Extending the Temperature Range of (U-Th)/ He Thermochronology

在地质时间尺度上探索地壳热结构的一种方法是明智地应用校准良好的同位素温度计。由于上地壳的热结构反映地表地形，低温热年代学也为研究古地形，从而研究山地地形的景观演化提供了有力的工具。（U-Th）/He磷灰石热时计的标称闭合温度为~70℃，最近在应用方面的爆发，催化了将构造地貌学与同位素地球化学相结合的研究的新时代。然而，要在地质时间尺度上高保真地重建地形演变，需要比现有的温度计更全面地了解时间-温度历史。特别是，我们有相对较少的方法来恢复200-300摄氏度区间的热历史。本提案提出了一个实验室研究计划，旨在评估两种可能改善这种情况的新温度计：(U-Th)/He独居石和xenotime。稀土磷酸盐独居石和xenotime通常含有高浓度的U和Th，因此具有在短时间尺度（104-105年）积累大量放射性成因的4He的潜力。利用矿物中稀有气体扩散的离子孔隙模型计算这些矿物的标称(U-Th)/He闭合温度表明，这些体系可能是~200-350℃温度范围内有用的温度表。幸运的是，我们在麻省理工学院拥有大量的独居石和xenotime，这是一个积极的U-Pb地质年代学研究项目的结果，因此我们能够提出一系列基本的4He扩散实验，这些实验将在代表广泛成分的自然产生的样品上进行。其中一些实验将关注扩散率作为晶粒尺寸的函数，另一些则关注成分变异性的影响。考虑到这些矿物中U和Th的高浓度，一个重要的问题是它们的4He扩散系统受到辐射损伤的影响程度，这将通过一系列实验进行明确评估。基于对独居石和xenotime显微取样程序的内部经验，我们还设计了一套通常用于校正α喷射(U-Th)/He日期的计算方法的经验测试。总的来说，这些研究应该允许对稀土磷酸盐(U-Th)/He热时计的可行性进行全面评价。

HodgesEAR-0087382One way to explore the thermal structure of the Earth's crust over geologic timescales is by the judicious application of well-calibrated isotopic thermochronometers. Because the thermal structure of the upper crust reflects surface topography, low-temperature thermochronology also provides a powerful tool for studying paleotopography, and thus landscape evolution in mountainous terrains. A recent explosion in the application of the (U-Th)/He apatite thermochronometer, which has a nominal closure temperature of ~70 C, has catalyzed a new era of research that integrates tectonic geomorphology with isotope geochemistry. However, high-fidelity reconstruction of topographic evolution over geologic timescales requires a more complete understanding of time-temperature histories than is currently provided by existing thermochronometers. In particular, we have relatively few ways of recovering thermal histories over the 200-300 degrees C interval. This proposal presents a program of laboratory investigations aimed at evaluating two new thermochronometers that may improve the situation: (U-Th)/He monazite and xenotime. The rare-earth phosphates monazite and xenotime typically contain high concentrations of U and Th and thus have the potential of accumulating substantial radiogenic 4He over short timescales (104-105 yr). Calculations of nominal (U-Th)/He closure temperatures for these minerals using the ionic porosity model for rare-gas diffusion in minerals suggests that these systems may be useful thermochronometers over the ~200-350 degrees C temperature range. We are fortunate to have extensive collections of both monazite and xenotime at MIT as a consequence of a vigorous research program in U-Pb geochronology, and thus we are able to propose a series of basic 4He diffusion experiments that would be conducted on naturally occurring samples representing a broad range of compositions. Some of these experiments will focus on diffusivity as a function of grain size, others on the influence of compositional variability. A significant question, given the high concentrations of U and Th in these minerals, is the extent to which their 4He diffusion systematics is affected by radiation damage, and this will be evaluated explicitly through a series of experiments. Based on in-house experience with monazite and xenotime microsampling procedures, we also have designed a set of empirical tests of the computational method commonly used to correct (U-Th)/He dates for alpha ejection. Collectively, these studies should permit a comprehensive evaluation of the viability of rare-earth phosphates (U-Th)/He thermochronometry.