Crustal growth and mantle differentiation through Earth’s history are often traced using two radiogenic isotope systems - ¹⁷⁶Lu-¹⁷⁶Hf and ¹⁴⁷Sm-¹⁴³Nd. Unlike most post-Archean igneous rocks that show correlated initial Hf and Nd isotopic compositions, many ancient rocks have broadly chondritic zircon initial εHf values but highly variable whole-rock initial εNd values. These features have classically been interpreted as differences in the behavior of the Lu-Hf and Sm-Nd isotope systems during either deep magma ocean crystallization, subduction zone processes, or post-crystallization metamorphism. To clarify the cause of early Archean Hf-Nd isotope relationships, which are essential for understanding early Earth’s evolution, we investigated the in situ U-Th-Pb and Sm-Nd isotope systematics of co-existing titanite, apatite, and allanite—the major Sm-Nd carriers in early Archean felsic rocks—in a representative early Archean (3.5–3.4 Ga) tonalite-trondhjemite-granodiorite (TTG) suite from the Minnesota River Valley (MRV) terrane, northern USA. These rocks exhibit multiple generations of closed-system zircon growth with chondritic initial zircon Hf isotope signatures, and apparent decoupled zircon initial Hf and whole-rock Nd isotopic compositions, and thus serve as an useful test of the role of accessory minerals in controlling the whole rock isotopic signatures.

Our new U-Pb results show that the REE-rich accessory phases were reset during the 2.60–2.58 Ga Sacred Heart Orogeny, representing the accretion of the MRV terrane to the southern margin of the Superior Craton. Additionally, two samples contain apatite that yields significantly younger U-Pb dates of 1.9–1.8 Ga, suggesting that a portion of the MRV basement was thermally overprinted by the ∼1.85 Ga Penokean Orogeny. The ∼2.6 Ga Sm-Nd array recorded by apatite and titanite in these rocks suggest terrane-wide Sm-Nd isotopic re-equilibration in the MRV during the Sacred Heart Orogeny, with no Penokean disturbance of the Sm-Nd isotope systematics. Nevertheless, allanite in one sample has survived the post-crystallization ¹⁴⁷Sm-¹⁴³Nd resetting event, and yielded a Sm-Nd isochron date broadly consistent with the crystallization age of the host rock (∼3.4 Ga), with chondritic initial εNd. Therefore, allanite appears to be an important target in order to obtain the primary Nd isotopic signature of early Archean rocks in ancient terranes. In contrast to the ambiguous whole-rock data, the Nd isotopic compositions of the coexisting apatite, titanite and allanite collectively reconstruct a clear crustal evolution history in which the crust was repeatedly re-melted in a closed-system since formation at 3.5 Ga, initially from a source evolving with broadly chondritic Lu-Hf and Sm-Nd systematics. This suggests a minimally differentiated, or already rehomogenized, mantle at 3.5 Ga. Our study highlights the importance of using REE-rich accessory phases to obtain a clear Nd isotopic record to constrain the history of continent formation on the early Earth.

地球历史上的地壳生长和地幔分化通常使用两种放射性同位素系统进行追踪 - ¹⁷⁶ Lu- ¹⁷⁶ Hf 和¹⁴⁷ Sm- ¹⁴³钕 与显示相关初始 Hf 和 Nd 同位素组成的大多数后太古代火成岩不同，许多古代岩石具有广泛的球粒状锆石初始 εHf 值，但全岩初始 εNd 值变化很大。这些特征经典地被解释为在深部岩浆海洋结晶、俯冲带过程或结晶后变质作用期间 Lu-Hf 和 Sm-Nd 同位素系统行为的差异。为了阐明早期太古代 Hf-Nd 同位素关系的原因，这对于了解早期地球的演化至关重要，我们调查了原位U-Th-Pb 和 Sm-Nd 同位素系统共存的钛铁矿、磷灰石和红榴石——早期太古代长英质岩石中的主要 Sm-Nd 载体——在一个代表性的早期太古代 (3.5-3.4 Ga) 闪长岩-长闪长岩-花岗闪长岩中(TTG) 套房来自美国北部的明尼苏达河谷 (MRV) 地层。这些岩石表现出多代闭合系统锆石生长，具有球粒状初始锆石 Hf 同位素特征，以及明显的解耦锆石初始 Hf 和全岩 Nd 同位素组成，因此可作为辅助矿物在控制整体的作用的有用测试岩石同位素特征。

我们新的 U-Pb 结果表明，在 2.60-2.58 Ga 圣心造山运动期间，富含 REE 的附属相被重置，这代表了 MRV 地体向上克拉通南缘的增生。此外，两个样品含有磷灰石，可产生 1.9-1.8 Ga 的显着更年轻的 U-Pb 日期，这表明 MRV 基底的一部分被 ~1.85 Ga Penokean 造山运动热压印。由这些岩石中的磷灰石和钛铁矿记录的~2.6 Ga Sm-Nd 阵列表明在圣心造山运动期间 MRV 中地体范围的 Sm-Nd 同位素重新平衡，没有对 Sm-Nd 同位素系统的 Penokean 扰动。尽管如此，一个样品中的铝榴石在结晶后¹⁴⁷ Sm- ¹⁴³Nd 重置事件，并产生 Sm-Nd 等时线日期，与主岩的结晶年龄（~3.4 Ga）大体一致，球粒状初始 εNd。因此，为了获得古地体中早期太古代岩石的主要 Nd 同位素特征，allanite 似乎是一个重要的目标。与含糊不清的全岩数据相反，共存的磷灰石、钛铁矿和红榴石的 Nd 同位素组成共同重建了清晰的地壳演化历史，其中地壳自 3.5 Ga 形成以来在封闭系统中反复重熔，最初来自广泛球粒状 Lu-Hf 和 Sm-Nd 系统学演化的来源。这表明 3.5 Ga 的地幔差异最小，或已经重新均质化。