Noble gas state of the Precambrian mantle

前寒武纪地幔的稀有气体状态

• 批准号: 424108993
• 负责人: Privatdozent Dr. Jens Hopp
• 金额: --
• 依托单位: Institut für Geowissenschaften
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2019
• 资助国家: 德国
• 起止时间: 2018-12-31 至 2021-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/424108993?language=en
• 关键词: Noble; gas; state; Precambrian; mantle

A significant fraction of the terrestrial atmosphere has formed by degassing of the Earth´s mantle very early in Earth’s history. This fundamental conclusion is mainly derived from the excess of radiogenic isotopes in mantle sources (MORB, OIB), e.g. 40Ar from 40K and more stringently 129Xe from decay of short-lived 129I decay, along with fission Xe from 238U and / or 244Pu. While the present-day mantle isotopic state is quite well known, there is hardly information about the Precambrian mantle. The ancient mantle is expected to contain a higher proportion of primordial and lower proportion of radiogenic nuclides, depending on - and therefore constraining - the degassing history and lithophile parent nuclide depletion of Earth´s mantle. However, there are only a few helium isotopic data on Neo-Archean mantle rocks, and it is uncertain to what degree cosmogenic nuclides may compromise isotopic data. Here I propose to analyze different suites of mantle rocks: Archean ultramafic rocks (peridotite lenses of the Itsaq Gneiss Complex south of the Isua Supracrustal belt, nominally 3.7 Ga old); komatiites (Barberton Greenstone Belt, South Africa and Pilbara craton, Western Australia, age range 3.6-2.9 Ga,); Indian Proterozoic carbonatites (Hogenakkal, 2.4 Ga; Newania, 1.55 Ga; Sevattur, 800 Ma) spanning an age range from 3.7 Ga to 800 Ma. More recent ultramafic rocks and carbonatites have turned out to host mantle noble gases in fluid inclusions trapped from their host magmas or from metasomatizing mantle fluids. These still have preserved well-defined abundances of mantle gases (e.g. in 380 Ma old carbonatites from the Kola Peninsula, Russia). In Precambrian samples major obstacles for identifying mantle gases are incorporation of crustal gases during greenschist facies metamorphism, addition of in situ produced radiogenic or cosmogenic nuclides, and alteration-induced atmospheric gases. The applicant plans to apply well-established stepwise crushing noble gas extraction that largely minimizes interference of in situ radiogenic and cosmogenic components. In addition, application of microthermometry of fluid inclusions will enable identifying trails and groups with similar trapping conditions. Individual fluid inclusions or groups can be opened using a laser probe and subsequently analyzed by a highly sensitive Noblesse noble gas mass spectrometer especially designed for small sample analyses. Detection of several isotope systems further facilitates quantifying non-mantle components, in particular of crustal origin.

地球大气层的很大一部分是在地球历史的早期由地幔脱气形成的。这一基本结论主要来源于地幔源中过量的放射性成因同位素（MORB， OIB），例如40K中的40Ar和129Xe，以及238U和/或244Pu中的裂变Xe。虽然现今的地幔同位素状态是众所周知的，但关于前寒武纪地幔的信息却很少。预计古地幔中含有较高比例的原始核素和较低比例的放射性核素，这取决于——因此也限制了——地球地幔的脱气历史和亲石母核素耗损。然而，新太古代地幔岩石的氦同位素数据很少，宇宙核素对同位素数据的影响程度尚不确定。在这里，我建议分析不同的套幔岩：太古宙超基性岩（伊苏亚上地壳带以南伊萨克片麻岩杂岩的橄榄岩透镜体，名义年龄3.7 Ga）；科马长岩（南非巴伯顿绿岩带和西澳大利亚皮尔巴拉克拉通，年龄范围3.6-2.9 Ga）；印度元古代碳酸盐岩（Hogenakkal, 2.4 Ga; Newania, 1.55 Ga; Sevattur, 800 Ma）年龄跨度为3.7 Ga - 800 Ma。最近的超镁铁质岩石和碳酸盐岩被证明含有地幔惰性气体，这些气体来自于它们的岩浆宿主或地幔流体的交代包裹体。它们仍然保留了明确的地幔气体丰度（例如，在俄罗斯科拉半岛380 Ma的碳酸盐岩中）。在前寒武纪样品中，识别地幔气体的主要障碍是绿片岩相变质过程中地壳气体的掺入、原位产生的放射性核素或宇宙成因核素的添加以及蚀变诱发的大气气体。申请人计划采用成熟的逐步破碎稀有气体提取，这在很大程度上减少了原位放射性成因和宇宙成因成分的干扰。此外，流体包裹体显微测温技术的应用将有助于识别具有相似捕获条件的痕迹和群体。单个流体包裹体或组可以使用激光探头打开，随后通过高灵敏度Noblesse惰性气体质谱仪进行分析，该质谱仪专为小样本分析而设计。几个同位素系统的检测进一步有助于量化非地幔成分，特别是地壳起源。