Noble gas state of the Precambrian mantle

点击翻译按钮获取中文摘要

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.