Abstract Spheroidally weathered corestones, which are remnant pieces of bedrock surrounded by progressively weathered saprolite, preserve an ideal, small-scale natural interface for examining incipient to intermediate weathering reactions. Ancient corestones are preserved in some Precambrian paleosols but have remained surprisingly understudied. Here detailed mineral-chemical trends are examined across corestone-saprolith interfaces in ca. 1.85 Ga dolerite-hosted paleosol from the Flin Flon-Creighton area (Manitoba and Saskatchewan, Canada). The study presents the first comprehensive paleo-redox tracer suite (Fe-Mn-Mo-U-V-Cr-Ce) for the classic Flin Flon paleosol, which formed during a crucial period in the Paleoproterozoic for which marine sedimentary archives infer a return to low oxygen levels after the Great Oxidation Event (GOE). The textural and mineralogical progression across corestone-saprolith interfaces documented with petrography and scanning electron microscope-mineral liberation analysis (SEM-MLA) show many features (e.g., development of weathering rindlets and solution channels) strikingly reminiscent of modern mafic rock-hosted saprolite, despite later overprint. Albite-dominated cores preferentially preserving carbonate and sulfide (pyrite, chalcopyrite) are progressively altered outwards to a finer-grained saprolith rich in chlorite, illite, and muscovite, with embaying rindlets bearing cryptocrystalline hematite-quartz-illite. These mineral-textural observations guided sub-sampling for bulk Fe(II) and solution ICP-MS ultra-trace and major element analysis, and mineral-scale LA-ICP-MS analysis. Many high-field-strength elements (Al, Ti, Zr, Nb, Hf, Ta, and Th), remained immobile across the interface and ascertain the homogeneity of the parent dolerite. Progressive weathering from corestone to saprolith was quantified with chemical index of alteration minus K (CIA-K) values that fall into three zones (incipient (1): 45–55; modest (2): 55–65; and moderate (3): 65–75). Mass balance and spatial geochemical analysis revealed the following features: outward migration of Fe(II) and Mn from corestone to saprolith, with partial oxidation of Fe(II) to Fe(III) in rindlets; minimal REE mobility with no pronounced Ce anomalies, but well-developed, unidirectional Y/Ho fractionation; significant Cr and V mobility from corestones outwards with enrichment in saprolith; minimal dm-scale cycling and limited loss of Mo and U from saprolith; and post-depositional enrichment of K, Rb, Cs, Tl, Ba, Be, and W. Using semi-quantitative LA-ICP-MS elemental maps, it was possible to contextualize the mineralogical controls on chemical weathering reactions. Iron, Mg, and Mn are coupled within chlorite (representing former pedogenic phyllosilicates), which also scavenged Cr. The REEs are predominantly hosted by apatite and titanite. The Ti-phases ilmenite and titanite are the predominant U- and Mo-bearing hosts. Collectively, the insights from the Flin Flon paleosol converge with those from other ca. 1.90–1.85 Ga terrestrial and marine deposits in inferring an oxygen-limited atmosphere capable of efficiently oxidizing Fe and S, but not Mn, in terrestrial environments. The Cr and V distributions are most consistent with small-scale solubilization and redeposition that appears to be linked to the weatherability of protolith minerals and locally-generated acid- and/or ligand-rich conditions rather than oxidation. Although normally sensitive indicators of oxidative weathering, the inhibited release of Mo and U from the corestone was instead primarily dictated by the weathering resistance of their host minerals. Oxygen-limited weathering in saprolite supplied sulfate to the oceans, but the continental flux of redox-sensitive trace elements from this zone as well as more weathered substrates was limited, ultimately contributing to sustaining a largely redox-dynamic, weakly buffered, and nutrient-limited ocean.

中文翻译：

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富含 1.85 Ga 核心石的腐泥揭示了古元古代地表条件的新见解

摘要 球状风化芯石是被逐渐风化的腐泥土包围的残余基岩块，保留了理想的小规模自然界面，用于检查初期到中间风化反应。古代核心石保存在一些前寒武纪古土壤中，但出人意料地仍未得到充分研究。在这里，详细的矿物化学趋势在约 来自 Flin Flon-Creighton 地区（加拿大马尼托巴省和萨斯喀彻温省）的 1.85 Ga 辉绿岩托管古土壤。该研究展示了经典 Flin Flon 古土壤的第一个综合性古氧化还原示踪剂套件 (Fe-Mn-Mo-UV-Cr-Ce)，该古土壤形成于古元古代的关键时期，海洋沉积档案推断出回归到低大氧化事件 (GOE) 后的氧气水平。岩相学和扫描电子显微镜-矿物解离分析 (SEM-MLA) 记录的岩心 - 腐泥土界面的结构和矿物学进展显示出许多特征（例如，风化细纹和溶液通道的发展），使人联想到现代基性岩石腐泥土，尽管后来叠印。钠长石占主导地位的岩心优先保留碳酸盐和硫化物（黄铁矿、黄铜矿），逐渐向外转变为富含绿泥石、伊利石和白云母的细粒腐泥，带有隐晶质赤铁矿-石英-伊利石的凹形细纹。这些矿物结构观察指导了块状 Fe(II) 和溶液 ICP-MS 超痕量和主要元素分析以及矿物级 LA-ICP-MS 分析的子采样。许多高场强元素（Al、Ti、Zr、Nb、Hf、Ta、和 Th)，在界面上保持不动并确定母辉绿岩的均匀性。从岩心石到腐泥土的渐进风化用化学蚀变指数减去 K (CIA-K) 值量化，分为三个区域（初期 (1)：45-55；中等 (2)：55-65；和中等 (3)） : 65–75)。质量平衡和空间地球化学分析揭示了以下特征：Fe(II)和Mn从芯岩向外迁移到腐泥土中，Fe(II)部分氧化为Fe(III)在毛皮中；REE 迁移率最小，没有明显的 Ce 异常，但发育良好，单向 Y/Ho 分馏；Cr 和 V 从岩心向外显着迁移，腐泥中富集；最小的 dm 尺度循环和有限的 Mo 和 U 从腐泥中损失；K、Rb、Cs、Tl、Ba、Be 和 W 的沉积后富集。使用半定量 LA-ICP-MS 元素图，可以将矿物学控制对化学风化反应进行背景化。铁、镁和锰与绿泥石（代表以前的成土层状硅酸盐）结合，后者也清除了 Cr。稀土元素主要由磷灰石和钛铁矿组成。钛相钛铁矿和钛铁矿是主要的含 U 和 Mo 的主体。总的来说，来自弗林弗隆古土壤的见解与来自其他约的见解相融合。1.90–1.85 Ga 陆地和海洋沉积物，推断在陆地环境中能够有效氧化 Fe 和 S 而不是 Mn 的限氧气氛。Cr 和 V 分布与小规模增溶和再沉积最一致，这似乎与原岩矿物的耐候性和局部产生的富含酸和/或配体的条件而不是氧化有关。虽然通常是氧化风化的敏感指标，但核心石中 Mo 和 U 的抑制释放主要取决于其宿主矿物的耐候性。腐泥土中的限氧风化为海洋提供了硫酸盐，但来自该区域的氧化还原敏感微量元素以及更多风化底物的大陆通量有限，最终有助于维持主要是氧化还原动力学、弱缓冲和营养物质-有限的海洋。虽然通常是氧化风化的敏感指标，但核心石中 Mo 和 U 的抑制释放主要取决于其宿主矿物的耐候性。腐泥土中的限氧风化为海洋提供了硫酸盐，但来自该区域的氧化还原敏感微量元素以及更多风化底物的大陆通量有限，最终有助于维持主要是氧化还原动力学、弱缓冲和营养物质-有限的海洋。虽然通常是氧化风化的敏感指标，但核心石中 Mo 和 U 的抑制释放主要取决于其宿主矿物的耐候性。腐泥土中的限氧风化为海洋提供了硫酸盐，但来自该区域的氧化还原敏感微量元素以及更多风化底物的大陆通量有限，最终有助于维持很大程度上氧化还原动力学、弱缓冲和营养物质-有限的海洋。