Coupled Calibration and Application of the Extinct 107Pd-107Ag and 205Pb-205Tl Decay Systems

太阳系中的行星体最早的历史包括天体的吸积，以及它们分化为硅酸盐地幔和金属核。随后，新形成的物体冷却并凝固。最近在建模和分析技术方面的发展极大地提高了我们对其中一些过程的时间和持续时间的理解。然而，分化的小行星冷却和它们的金属核心结晶的时间尺度仍然没有得到很好的限制。在这个研究项目中，我们将通过研究唯一可供直接分析的行星核样本--铁陨石来解决这一缺点。该方法将测量这些陨石中短暂的钯-银(107Pd-107Ag)和铅-铊(205Pb-205Tl)放射性核素体系的衰变产物。这两种衰变方案的半衰期只有10 Myr左右，因此它们可以提供非常精确的(+/-几Myr)“金属结晶”年龄。这些年龄定义了最初以液体形式存在的金属冷却形成固体富铁核心的时间。为了获得这样的年龄，研究项目涉及以下几个方面的研究：1)首先需要对Pd-Ag和Pb-Tl衰变系统进行标定，然后才能提供绝对年龄信息。为了获得这样的校准，我们将分析已有准确绝对年龄的陨石样本。一旦完成，校准将具有长期的价值，因为它为使用Pd-Ag和Pb-Tl测年系统作为早期太阳系过程的精确绝对时钟提供了基本基础。2)利用这个定标，我们将分析铁陨石中Pd-Ag和Pb-Tl钟的衰变产物。这些分析将提供小行星金属核心的准确年龄，铁陨石就是从这些核心衍生、冷却和结晶的。3)这一信息将与以前的研究结果结合起来，这些研究确定了“金属偏析”的时代。在这个时候，一颗原本原始的小行星被加热到熔融的温度，以至于它分化成一个由硅酸盐组成的外地幔和一个主要由液态金属铁组成的核心。通过比较“金属偏析”和“金属结晶”年龄，我们可以推断出小行星的金属核心冷却的速度。由于这个“降温速率”主要是大小的函数(较大的天体比较小的天体冷却较慢)，我们可以利用这一信息来估计铁陨石的小行星前体的直径。这意味着，我们的数据将第一次使我们能够将吸积持续时间与给定小行星母体的大小联系起来。在这项研究中，我们将分析不同类型的铁陨石，它们来自不同的小行星。我们将获得的这些样品的年龄信息，加上之前的结果，将提供大多数铁陨石母体小行星的全面年代学记录。这些记录将跨越从凝聚和伴随的核形成到金属核的冷却和结晶的时期。综上所述，这些信息将大大扩大我们对太阳系早期小行星的化学、物理和热演化的理解。

The earliest history of planetary bodies in the solar system encompasses the accretion of the objects and their differentiation into silicate mantles and metallic cores. Subsequently the newly formed bodies cooled and solidified. Recent developments in modelling and analytical techniques have significantly improved our understanding of the timing and duration of some of these processes. The timescales over which differentiated asteroids cooled and their metallic cores crystallized remain poorly constrained, however. In this research project, we will address this shortcoming by studying the only samples of planetary cores that are available for direct analyses - iron meteorites. The approach will be to measure the decay products of the short-lived palladium-silver (107Pd-107Ag) and lead-thallium (205Pb-205Tl) radionuclide systems in such meteorites. These two decay schemes have half-lives of only about 10 Myr, and they can thus provide very precise (+/- a few Myr) ages of 'metal crystallization'. These ages define the time at which the metal, which was originally present in liquid form, had cooled to form a solid iron-rich core. To obtain such ages, the research project involves the following investigations: 1) The Pd-Ag and Pb-Tl decay systems first need to be calibrated before they can provide absolute age information. To obtain such a calibration, we will analyze meteorite samples for which precise absolute ages are already available. Once completed, the calibration will be of long-lasting value because it provides the basic foundation for the use of the Pd-Ag and Pb-Tl dating systems as precise absolute 'clocks' of processes that took place in the early solar system. 2) With this calibration, we will analyze the decay products of the Pd-Ag and Pb-Tl clocks in iron meteorites. These analyses will provide the precise ages at which the metallic cores of the asteroids, from which the iron meteorites were derived, cooled and crystallized. 3) This information will then be combined with the results of previous studies, which dated the age of 'metal segregation'. This is the time at which an originally primitive asteroid was heated to melting temperatures, such that it differentiated into an outer mantle composed of silicates and a core composed mainly of liquid metallic iron. By comparing the 'metal segregation' with the 'metal crystallization' age, we can infer the rate at which the metal core of an asteroid cooled. As this 'cooling rate' is primarily a function of the size (with larger bodies cooling slower than smaller ones), we can use this information to estimate the diameter of the asteroidal precursor of an iron meteorite. This implies that our data will enable us, for the first time, to relate the duration of accretion to the size of a given asteroidal parent body. In this study, we will analyze various groups of iron meteorites, which are derived from distinct asteroids. The age information that we will obtain for these samples, together with previous results, will provide comprehensive chronological records for the parent asteroids of most iron meteorites. These records will span the period from accretion and concomitant core formation to the cooling and crystallization of the metallic cores. Taken together, this information will significantly expand our understanding of the chemical, physical and thermal evolution of asteroids in the early solar system.