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: 2018
• 资助国家: 加拿大
• 起止时间: 2018-01-01 至 2019-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=658132
• 关键词: Experimental; studies; Earth; mantle

Diamonds form in the Earth's mantle at depths of > ~130 km and temperatures > ~ 900°C. 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.

钻石形成于地幔深处> ~130 km，温度> ~ 900°C。但是，关于它们如何形成以及为什么形成，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作为金刚石而不是结晶碳酸盐或溶解在流体或熔体中的稳定条件。通过在实验室中重现钻石稳定条件下地幔中的压力和温度，我们可以以一种补充其他研究人员对天然钻石及其流体和矿物包裹体的观察的方式来探索这些问题。我们最近提出了一个钻石形成的模型，其中的关键因素是富含水的流体如何与构成地球上地幔的不同岩石类型在钻石稳定的条件下相互作用。在某些岩石类型中，这种流体会引发熔化，钻石必须在这种熔体存在的情况下形成。然而，在与钻石有关的最常见的岩石类型中，不会发生熔融，并且流体本身可以通过冷却或上升来沉淀钻石。这个模型是对钻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钻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。* 这些液体是从哪里来的？一个可能的来源是被改变的洋壳和地幔，这些洋壳和地幔在俯冲带中再循环到地幔中，并在俯冲时释放出流体，直到200公里或更深。这些流体会将H2O、CO2、Cl和其他挥发物添加回地幔。前两个已经被广泛研究，但没有Cl和其他。*** 我们想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？我们还将研究钻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有效性。* 最后，有些钻石含有高达5000 ppm的氮。这是否意味着在某些金刚石形成流体或熔体中有大量或少量的氮？我们将在含氮流体和熔体中生长金刚石，以了解氮如何在这些介质和生长的金刚石之间分配。这项研究从学术角度来看都很重要-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球的地幔-而且从更实用的角度来看。钻石是加拿大北部的主要经济驱动力，如果我们能够更好地了解钻石的形成方式，这将为模型提供信息，以帮助寻找新的矿床。