3:30 PM - 3:45 PM
[SMP26-06] Raman shift of coesite inclusions:
A reappraisal of UHP inclusion mineralogy of Dora-Maira whiteschist
Keywords:coesite, Raman spectroscopy, ultrahigh-pressure metamorphism, whiteschist
Due to differences in thermal expansion and volumetric modulus with surrounding host minerals, mineral inclusions in metamorphic minerals can retain residual stresses. The residual stress is visible as Raman shifts in Raman spectra. In fact, the residual stress for quartz inclusion in garnet can be used for geobarometry. On the other hand, the residual stresses of coesite, a high-pressure polymorph of quartz, have not been intensively studied in regional metamorphic rocks. The occurrence of coesite in supracrustal rock is synonymous with ultrahigh-pressure (UHP) metamorphism, which refers to metamorphism of crustal rocks at depths as great as >~100 km. Trace amount of coesite can be partially preserved in strong 'container' minerals such as garnet, zircon and kyanite in world-wide UHP terranes. However, the occurrence of intergranular coesite in matrix is extremally rare; there are only a few reports from Dora-Maira massif, Lago di Cignana UHP unit, and Sulu UHP terrane.
In order to characterize Raman shifts of the major Raman peaks of coesite inclusion, we separated ~400 grains of pyrope garnet, 88 grains of kyanite, and 40 grains of zircon, and ~400 grains of rutile from a Dora-Maira whiteschist sample, to investigate Raman spectrum of their ~120 coesite inclusions. We confirmed that the kyanite-hosted coesite inclusions (~4–58 µm in size) show pressure-dependent Raman shifts, with a maximum value of 524.4 cm-1. The shift of the peak at ~176 cm-1 was also significant, suggesting the maximum residual stress of 1.74 GPa. Coesite inclusions in kyanite retain high residual stress even near the polished surface of the crystal. Coesites within kyanite are classified into three morphological types: rounded, pseudohexagonal, and irregular shapes. Pseudohexagonal and irregular-shaped coesites show higher Raman shifts than rounded ones; this might be due to higher interfacial energy of the pseudohexagonal and irregular-shaped inclusions. In this contribution, we report some new insights on the relationship among Raman shift (~521 and ~176 cm-1 peaks), host minerals, and morphology of coesite inclusions.
In order to characterize Raman shifts of the major Raman peaks of coesite inclusion, we separated ~400 grains of pyrope garnet, 88 grains of kyanite, and 40 grains of zircon, and ~400 grains of rutile from a Dora-Maira whiteschist sample, to investigate Raman spectrum of their ~120 coesite inclusions. We confirmed that the kyanite-hosted coesite inclusions (~4–58 µm in size) show pressure-dependent Raman shifts, with a maximum value of 524.4 cm-1. The shift of the peak at ~176 cm-1 was also significant, suggesting the maximum residual stress of 1.74 GPa. Coesite inclusions in kyanite retain high residual stress even near the polished surface of the crystal. Coesites within kyanite are classified into three morphological types: rounded, pseudohexagonal, and irregular shapes. Pseudohexagonal and irregular-shaped coesites show higher Raman shifts than rounded ones; this might be due to higher interfacial energy of the pseudohexagonal and irregular-shaped inclusions. In this contribution, we report some new insights on the relationship among Raman shift (~521 and ~176 cm-1 peaks), host minerals, and morphology of coesite inclusions.