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

机译：U-Pb热年代学：创建岩石圈热演化的时间记录

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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 ~(238)U and ~(235)U results in the time-variable production of Pb isotopes ~(206)Pb and ~(207)Pb, 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.
机译：U-Pb辅助矿物热年代学的一种新方法允许从下地壳异岩中提取高分辨率的时间-温度历史。 U-Pb系统的双重衰减方案与温度相关的Pb扩散的影响相结合，可以在辅助相中产生对Pb产生/扩散的时间敏感记录。母同位素〜（238）U和〜（235）U的半衰期差异导致Pb同位素〜（206）Pb和〜（207）Pb随时间变化的产生，而Pb扩散会导致较大的变化在不同尺寸的晶粒之间发生Pb保留的时间尺度上。两个系统之间可变生产率和扩散之间的综合影响，导致共鸣图上的数据拓扑结构得以区分慢速t-T路径和再加热t-T路径。在缓慢冷却的系统中，可变大小的晶粒的Pb保留时间差异产生了部分保留区（PRZ）停留时间的量度，并提供了通过PRZ的冷却速率的可靠量度。在蒙大纳州，三个较低的地壳异种岩，每个来自不同的深度，产生的U-Pb金红石数据记录了将地形合并到北美后向稳态地热梯度持续（> 1 Ga）和缓慢冷却的历史。最浅的样品记录了一个传导性地热梯度的初始恢复，并在〜500 Ma内以<0.25°C / Ma的速率冷却通过PRZ矿物。在相似的时间尺度和速率下，较深的异种岩层在更年轻的时期记录到冷却。这种多深度的热历史提供了岩石圈冷却和稳定的长期记录。

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