Microstructure Defects in Ultrahigh-pressure Metamorphic Rock: Evidence from TEM and FTIR Studies

Abstract

Defect microstructures of minerals are a potential indicator of extremely rapid and episodic exhumation of ultrahigh-pressure (UHP) metamorphic rock. The microstructure defects in the minerals from UHP jadeite quartzite at Shuanghe, Dabie Mountains, China, have been investigated using transmission electron microscope (TEM) and Fourier transform infrared spectroscopy (FTIR). TEM observations indicate that three polymorphs, namely, submicron-scale nature monalbite (MA, C2/ m) (about 0.8 μm), high albite (HA, CĪ), and low albite (LA, CĪ) occur in jadeite inclusions from jadeite quartzite. The existence of monalbite (MA) implies that high-temperature metamorphism (>930 °C) takes place during exhumation and rapid cooling, during retrogression. The occurrence of nanoscale coesite (about 80 nm) and halite daughter minerals in quartz inclusions provide evidence for the transformation of the original coesite to quartz and the existence of high-salty fluids at peak metamorphic conditions. Structural water (OH/H 2O) in nominally anhydrous minerals (NAMs) exists in the structures of these minerals as defects. The FTIR results show that the average contents of structural water in jadeite, garnet, rutile, and quartz are 1000 × 10 −6, (900–1600) × 10 −6, > 2000 × 10 −6, and < 4 × 10 −6, respectively, and the estimated whole-rock water content is (490–600) × 10 −6. The results reveal that the water in crust or protolith can transport into the earth's depth through NAMs during HP-UHP metamorphism. These unique defect microstructures can be rationalized by a high strain rate at local weakening and deep faulting of continental collision orogens.