Elucidating planet formation using chondrule oxygen fugacity

背景最近，在阿尔马望远镜拍摄到令人难以置信的图像之后，对行星形成和星周盘的理解发生了巨大的转变。他们彻底改变了对行星形成的理解，并提出了一套新的理论。例如，Johansen等人在2014年提出，尘埃和气体压力增加的区域（压力最大值）必须存在于原行星盘中，以形成第一个行星体。最大压力的起源和性质知之甚少，尽管它们在行星形成中具有潜在的重要性。雪线，水从固体转变为蒸汽，因为磁盘温度，已被认为是一个可能的原因压力最大。如果这是真的，那么行星的形成是盘演化的自然结果。目的雪线处水的冷凝和蒸发通过改变气体中的H2：H2O来影响氧逸度。因此，来自原行星盘的fO 2记录可以使我们能够研究压力最大值的性质和起源，以及它们的后果，其深度和清晰度在行星科学研究中是前所未有的。球粒陨石由来自盘的毫米大小的固体组成，因此携带着可以追溯到早期太阳系的fO 2记录。萨顿等人（2017）证明了这一点，他们表明碳质碳酸盐（CC）记录的fO 2比非碳质碳酸盐（NC）更正值。这是由于母体氧逸度，因为CC比NC体结合更多的固体水。为了发现实际盘的fO 2，我们需要记录单个球粒的数据，范围从100- 1000 μ m。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。在该项目中，我希望使用金属，硅酸盐和硫化物之间的微量元素的可变分配作为单个球粒中fO 2值的代理。这将使我能够了解陨石球粒形成的环境，对雪线及其在太阳系中的后果以及相关的压力最大值有新的理解。因此，该项目可以解开几个关键的新见解行星建设过程。研究方法元素包括铁，铬，钨，钛，和V的氧化还原状态随fO 2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由它们的氧化还原状态控制，因此fO 2。因此，我们可以通过观察这些元素在金属、硅酸盐和硫化物中的相对浓度来创建单个球粒中氧逸度的代理。在本项目中，将通过比较球粒内的浓度与在一系列受控fO 2条件下创建的人工样品中测量的值来估计值。将使用高分辨率技术测量浓度，包括激光烧蚀电感耦合等离子体质谱法（LA-ICP-MS）、电子探针显微分析（EMPA）和金刚石光源的微X射线荧光（uXRF）。这里讨论的代理人已经在牛津开发和使用了几十年，应用于陆地和火星岩石，但以前从未在其他地外物质上使用过，突出了这个项目的潜力，以解锁有关行星建设过程的新信息。

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.