Komatiites and sedimentary rocks sampled during the International Continental Drilling Program (BARB1-2–3-4–5) in the Barberton greenstone belt, South Africa, were analyzed for ¹⁴⁶Sm-¹⁴²Nd systematics. Resolved negative μ¹⁴²Nd values (down to −7.7 ± 2.8) were identified in komatiites from the 3.48 Ga Komati Formation and this signature correlates with low Hf/Sm ratios measured in these samples. The negative μ¹⁴²Nd point to a source with subchondritic Sm/Nd ratio which formed during the Hadean. No analytically resolvable ¹⁴²Nd anomalies were measured in crustal detritus-rich, Si-rich, Ca-Fe-rich sediments and cherts from the Buck Reef (3.42 Ga) and the Fig Tree Group (3.23–3.28 Ga). Our new measurements are incorporated into a larger set of ¹⁴⁷Sm-¹⁴³Nd and ¹⁷⁶Lu-¹⁷⁶Hf data to better understand the ^142,143Nd-¹⁷⁶Hf isotope signatures in the mantle source at the time of komatiite crystallization. Our calculations show that the ^142,143Nd-¹⁷⁶Hf isotope signatures and Hf/Sm ratios cannot be produced by recycling into the komatiite source of detrital sediments like those sampled in the Barberton area. Only cherts have the required trace element characteristics – low Hf/Sm, radiogenic ε¹⁷⁶Hf –but the trace element concentrations in the cherts are so low that unrealistic amounts of chert would need to be added.

We propose a four-stage model for the formation of these rocks. Negative μ¹⁴²Nd and low Hf/Sm ratios developed during the crystallization of a deep magma ocean soon after Earth accretion. The material that ultimately became the source of komatiites was a residual liquid produced by 50% crystallization leaving a bridgmanite/ferropericlase/Ca-perovskite cumulate. The Lu-Hf and Sm-Nd isotope systems were decoupled at this stage. After extinction of ¹⁴⁶Sm around 4 Ga, parent/daughter ratios fractionated during a melt extraction event. With this model we explain the positive ε¹⁷⁶Hf and slightly negative ε¹⁴³Nd in these samples. The 3.55 Ga Schapenburg komatiites in another part of the Barberton belt share similar chemical signatures, supporting our model of fractionation in a deep magma ocean early in Earth's history.

在南非Barberton绿岩带的国际大陆钻探计划（BARB1-2–3-4–5）期间采样的科马提岩和沉积岩进行了¹⁴⁶ Sm- ¹⁴² Nd系统分析。在来自3.48 Ga Komati地层的科马提岩中，分辨出负的¹⁴² Nd负值（低至-7.7±2.8），并且该特征与这些样品中测得的低Hf / Sm比相关。负μ ¹⁴²的Nd点与subchondritic SM /钕比其中冥古过程中形成的源极。不能解析解析¹⁴²在来自巴克礁（3.42 Ga）和无花果树组（3.23-3.28 Ga）的富含地壳碎屑，富含硅，富含Ca-Fe的沉积物和石中测量了Nd异常。我们的新测量值被合并到一组更大的¹⁴⁷ Sm- ¹⁴³ Nd和¹⁷⁶ Lu- ¹⁷⁶ Hf数据中，以便更好地了解在^孔铁矿结晶时地幔源中的^142,143 Nd- ¹⁷⁶ Hf同位素特征。我们的计算表明^142,143 Nd- ¹⁷⁶像巴尔巴顿地区那样，通过将碎屑沉积物的钾铁矿资源回收到科马提石中无法产生Hf同位素特征和Hf / Sm比。只有硅质岩具有所需的微量元素特征-低HF / SM，放射性ε ¹⁷⁶铪-但在硅质岩微量元素含量都非常低，燧石不现实的数额都需要添加。

我们为这些岩石的形成提出了一个四阶段模型。负μ ¹⁴² Nd和深岩浆海洋的后不久地球吸积的结晶过程中开发的低HF /钐比率。最终成为高锰铁矿来源的材料是残留的液体，它是通过50％结晶产生的，剩下的是桥锰铁矿/铁锂锰矿石/钙钙钛矿累积物。在这一阶段，Lu-Hf和Sm-Nd同位素系统解耦了。在4 Ga附近熄灭¹⁴⁶ Sm之后，在熔体萃取过程中母体/母体比例分馏。有了这个模型中，我们解释了积极ε ¹⁷⁶ Hf和轻微的负ε ¹⁴³这些样品中的钕。Barberton带另一部分的3.55 Ga Schapenburg科马提岩具有相似的化学特征，支持了我们在地球历史早期的深部岩浆海中进行分馏的模型。