U-Pb thermochronology: creating a temporal record of lithosphere thermal evolution

Abstract

A new approach to U-Pb accessory mineral thermochronology allows high-resolution time-temperature histories to be extracted from lower crustal xenoliths. The combination of the U-Pb system’s dual decay scheme with the effects of temperature dependent Pb-diffusion can yield a time sensitive record of Pb production/diffusion within accessory phases. The difference in half-life for parent isotopes 238U and 235U results in the time-variable production of Pb isotopes 206Pb and 207Pb, while Pb diffusion can result in large variations in the time-scales at which Pb retention occurs between grains of different sizes. The combined effects of variable production rates between the two systems and diffusion result in data topologies on a concordia diagram that permit distinction between slow cooling and reheating t-T paths. In slowly cooled systems, the difference in time for Pb retention for grains of variable size yields a measure of partial retention zone (PRZ) residence time, and provides a robust measure of cooling rate through the PRZ. In Montana, three lower crustal xenoliths, each from a different depth, yield U-Pb rutile data that record a prolonged (>1 Ga) and slow cooling history towards a steady state geothermal gradient following the amalgamation of the terrain onto North America. The shallowest samples record the initial recovery of a conductive geothermal gradient and cool through the mineral PRZ at rates of <0.25°C/Ma over ~500 Ma. Deeper xenoliths record cooling at younger times over similar time scales and rates. This multi-depth thermal history provides a long-term record of lithosphere cooling and stabilization.