Spherule layers in the 2011 ICDP drilling in the Barberton Mountain Land: Early impact record on Earth

2011年ICDP钻探“巴伯顿山地”钻探了相对未风化的稀有晚太古代火山岩和沉积岩，包括地球上已知最古老的撞击喷射层。其中一些高达数十厘米厚的太古代球体层的化学特征（高铱浓度，铬同位素比率）表明，这些喷出物沉积物主要是由外星物质形成的。喷出层包含毫米大小的球体，与显生宙沉积物中已知的任何撞击喷出层相比，这些球体更大，形成的层也更厚，包括希克苏鲁伯撞击事件的全球喷出层，该事件结束了地球历史上的中生代。我们建议进行1)主要元素和微量元素的大量化学分析，2)撞击喷出层的岩石学、微化学和矿物学表征，以及3)铂族元素（PGE）分布的LA-ICP-MS元素制图。其目的是：1)对喷出物层进行表征；2)识别具有地外PGE特征的阶段；3)从沉积前后蚀变的特征中区分撞击事件的主要地质证据。我们想利用地质证据来提取关于撞击太古代地球的弹丸的大小、类型和频率的关键信息。

The 2011 ICDP drilling “Barberton Mountain Land” drilled relatively unweathered rare late Archean volcanic and sedimentary rocks including the oldest known impact ejecta layers on Earth. The chemical signature (high Iridium concentrations, Chromium isotopic ratios) of some of these up to tens of cm thick Archean spherule layers advocate that these ejecta deposits formed dominantly from extraterrestrial material. The ejecta layers contain millimeter sized spherules that are larger and form thicker layers compared to any impact ejecta layer known from Phanerozoic sediments, including the global ejecta layer of the Chicxulub impact catering event that terminated the Mesozoic era of Earth’s history. We propose to conduct 1) bulk chemical analyses of major and trace elements, 2) petrographic, micro-chemical and mineralogical characterization of the impact ejecta layers, and 3) LA-ICP-MS elemental mapping of platinum group element (PGE) distributions. This aims at 1) characterization of the ejecta layers, 2) identification of the phases hosting the extraterrestrial PGE signature, 3) discrimination of the primary geological evidence of the impact event from those characteristics that resulted from syn- and post-sedimentary alteration. We want to exploit the geological evidence for extracting key information regarding size, type and frequency of projectiles impacting the Archean Earth.