The peak-ring of the 66 Ma, ~180 km Chicxulub impact structure in the northern Yucatán peninsula and southern Gulf of Mexico was sampled during the International Ocean Discovery Program and International Continental Scientific Drilling Program (IODP–ICDP) Expedition 364 at Site M0077 (21.45° N, 89.95° W). Secondary clay minerals are pervasive throughout the upper peak-ring lithologies as a product of ubiquitous altered glass present throughout the impact melt and melt-bearing breccia sequence. Here we present the first detailed study of the clay mineralogy (microprobe, pXRD, spectral reflectance from 350 to 2500 nm) and isotope geochemistry (δ²H and δ¹⁸O) of the <0.2 μm size-fraction from upper peak-ring lithologies. The clay mineralogy is dominated by smectitic clay minerals, whose composition varies with stratigraphic position. Trioctahedral MgFe smectite (var. saponite) is most common in Units or Subunits 2A, 2C, 3 and 4, while a section of Subunit 2B contains a more dioctahedral, Al-rich smectite. Higher porosity regions of the lower to mid, dioctahedral smectite-dominated intervals have higher δ¹⁸O (+14.2 to +18.6‰) whereas intervals dominated by trioctahedral smectite have lower δ¹⁸O (+10.4 to +14.1‰). The range of smectite δ²H (−105 to −87‰), in comparison to that of oxygen isotopes, is proportionally much less variable and unrelated to smectite mineralogy. When combined, the oxygen and hydrogen isotope compositions of the smectitic clay minerals suggest low temperature (~20 to 50 °C) formation from meteoric water-dominated fluids. The lower end of this temperature range is below current ambient conditions, which conceivably could suggest smectite formation before much of the overlying sedimentary rocks were deposited (~56 Ma?). Calculated temperatures are generally lower than those associated with impact-generated hydrothermal alteration.

Calculated δ¹⁸O and δ²H of meteoric water-dominated fluids associated with low-temperature formation of these clay minerals are lower than known for modern meteoric water in the Yucatán region. The simplest explanation for the source of these ancient fluids is meteoric water-dominated Gulf Coast brines. A more remote possibility is orogenically-driven, long-distance transport of groundwater from highlands to the east via an artesian aquifer formed in part by fractured Mesozoic rocks extending laterally beneath the impact structure.

在国际海洋发现计划和国际大陆科学钻探计划 (IODP-ICDP) 364 号远征队期间，在 M0077 站点对 66 Ma、约 180 公里希克苏鲁伯撞击结构的峰环进行了采样（ 21.45° 北，89.95° 西）。次生粘土矿物在整个峰环上部岩性中无处不在，作为存在于整个冲击熔体和含熔体角砾岩序列中的无处不在的蚀变玻璃的产物。在这里，我们首次详细研究了粘土矿物学（微探针、pXRD、350 到 2500 nm 的光谱反射率）和同位素地球化学（δ ² H 和δ ¹⁸O) 来自上部峰环岩性的 <0.2 μm 尺寸部分。粘土矿物以蒙脱石粘土矿物为主，其成分随地层位置而异。三八面体镁铁蒙脱石（皂石变种）在单元或亚单元 2A、2C、3 和 4 中最常见，而亚单元 2B 的一部分包含更多二八面体、富含铝的蒙脱石。的较低到中等孔隙率较高的区域，双八面体绿为主的间隔具有更高的δ ¹⁸ O（14.2到18.6 +‰），而时间间隔由主导三八面体绿土具有较低的δ ¹⁸ O（10.4到14.1 +‰）。蒙脱石δ ²的范围与氧同位素相比，H（-105 至 -87‰）在比例上的可变性要小得多，并且与蒙脱石矿物学无关。结合起来，蒙脱石粘土矿物的氧和氢同位素组成表明，以大气水为主的流体在低温（~20 至 50 °C）形成。这个温度范围的下限低于当前的环境条件，这可能表明在大部分上覆沉积岩沉积之前（~56 Ma？）就形成了蒙脱石。计算出的温度通常低于与撞击产生的热液蚀变相关的温度。

与这些粘土矿物的低温形成相关的大气水主导流体的计算δ ¹⁸ O 和δ ² H 低于尤卡坦地区现代大气水的已知值。对于这些古老流体的来源，最简单的解释是以大气水为主的墨西哥湾沿岸卤水。更遥远的可能性是造山带驱动的地下水通过自流含水层从高地长距离输送到东部，该含水层部分由在撞击结构下方横向延伸的中生代断裂岩石形成。