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Glauconite is an authigenic clay mineral that is common in marine sedimentary successions. Dating of glauconite to determine the depositional age of sedimentary sequences has a long history but has fallen into disfavor due to the difficulty of obtaining “pure” glauconite separates. Recent advances in sedimentary petrography and reaction cell mass spectrometry permit rapid in situ Rb‐Sr dating of carefully screened glauconite grains. However, glauconite remains susceptible to burial alteration so that successful application of in situ Rb‐Sr glauconite geochronology requires improved, microscale constraints on the impact of postdepositional alteration on glauconite Rb‐Sr systematics and articulation of robust criteria for identifying grains suitable for geochronology. Here, we address these questions by combining SEM‐EDS mineral mapping, geochemical characterization, and in situ Rb‐Sr dating of glauconite grains in partially altered lower Cambrian sedimentary sequences from the Arrowie and Amadeus basins in Australia. Our approach provides information at high spatial resolution, representing new insights into the interplay between source material, burial fluids, and diagenetic processes. Among the different glauconite classes, which we classify based on alteration and inclusion type, only the primary apatite‐bearing “pristine” glauconite returns an age within the error of the expected stratigraphic age. We attribute the preservation of a depositional Rb‐Sr age to the influence of Sr‐rich, alteration‐resistant apatite and the limited permeability of the clay‐rich strata hosting these grains. We conclude that our combined petrographic–geochemical screening approach holds considerable potential for identifying the best preserved glauconite grains for in situ Rb‐Sr geochronology. Key Points New technology permits in situ Rb‐Sr dating of carefully screened glauconite grains Apatite resists Sr exchange, and apatite‐bearing glauconite preserves primary Rb‐Sr age Young and old ages are due to Sr exchange with burial fluids and uptake of radiogenic Sr in secondary carbonate inclusions, respectively

The scarcity of well-preserved and directly dateable sedimentary sequences is a major impediment to inferring the Earth’s paleo-environmental evolution. The authigenic mineral glauconite can potentially provide absolute stratigraphic ages for sedimentary sequences and constraints on paleo-depositional conditions. This requires improved approaches for measuring and interpreting glauconite formation ages. Here, glauconite from a Cretaceous shelfal sequence (Langenstein, northern Germany) was characterized using petrographical, geochemical (EMP), andmineralogical (XRD) screening methods before in situ Rb-Sr dating via LA-ICP-MS/MS. The obtained glauconite ages (~101 to 97 Ma) partly overlap with the depositional age of the Langenstein sequence (±3 Ma), but without the expected stratigraphic age progression, which we attribute to detrital and diagenetic illitic phase impurities inside the glauconites. Using a novel age deconvolution approach, which combines the new Rb-Sr dataset with published K-Ar ages, we recalculate the glauconite bulk ages to obtain stratigraphically significant ‘pure’ glauconite ages (~100 to 96 Ma). Thus, our results show that pristine ages can be preserved in mineralogically complex glauconite grains even under burial diagenetic conditions (T < 65 °C; <1500 m depth), confirming that glauconite could be a suitable archive for paleo-environmental reconstructions and direct sediment dating.

选择吕梁群中原岔上群北部地层的蚀变火山岩进行锆石U-Pb年代学和Hf同位素研究.锆石U-Pb测试获得两组年龄结果, 较年轻的谐和年龄为 1813±6 Ma (n=7), 较老的 207Pb/206Pb 加权平均年龄为 2516±31 Ma (n=2), 前者为火山岩喷发时代, 后者代表捕获锆石年龄.年轻锆石的 εHf(t)值为?10.8~?2.3, TDM1 值为 2308~2655 Ma; 捕获锆石的 εHf(t)值为+10.0~+13.1.年龄约为 2.5 Ga 锆石的 εHf(t)值高于亏损地幔演化线, 考虑到 U-Pb 同位素和 Hf 同位素测点位置不完全相同, 说明所获得的 Hf 同位素组成为无地质意义的混合数值; 年龄约为 1.8 Ga 锆石的 Hf 同位素特征反映其可能源于富集地幔或受地壳物质混染的亏损地幔.结合前人的研究成果, 推断岩浆作用事件发生在约 1.81 Ga 的碰撞后阶段.

为了获得南海北缘琼南缝合带南北两侧地块的碰撞时间,对九所陵水东段采集的中高级变质石榴石石英片岩和花岗质片麻岩进行了LA-ICP-MS锆石U-Pb定年。实验结果发现:原岩为沉积岩的石榴石石英片岩形成于晚泥盆纪;3个锆石变质壳谐和/近谐和年龄表明碰撞事件发生在(233±4) Ma左右;花岗质片麻岩原岩为同构造侵位的花岗岩,其8个锆石增生年龄显示的是深熔作用的时间,为(206±3) Ma。这些研究成果对重建南海北缘琼南缝合带在研究区内的演变历史具有重要意义。