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

太阳系中行星体的最早历史包括天体的吸积和它们分化成硅酸盐地幔和金属核。随后，新形成的物体冷却并固化。最近在建模和分析技术方面的发展大大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，我们将通过研究可用于直接分析的行星核心的唯一样本-铁陨石来解决这个缺点。该方法将测量这些陨石中短寿命钯-银（107 Pd-107 Ag）和铅-铊（205 Pb-205 Tl）放射性核素系统的衰变产物。这两种衰变方案的半衰期只有大约1000万年，因此它们可以提供非常精确的（+/-几个百万年）“金属结晶”的年龄。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，该研究项目涉及以下研究：1）Pd-Ag和Pb-Tl衰变系统首先需要校准，然后才能提供绝对年龄信息。为了获得这样的校准，我们将分析已经获得精确绝对年龄的陨石样品。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中发生的过程的精确绝对“时钟”提供了基本基础。2)通过这种校准，我们将分析铁陨石中Pd-Ag和Pb-Tl时钟的衰变产物。这些分析将提供小行星金属核冷却和结晶的精确年龄，铁陨石就是从这些金属核中产生的。3)然后，这些信息将与以前的研究结果相结合，这些研究确定了“金属分离”的年龄。这是一个原始的小行星被加热到熔化温度的时候，这样它就分化成了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“金属分离”和“金属结晶”的年龄，我们可以推断出小行星金属核心冷却的速度。由于这种“冷却速率”主要是大小的函数（较大的天体比较小的天体冷却得慢），我们可以使用这些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使我们能够第一次将吸积的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，我们将分析来自不同小行星的各种铁陨石群。我们将从这些样品中获得的年龄信息，加上以前的结果，将为大多数铁陨石的母小行星提供全面的年代记录。这些记录将涵盖从吸积和伴随的核心形成到金属核心冷却和结晶的时期。这些信息将大大扩展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.