The oldest well-preserved sedimentary rocks on Earth, which crop out around the North Pole Dome area of the Pilbara Craton, Australia, are an invaluable archive that provides clues about environments, life and the chemistry of Earth's surface up to 3.49 billion years ago. However, despite their remarkable preservation, they have undergone a long history of post-depositional modifications during diagenesis, hydrothermal alteration, low-grade metamorphism, deformation and weathering. Post-depositional overprinting of the geochemical depositional environment is a common problem (e.g., Slotznick et al., 2022), even for modern sediments, leading Berner (1981) to caution that “In order to unravel multienvironmentally overprinted rocks, careful petrographic and paragenetic study is… necessary.”

The recent paper by Johnson et al. (2022) reports on the geochemical environment of deposition of banded red cherts (jaspilites) from the 3.49 billion-year-old Dresser Formation and Mt. Ada Basalt in the North Pole Dome area. New Fe-isotope and trace element data from the jaspilites are used to make inferences about biogeochemical iron cycling and nutrient availability across a purported land-sea transition. The interpretations are based on the underlying assumption that fine-grained hematite preserved in these cherts is the only primary iron phase. In support of a primary origin, the authors report observations from only “plane” and “polarized” light microscopy of standard thin sections. It is suggested that the “prominent red color in plane … and polarized … light indicates hematite grains are on the order of 100 nm in size”, and that the “fine grained nature and even distribution of hematite indicates a near-primary origin and not late-stage replacement” (Johnson et al., 2022).

The presence of “mosaic-type” structures, in which white chert surrounds hematite “clots”, is interpreted to reflect dewatering of an initial Si ferrihydrite precipitate that was converted to hematite during very early diagenesis (Johnson et al., 2022). Similar textures were first described by Spencer and Percival (1952), and subsequently many others (e.g., Fig. 11 in Rasmussen et al. 2017), who interpreted them to be shrinkage/dewatering structures. Based on light petrography, Johnson et al. (2022) conclude that the hematite was derived from ferrihydrite, which resulted from the oxidation of Fe(II) to Fe(III) in an oxygen-deficient water column. As with jaspilite in the overlying and nearby 3.46 Ga Marble Bar Chert (Li et al., 2013), Fe(II)(aq) oxidation mechanisms involving photo-oxidation and oxygenic photosynthesis are discounted in favour of photoferrotrophy.

A potential flaw in the study is the choice of methodology (light microscopy) used to investigate the primary phases in the jaspilites. Whereas the presence of fine-grained hematite is not in dispute (“dusty” hematite is well-known from many previous studies of jaspilite), it may obscure the presence of nanoparticles of other minerals. As the main criterion used to identify hematite as a primary phase is its size, purported to be 100 nm, it is essential to determine whether other fine-grained phases are present in the rocks. While plane-polarized and cross-polarized light imaging is of limited use in identifying fine-grained particles, polarized reflected and incident light techniques, particularly at magnifications of 200–500 times, may be more helpful. However, even at high-magnification, light microscopy is not sufficient to identify the mineralogy of non-opaque nanoparticles.

Based on petrographic work on jaspilite from the 3.46 Ga Marble Bar Chert (Rasmussen et al., 2014b; Muhling and Rasmussen, 2020), we suggest that the use of high-magnification light and scanning electron microscopy (SEM) techniques on the 3.49 Ga Dresser jaspilites may reveal a more diverse picture of the original mineralogy of the chemical sediments. Indeed, if jaspilites in the 3.49 Ga Dresser Formation are similar to those of the nearby and slightly younger Marble Bar Chert, as suggested by Johnson et al. (2022), then the Dresser jaspilites may also preserve enclaves of Fe(II)-rich nanoparticles sealed within microcrystalline quartz crystals.

地球上保存完好的最古老的沉积岩出现在澳大利亚皮尔巴拉克拉通的北极圆顶地区周围，是一个宝贵的档案，它提供了有关 34.9 亿年前地球表面环境、生命和化学的线索。然而，尽管它们保存得很好，但它们在成岩作用、热液蚀变、低级变质作用、变形和风化过程中经历了长期的沉积后修饰。地球化学沉积环境的沉积后叠印是一个常见问题（例如，Slotznick 等人，2022 年），即使对于现代沉积物也是如此，导致 Berner（1981）警告说：“为了解开多环境叠印的岩石，仔细的岩相学和共生学习是……必要的。”

Johnson 等人最近的论文。（2022 年）报告了北极圆顶地区 34.9 亿年前的德莱塞组和艾达山玄武岩中带状红色燧石（碧玉）沉积的地球化学环境。来自碧玉的新铁同位素和微量元素数据用于推断在所谓的陆海过渡期间的生物地球化学铁循环和养分可用性。这些解释基于以下假设，即保存在这些燧石中的细粒赤铁矿是唯一的原生铁相。为了支持主要来源，作者报告了仅来自标准薄切片的“平面”和“偏振”光学显微镜的观察结果。建议“平面上显着的红色……和偏光……光表明赤铁矿晶粒大小在 100 nm 量级”，并且“赤铁矿的细粒性质和均匀分布表明接近初级起源而不是后期替换”（约翰逊等人，2022）。

“镶嵌型”结构的存在，其中白色燧石围绕赤铁矿“凝块”，被解释为反映在早期成岩作用期间转化为赤铁矿的初始水铁矿硅沉淀物的脱水（Johnson 等人，2022 年）。Spencer 和 Percival（1952 年）首先描述了类似的纹理，随后还有许多其他人（例如，Rasmussen 等人 2017 年的图 11）将它们解释为收缩/脱水结构。基于光岩相学，Johnson 等人。(2022) 得出结论，赤铁矿来源于水铁矿，这是由于在缺氧水柱中 Fe(II) 氧化为 Fe(III) 所致。与上覆和附近 3.46 Ga Marble Bar Chert 中的碧玉一样（Li et al., 2013），

该研究中的一个潜在缺陷是选择用于研究碧玉初级阶段的方法（光学显微镜）。尽管细粒赤铁矿的存在是没有争议的（“尘土飞扬”的赤铁矿在许多先前对碧玉的研究中都是众所周知的），但它可能会掩盖其他矿物纳米颗粒的存在。由于用于将赤铁矿识别为初级相的主要标准是其尺寸（据称为 100 nm），因此必须确定岩石中是否存在其他细粒相。虽然平面偏振和交叉偏振光成像在识别细粒粒子方面的用途有限，但偏振反射和入射光技术，特别是在 200-500 倍的放大倍率下，可能更有帮助。然而，即使在高倍率下，

基于对 3.46 Ga Marble Bar Chert 的 jaspilite 的岩相学工作（Rasmussen 等人，2014b；Muhling 和 Rasmussen，2020），我们建议在 3.49 Ga 上使用高倍率光学和扫描电子显微镜 (SEM) 技术Dresser jaspilites 可以揭示化学沉积物原始矿物学的更加多样化的图景。事实上，如果 3.49 Ga Dresser 组中的 jaspilites 与附近和稍微年轻的 Marble Bar Chert 相似，正如 Johnson 等人所建议的那样。（2022 年），那么 Dresser jaspilites 还可以保存密封在微晶石英晶体中的富含 Fe (II) 的纳米粒子的飞地。