Spherule layers from South Africa as windows into the Paleoarchean meteorite bombardment of the Earth-Evidences from coupled Os-W isotope systematics and siderophile elements

来自南非的球粒层是了解古太古代陨石轰击地球的窗口——来自耦合 Os-W 同位素系统学和亲铁元素的证据

• 批准号: 276770016
• 负责人: Dr. Toni Schulz
• 金额: --
• 依托单位: Department für Lithosphärenforschung
• 依托单位国家: 德国
• 项目类别: Priority Programmes
• 财政年份: 2015
• 资助国家: 德国
• 起止时间: 2014-12-31 至 2019-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/276770016?language=en
• 关键词: Spherule; layers; South; Africa; windows

Much of the Archean impact record remains shrouded in mystery. Large asteroid fragments have undoubtedly hit the Earth during and in the wake of the late heavy bombardment, but the scars of these impact events have long since been obliterated. However, spherules, ejected during the impact event, provide information about an impact event even when the source crater cannot be found. Based on their Cr isotope signatures three spherule layers of Paleoarchean age (exclusively preserved in the Barberton Greenstone Belt, South Africa) unambiguously represent the oldest impact remnants identified so far. Two recently recovered drill cores from the Barberton area with possibly up to 21 new spherule layers of Paleoarchean age provide an outstanding opportunity to gain new insights into the meteorite bombardment of the Early Earth. This proposal aims to (i) assess possible correlations between the closely spaced spherule beds from both drill cores, (ii) determine the magnitude and spatial distribution of meteoritic admixtures within them, (iii) characterize impactor materials involved, and (iv) assess the number of cratering events they represent. These objectives will be achieved by a combination of in-situ and whole rock geochemical and petrological approaches, including detailed mineralogical investigations, the first high precision Os and W isotope analysis and isotope dilution generated concentrations of highly siderophile elements. Moreover, this study intends to analyze pure spherule separates instead of spherule-matrix aggregates as commonly utilized. The combination of these approaches opens a promising new tool-box for the investigation of Archean impact deposits, leading ultimately to new insights into the element delivery to the Archean crust, possibly global tectonic reorganizations resulting from large impacts and the characterization of conditions on Earth that prevailed in an era from which the first signs of life were discovered.

太古宙撞击记录的很大一部分仍然笼罩在神秘之中。毫无疑问，在最近的猛烈轰炸期间和之后，巨大的小行星碎片撞击了地球，但这些撞击事件留下的伤疤早已被抹去。然而，即使在找不到源陨石坑的情况下，在撞击事件期间抛出的球体也会提供有关撞击事件的信息。根据它们的铬同位素特征，三个古太古代球状层(仅保存在南非巴伯顿绿岩带中)明确代表了迄今为止已发现的最古老的撞击残留物。最近从巴伯顿地区发现的两个钻芯，可能有多达21个新的古太古代球状层，这为我们提供了一个绝佳的机会，以获得对早期地球的陨石轰击的新见解。这项建议旨在(1)评估两个钻芯的紧密间隔球层之间的可能相关性，(2)确定它们内部陨石混合物的大小和空间分布，(3)表征所涉及的冲击器材料，以及(4)评估它们所代表的陨石坑事件的数量。这些目标将通过现场和全岩地球化学和岩石学相结合的方法实现，包括详细的矿物学调查，第一次高精度的Os和W同位素分析，以及同位素稀释产生的高度亲铁元素的浓度。此外，这项研究打算分析纯球体分离，而不是通常使用的球体基质聚集体。这些方法的结合为太古宙撞击沉积的研究开辟了一个很有前途的新工具箱，最终导致对太古宙地壳元素输送的新见解，可能是大规模撞击导致的全球构造重组，以及在发现第一批生命迹象的时代对地球条件的描述。