There is a paucity of evidence preserved in the rock record regarding Earth’s earliest enriched crust and its complementary depleted mantle during the Hadean. In recent years, vestiges of these early reservoirs have been inferred by examination of Hf isotope systematics compiled from zircons. The Singhbhum craton of Eastern India, for example, preserves only the existence of an enriched (εHf <0) crustal reservoir during the Hadean–Eoarchean, with the notable absence of a depleted mantle reservoir signature (εHf >0) until ca. 3.5 Ga. Here we report a new Sm-Nd isochron for the Lower Lava greenstones of the western Iron Ore Group from the Singhbhum craton, confirming a 3.42 ± 0.14 Ga crystallization age with an initial εNd of +5.7 ± 2.5. This is the highest positive εNd value derived from an isochron of this age. We infer that this depleted mantle source is a vestige complementary to the primary crust following planetary differentiation. Furthermore, we present U-Pb zircon ages for a 3.39 ± 0.02 Ga tuff that lies stratigraphically above the Lower Lava and <30 cm below an extensive conformable banded iron formation (BIF). This age implies that the western Iron Ore Group’s BIF is the largest economic-grade iron formation of its Paleoarchean age, suggesting that free atmospheric oxygen existed as more than just whiffs at this time.While there is evidence to suggest that Earth differentiated into an enriched crust and a depleted mantle reservoir early (>4.5 Ga; e.g., Harper and Jacobsen, 1992), the fate of these complementary reservoirs and the role they have played in the evolution of Earth remains a subject of great interest. Over the decades, several studies have reported 142Nd anomalies, with some authors proposing the earliest enriched crust became reworked to form cratons (O’Neil and Carlson, 2017) while also possibly remaining stored at the base of the lower mantle (Carlson and Boyet, 2008). However, although 143Nd systematics provides traces for an extremely depleted mantle reservoir in the Archean (Collerson et al., 1991), there are few proposals for the potential survival of a >4.5 Ga depleted mantle source (e.g., Jackson et al., 2010; Caro and Bourdon, 2010). The inaccessibility of Earth’s earliest depleted mantle, the lack of well-preserved mantle-derived Archean rocks, and overprinting by continuous surface recycling have all contributed to our limited understanding of these ancient reservoirs (e.g., Armstrong, 1991), although recently, the most ancient accessible reservoirs of Earth’s mantle have been implicated in studies in basalts of Baffin Island (northern Canada) and West Greenland (see Caro and Bourdon, 2010; Jackson et al., 2010).The purpose of this study is to apply the 147Sm-143Nd isotopic system (based on the long-lived decay of 147Sm to 143Nd, decay constant λSm = 6.54 × 10−12 yr−1) on well-preserved greenstones from the western Iron Ore Group greenstone belt, Eastern India, in order to establish their age as well as to evaluate the significance of their initial 143Nd/144Nd ratios with respect to the evolution of the depleted mantle. Located in the Singhbhum craton, the western Iron Ore Group is part of a regional (55 × 35 km) NNE-plunging asymmetric synclinorium structure in the Jamda-Koira Valley (Fig. 1; Fig. S1 in the Supplemental Material1). Greenstones make up two significant lithologies within the western Iron Ore Group stratigraphy, with the lowermost stratum designated as the “Lower Lava” and the uppermost stratum the “Upper Lava” (Fig. S2). Together, these greenstones encompass an apparent 8-km-thick volcano-sedimentary and economic-grade banded iron formation (BIF)–bearing greenstone belt succession (e.g., Basu et al., 2008). Samples were collected from both the eastern and western limbs of the Lower Lava western Iron Ore Group synclinorium as well as the overlying Upper Lava deposited on an unconformity (Fig. 1; Fig. S2). All the western Iron Ore Group greenstones are characterized by their low-grade greenschist facies (quartz + albite + chlorite), primary igneous augite and pigeonite, lack of penetrative deformation, preservation of original volcanic textures (i.e., hydrothermal metamorphism), dominant calc-alkaline affinities with subordinate tholeiite, massive-pillowed morphologies, and average basaltic-andesite composition.Here we report the western Iron Ore Group Lower Lava to preserve the most-depleted initial ε143Nd signature derived from a Paleoarchean isochron. We also present new U-Pb zircon geochronology for a stratigraphically overlying tuff whose age agrees with the Lower Lavas isochron age. Lastly, we examine the significance of the new 143Nd results presented in this study within the evolutionary context of Singhbhum’s Hfzircon isotopic record.The data used to construct the Sm-Nd isochrons for both the Lower and Upper Lava units are provided in Table 1. See the Supplemental Material for the isotopic methodology. The Lower Lava of the eastern and western limbs defines a ten-sample whole-rock isochron registering an age of 3420 ± 140 Ma and an initial 143Nd/144Nd ratio of 0.50848 ± 0.00013 (initial εNd = +5.7 ± 2.5), with a low value of 0.98 for the mean square of weighted deviates (MSWD) (Fig. 2A). Sm/Nd ratios for these lavas are subchondritic, which is a result of the bulk rock being light rare earth element (LREE) enriched—possibly from fractionation during melting of the mantle source. A relatively small range of 147Sm/144Nd from 0.12 to 0.17 (Table 1) for the Lower Lava derives from the variable amounts of feldspar, altered glass, and primary clinopyroxene, and a few western limb samples having minor secondary amphibole.Precise secondary ion microprobe U-Pb ages of 22 zircons (grains ~350 µm long) were measured from a tuff unit that lies above the Lower Lava but just 30 cm conformably below the BIF (Fig. 1; Fig. S2; sample 4/03). The concordant 3392 ± 29 Ma tuff age (Fig. 2B; Table S2 [see footnote 1]) confirms the BIFs Paleoarchean antiquity (see Basu et al., 2008), stratigraphically agreeing with the Lower Lava’s 3.42 Ga age. This tuff age is remarkable in being close to that of the BIF that exceeds a 220-m thickness in the type area with single ore bodies as much as 3 km long along strike and several hundred meters wide. This makes the western Iron Ore Group BIF the largest for its age (~5 × 1010 tons), and it remains a significant economic-grade iron-ore deposit (>60 wt% Fe2O3; Beukes et al., 2008).In addition to the Lower Lava isochron age agreeing with the tuff age, it is also consistent with the age of the younger Bonai granite, ca. 3.37 Ga (see Asokan et al., 2021), which has an intrusive relationship with the western limb of the western Iron Ore Group syncline (Fig. 1). The 3.42 ± 0.14 Ga age of the Lower Lava Sm-Nd isochron and the U-Pb zircon tuff age place the antiquity of the western Iron Ore Group into the same emplacement timeline as the 3.51 Ga U-Pb zircon age for the southern Iron Ore Group (Mukhopadhyay et al., 2008) and eastern Iron Ore Group (Jodder et al., 2021) greenstone belts in Singhbhum (Fig. S1). The Upper Lava greenstones, collected on a 4 km traverse, lie on a five-sample whole-rock isochron that yields an age of 2654 ± 104 Ma and an initial 143Nd/144Nd ratio of 0.509332 ± 0.000097 (initial εNd = +2.7 ± 1.9; Fig. S3). This isochron also displays a small range of Sm/Nd ratios from 0.12 to 0.17 (Table 1) with a MSWD of 0.57.The mantle source for the Lower Lava greenstones represents the highest inferred time-integrated 143Nd/144Nd ratio (with respect to chondrite) of any Archean suite reported to date, using the “real age” isochron technique and not individually calculated model εNd values using other chronometers for the age at the time of its initial value (Fig. 2A). Assuming planetary fractionation occurred at 4.5 Ga, the early depleted mantle reservoir of the Lower Lavas was derived from an initial εNd value of +5.7 at 3.42 Ga that has an estimated present-day 143Nd/144Nd = 0.513817 and 147Sm/144Nd = 0.236 ratio that projects to a modern-day εNd of +23 ± 10. Straddling this evolution line are ultramafics from Labrador (northern Canada) (Collerson et al., 1991) and West Greenland (van de Löcht et al., 2020) that depict the presence of an existing terrestrial mantle reservoir with a similar Sm-Nd fractionation history. Given that a younger mantle separation age requires the existence of increasingly higher present-day εNd values, we propose εNd = +23 to be more reasonable as observed by our present data. Thus, the Lower Lava’s initial εNd value (+5.7) argues for having been derived from a long-lived (>1 Ga) mantle source that evolved within a closed system. This observation is consistent with previous studies that reported Nd isotopic data supporting the presence of chemical heterogeneities in Earth’s early mantle that persisted for at least the first billion years of Earth history, as well as highly depleted mantle reservoirs (e.g., Bennett et al., 1993, 2007; Hoffmann et al., 2010).It has been problematic to compare the depleted ε143Nd signature reported from the Singhbhum craton by other studies to that recorded by the western Iron Ore Group Lower Lava in the current study. This is because these previously reported ages and initial εNd values were either not derived from an isochron (e.g., Chaudhuri et al., 2017; Pandey et al., 2019; Asokan et al., 2023), derived from an errorchron (e.g., Pandey et al., 2019; Maltese et al., 2022), from samples significantly affected by crustal contamination (e.g., Adhikari 2021a, 2021b; Chaudhuri et al., 2017), or associated with an isochron-derived age that does not agree with other independent stratigraphic chronometers (e.g., Basu et al., 1981; Adhikari et al., 2021b; Adhikari and Vadlamani, 2022). It is important to specify that the assumed initial εNd value of +5.2 for a single Singhbhum granitic sample accepted by Pandey et al. (2019) and reused by Maltese et al. (2022) was calculated not from a reliable isochron but from using a single U-Pb zircon age whose 147Sm-143Nd data also formed an errorchron (very high MSWD) according to Pandey et al. (2019). The results from these studies suggest the samples were not cogenetic and have not remained in a closed system, or they were the result of a mixing line between two or more temporary distinct magmatic events. The strength of the current study is that the initial εNd value (+5.7) and age (3.42 Ga) of the Lower Lava are derived from a well-constrained isochron (closed-system behavior) whose age agrees with two independent stratigraphic chronometers. Plotting 143Nd/144Nd versus 1/Nd (ppm) for both the Lower and Upper Lava does not exhibit correlation, suggesting the Lower Lava and Upper Lava isochrons are not mixing lines. Lastly, the calculated initial εNd values for the individual samples of both the Lower Lava (±0.3) and Upper Lava (±0.1) exhibit low deviations from their isochron-derived initial εNd values, indicating their cogenetic derivation.Hafnium isotopes can provide an independent examination for the plausibility of the Nd data given that the behavior of the 147Sm-143Nd system parallels that of the 176Lu-176Hf system. The current Hfzircon isotopic record from the Singhbhum craton (Fig. 3B) suggests that an enriched primary crust (implied from the presence of only negative εHf values) evolved not from the addition of juvenile magma from the mantle but rather from extensive reworking of an older existing enriched crust between the Hadean (4.2 Ga) to the Eoarchean (ca. 3.5 Ga) (e.g., Bauer et al., 2020). This older Hadean enriched crust suggests the existence of an isolated complementary depleted mantle reservoir equivalent in age. The simultaneous occurrence of both positive and negative εHf values abruptly beginning at ca. 3.5 Ga signals the sustained emergence of the addition of a juvenile mantle signature (e.g., Sreenivas et al., 2019). This excursion of positive εHf values coincides with the 3.42 ± 0.14 Ga Sm-Nd whole-rock emplacement age of the Lower Lava greenstones. Therefore, the 1-billion-year interval from 4.5 to 3.5 Ga provides a reasonable geological time scale for the closed-system ingrowth of 143Nd to have occurred in order to have imparted their distinctly positive initial (εNd = +5.7) value (Fig. 1A). An early Earth differentiation event for the Singhbhum craton is consistent with the inferred 4.2–4.5 Ga separation model ages (Chaudhuri et al., 2018; Maltese et al., 2022). The emergence of a zircon Hf juvenile signature at 3.5 Ga, with many samples having initial εHf >+6 above the proposed Hf evolution curve for the depleted mantle (Fig. 3B), may indicate that the highly depleted nature of the mantle as recorded by Nd isotopes from the western Iron Ore Group Lower Lava greenstones and Hfzircon isotopes is a characteristic of the Singhbhum craton preserved in both the Nd and Hf isotopic systems.The difference of three ε143Nd units over the ~770 m.y. period between the emplacement of the Lower Lava (5.7 at 3.42 Ga) and that of the Upper Lava (2.7 at 2.65 Ga) could be interpreted to reflect either the sampling of different mantle sources or the progressive recycling of LREE-enriched material into the mantle source (e.g., Frost et al., 2023). If the latter is correct, then the ε143Nd became suppressed, preventing the growth of increasingly positive values. Such time-integrated closed system behavior is observed by many mantle-derived samples from other silicate bodies (e.g., Moon and Mars; Borg et al., 2011; Lapen et al.,2017) that lack an active surface recycling plate tectonic regime (e.g., Armstrong, 1991; Bowering and Housh, 1995). The difference between the ε143Nd values of the inferred present-day western Iron Ore Group depleted mantle source for the Lower Lava (+23) and those of present-day mid-ocean-ridge basalts (MORBs) (+10) suggests that crustal recycling has suppressed Earth’s mantle by a minimum of 13 εNd units over 3.4 b.y. of geologic time. However, this assumes that the depletion recorded in the Singhbhum craton is representative of a global phenomenon that is a mantle vestige of the early differentiation of the primary crust.We obtained a 3.42 Ga Sm-Nd isochron age for the Lower Lava, which agrees with the 3.39 Ga zircon U-Pb age obtained for the stratigraphically overlying tuff. The Lower Lava’s initial 143Nd suggests derivation from a source that represents one of the best-preserved examples of early Earth’s Hadean LREE-depleted mantle reservoir, whose 143Nd evolution mirrors that of the 176Hf-enriched crust as preserved in Singhbhum’s zircon record. The new Nd isochrons reported for the Lower Lava and the unconformable Upper Lava bracket the depositional age of the western Iron Ore Group basin between 3.42 and 2.65 Ga. Lastly, the 3.39 Ga zircon tuff age immediately below the conformably overlying huge BIF deposit indicates large variations of free atmospheric oxygen during the Paleoarchean.We thank Mouhcine Gannoun for measuring the Sm-Nd isotopic compositions. Basu collected the western Iron Ore Group samples in the field with help from P.K. Bandyopadhyay of Presidency College, Kolkata, and D.K. Bose of the iron ore mines of Bihar-Orissa. H. Zou aided in the U-Pb data acquisition of the zircons from the tuff at the University of California, Los Angeles (UCLA). R. Chakrabarti helped in final picking of these zircons at the University of Rochester (New York). We thank Robert T. Gregory at SMU for providing manuscript edits. Wright received support from the Roy M. Huffington Scholarship account, the Stable Isotope Laboratory of Southern Methodist University, the Clifford W. Matthews chair account, the American Federation of Mineralogical Societies, and the Society of Independent Professional Earth Scientists. Basu partially supported this study while at the University of Rochester for field sampling and U-Pb zircon analyses with the UCLA ion probe. We thank Rob Strachan for the handling of this manuscript and the anonymous reviewers for their valuable comments that improved this manuscript.

中文翻译：

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地球最早枯竭的地幔储层的遗迹


岩石记录中保存的有关冥古宙期间地球最早富集的地壳及其补充的贫化地幔的证据很少。近年来，通过对锆石中的 Hf 同位素系统学检查，推断出这些早期储层的遗迹。例如，印度东部的 Singhbhum 克拉通在冥古宙-太太古代期间仅保留了富集 (εHf <0) 地壳储层的存在，而直到大约 1977 年才显着缺乏贫化地幔储层特征 (εHf >0)。 3.5 Ga。在此，我们报告了 Singhbhum 克拉通西部铁矿群下熔岩绿岩的新 Sm-Nd 等时线，确认了 3.42 ± 0.14 Ga 结晶年龄，初始 εNd 为 +5.7 ± 2.5。这是从该年龄的等时线得出的最高正 εNd 值。我们推断，这种耗尽的地幔源是行星分异后与原始地壳互补的遗迹。此外，我们还提供了 3.39 ± 0.02 Ga 凝灰岩的 U-Pb 锆石年龄，该凝灰岩在地层上位于下熔岩上方，且在广泛的整合带状铁地层 (BIF) 下方 <30 厘米。这个年龄意味着西部铁矿群的 BIF 是其古太古代时代最大的经济级铁矿层，这表明此时大气中的游离氧不仅仅是存在的。虽然有证据表明地球已分化为富集的由于早期地壳和耗尽的地幔储层（>4.5 Ga；例如，Harper 和 Jacobsen，1992），这些互补储层的命运以及它们在地球演化中所发挥的作用仍然是人们非常感兴趣的主题。 几十年来，一些研究报告了 142Nd 异常，一些作者提出最早的富集地壳被改造形成了克拉通（O'Neil 和 Carlson，2017），同时也可能仍然储存在下地幔底部（Carlson 和 Boyet， 2008）。然而，尽管 143Nd 系统学提供了太古宙中极度贫化地幔储层的痕迹（Collerson 等，1991），但对于 >4.5 Ga 贫化地幔源的潜在生存却鲜有建议（例如，Jackson 等，2010）卡罗和波登，2010）。地球最早枯竭的地幔难以接近，缺乏保存完好的源自地幔的太古宙岩石，以及持续的地表再循环造成的叠印，这些都导致我们对这些古代储层的了解有限（例如，Armstrong，1991），尽管最近，最巴芬岛（加拿大北部）和西格陵兰玄武岩的研究涉及地幔的古代可开采储层（参见 Caro 和 Bourdon，2010 年；Jackson 等人，2010 年）。这项研究的目的是应用 147Sm-印度东部铁矿石群西部绿岩带保存完好的绿岩上的 143Nd 同位素系统（基于 147Sm 到 143Nd 的长寿命衰变，衰变常数 λSm = 6.54 × 10−12 yr−1），以便建立其年龄以及评估其初始 143Nd/144Nd 比率对于贫化地幔演化的意义。西部铁矿群位于 Singhbhum 克拉通，是 Jamda-Koira 河谷区域性（55 × 35 km）NNE 倾斜不对称向斜构造的一部分（图 1；补充材料 1 中的图 S1）。 绿岩构成了西部铁矿群地层中的两个重要岩性，最下面的地层被称为“下熔岩”，最上面的地层被称为“上熔岩”（图S2）。这些绿岩共同构成了一个明显的 8 公里厚的火山沉积岩和经济级带状铁矿层 (BIF) 绿岩带序列（例如，Basu 等人，2008 年）。样品采集自下熔岩西部铁矿群向斜层的东翼和西翼以及沉积在不整合面上的上熔岩（图 1；图 S2）。所有西部铁矿群绿岩的特征都是低品位绿片岩相（石英+钠长石+绿泥石）、原生火成辉长石和辉长岩、缺乏穿透变形、保留了原始火山结构（即热液变质作用）、占主导地位的钙质。与次级拉斑玄武岩的碱性亲和力、块状枕状形态和平均玄武岩-安山岩成分。在这里，我们报告了西部铁矿群下部熔岩，以保留源自古太古代等时线的最贫化的初始 ε143Nd 特征。我们还提出了新的 U-Pb 锆石地质年代学，用于地层上覆凝灰岩的年龄与下熔瓦斯等时线年龄一致。最后，我们在 Singhbhum 的 Hfzircon 同位素记录的演化背景下检验了本研究中提出的新 143Nd 结果的重要性。表 1 中提供了用于构建下熔岩单元和上熔岩单元的 Sm-Nd 等时线的数据。同位素方法的补充材料。东肢和西肢的下熔岩定义了一个十个样本的全岩等时线，记录的年龄为 3420 ± 140 Ma，初始 143Nd/144Nd 比率为 0.50848 ± 0。00013（初始εNd = +5.7 ± 2.5），加权偏差均方（MSWD）的低值为0.98（图2A）。这些熔岩的 Sm/Nd 比率是亚球粒状的，这是由于块状岩石富含轻稀土元素（LREE）的结果——可能来自地幔源熔化过程中的分馏。下部熔岩的 147Sm/144Nd 范围相对较小，从 0.12 到 0.17（表 1），源自不同数量的长石、蚀变玻璃和原生单斜辉石，以及一些具有少量次生角闪石的西边样品。精确的二次离子微探针22 颗锆石（颗粒长约 350 µm）的 U-Pb 年龄是从位于下熔岩上方、但位于 BIF 下方仅 30 厘米的凝灰岩单元中测量的（图 1；图 S2；样本 4/03）。一致的 3392 ± 29 Ma 凝灰岩年龄（图 2B；表 S2 [见脚注 1]）证实了 BIF 的古太古代（见 Basu 等，2008），在地层学上与下熔岩的 3.42 Ga 年龄一致。该凝灰岩年龄与典型区厚度超过220米的BIF年龄非常接近，单矿体沿走向长3公里，宽数百米。这使得西部铁矿群 BIF 成为其年龄中最大的铁矿群（约 5 × 1010 吨），并且它仍然是一个重要的经济级铁矿床（> 60 wt% Fe2O3；Beukes 等，2008）。此外下熔岩等时线年龄与凝灰岩年龄一致，也与较年轻的博奈花岗岩的年龄一致。 3.37 Ga（见Asokan等，2021），与西铁矿群向斜西翼有侵入关系（图1）。 3.42±0。下熔岩 Sm-Nd 等时线的 14 Ga 年龄和 U-Pb 锆石凝灰岩年龄将西部铁矿群的古代置于与南部铁矿群 (Mukhopadhyay) 的 3.51 Ga U-Pb 锆石年龄相同的侵位时间线中等，2008）和 Singhbhum 东部铁矿群（Jodder 等，2021）绿岩带（图 S1）。上熔岩绿岩是在 4 公里的横断中收集的，位于五样本全岩等时线上，得出的年龄为 2654 ± 104 Ma，初始 143Nd/144Nd 比率为 0.509332 ± 0.000097（初始 εNd = +2.7 ± 1.9图S3）。该等时线还显示了从 0.12 到 0.17 的小范围 Sm/Nd 比率（表 1），MSWD 为 0.57。下熔岩绿岩的地幔源代表了推断的最高时间积分 143Nd/144Nd 比率（相对于球粒陨石） ）迄今为止报告的任何太古宙套件，使用“真实年龄”等时线技术，而不是使用其他天文钟单独计算模型εNd值来计算其初始值时的年龄（图2A）。假设行星分馏发生在 4.5 Ga，下熔岩的早期贫化地幔储层源自 3.42 Ga 时 +5.7 的初始 εNd 值，估计当今的 143Nd/144Nd = 0.513817 和 147Sm/144Nd = 0.236 比率为预计现代 εNd 为 +23 ± 10。跨越这条演化线的是来自拉布拉多（加拿大北部）（Collerson 等人，1991）和西格陵兰（van de Löcht 等人，2020）的超镁铁质，描绘了存在具有类似 Sm-Nd 分馏历史的现有陆地地幔储层。鉴于较年轻的地幔分离年龄需要存在越来越高的当前 εNd 值，根据我们目前的数据观察，我们建议 εNd = +23 更为合理。 因此，下熔岩的初始 εNd 值 (+5.7) 认为源自在封闭系统内演化的长寿 (>1 Ga) 地幔源。这一观察结果与之前的研究一致，这些研究报告的 Nd 同位素数据支持地球早期地幔中存在化学异质性，这种异质性至少持续了地球历史的前十亿年，并且地幔储层高度枯竭（例如，Bennett 等人， 1993, 2007; Hoffmann et al., 2010）。将其他研究报告的 Singhbhum 克拉通的耗尽 ε143Nd 特征与当前研究中西部铁矿群下熔岩记录的特征进行比较是有问题的。这是因为这些先前报告的年龄和初始 εNd 值要么不是从等时线得出（例如，Chaudhuri 等人，2017 年；Pandey 等人，2019 年；Asokan 等人，2023 年），而是从错误时间得出（例如， Pandey 等人，2019；Maltese 等人，2022），来自受地壳污染显着影响的样本（例如，Adhikari 2021a，2021b；Chaudhuri 等人，2017），或与等时线推导的年龄不一致与其他独立的地层计时器（例如，Basu 等人，1981；Adhikari 等人，2021b；Adhikari 和 Vadlamani，2022）。重要的是要指定 Pandey 等人接受的单个 Singhbhum 花岗岩样品的假设初始 εNd 值为 +5.2。 (2019) 并被 Maltese 等人重复使用。 Pandey 等人 (2022) 不是根据可靠的等时线计算的，而是根据单个 U-Pb 锆石年龄计算的，其 147Sm-143Nd 数据也形成了误差年代（非常高的 MSWD）。 （2019）。 这些研究的结果表明，这些样本不是同生的，也没有保留在一个封闭的系统中，或者它们是两个或多个临时的不同岩浆事件之间混合线的结果。当前研究的优势在于，下熔岩的初始 εNd 值 (+5.7) 和年龄 (3.42 Ga) 来自良好约束的等时线（封闭系统行为），其年龄与两个独立的地层计时仪一致。绘制下熔岩和上熔岩的 143Nd/144Nd 与 1/Nd (ppm) 并没有表现出相关性，这表明下熔岩和上熔岩等时线不是混合线。最后，下熔岩 (±0.3) 和上熔岩 (±0.1) 的各个样本的计算初始 εNd 值与其等时线推导的初始 εNd 值的偏差较小，表明它们是同生推导的。铪同位素可以提供独立的鉴于 147Sm-143Nd 系统的行为与 176Lu-176Hf 系统的行为相似，对 Nd 数据的合理性进行检查。 Singhbhum 克拉通目前的 Hf 锆石同位素记录（图 3B）表明，富集的原生地壳（仅存在负 εHf 值暗示）不是由地幔中的新生岩浆的添加演化而来，而是由较古老的岩浆的广泛改造演化而来。冥古宙（4.2 Ga）到太太古代（约 3.5 Ga）之间存在丰富的地壳（例如，Bauer 等人，2020）。这个较古老的冥宙富集地壳表明，存在一个年龄相当的孤立的互补贫化地幔储层。正负 εHf 值同时出现，从大约 10 点突然开始。 3.5 Ga 标志着幼年地幔特征的持续出现（例如，Sreenivas 等，2019）。 正 εHf 值的偏移与下熔岩绿岩的 3.42 ± 0.14 Ga Sm-Nd 全岩侵位年龄一致。因此，从 4.5 到 3.5 Ga 的 10 亿年间隔为 143Nd 的封闭系统向内生长提供了合理的地质时间尺度，以便赋予其明显的正初始值 (εNd = +5.7)（图 1）。 1A）。 Singhbhum 克拉通的早期地球分异事件与推断的 4.2–4.5 Ga 分离模型年龄一致（Chaudhuri 等，2018；Maltese 等，2022）。在 3.5 Ga 处出现了锆石 Hf 幼体特征，许多样本的初始 εHf >+6 高于所提出的贫化地幔 Hf 演化曲线（图 3B），这可能表明地幔的高度贫化性质，如来自西部铁矿群下部熔岩绿岩的 Nd 同位素和 Hf 锆石同位素是 Singhbhum 克拉通在 Nd 和 Hf 同位素系统中保存的特征。三个 ε143Nd 单元在 ~770 m.y 范围内的差异。下熔岩（3.42 Ga 时为 5.7）与上熔岩（2.65 Ga 时为 2.7）之间的周期可以解释为反映了不同地幔来源的采样或富含 LREE 的物质逐渐循环到地幔中来源（例如，Frost 等人，2023）。如果后者是正确的，那么 ε143Nd 就会受到抑制，从而阻止正值的增长。这种时间积分的封闭系统行为可以通过许多来自其他硅酸盐体（例如月球和火星；Borg 等人，2011；Lapen 等人，2017）的地幔衍生样本观察到，这些硅酸盐体缺乏活跃的表面再循环板块构造体系（例如，阿姆斯特朗，1991；鲍林和胡什，1995）。 推断的现今西部铁矿群下熔岩贫化地幔源的 ε143Nd 值 (+23) 与现今洋中脊玄武岩 (MORB) 的 ε143Nd 值 (+10) 之间的差异表明地壳循环在 3.4 年的时间里，地幔被抑制了至少 13 εNd 单位。地质时期。然而，这假设Singhbhum克拉通记录的损耗代表了一种全球现象，即原始地壳早期分化的地幔遗迹。我们获得了下熔岩的3.42 Ga Sm-Nd等时线年龄，这与为地层上覆凝灰岩获得的 3.39 Ga 锆石 U-Pb 年龄。下熔岩最初的 143Nd 表明其来源代表了早期地球冥古宙低稀土元素耗尽地幔储层保存最完好的例子之一，其 143Nd 演化反映了 Singhbhum 锆石记录中保存的富含 176Hf 地壳的演化。新的下熔岩和不整合上熔岩支架的 Nd 等时线报告了西部铁矿群盆地的沉积年龄在 3.42 至 2.65 Ga 之间。最后，紧邻整合覆盖的巨大 BIF 矿床下方的 3.39 Ga 锆石凝灰岩年龄表明存在很大的变化古太古代期间大气中的游离氧。我们感谢 Mouhcine Gannoun 测量了 Sm-Nd 同位素组成。巴苏在 P.K. 的帮助下，在现场采集了西部铁矿群样品。加尔各答总统学院的 Bandyopadhyay 和 D.K.比哈尔-奥里萨邦铁矿的博斯。 H. Zou 协助加州大学洛杉矶分校 (UCLA) 采集凝灰岩中的锆石 U-Pb 数据。 R. Chakrabarti 在罗切斯特大学（纽约）帮助最终挑选了这些锆石。我们感谢罗伯特·T. SMU 的 Gregory 提供手稿编辑。赖特获得了罗伊·M·赫芬顿奖学金账户、南卫理公会大学稳定同位素实验室、克利福德·W·马修斯主席账户、美国矿物学会联合会和独立专业地球科学家协会的支持。 Basu 在罗切斯特大学使用 UCLA 离子探针进行现场采样和 U-Pb 锆石分析时部分支持了这项研究。我们感谢罗布·斯特拉坎 (Rob Strachan) 对本手稿的处理，以及匿名审稿人对改进本手稿提出的宝贵意见。