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

HodgesEAR-0087382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时器。由于上地壳的热结构反映了地表地形，低温热年代学也为研究古地形地貌，从而研究山地地形地貌演化提供了有力的工具。标称闭合温度为~70℃的(U-Th)/He磷灰石温度计时仪最近在应用中的爆炸性增长，催生了一个将构造地貌学与同位素地球化学相结合的研究新时代。然而，要高保真地重建地质时间尺度上的地形演化，需要比现有的温度计时器提供的对时间-温度历史的更完整的理解。特别是，我们在200-300摄氏度范围内恢复热历史的方法相对较少。这项建议提出了一项实验室调查计划，旨在评估可能改善这种情况的两种新的温度计时器：(U-Th)/He独居石和Xe-Time。稀土磷酸盐、独居石和辉石通常含有高浓度的U和Th，因此有可能在短时间尺度(104-105年)内积累大量的放射性成因4He。用稀有气体在矿物中扩散的离子孔隙度模型计算了这些矿物的标称(U-Th)/He闭合温度，表明这些体系在~200-350℃的温度范围内可能是有用的温度计时器。我们很幸运地在麻省理工学院拥有大量独居石和Xe-Time，这是一个强有力的U-Pb年代学研究计划的结果，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发生的样品上进行。其中一些实验将重点放在扩散系数作为颗粒大小的函数，其他实验将集中在成分可变性的影响上。考虑到这些矿物中U和Th的浓度很高，一个重要的问题是它们的4He扩散系统受到辐射损伤的程度如何，这将通过一系列实验来明确评估。基于独居石和Xe-time微量取样程序的内部经验，我们还设计了一套常用的计算方法的经验测试，以修正(U-Th)/He的阿尔法射出日期。总而言之，这些研究应该能够全面评估稀土磷酸盐(U-Th)/He热计时法的可行性。

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.