Understanding noble gases in the context of mantle dynamics

在地幔动力学背景下理解惰性气体

• 批准号: 2672382
• 金额: --
• 依托单位: CARDIFF UNIVERSITY
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2021
• 资助国家: 英国
• 起止时间: 2021 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2672382
• 关键词: Understanding; noble; gases; context; mantle

Mantle convection drives critical processes such as seismicity, volcanoes and mountain building. Noble gases provide key constraints on this poorly understood, but fundamental, mantle dynamics. Models have struggled to fully incorporate these constraints.These constraints include:- Ocean Island Basalts have higher 3He/4He ratios than mid-ocean ridge basalts, and so must be supplied by a reservoir with much higher ratios of primordial 3He to U+Th. This is generally interpreted to be due to retention of high concentrations of primordial gases (Porcelli and Elliott, 2008).- The amount of 40Ar in the atmosphere and upper mantle appears to account for only half of the total produced by 40K, requiring another, deep storage reservoir for Ar.- Variations in the ratios of Xe isotopes produced by short-lived nuclides only present during early Earth history require early separation of noble gas reservoirs.Establishing the necessary reservoirs for noble gases within the convecting mantle has been difficult, and the nature and location of these reservoirs has been hotly debated. Possibilities include domains within the convecting mantle, convective isolation of mantle layers, a separate mantle reservoir at the Core Mantle Boundary (CMB), and the core. This studentship will engage in using the convecting mantle models developed in the MC^2 NERC Large Grant project to explore how the noble gas observational data can be successfully explained.The approach is to expand numerical mantle circulation models (MCM) to include the tracking of noble gases (van Heck et al., 2016). Like other chemical and isotopic characteristics, the noble gases will be tracked on particles that are advected by the flow. The model is capable of partitioning the noble gases into melts and outgassing them to external reservoirs, they can also be input from the core. The student will be able to adjust and investigate the result of all such choices. Tomography identifies two large low shear wave velocity provinces (LLSVP) above the CMB. They are hypothesized to be poorly resolved thermal upwellings, or a primordial layer left over from earliest Earth history, or a graveyard of subducted lithosphere, or a combination thereof. The LLSVP could be the separate reservoir above the CMB mentioned above that can help to reconcile the enigmatic noble gas observations. The MCMs can address all these hypotheses.In addition, the studentship will take the novel approach of explicitly supplementing these expensive dynamical models with analytical models. These analytical models will allow the student to interrogate a wider range of possible implications of the convection models for the noble gases.

地幔对流驱动着地震活动、火山和造山等关键过程。稀有气体对这种知之甚少但基本的地幔动力学提供了关键限制。模型一直在努力完全纳入这些限制。这些限制包括： - 大洋岛玄武岩的 3He/4He 比率高于洋中脊玄武岩，因此必须由原始 3He 与 U+Th 比率高得多的储层提供。这通常被解释为由于保留了高浓度的原始气体（Porcelli 和 Elliott，2008）。- 大气和上地幔中 40Ar 的量似乎只占 40K 产生总量的一半，需要另一个深层 Ar 储存库。- 仅在地球历史早期存在的短命核素产生的 Xe 同位素比率的变化需要早期分离稀有气体 在对流地幔内建立必要的稀有气体储层一直很困难，并且这些储层的性质和位置一直存在激烈争论。可能性包括对流地幔内的区域、地幔层的对流隔离、核心地幔边界（CMB）处的独立地幔储层和地核。该学生奖学金将致力于使用 MC^2 NERC 大笔资助项目中开发的对流地幔模型来探索如何成功解释惰性气体观测数据。该方法是扩展数值地幔环流模型（MCM）以包括惰性气体的跟踪（van Heck 等人，2016）。与其他化学和同位素特征一样，稀有气体将在由流动平流输送的颗粒上被追踪。该模型能够将稀有气体分配到熔体中并将其排出到外部储层，它们也可以从堆芯输入。学生将能够调整和调查所有此类选择的结果。断层扫描识别出 CMB 上方两个较大的低剪切波速区 (LLSVP)。它们被假设为难以解析的热上升流，或地球最早历史遗留下来的原始层，或俯冲岩石圈的墓地，或它们的组合。 LLSVP 可能是上述 CMB 上方的独立储层，有助于协调神秘的惰性气体观测结果。 MCM 可以解决所有这些假设。此外，学生将采取新颖的方法，用分析模型明确补充这些昂贵的动力学模型。这些分析模型将使学生能够探究稀有气体对流模型的更广泛的可能影响。