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

太阳系中行星体最早的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凝固。建模和分析技术的最新发展显著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，不同的小行星冷却和金属核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用于直接分析的行星核心样本-铁陨石来解决这一缺点。该方法将测量这些陨石中短寿命的钯银（107Pd-107Ag）和铅铊（205Pb-205Tl）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10迈，因此它们可以提供非常精确的“金属结晶”年龄（+/-几个迈）。这些年龄定义了最初以液态存在的金属冷却形成富含铁的固体核心的时间。为了获得这样的年龄，研究项目包括以下调查：1)首先需要校准Pd-Ag和Pb-Tl衰变系统，然后才能提供绝对年龄信息。为了获得这样的校准，我们将分析已经有精确绝对年龄的陨石样本。一旦完成，校准将具有持久的价值，因为它为使用Pd-Ag和Pb-Tl测年系统作为早期太阳系发生的过程的精确绝对“时钟”提供了基础。2)利用这一校准，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（铁陨石的来源）冷却和结晶的精确年龄。这一信息将与先前的研究结果相结合，这些研究确定了“金属偏析”的年代。在这个时期，一颗原始的小行星被加热到熔化的温度，这样它就分化成由硅酸盐组成的外地幔和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金属核心冷却的速度。由于这个“冷却速率”主要是大小的函数（较大的物体比较小的物体冷却得慢），我们可以利用这个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第一次能够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。我们将从这些样本中获得的年龄信息，连同之前的结果，将为大多数铁陨石的母小行星提供全面的时间记录。这些记录将跨越从吸积和伴随的岩心形成到金属岩心的冷却和结晶的时期。综合起来，这些信息将极大地扩展我们对早期太阳系小行星的化学、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.