Beyond Isoplot: new software for better geochronology.

放射年代学的发展是20世纪地球科学最伟大的成就之一。地质年代学是地球历史研究的基础，并对生物进化的速度提出了基本的限制。大量的资源投入到精密质谱仪的开发中，这些质谱仪能够以不断提高的分辨率和灵敏度测量同位素比率。不幸的是，质谱仪数据的统计处理没有跟上这些硬件的发展，这破坏了放射性地质年代学的可靠性。该提案旨在通过建立一个内部一致的计算机程序生态系统来解释样本间误差相关性，从而在地质年代学中创造一场“软件革命”。这些对地质年代学的精度和准确性具有一级影响，但在很大程度上被当前的地质年代学数据处理方案所忽略。拟议中的软件将修改现有的数据简化平台，并创建全新的平台。它将实现一种数据交换格式，将来自多个计时器的数据集组合在一起，同时跟踪它们之间的相关不确定性。新算法将应用于五个重要的地质问题。太阳系的年龄目前被限制在4567.30 +/- 0.16 Ma。陨石数据在分析不确定度方面是“欠分散”的。强样本间误差相关性的存在很可能是这种分散不足的罪魁祸首。考虑到这些相关性将显著提高这一标志性年龄估计的准确性和精密度。白垩纪-古近纪的分界线标志着地球历史上最臭名昭著的大灭绝中恐龙的消失。我们将使用高精度的40Ar/39Ar地质年代学重新评估这一边界附近关键事件的时间。其他样本的初步结果表明，40Ar/39Ar数据倾向于强（r^2 > 0.9）样本间误差相关性，并且这些对加权平均年龄估计的精度和准确性具有一阶影响。一项灵敏度测试表明，这可能会使大灭绝的时间至多改变200ka.3。“陶孩子”是1924年在南非洞穴中发现的著名古人类化石。它被认为是世界上第一个南方古猿，但尚未确定年代。我们在原始人以上收集的凝灰岩中有一个良好的未发表的U-Pb年龄为1.99 +/- 0.05 Ma，而在其以下的钙质沉积物中有一个不精确的年龄上限为1.4 +/- 2.7 Ma。将新算法应用于后期数据，将大大提高其精度。这将进一步改善额外的测量，及时为100周年的发现黄童。金红石和磷灰石中U-Pb年龄的深度剖面为约束下地壳岩石的热演化提供了令人兴奋的新途径。然而，用于本研究的激光烧蚀数据容易出现较强的误差相关性，这是当前数据简化协议无法解释的。这些协议将使用新的软件进行修订，以便更好地分辨推断出的t-T路径。(5)放射性稀有气体如40Ar（来自40K）、4He（来自U、Th和Sm）和129Xe（来自129I）在高温下因体积扩散而损失。修订后的回归算法实施的研究计划将应用于逐步加热‘阿伦尼乌斯’实验。这将改进这些气体扩散系数的计算，从而进一步改进（惰性气体）热年代学。

The development of radiometric geochronology is one of the greatest triumphs of 20th century geoscience. Geochronology underpins the study of Earth history and puts fundamental constraints on the rate of biological evolution. Tremendous resources are invested in the development of sophisticated mass spectrometers capable of measuring isotopic ratios with ever increasing resolution and sensitivity. Unfortunately, the statistical treatment of mass spectrometer data has not kept up with these hardware developments and this undermines the reliability of radiometric geochronology.This proposal aims to create a 'software revolution' in geochronology, by building an internally consistent ecosystem of computer programs to account for inter-sample error correlations. These have a first order effect on the precision and accuracy of geochronology but are largely ignored by current geochronological data processing protocols. The proposed software will modify existing data reduction platforms and create entirely new ones. It will implement a data exchange format to combine datasets from multiple chronometers together whilst keeping track of the correlated uncertainties between them. The new algorithms will be applied to five important geological problems.1. The age of the Solar System is presently constrained to 4567.30 +/- 0.16 Ma using primitive meteorites. The meteorite data are 'underdispersed' with respect to the analytical uncertainties. The presence of strong inter-sample error correlations is one likely culprit for this underdispersion. Accounting for these correlations will significantly improve the accuracy and precision of this iconic age estimate.2. The Cretaceous-Palaeogene boundary marks the disappearance of the dinosaurs in the most notorious mass extinction of Earth history. We will re-evaluate the timing of critical events around this boundary using high precision 40Ar/39Ar geochronology. Preliminary results from other samples show that 40Ar/39Ar data are prone to strong (r^2 > 0.9) inter-sample error correlations, and that these have a first order effect on the precision and accuracy of weighted mean age estimates. A sensitivity test indicates that this may change the timing of the mass extinction by up to 200ka.3. The 'Taung Child' is a famous hominin fossil that was discovered in a South African cave in 1924. It is considered to be the world's first Australopithecine, but has not yet been dated. We have a good unpublished U-Pb age of 1.99 +/- 0.05 Ma from a tufa collected above the hominid, and an imprecise upper age limit of 1.4 +/- 2.7 Ma on a calcrete deposit below it. Applying the new algorithms to the latter date will greatly improve its precision. This will be further improved with additional measurements, in time for the 100th anniversary of the Taung Child's discovery.4. Depth profiling of the U-Pb ages in rutile and apatite provides an exciting new way to constrain the thermal evolution of lower crustal rocks. However, the laser ablation data used for this research are prone to strong error correlations that are not accounted for by current data reduction protocols. These protocols will be revised using the new software, permitting better resolution of the inferred t-T paths.(5) Radiogenic noble gases such as 40Ar (from 40K), 4He (from U, Th and Sm), and 129Xe (from 129I) are lost by volume diffusion at high temperatures. The revised regression algorithms implemented by the research programme will be applied to step-heating 'Arrhenius' experiments. This will improve the calculation of diffusion coefficients for these gas species, resulting in further improvement of (noble gas) thermochronology.