School of Earth Sciences, University of Melbourne, Melbourne, Victoria, Australia.
Department of Earth and Planetary Sciences, Macquarie University, Sydney, New South Wales, Australia.
Nature. 2019 Sep;573(7775):578-581. doi: 10.1038/s41586-019-1574-8. Epub 2019 Sep 25.
The widely accepted paradigm of Earth's geochemical evolution states that the successive extraction of melts from the mantle over the past 4.5 billion years formed the continental crust, and produced at least one complementary melt-depleted reservoir that is now recognized as the upper-mantle source of mid-ocean-ridge basalts. However, geochemical modelling and the occurrence of high He/He (that is, primordial) signatures in some volcanic rocks suggest that volumes of relatively undifferentiated mantle may reside in deeper, isolated regions. Some basalts from large igneous provinces may provide temporally restricted glimpses of the most primitive parts of the mantle, but key questions regarding the longevity of such sources on planetary timescales-and whether any survive today-remain unresolved. Kimberlites, small-volume volcanic rocks that are the source of most diamonds, offer rare insights into aspects of the composition of the Earth's deep mantle. The radiogenic isotope ratios of kimberlites of different ages enable us to map the evolution of this domain through time. Here we show that globally distributed kimberlites originate from a single homogeneous reservoir with an isotopic composition that is indicative of a uniform and pristine mantle source, which evolved in isolation over at least 2.5 billion years of Earth history-to our knowledge, the only such reservoir that has been identified to date. Around 200 million years ago, extensive volumes of the same source were perturbed, probably as a result of contamination by exogenic material. The distribution of affected kimberlites suggests that this event may be related to subduction along the margin of the Pangaea supercontinent. These results reveal a long-lived and globally extensive mantle reservoir that underwent subsequent disruption, possibly heralding a marked change to large-scale mantle-mixing regimes. These processes may explain why uncontaminated primordial mantle is so difficult to identify in recent mantle-derived melts.
地球地球化学演化的广泛接受的范式表明，过去 45 亿年中，地幔中连续提取的熔体形成了大陆地壳，并产生了至少一个互补的熔体亏损储层，现在被认为是大洋中脊玄武岩的上地幔源区。然而，地球化学模拟和一些火山岩中高氦/氦（即原始）特征的出现表明，相对未分化的地幔体积可能存在于更深、更孤立的区域。一些大火成岩省的玄武岩可能提供了对地幔最原始部分的暂时限制的了解，但关于这些源在地幔时间尺度上的持久性的关键问题——以及今天是否有任何源存在——仍未解决。金伯利岩是大多数钻石的源岩，是研究地球深部地幔成分的重要窗口。不同年龄的金伯利岩的放射性同位素比值使我们能够通过时间来绘制该区域的演化图。在这里，我们表明，全球分布的金伯利岩起源于一个单一的均匀储层，其同位素组成表明一个均匀和原始的地幔源，在地球历史上至少隔离演化了 25 亿年——据我们所知，这是迄今为止唯一确定的储层。大约 2 亿年前，同一来源的大量物质受到干扰，可能是由于外源物质的污染。受影响的金伯利岩的分布表明，这一事件可能与泛大陆超大陆边缘的俯冲有关。这些结果揭示了一个长期存在且广泛分布的地幔储层，该储层随后受到干扰，可能预示着大规模地幔混合机制发生了显著变化。这些过程可能解释了为什么在最近的地幔衍生熔体中很难识别未受污染的原始地幔。
