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罕见的太古代晚期火山岩和沉积岩，包括地球上已知最古老的撞击喷出物层。其中一些厚达数十厘米的太古代球粒层的化学特征（高铱浓度，铬同位素比值）表明，这些喷出物沉积物主要由外星物质形成。喷出物层包含毫米大小的球粒，与任何已知的中生代沉积物的撞击喷出物层相比，这些球粒更大，形成的层更厚，包括结束地球中生代历史的希克苏鲁布撞击事件的全球喷出物层。我们建议进行1）主要和微量元素的大量化学分析，2）冲击喷出物层的岩相学，微观化学和矿物学特征，和3）LA-ICP-MS元素绘图铂族元素（PGE）的分布。其目的是：1）确定喷出物层的特征，2）确定地球外铂族元素特征的相态，3）区分撞击事件的主要地质证据和沉积后和沉积期蚀变的特征。我们希望利用地质证据来提取有关撞击太古代地球的抛射体的大小，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.