Experimental studies of the Earth's mantle

地幔的实验研究

• 批准号: RGPIN-2017-03914
• 负责人: Luth, Robert
• 金额: $ 1.97万
• 依托单位: University of Alberta
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2020
• 资助国家: 加拿大
• 起止时间: 2020-01-01 至 2021-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=710609
• 关键词: Experimental; studies; Earth; mantle

Diamonds form in the Earth's mantle at depths of > ~130 km and temperatures > ~ 900C. But there remain many questions about how and why they form, and what they tell us about the mantle in which they form. My research aims to address these questions, and addresses the behaviour of carbon in the mantle more broadly, because we need to understand the conditions in which carbon will be stable as diamond rather than as crystalline carbonate or dissolved in a fluid or melt. By reproducing the pressures and temperatures in the Earth's mantle at diamond-stable conditions in the laboratory, we can explore these questions in a way complementary to the observations that other researchers make on natural diamonds and their fluid and mineral inclusions. We proposed recently a model for diamond formation in which the key element is how a water-rich fluid interacts with the different rock types that make up the Earth's upper mantle at diamond-stable conditions. In some rock types, this fluid would trigger melting, and diamonds would have to form in the presence of that melt. In the most common rock type associated with diamonds, however, melting would not occur, and the fluid itself could precipitate diamond by cooling or ascending. This model is a fundamental shift in thinking about how diamond may form in the mantle, but we need to know more about how diamond behaves in these fluids and melts, and even what kind of fluids and melts could be present in the mantle at these conditions. Where do these fluids come from? One likely source is altered oceanic crust and mantle that is recycled into the mantle in subduction zones and gives off fluids as it subducts, down to 200 km depth or more. These fluids would add H2O, CO2, Cl, and other volatiles back into the mantle. The first two have been studied extensively, but not Cl and others. We want to study what happens when these fluids interact with the different rock types of the Earth's upper mantle. When will they trigger melting? How does the presence of Cl change the melting behaviour? We will also study how diamond dissolves in melts that are produced by the interaction of these fluids with mantle rocks, and how that solubility changes with pressure and temperature. This will tell us how effective these melts would be in forming diamonds. Finally, some diamonds contain up to 5000 ppm nitrogen. Does this mean that there is a lot or only a little nitrogen in some diamond-forming fluids or melts? We will grow diamond in N-bearing fluids and melts to understand how nitrogen partitions between those media and the growing diamond. This research is important both from an academic perspective helping us to understand how diamond forms and what sort of fluids and melts have modified the Earth's mantle but also from a more practical perspective. Diamonds are a key economic driver in Canada's north, and if we can develop a better understanding of how diamonds form, that will inform models to help find new deposits.

钻石形成于地幔深度 > ~ 130 公里且温度 > ~ 900 C 的地幔中。但关于它们如何形成、为何形成，以及它们告诉我们关于它们形成的地幔的信息，仍然存在许多问题。我的研究旨在解决这些问题，并更广泛地解决碳在地幔中的行为，因为我们需要了解碳在什么条件下会像金刚石一样稳定，而不是像结晶碳酸盐一样稳定或溶解在流体或熔体中。通过在实验室中重现钻石稳定条件下地幔的压力和温度，我们可以以与其他研究人员对天然钻石及其流体和矿物包裹体进行的观察相补充的方式探索这些问题。 我们最近提出了一个钻石形成模型，其中的关键要素是富含水的流体如何与在钻石稳定条件下构成地球上地幔的不同岩石类型相互作用。在某些岩石类型中，这种流体会引发熔化，钻石必须在熔化的情况下形成。然而，在与钻石相关的最常见的岩石类型中，不会发生熔化，并且流体本身可以通过冷却或上升来沉淀钻石。这个模型是对金刚石如何在地幔中形成的思考的根本转变，但我们需要更多地了解金刚石在这些流体和熔体中的行为，甚至在这些条件下地幔中可能存在什么样的流体和熔体。 这些液体从哪里来？一个可能的来源是改变的海洋地壳和地幔，它们被循环到俯冲带的地幔中，并在俯冲到 200 公里或更深的深度时释放出流体。这些流体会将水、二氧化碳、氯和其他挥发物带回到地幔中。前两者已得到广泛研究，但 Cl 和其他物质尚未得到广泛研究。 我们想要研究当这些流体与地球上地幔的不同岩石类型相互作用时会发生什么。它们什么时候会引发融化？ Cl 的存在如何改变熔化行为？我们还将研究金刚石如何溶解在这些流体与地幔岩石相互作用产生的熔体中，以及溶解度如何随压力和温度变化。这将告诉我们这些熔体在形成钻石方面的效果如何。 最后，一些钻石的氮含量高达 5000 ppm。这是否意味着某些金刚石形成流体或熔体中含有大量或仅有少量氮？我们将在含氮流体和熔体中生长钻石，以了解氮如何在这些介质和生长的钻石之间分配。 这项研究不仅从学术角度来看很重要，可以帮助我们了解钻石是如何形成的以及什么样的流体和熔体改变了地幔，而且从更实际的角度来看也很重要。钻石是加拿大北部的关键经济驱动力，如果我们能够更好地了解钻石的形成方式，这将为帮助寻找新矿床的模型提供信息。