High Pressure Response of Rutile Polymorphs and Its Significance for Indicating the Subduction Depth of Continental Crust

Abstract

Abstract: α‐PbO2‐type TiO2 (TiO2‐II) is an important index mineral for ultrahigh‐pressure metamorphism. After the discovery of a natural high‐pressure phase of titanium oxide with α‐PbO2‐structure in omphacite from coesite‐bearing eclogite at Shima in the Dabie Mountains, China, a nano‐scale (<2 nm) α‐PbO2‐type TiO2 has been identified through electron diffraction and high‐resolution transmission electron microscopy in coesite‐bearing jadeite quartzite at Shuanghe in the Dabie Mountains. The crystal structure is orthorhombic with lattice parameters a = 4.58times10−1 nm, b = 5.42times10−1 nm, c = 4.96times10−1 nm and space group Pbcn. The analysis results reveal that rutile {011}R twin interface is a basic structural unit of α‐PbO2‐type TiO2. Nucleation of α‐PbO2‐type TiO2 lamellae is caused by the displacement of one half of the titanium cations within the {011}R twin slab. This displacement reduces the Ti‐O‐Ti distance and is favored by high pressure. The identification of α‐PbO2‐type TiO2 in coesite‐bearing jadeite quartzite from Shuanghe, Dabie Mountains, provides a new and powerful evidence of ultrahigh‐pressure metamorphism at 4–7 GPa, 850°C‐900°C, and implies a burial of continental crustal rocks to 130–200 kilometers depth or deeper. The α‐PbO2‐type TiO2 may be a useful indicator of the pressure and temperature in the diamond stability field.