Nanometer-size alpha-PbO2-type TiO2 in garnet: A thermobarometer for ultrahigh-pressure metamorphism

A high-pressure phase of titanium dioxide (TiO2) with an alpha-PbO2-type st ructure has been identified in garnet of diamondiferous quartzofeldspathic rocks from the Saxonian Erzgebirge, Germany. Analytical electron microscopy indicates that this alpha-PbO2-type TiO2 occurred as an epitaxial nanomete r-thick slab between twinned rutile bicrystals. Given a V-shaped curve for the equilibrium phase boundary of alpha-PbO2-type TiO2 to rutile, the stabi lization pressure of alpha-PbO2-type TiO2 should be 4 to 5 gigapascals at 9 00 degrees to 1000 degrees C. This suggests a burial of continental crustal rocks to depths of at Least 130 kilometers. The alpha-PbO2-type TiO2 may b e a useful pressure and temperature indicator in the diamond stability fiel d.