Geochemistry of Chixulub ejecta in ODP Leg 207 holes

An up to 2-cm thick Chicxulub ejecta deposit marking the Cretaceous-Paleogene (K-Pg) boundary (the "K-T" boundary) was recovered in six holes drilled during ODP Leg 207 (Demerara Rise, tropical western Atlantic). Stunning features of this deposit are its uniformity over an area of 30 km2 and the total absence of bioturbation, allowing documentation of the original sedimentary sequence. High-resolution mineralogical, petrological, elemental, isotopic (Sr-Nd), and rock magnetic data reveal a distinct microstratigraphy and a range of ejecta components. The deposit is normally graded and composed predominantly of rounded, 0.1- to max. 1-mm sized spherules. Spherules are altered to dioctahedral aluminous smectite, though occasionally relict Si-Al-rich hydrated glass is also present, suggesting acidic precursor lithologies. Spherule textures vary from hollow to vesicle-rich to massive; some show in situ collapse, others include distinct Fe-Mg-Ca-Ti-rich melt globules and lath-shaped Al-rich quench crystals. Both altered glass spherules and the clay matrix (Site 1259B) display strongly negative epsilon-Nd (T=65Ma) values (-17) indicating uptake of Nd from contemporaneous ocean water during alteration. Finally, Fe-Mg-rich spherules, shocked quartz and feldspar grains, few lithic clasts, as well as abundant accretionary and porous carbonate clasts are concentrated in the uppermost 0.5-0.7 mm of the deposit. The carbonate clasts display in part very unusual textures, which are interpreted to be of shock-metamorphic origin. The preservation of delicate spherule textures, normal grading with lack of evidence for traction transport, and sub-millimeter scale compositional trends provide evidence for this spherule deposit representing a primary air-fall deposit not affected by significant reworking.
The ODP Leg 207 spherule deposit is the first known dual-layer K-Pg boundary in marine settings; it incorporates compositional and stratigraphic aspects of both proximal and distal marine sites. Its stratigraphy strongly resembles the dual-layer K-Pg boundary deposits in the terrestrial Western Interior of North America (although there carbonate phases are not preserved). The occurrence of a dual ejecta layer in these quite different sedimentary environments - separated by several thousands of kilometers - provides additional evidence for an original sedimentary sequence. Therefore, the layered nature of the deposit may document compositional differences between ballistic Chicxulub ejecta forming the majority of the spherule deposit, and material falling out from the vapor (ejecta) plume, which is concentrated in the uppermost part.

在热带西大西洋德梅拉拉海隆开展的大洋钻探计划（Ocean Drilling Program, ODP）第207航次的6个钻孔中，采获了一层厚度可达2厘米的希克苏鲁伯冲击碎屑沉积（Chicxulub ejecta deposit），该沉积标识了白垩纪-古近纪（Cretaceous-Paleogene, K-Pg）界线（即“K-T”界线）。该沉积的显著特征为：在30平方千米范围内保持高度均一且完全未受生物扰动（bioturbation），从而完整保留了原始沉积序列。高分辨率矿物学、岩石学、元素地球化学、同位素（Sr-Nd）及岩石磁学分析数据揭示了清晰的微观地层结构与多种冲击碎屑组分。该沉积具正粒序结构，主体由直径0.1毫米至最大1毫米的浑圆状球形颗粒（spherules）组成。球形颗粒已蚀变为二八面体铝蒙脱石，偶见残留的富硅铝水化玻璃，据此可推断其前驱岩性为酸性岩石。球形颗粒的结构形态多样，从空心状、富囊泡状至致密块状均有分布；部分颗粒呈现原位坍塌特征，部分则包含清晰的富铁-镁-钙-钛熔融球粒与板状富铝淬火晶体。蚀变玻璃质球形颗粒与黏土基质（1259B站位）均呈现极强的负εNd（epsilon-Nd，T=65Ma）值（-17），表明其在蚀变过程中从同期海水中获取了钕元素。最后，富铁-镁球形颗粒、冲击石英（shocked quartz）与长石颗粒、少量岩屑以及大量增生状多孔碳酸盐岩屑集中分布于该沉积的最上部0.5~0.7毫米层位。部分碳酸盐岩屑呈现极为特殊的结构，被认定为冲击变质作用（shock-metamorphic）的产物。精细完好的球形颗粒结构、无牵引搬运（traction transport）证据的正粒序特征，以及亚毫米级的组分变化趋势，均证实该球形颗粒沉积为未受显著再改造（reworking）作用的原生大气降落沉积（air-fall deposit）。
大洋钻探计划第207航次发现的球形颗粒沉积是目前已知首个发育双层结构的海相白垩纪-古近纪界线沉积，兼具近海相与远海相沉积的组分与地层特征。其地层结构与北美西部内陆陆相区的双层白垩纪-古近纪界线沉积高度相似（尽管该陆相沉积未保存碳酸盐相）。在相距数千公里的迥异沉积环境中均发现双层冲击碎屑层，为原始沉积序列的存在提供了额外佐证。因此，该沉积的层状结构可能反映了构成主体球形颗粒沉积的弹道式希克苏鲁伯冲击碎屑，与集中于最上部层位的蒸汽（冲击碎屑）羽流沉降物质之间的组分差异。