U-PB CLOSURE TEMPERATURES AND METAMORPHIC COOLING RATES FROM COEXISITING RUTILE AND TITANITE

U-Pb ages of coexisting titanite and rutile can constrain the timing and rate of metamorphic cooling if Pb diffusion properties are well understood. Experimental, comparative geochronologic, and PT-t studies arrive at varying closure temperatures for these minerals. Estimates of titanite closure are generally in the range 550-600°C, but inherited titanite in igneous rocks, titanite ages identical to pluton crystallization ages, and phase equilibria suggest a higher temperature. Experimentally determined diffusion parameters for rutile indicate a closure temperature similar to that of titanite if volume diffusion is the main mechanism of Pb-transport. In general, however, titanite ages are older than rutile ages, but about the same as or slightly older than hornblende Ar/Ar ages. Rutile ages are more comparable to muscovite Ar/Ar ages.

New titanite and rutile U-Pb ages illustrate the dilemma. A retrograded eclogite (Newberry, South Carolina) yielded ages of 411±3 Ma from lenticular titanite of two sizes (~225, ~125µm), and 383±3 Ma for fine-grained clusters. Lenticular titanite from an Eastern Blue Ridge Province amphibolite yielded ages of 406±1 Ma (~300µm) and 388±3 Ma (~100µm). Titanite from a nearby amphibolitized eclogite is 393.5±3.8 regardless of grain size, and three size fractions (>500, ~200, ~80µm) of rutile are indistinguishable at ca. 334 Ma, identical to the age of rutile in nearly pristine eclogite three meters away.

U-Pb ages of coexisting rutile and titanite suggest significantly different closure conditions for the two minerals. Fluids, stress, fractures, cleavage, mineral composition, exsolution, fission damage, and recrystallization could affect closure systematics, but are not readily incorporated in Dodson-type models. In natural systems, volume diffusion may not be the principle method of Pb transport and grain size may not be the same as diffusion domain size. The idea of a single closure temperature characterizing each mineral, and applicable in all geological situations, is probably an overly simplified concept. Understanding these complexities is a prerequisite to full utilization of titanite and rutile thermochronometers.