Volatile accretion and impact histories of early-formed planetesimals

产品描述：大的，可能是火星大小的行星体，形成得非常早，有些比太阳系中最早的固体早1到2 Ma [1]。无球粒陨石中的角闪石类无疑是这些早期形成的分化物质中最神秘的[2]。相互矛盾的证据意味着我们目前对它们的母小行星的大小或它们形成过程中撞击过程的作用知之甚少。大多数安格利特表面上缺乏广泛的激波特征，这使得一些人认为它们起源于小行星，因此逃脱了早期影响较大天体的灾难性撞击[3]。另一方面，在粗粒英格利特中发现的变质结构被解释为有利于行星大小的母体[4]。最近发现的橄榄石捕虏晶的变形特征，从一些更细的晶粒“淬火”角闪石提出了可能性，他们代表的冲击熔融岩石[5]。开放大学收集的初步氧同位素数据表明，在一些淬灭的角闪石中，基质和捕虏橄榄石之间存在同位素不平衡，这些证据可以解释为有利于这些样本的撞击起源。角闪石陨石的另一个令人困惑的属性是它们的极端碱耗尽的签名与它们相对富集的高挥发性元素（如H和C）的签名形成对比[7-8]。这导致了将位于雪线内的挥发性极低的物质与来自雪线外的挥发性丰富的物质混合的建议[7]。然而，这些推论是基于一个相对较小的数据集，来自两个安格利特[7-8]，因此，需要额外的工作，以充分评估太阳系早期形成的星子的形成和演化。本博士项目将主要利用稳定同位素分析技术研究安格利特的起源（激光扫描，NanoSIMS），辅以微束分析（SEM，EPMA）。也可能有机会与我们在日本的合作伙伴一起工作，可能包括在国家极地研究所（NIPR）进行一些研究，作为该项目的一部分。氧同位素分析表明，安山岩和异常无球粒陨石Ibitira之间可能存在联系[6]。这种关系的性质，如果有的话，将作为本研究的一部分进行调查，连同一般的异常无球粒陨石的起源中的影响过程的作用的评估。

Description: Large, possibly Mars-sized planetary bodies, formed extremely early, some as little as 1 to 2 Ma after the earliest dated solids in the Solar System [1]. The angrite group of achondritic meteorites are without doubt the most enigmatic of these early-formed, differentiated materials [2]. Contradictory evidence means that we currently have little understanding as to the size of their parent asteroid, or the role of impact processes during their formation. The apparent lack of widespread shock features in most angrites has led some to argue that they originated on small asteroids and so escaped the early catastrophic bombardment that affected larger bodies [3]. On the other hand, metamorphic textures found in coarser-grained angrites have been interpreted in favour of a planet-sized parent body [4]. The recent discovery of deformation features in olivine xenocrysts from a number of finer-grained "quenched" angrites has raised the possibility that they represent impact-melt rocks [5]. Preliminary oxygen isotope data collected at the Open University indicates an isotopic disequilibrium between matrix and xenocrystic olivines, in some quenched angrites, evidence that can be interpreted in favour of an impact origin for these samples. Another puzzling attribute of angrite meteorites is their extreme alkali-depleted signature contrasting with their relatively enriched signatures for highly-volatile elements such as H and C [7-8]. This has led to the suggestion for mixing of extremely volatile-depleted material, located well inside the snow line, with volatile-rich material derived from outside the snow line [7]. However, these deductions are based upon a relatively small dataset, derived from two angrites [7-8] and, therefore, additional work is required in order to fully evaluate the formation and evolution of early-formed planetesimals in the Solar System.This PhD project will investigate the origin of angrites, primarily making use of stable isotope analysis techniques (laser fluorination, NanoSIMS), supplemented by microbeam analysis (SEM, EPMA). There also may be an opportunity to work alongside our established collaborators in Japan, possibly including some study at the National Institute of Polar Research (NIPR) as part of this project. Oxygen isotope analysis has suggested a possible link between angrites and the anomalous achondrite Ibitira [6]. The nature of this relationship, if any, will be investigated as part of this study, together with an assessment of the role of impact processes in the origin of anomalous achondrites in general.