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

太古宙的大部分撞击记录仍然笼罩在神秘之中。毫无疑问，在最近的猛烈撞击期间和之后，大型小行星碎片撞击了地球，但这些撞击事件的伤痕早已被抹去。然而，在撞击事件期间喷出的小球体，即使在无法找到撞击源的情况下，也能提供有关撞击事件的信息。根据它们的铬同位素特征，三个古太古代球形层（仅保存在南非巴伯顿绿岩带）无疑代表了迄今为止发现的最古老的撞击残留物。最近从巴伯顿地区恢复的两个钻孔岩心可能有多达21个新的古太古代球体层，为获得早期地球陨石轰击的新见解提供了一个绝佳的机会。本提案旨在(i)评估两个岩心中间隔紧密的球体床之间可能的相关性，（ii）确定其中的陨石混合物的大小和空间分布，（iii）表征所涉及的撞击物材料，以及（iv）评估它们所代表的陨石坑事件的数量。这些目标将通过结合原位和全岩地球化学和岩石学方法来实现，包括详细的矿物学调查，首次高精度的Os和W同位素分析以及同位素稀释产生的高亲铁元素浓度。此外，本研究打算分析纯球粒分离物，而不是通常使用的球粒基质聚集体。这些方法的结合为太古宙撞击沉积物的研究打开了一个很有前途的新工具箱，最终导致对太古宙地壳元素传递的新见解，可能是由大撞击引起的全球构造重组，以及在发现第一个生命迹象的时代盛行的地球条件的特征。

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.