Abstract
The equation of state and stability of topaz at high-pressure/high-temperature conditions have been investigated by in situ synchrotron X-ray diffraction (XRD) and Raman spectroscopy in this study. No phase transition occurs on topaz over the experimental pressure–temperature (P-T) range. The pressure–volume data were fitted by the third-order Birch–Murnaghan equation of state (EoS) with the zero-pressure unit–cell volume V0 = 343.86 (9) Å3, the zero-pressure bulk modulus K0 = 172 (3) GPa, and its pressure derivative K’0 = 1.3 (4), while the obtained K0 = 155 (2) GPa when fixed K’0 = 4. In the pressure range of 0–24.4 GPa, the vibration modes of in-plane bending OH-groups for topaz show non-linear changes with the increase in pressure, while the other vibration modes show linear changes. Moreover, the temperature–volume data were fitted by Fei’s thermal equation with the thermal expansion coefficient α300 = 1.9 (1) × 10−5 K−1 at 300 K. Finally, the P-T stability of topaz was studied by a synchrotron-based single-crystal XRD at simultaneously high P-T conditions up to ~10.9 GPa and 700 K, which shows that topaz may maintain a metastable state at depths above 370 km in the upper mantle along the coldest subducting slab geotherm. Thus, topaz may be a potential volatile-carrier in the cold subduction zone. It can carry hydrogen and fluorine elements into the deep upper mantle and further affect the geochemical behavior of the upper mantle.
Highlights
Plate subduction is an important way to carry water into the deep Earth, and hydrous minerals in subduction zones are important carriers of water into the Earth’s interior [1,2,3,4]
P-T) conditions are the keys to understanding the geodynamic processes in subduction zones [10,11,12,13,14,15,16]
The stability of topaz under relatively low-temperature and high-pressure the temperature range up to 1173 K, which may seriously affect the fitting accuracy of the thermal conditions in the cold subduction zone remains unclear
Summary
Plate subduction is an important way to carry water into the deep Earth, and hydrous minerals in subduction zones are important carriers of water into the Earth’s interior [1,2,3,4]. Minerals 2020, 10, 780 subduction depth, hydrous minerals will dehydrate, and the produced fluids will react with mantle wall-rocks inducing their melting to form arc magma in subduction zones [5]. There are many studies, including experiments and theoretical calculations, about the equation of state (EoS) on topaz, where the highest pressure condition is 60 GPa [19,30,32,33,34]. Komatsu et al (2008) obtained K0 = 145 (4) GPa of topaz with composition Al2 SiO4 (OD1.45 , OH0.55 )Σ2.0 using neutron powder diffraction method, which is 12% lower than that of Gatta et al [33]. Theoretical studies about the EoS ofet topaz
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