A new approach to (U-Th)/He thermochronometry: exploiting the natural dispersion of single grain ages to obtain robust thermal history information

地质年代学是一门测定岩石和化石年代并确定地球历史上事件时间顺序的科学，它是现代定量地质学的基础。传统的古生物学和生物地层对比法是地质学家最常用的相对测年方法。但是，为了确定过程的定量日期和速率，需要一个绝对的时间框架。绝对年代测定法或数字年代测定法涉及以时间单位（通常是年）确定化石、岩石或地质事件的地质年代的方法。这些绝对方法通过测量样品中特定放射性同位素（母体）的数量以及放射性衰变产生的稳定产物（子同位素）的数量来确定样品的年龄。例如，U-Pb技术测量238-U（母体）的存在量和238-U的放射性衰变形成206-Pb（子体）的量，通过了解238-U的放射性衰变速率，我们可以计算样品的年龄。热时学是对地质年代学的扩展，通过测定岩石样品在过去某一特定时间或几次经历的温度，即岩石的热历史。由于地下温度随着地球内部深度的增加而系统地增加，因此在地表收集的样品的热历史记录了样品从深度到地表的轨迹。因此，热时测定法使地质学家能够研究和量化对了解地球如何演化非常重要的一系列过程，例如山脉形成和毁灭的时间和速度，或者量化从大陆到海洋的固体和化学物质的质量。最广泛使用的热测时技术之一是基于测量由一种叫做磷灰石的矿物中238-U和232-Th的放射性衰变产生的氦（子产物）的量，它被称为(U-Th)/He技术。由于该技术对相对较低的温度（摄氏40-70度）非常敏感，因此现在通常用于在非常广泛的地质环境中确定地球科学调查中岩石的热历史。由于一系列原因，现在分析从样品中提取的磷灰石的单个颗粒是标准做法，当同一样品中的2或3个（或更多）颗粒年龄在统计上相等时，即单个颗粒年龄相同时，分析被认为是可靠的（一旦考虑到尺寸和成分）。然而，许多研究表明，同一样品的单粒年龄通常不相同，实际上是高度分散的。这被认为是由于U和Th在晶粒内的不均匀分布引起的，He从晶界外被添加到晶粒中（所谓的过量He），或者是由于晶粒尺寸的差异，或者是由于磷灰石晶体辐射损伤的积累引起的在任何给定温度下He从晶粒中扩散的速率的差异。然而，在许多情况下，观测到的色散与这些已知的效应无关。我们相信我们已经发现了这种分散发生的根本原因，我们的初步实验表明，这是对岩石破碎和矿物分离过程中破碎的晶体进行分析的自然结果。我们发现的令人兴奋的结果是，这种色散包含了样品热历史的宝贵信息，而不是技术的障碍。在这个项目中，我们的目标是展示和充分测试一种新的方法来提取热历史信息，并应用这种广泛使用的热测时技术，我们相信这将解决几十年来关于单粒年龄“神秘”分散起源的不确定性，并将极大地扩展这种强大的分析方法的适用性。

Geochronology, the science of dating rocks and fossils and determining the time sequence of events in the history of the Earth, underpins modern, quantitative geology. Traditional paleontological and biostratigraphic correlation methods are the most common relative dating methods used by geologists. But, to establish quantitative dates and rates of processes an absolute time frame is required. Absolute, or numeric, dating involves methods of determining the geologic age of a fossil, rock, or geologic event in units of time, usually years. These absolute methods establish the ages of samples by measuring the amount of a specific radioactive isotope (the parent) within the sample as well as the amount of the stable product arising from the radioactive decay (the daughter isotope). For example, the U-Pb technique measures how much 238-U (the parent) is present and how much 206-Pb (the daughter) has been formed by radioactive decay of 238-U, and by knowing the rate of radioactive decay of 238-U we can calculate the age of the sample.The science of thermochronometry extends the practice of geochronology by determining the temperature a rock sample experienced at a particular time, or times, in the past, i.e. the rock's thermal history. Because subsurface temperatures increase systematically with depth within the Earth the thermal history of a sample collected at the surface records the sample's trajectory from depth to the surface. Thermochronometry thus enables geologists to study and quantify a whole range of processes that are important to understanding how the Earth evolved, such as when and how fast mountain ranges are created and destroyed or quantifying the mass of solid and chemical material transported from continents to the oceans by rivers. One of the most widely used thermochronometry techniques is based on measuring the amount of Helium (the daughter product) resulting from the radioactive decay of 238-U and 232-Th in a mineral called apatite and it is known as the (U-Th)/He technique. This technique is now commonly used to determine thermal histories of rocks in geoscience investigations across an extremely wide range of geological settings because of it is sensitive to relatively low temperatures (c. 40-70 C).For a range of reasons it is now standard practice to analyse individual grains of apatite extracted from a sample, and an analysis is deemed reliable when 2 or 3 (or more) grain ages from the same sample are statistically equivalent, i.e. the individual grain ages are the same (once size and composition are accounted for). In many studies though it has been shown that single grain ages from the same sample are commonly not the same, and in fact are highly dispersed. This is thought to arise from heterogeneous U and Th distribution within grains, He being added to the grain for from outside grain boundaries (so called excess He) or from differences in the size of grains or in the rate that He diffuses out of the grains at any given temperature caused by accumulation of radiation damage to the apatite crystals. However, in many cases, the observed dispersion is shown to be unrelated to these known effects.We believe we have discovered the underlying reason for why this kind of dispersion occurs, and our initial experiments indicate it is a natural consequence of analysing crystals that have been broken during the rock crushing and mineral separation process. The exciting consequence of our discovery is that, rather than a hindrance to the technique, this dispersion contains valuable information about a sample's thermal history. In this project we aim to demonstrate and fully test a novel new approach to extracting the thermal history information and applying this widely used thermochronometry technique which we believe will resolve decades of uncertainty about the origin of the 'cryptic' dispersion of single grain ages, and will vastly extend the applicability of this powerful analytical method.