Diamond formation in Earth's mantle

地幔中的钻石形成

• 批准号: RGPIN-2017-06041
• 负责人: Stachel, Thomas
• 金额: $ 3.42万
• 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=712014
• 关键词: Diamond; formation; Earth; mantle

Diamond exploration and mining is the principal economic driving force in Canada's remote North. However, current levels of production will be unsustainable unless new deposits are discovered within the next few years. To help develop more efficient exploration strategies, we will geochemically fingerprint the conditions deep in the Earth's mantle that enhanced or prevented diamond formation in the distant past. The investigations outlined below are expected to produce not only ground-breaking basic research but also new and better methods for diamond exploration. 1. Processes of diamond formation The layered internal structure of diamond can be compared to tree rings in that both can reveal complex and extended growth histories. I direct a multi-collector ion microprobe facility, which will enable us to obtain profiles of carbon and nitrogen isotopic compositions and nitrogen contents at high spatial resolution across diamond growth zones. These profiles allow us to deduce the chemical character of the original diamond-forming fluids or melts and reveal if they were methane- or carbonate-bearing. By studying diamonds with known ages of crystallization, we will be able to follow variations in the preferred modes of diamond precipitation through time. 2. Unconventional diamond deposits Archean cratons are large, stable blocks of the Earth's crust that are older than 2.5 billion years. The lithospheric mantle beneath such cratons is the source of >99% of current diamond production, and the rock type harzburgite is its key diamond substrate. However, younger areas of the crust can also host world-class diamond deposits. The rock type eclogite is a key diamond substrate in such unconventional deposits, but our preliminary results suggest that lherzolite may also be important. We will investigate two such diamond-bearing areas beneath the Canadian Shield (sampled by kimberlite volcanoes at Fort à la Corne and the Victor Mine) that were strongly affected by tectonic/thermal events less than two billion years ago. By studying the diamonds, their inclusions, and the associated mantle rocks, we aim to determine the diamond-forming conditions in the lithospheric mantles of these areas through time. 3. New Canadian diamond deposits Diamonds from two new deposits will be used to probe the mantle roots of little-studied deep sections of the Canadian Shield: (1) The Chidliak kimberlite field (Baffin Island). This area is underlain by a small cratonic fragment of unclear origin, and its diamond source rocks have not yet been investigated. (2) The Gahcho Kue diamond mine. This mine provides unique access to an exceptionally thick section of lithospheric mantle in the southern Slave Craton. The knowledge gained in all three study themes will be crucial to future exploration for new diamond deposits in Canada.

钻石勘探和开采是加拿大偏远北部地区的主要经济驱动力。然而，目前的产量水平将是不可持续的，除非在未来几年内发现新的矿藏。为了帮助制定更有效的勘探战略，我们将在地球地幔深处用地球化学方法确定在遥远的过去促进或阻止钻石形成的条件。 下文概述的调查预计不仅将产生开创性的基础研究，而且还将产生新的和更好的钻石勘探方法。 1.钻石的形成过程 钻石的分层内部结构可以比作树轮，因为两者都可以揭示复杂而漫长的生长历史。我指导了一个多收集器离子微探针设施，这将使我们能够在高空间分辨率下获得钻石生长区域的碳和氮同位素组成和氮含量的概况。这些剖面使我们能够推断原始钻石形成流体或熔体的化学性质，并揭示它们是含甲烷的还是含碳酸盐的。通过研究已知结晶年龄的钻石，我们将能够跟踪钻石沉淀的首选模式随时间的变化。 2.非常规钻石矿床 太古宙克拉通是地壳的大而稳定的块体，其历史超过25亿年。这些克拉通之下的岩石圈地幔是目前钻石产量的99%的来源，而岩石类型的方辉橄榄岩是其关键的钻石底板。然而，地壳的年轻地区也可以拥有世界级的钻石矿藏。岩石类型的榴辉岩是这些非常规矿床的关键金刚石基质，但我们的初步结果表明，二辉橄榄岩也可能是重要的。我们将调查加拿大盾下的两个这样的钻石产区(由康纳堡和维克多矿的金伯利火山采集的样本)，它们在不到20亿年前受到构造/热事件的强烈影响。通过对钻石及其包裹体和伴生地幔岩石的研究，我们旨在确定这些地区岩石圈地幔随时间的形成条件。 3.加拿大新的钻石矿藏 来自两个新矿床的钻石将用于探测加拿大地盾鲜为人知的深部的地幔根部： (1)Chidliak金伯利岩田(巴芬岛)。该地区被一个成因不明的小克拉通碎块覆盖，其钻石源岩尚未调查。 (2)加乔库钻石矿。这个矿场提供了通往南部奴隶克拉通异常厚实的岩石圈地幔的独特通道。 在所有三个研究主题中获得的知识将对今后在加拿大勘探新的钻石矿藏至关重要。