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

(U-Th)/He 测温新方法：利用单晶年龄的自然分散来获取可靠的热历史信息

• 批准号: NE/J013242/1
• 负责人: Roderick Brown
• 金额: $ 5.34万
• 依托单位: University of Glasgow
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2012
• 资助国家: 英国
• 起止时间: 2012 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FJ013242%2F1
• 关键词: new; approach; Th; He; thermochronometry

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.

地质年代学是测定岩石和化石年代并确定地球历史事件时间顺序的科学，是现代定量地质学的基础。传统的古生物和生物地层对比方法是地质学家最常用的相对定年方法。但是，要确定进程的数量日期和速度，就需要一个绝对的时间框架。绝对或数字定年法涉及以时间单位（通常是年）确定化石、岩石或地质事件的地质年龄的方法。这些绝对方法通过测量样品中特定放射性同位素（母体）的量以及放射性衰变产生的稳定产物（子体同位素）的量来确定样品的年龄。例如，U-Pb技术测量238-U（母体）是否存在以及206-Pb（子体）是由238-U的放射性衰变形成的，通过知道238的放射性衰变率我们可以计算出样品的年龄。热年代测定学通过测定岩石样品在某一特定温度下所经历的温度来扩展地质年代学的实践。时间，或时间，在过去，即岩石的热历史。由于地下温度随着地球内部的深度而系统地增加，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，热年代学使地质学家能够研究和量化对了解地球如何演变至关重要的一系列过程，例如山脉何时以及以多快的速度形成和毁灭，或者量化河流从大陆运输到海洋的固体和化学物质的质量。最广泛使用的热计时技术之一是基于测量由238-U和232-Th在称为磷灰石的矿物中的放射性衰变产生的氦（子产物）的量，并且它被称为（U-Th）/He技术。这种技术现在通常用于在地质科学调查中确定岩石的热历史，因为它对相对较低的温度（c）敏感。由于一系列原因，现在标准的做法是分析从样品中提取的磷灰石的单个颗粒，并且当来自同一样品的2个或3个（或更多个）颗粒年龄在统计学上相等时，即单个颗粒年龄相同（一旦考虑尺寸和组成），分析被认为是可靠的。然而，许多研究表明，来自同一样品的单个颗粒的年龄通常是不相同的，事实上是高度分散的。这被认为是由于U和Th在晶粒内的不均匀分布引起的，He从外部晶界（所谓的过量He）或由于晶粒尺寸的差异或由于对磷灰石晶体的辐射损伤的累积引起的He在任何给定温度下扩散出晶粒的速率的差异而被添加到晶粒中。然而，在许多情况下，观察到的分散显示与这些已知的影响无关。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，这是分析岩石破碎和矿物分离过程中破碎的晶体的自然结果。我们的发现令人兴奋的结果是，这种分散包含了关于样品热历史的有价值的信息，而不是对技术的阻碍。在这个项目中，我们的目标是展示和充分测试一种新的新方法来提取热历史信息，并应用这种广泛使用的热计时技术，我们相信这将解决几十年的不确定性的起源的“神秘”分散的单颗粒年龄，并将大大扩展这种强大的分析方法的适用性。

项目成果

How local crustal thermal properties influence the amount of denudation derived from low-temperature thermochronometry

局部地壳热特性如何影响低温测时法得出的剥蚀量

• DOI: 10.1130/g39036.1
• 发表时间: 2017
• 期刊: Geology
• 影响因子: 5.8
• 作者: Luszczak K

Natural age dispersion arising from the analysis of broken crystals. Part I: Theoretical basis and implications for the apatite (U-Th)/He thermochronometer

• DOI: 10.1016/j.gca.2013.05.041
• 发表时间: 2013-12-01
• 期刊: GEOCHIMICA ET COSMOCHIMICA ACTA
• 影响因子: 5
• 作者: Brown, Roderick W.;Beucher, Romain;Fitzgerald, Paul
• 通讯作者: Fitzgerald, Paul