystal defects, such as dislocations, stacking faults, or microcracks observed within either coesite or garnet. The high-resolution TEM image displays the lattice fringes of coesite along the [100] direction, corresponding to the lattice spacings of the (020) plane for coesite (d values = 0.62 nm). Our TEM results show only the lattice fringes of either coesite or garnet, further indicating that no quartz crystallized at the nano-interfaces.
We suggest that common coesite-derived quartz pseudomorphs are less typical structures in UHP metamorphic rocks. The discovery of such intact coesite inclusions may fill the gaps in the predicted and observed abundance of coesite worldwide. In addition, abundant intact minuscule coesite inclusions may be hidden in many established UHP localities, and overlooked because of their size. Our findings would play an essential role in reappraisal of the extent of deeply subducted rocks and their rheology.