4He/3He laser microprobe analysis: a disruptive new technology for in-situ U-Th-He thermochronology

欧内斯特·卢瑟福因发现放射性是一种放射性元素(母体)自发分解成另一种元素(子体)的产物而获得1908年诺贝尔奖。他很快意识到，如果知道衰变率，这个系统就可以用来确定地质材料的年龄。这类似于沙漏，其中顶部的沙子是放射性母体的量，底部的沙子是导致放射性的子体的量。卢瑟福的想法首先应用于U和Th的放射性衰变，产生了铅和氦，产生了两个计时器。提出的方法旨在开发和应用一种全新的技术，其中两种计时器在显微样品上的结合时间比传统技术所需的时间要短。目前测量U-Th-He年龄的方法是：(1)用长波(IR或可见光)激光加热含U、Th矿物的整个颗粒，(2)用稀有气体质谱仪分析释放出的He，(3)将脱气的颗粒溶解在酸中，(4)在单独的(电感耦合等离子体质谱)质谱仪上测量它们的U、Th含量。这种方法的一个问题是耗时，特别是对锆石这种极难溶解的矿物。第二个问题是，它假设矿物具有均匀的U和Th组成，因为由于放射性衰变过程的能量性质，他与母体在空间上是分开的，而这种空间分离只能通过对母体同位素位置的假设来修正。为了解决这些问题，拟议的研究将开发一种全新的技术，在这种技术中，他不是通过长波激光脱气整个颗粒释放出来的，而是通过短波(UV)激光从抛光表面的一个小坑中提取出来的，并最终在稀有气体质谱仪上进行测量。然后使用电感耦合等离子体质谱以完全相同的方法分析U、Th(和Pb)。与以前的U-Th-He原位测年方法相比，该方法的关键创新之处在于，所有的测量都是相对于已知U-Th-He年龄的标准进行的，该标准与高能质子一起照射样品，产生内部均匀的3He(稀有的氦同位素)背景信号。新方法将使样品吞吐量增加两个数量级，同时使我们能够绘制出U、Th和He在单个颗粒内的空间分布，并默认产生(U-Th)/(Pb-He)‘双日期’。从而揭示了以往常规方法看不到的丰富地质信息。我们预计质子辐照样品的原位U-Th-He测年将在许多(尽管不是所有)应用中取代传统的全颗粒脱气和溶解，从而保证被贴上“颠覆性技术”的标签。我们正在寻求资金来开发这种新的测年方法，作为卢瑟福开创性研究的继续，并为纳税人支付适度的费用。

Ernest Rutherford received the 1908 Nobel Prize for the discovery that radioactivity is a product of the spontaneous disintegration of a radioactive element (the parent) into another element (the daughter). He soon realised that if the decay rate is known, this system can be used to determine the age of geological materials. This is analogous with an hourglass in which sand in the top is the amount of radioactive parent and sand in the bottom is the amount of radiogenic daughter. Rutherford's idea was first applied to the radioactive decay of U and Th, which produces Pb and He, resulting in two chronometers. The proposed method aims to develop and apply a radically new technology in which both chronometers are combined on microscopic samples in a fraction of the time required by conventional techniques.U-Th-He ages are currently measured by (1) heating entire grains of U,Th-bearing minerals such as apatite or zircon with a long wavelength (IR or visible light) laser, (2) analysing the released He with a noble gas mass spectrometer, (3) dissolving the degassed grains in acid and (4) measuring their U,Th-content on a separate (ICP-MS) mass spectrometer. One problem with this method is that it is time consuming, especially for zircon which is an extremely hard to dissolve mineral. A second problem is that it assumes minerals to have a uniform U and Th composition, because He is spatially separated from its parent due to the energetic nature of the radioactive decay process, and this spatial separation can only be corrected for by making assumptions regarding the location of the parent isotopes.To solve these problems, the proposed research will develop a radically new technology in which He is not released by degassing entire grains with a long wavelength laser, but is extracted from a small pit in a polished surface by ablation with a short wavelength (UV) laser, and ultimately measured on a noble gas mass spectrometer. U, Th (and Pb) are then analysed by exactly the same means using an ICP-MS. The key innovation of the proposed method over previous attempts at 'in-situ' U-Th-He dating is that all the measurements are done relative to a standard of know U-Th-He age, which is irradiated alongside the sample with high energy protons to produce an internally uniform background signal of 3He (which is a rare isotope of helium).The new method will deliver a two orders of magnitude increase in sample throughput while enabling us to map out the spatial distribution of U, Th and He within individual grains and producing (U-Th)/(Pb-He) 'double-dates' by default, thus revealing a wealth of geological information previously invisible to the conventional method. We anticipate in-situ U-Th-He dating of proton-irradiated samples to replace conventional whole-grain degassing and dissolution in many (albeit not all) applications, warranting the label of a 'disruptive technology'. We are seeking funds to develop this new dating method as a continuation of Rutherford's groundbreaking research, at a modest cost to the tax payer.