DataSheet1_Geodynamic seawater-sediment porewater evolution of the east central Atlantic Paleogene ocean margin revealed by U-Pb dating of sedimentary phosphates.DOCX

Emerging evidence suggests that U-Pb and Lu-Hf ages of sedimentary apatite group minerals are often younger than their biostratigraphic ages. However, U-Pb dating of exquisitely preserved carbonate fluorapatite (CFA) is rare. The Upper Cretaceous/Paleogene marine sedimentary rocks of the Moroccan High Atlas host phosphate-rich sediments bracketed by calcareous nannofossil Zones (NP4-NP9) of late Danian to Thanetian age. Here, we use a laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to decipher whether CFA minerals are suitable for U-Pb chronostratigraphy and whether they can reveal the sedimentary and seawater history from which they formed. U-Pb dating of the CFA grains yields ages of 42.9 ± 1.3 Ma (MSWD = 2.3) and 35.7 ± 2.8 Ma (MSWD = 1.3) from three distinct phosphate-rich beds, being >15 million years younger than the expected biostratigraphic age. Combined scanning electron microscopy, X-ray diffraction, and infrared spectroscopy analyses, associate the Mg-rich clay minerals sepiolite and palygorskite, with micro-CFA crystals, while LA-ICP-MS trace element, rare earth element, and yttrium content for primary CFA grains, collectively point to long-term early diagenetic adsorption from oxygenated seawater-dominated porewater fluids. Authigenic clay minerals display a seawater-like pattern, with negligible U concentrations suggesting limited clay mineral influence on U-Pb dating of the CFA crystals. Considering the absence of extensive post-depositional alteration, we propose that because of their large surface area, the µm-sized CFA crystallites facilitated real-time surface adsorption and desorption of elements and diffusion processes. These conditions generated long-term open system connection of sediments with overlying seawater, enabling continuous U-Pb exchange for 15–25 Myr after phosphate precipitation. The data suggest that system closure was potentially associated with sediment lithification and the Atlas orogeny, pointing to stable oxygenation of shallow marine waters along the eastern passive margin of the central Atlantic Ocean in the Paleogene.

新兴的证据表明，沉积岩中磷灰石族矿物的U-Pb和Lu-Hf年龄通常较其生物地层年龄年轻。然而，对精心保存的碳氟磷灰石（CFA）进行U-Pb测年较为罕见。摩洛哥高阿特拉斯地区的白垩纪/古近纪海洋沉积岩承载着富含磷的沉积物，这些沉积物被丹尼亚晚期至坦尼特年龄的钙质浮游有孔虫带（NP4-NP9）所包围。在此，我们利用激光剥蚀电感耦合等离子体质谱（LA-ICP-MS）技术，以解析CFA矿物是否适用于U-Pb年代地层学，以及它们能否揭示其形成的沉积物和海水的历史。对CFA颗粒的U-Pb测年结果显示，三个不同的富含磷层分别获得42.9 ± 1.3百万年（MSWD = 2.3）和35.7 ± 2.8百万年（MSWD = 1.3）的年龄，这些年龄比预期的生物地层年龄年轻超过1500万年。结合扫描电子显微镜、X射线衍射和红外光谱分析，将富含镁的粘土矿物（如菱沸石和坡缕石）与微-CFA晶体相联系，而LA-ICP-MS对初级CFA颗粒的微量元素、稀土元素和钇含量分析，共同表明了从氧化海水主导的孔隙水流体中发生的长期早期成岩吸附。自生粘土矿物呈现出类似海水样的模式，其中可忽略的U浓度表明粘土矿物对CFA晶体U-Pb测年的影响有限。考虑到缺乏广泛的沉积后变质作用，我们提出，由于CFA晶体具有较大的比表面积，微米级的CFA晶粒促进了元素的实际时间表面吸附和脱附以及扩散过程。这些条件产生了沉积物与上覆海水之间的长期开放系统联系，使得在磷酸沉积后的15-25百万年内能够持续进行U-Pb交换。数据表明，系统封闭可能与沉积物成岩作用和阿特拉斯造山运动有关，这指向了古近纪时期沿中大西洋东部被动大陆边缘浅海水的稳定氧化状态。