Elucidating planet formation using chondrule oxygen fugacity

最近，在ALMA望远镜拍摄的令人难以置信的图像之后，人们对行星形成和星周盘的理解发生了戏剧性的转变。它们彻底改变了人们对行星形成的认识，并催生了一系列新理论。例如，Johansen et al. 2014提出，原行星盘中必定存在尘埃和气体压力（压力最大值）增加的区域，以形成第一批行星体。尽管压力峰值在行星形成中具有潜在的重要性，但人们对其起源和性质知之甚少。由于磁盘温度，水从固体转变为蒸汽的雪线被认为是压力最大值的可能原因。如果这是真的，那么行星的形成就是行星盘演化的自然结果。目的雪线处水的凝结和蒸发通过改变气体中的H2:H2O影响氧逸度。因此，来自整个原行星盘的fO2记录可以使我们以行星科学研究中前所未有的深度和清晰度调查压力最大值的性质和起源，以及它们的后果。球粒陨石由来自圆盘的毫米大小的固体组成，因此携带的fO2记录可以追溯到太阳系早期。Sutton等人（2017）证明了这一点，他们表明碳质球粒陨石（CC）比非碳质球粒陨石（NC）记录了更多的正fO2。这是由于母体氧逸性，因为CCs比NC体吸收了更多的固体水。为了发现实际磁盘的fO2，我们需要记录来自单个球粒的数据，范围从100-1000um。它们的大小意味着需要高精度，高分辨率的技术来恢复值，这在以前从未以系统和集中的方式完成过。在这个项目中，我希望使用微量元素在金属、硅酸盐和硫化物之间的可变分配作为单个球粒中fO2值的代理。这将使我了解球粒形成的环境，对雪线及其对我们太阳系的影响，以及相关的压力最大值有了新的认识。因此，这个项目可以为行星形成过程提供几个关键的新见解。研究方法Fe、Cr、W、Ti、V等元素的氧化还原状态随fO2的变化而变化。这些元素在熔融金属、硅酸盐和硫化物中的热力学稳定性是由它们的氧化还原状态控制的，因此是由fO2控制的。因此，我们可以通过观察金属、硅酸盐和硫化物中这些元素的相对浓度，来创建单个球粒中氧逸度的代理。在这个项目中，将通过比较球粒内的浓度与在一系列受控的fO2条件下产生的人工样品中测量的值来估计值。浓度测量将使用高分辨率技术，包括激光烧蚀电感耦合等离子体质谱（LA-ICP-MS）、电子探针微分析（EMPA）和金刚石光源的微x射线荧光（uXRF）。人造样品将在地球科学系使用专门的熔炉制作。这里讨论的代理已经在牛津大学开发和使用了几十年，应用于陆地和火星岩石，但以前从未应用于其他地外物质，突出了这个项目在解开行星形成过程新信息方面的潜力。

BackgroundRecently, there has been a dramatic shift in understanding of planet formation and the circumstellar disk, following incredible images from the ALMA telescope. They have revolutionised understanding of planetary formation and caused a suite of new theories. For example, Johansen et al. 2014 suggested that regions with increased dust and gas pressure (pressure maxima) must have been present in the protoplanetary disk to form the first planetary bodies. The origin and nature of pressure maxima is poorly understood, despite their potential importance in planetary formation. The snow line, where water transitions from solid to vapour because of disk temperature, has been suggested as a possible cause of pressure maxima. If this is true, then planetary formation is a natural result of disk evolution.AimsCondensation and vaporisation of water at the snow line affect oxygen fugacity, through altering H2:H2O in the gas. fO2 records from throughout the protoplanetary disk therefore could enable us to investigate the nature and origin of pressure maxima, as well as their consequences, with depth and clarity unprecedented in planetary science research.Chondrites are made of mm sized solids from the disk, and therefore carry fO2 records dating from the early Solar System. This is demonstrated by Sutton et al. (2017), who showed that carbonaceous chondrites (CC) recorded a more positive fO2 than non-carbonaceous (NC). This is due to the parent body oxygen fugacity, as CCs incorporated more solid water than NC bodies. In order to discover the fO2 of the actual disk, we need to record data from individual chondrules, which range from 100-1000um. Their size means that high precision, high-resolution techniques are required to recover values, this has never been done before in a systematic and focussed way.In the project, I hope to use the variable partitioning of trace elements among metal, silicates and sulphides as a proxy for fO2 values in individual chondrules. This will allow me to understand the environments in which chondrules formed, giving new understanding about the snow line and its consequences in our Solar System, and the associated pressure maxima. As such, this project could unlock several key new insights into planet building processes.Research MethodologyThe redox state of elements including Fe, Cr, W, Ti, and V varies with fO2. The thermodynamic stability of these elements in molten metal, silicates and sulphides is controlled by their redox states, and therefore fO2. As a result, we can create a proxy for oxygen fugacity in individual chondrules by looking at relative concentrations of these elements within metal, silicate and sulphide. In this project, values will be estimated through a comparison of concentration inside a chondrule to the value measured in artificial samples created under a range of controlled fO2 conditions. Concentrations would be measured using high-resolution techniques, including laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), electron probe micro-analysis (EMPA), and micro X-ray fluorescence (uXRF) at Diamond Light Source.Artificial samples will be created in the Department of Earth Sciences using specialised furnaces. The proxies discussed here have been developed and used in Oxford for decades, applied to terrestrial and martian rocks, but never before on other extra-terrestrial material, highlighting the potential of this project to unlock new information on the process of planet building.