Nanometer-Size α-PbO_2-Type TiO_2 in Garnet: A Thermobarometer for Ultrahigh-Pressure Metamorphism

摘要

A high-pressure phase of titanium dioxide (TiO_2) with an α-PbO_2-type structure has been identified in garnet of diamondiferous quartzofeldspathic rocks from the Saxonian Erzgebirge, Germany. Analytical electron microscopy indicates that this α-PbO_2-type TiO_2 occurred as an epitaxial nanometer-thick slab between twinned rutile bicrystals. Given a V-shaped curve for the equilibrium phase boundary of α-PbO_2-type TiO_2 to rutile, the stabilization pressure of α-PbO_2-type TiO_2 should be 4 to 5 gigapascals at 900° to 1000℃ This suggests a burial of continental crustal rocks to depths of at least 130 kilometers. The α-PbO_2-type TiO_2 may be a useful pressure and temperature indicator in the diamond stability field.