Sedimentary phosphates, chiefly composed of apatite group minerals, contain exceptionally high concentrations (10–200 ppm) of U relative to other sedimentary precipitates, making them an attractive target for U–Pb dating. Phosphate-rich sedimentary rocks are common from the Neoproterozoic onwards and commonly occur at key points in the geological record, often coinciding with periods of significant environmental flux or facies change. There is thus a clear rationale for dating phosphatic rocks. In this study, we demonstrate how spatially-resolved U–Pb dating of sedimentary apatite by laser-ablation quadrupole inductively-coupled-plasma mass-spectrometry (LA-Q-ICPMS) can provide constraints on early phosphate diagenesis and reworking in ancient rocks. Our samples are from two locations in a well-characterized stratigraphic sequence containing phosphorites, and phosphate-rich carbonates and shales in western Ireland, which span the Mississippian–Pennsylvanian (mid-Carboniferous) boundary. Sedimentary phosphate preserves precise (<0.7% uncertainty) U–Pb ages that record early diagenesis, or sediment reworking, depending upon the facies sampled. These ages are consistent with age constraints derived from Carboniferous conodont biostratigraphy from the same sections. Phosphorite deposition, occurring on the sediment-starved margin of an epeiric basin, is constrained by U–Pb dating of apatite to the lower part of the Bashkirian stage at 320.7 ± 2.0 Ma (MSWD = 1). However, we identify a significant time-gap between the phosphorite and the immediately underlying carbonate stratigraphy, and thus attribute phosphorite accumulation to forced regression accompanying a widely recognized and significant glacial advance in Gondwana that took place at the mid-Carboniferous boundary. Our two sampling sites are several kilometers apart, broadly along strike of the basin margin, but preserve U–Pb isotopic and trace element compositional information that is strikingly consistent at the same stratigraphic levels. If used appropriately, integrated phosphate U–Pb dating and trace element analysis could provide age constraints for key parts of the Neoproterozoic and Phanerozoic, aiding reconstruction of evolutionary and paleoenvironmental records.

沉积磷酸盐主要由磷灰石族矿物组成，相对于其他沉积沉淀物，其含有极高浓度（10-200 ppm）的 U，使其成为 U-Pb 定年的有吸引力的目标。富含磷酸盐的沉积岩从新元古代开始就很常见，通常出现在地质记录的关键点，通常与环境通量或相变的显着时期相吻合。因此，对磷酸盐岩进行测年有明确的理由。在这项研究中，我们展示了通过激光烧蚀四极杆电感耦合等离子体质谱 (LA-Q-ICPMS) 对沉积磷灰石进行空间分辨 U-Pb 测年如何限制早期磷酸盐成岩作用和古代岩石的再加工。我们的样品来自爱尔兰西部一个特征良好的地层序列中的两个位置，其中包含磷灰岩、富含磷酸盐的碳酸盐和页岩，跨越密西西比-宾夕法尼亚（石炭纪中期）边界。沉积磷酸盐保留了精确的（<0.7% 不确定性）U-Pb 年龄，这些年龄记录了早期成岩作用或沉积物再加工，具体取决于采样的相。这些年龄与来自相同剖面的石炭纪牙形石生物地层的年龄限制一致。磷灰石沉积发生在表层盆地缺乏沉积物的边缘，受磷灰石 U-Pb 年代测定的限制，该年代在 320.7 ± 2.0 Ma (MSWD = 1) 时位于巴什基尔阶的下部。然而，我们发现磷矿和直接下伏的碳酸盐地层之间存在显着的时间间隔，因此，将磷矿的积累归因于伴随着冈瓦纳大陆中石炭纪边界发生的广为人知且显着的冰川推进的强制回归。我们的两个采样点相距几公里，大致沿着盆地边缘的走向，但保留了在相同地层水平上惊人一致的 U-Pb 同位素和微量元素组成信息。如果使用得当，综合磷酸盐 U-Pb 测年和微量元素分析可以为新元古代和显生宙的关键部分提供年龄限制，有助于重建进化和古环境记录。我们的两个采样点相距几公里，大致沿着盆地边缘的走向，但保留了在相同地层水平上惊人一致的 U-Pb 同位素和微量元素组成信息。如果使用得当，综合磷酸盐 U-Pb 测年和微量元素分析可以为新元古代和显生宙的关键部分提供年龄限制，有助于重建进化和古环境记录。我们的两个采样点相距几公里，大致沿着盆地边缘的走向，但保留了在相同地层水平上惊人一致的 U-Pb 同位素和微量元素组成信息。如果使用得当，综合磷酸盐 U-Pb 测年和微量元素分析可以为新元古代和显生宙的关键部分提供年龄限制，有助于重建进化和古环境记录。