Sampling the volatile-rich transition zone beneath Bermuda
Sampling the volatile-rich transition zone beneath Bermuda
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DOI：
10.1038/s41586-019-1183-6
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发表时间：
2019-05-16
期刊：
影响因子：
64.8
通讯作者：
Sobolev, Alexander V.
中科院分区：
文献类型：
--
作者：
Mazza, Sarah E.;Gazel, Esteban;Sobolev, Alexander V.
远离板块边界的板内岩浆区提供了地幔成分和不均匀性的直接取样。在地幔中观察到的化学不均匀性通常归因于俯冲过程中的再循环（1-3），这使得挥发物和不相容元素进入地幔。尽管许多板内火山都采集了深地幔的样本--可能是在核幔边界（4）--但并非所有的板内火山都是根深蒂固的（5），其他较浅的边界层中的储集层可能也参与了岩浆的生成。在这里，我们提出的证据表明，百慕大采样一个以前未知的地幔域，其特点是二氧化硅不饱和熔体，大大丰富的不相容元素和挥发物，和一个独特的，极端的同位素签名。据我们所知，百慕大记录了使用高精度方法在海洋盆地中记录的最具放射性的Pb-206/Pb-204同位素（Pb-206/Pb-204比率为19.9-21.7）。再加上低的Pb-207/Pb-204比值（15.5-15.6）和相对稳定的Sr、Nd和Hf同位素，数据表明这个来源的年龄必须小于6.5亿年。因此，我们将百慕大源解释为以前未知的瞬态地幔储集层，这是由于过渡区（上地幔和下地幔之间）中不相容元素和挥发物的再循环和储存6 -8，以及仅在该边界层中稳定的矿物（如K-硬硅钙石）中铅的分馏（9，10）。我们认为，最近回收到过渡区，有关的俯冲事件在泛大陆的形成，是这个水库只被发现在大西洋的原因。我们的地球动力学模型表明，这个边界层采样的扰动有关的地幔流。地震研究和金刚石夹杂物（6，7）表明，回收材料可以储存在过渡区（11）。据我们所知，我们首次展示了地球化学证据，表明这种储存是产生极端同位素域的关键，而这些极端同位素域以前被认为只与深度循环有关。
Intraplate magmatic provinces found away from plate boundaries provide direct sampling of the composition and heterogeneity of the Earth's mantle. The chemical heterogeneities that have been observed in the mantle are usually attributed to recycling during subduction(1-3), which allows for the addition of volatiles and incompatible elements into the mantle. Although many intraplate volcanoes sample deep-mantle reservoirs-possibly at the core-mantle boundary(4)-not all intraplate volcanoes are deep-rooted(5), and reservoirs in other, shallower boundary layers are likely to participate in magma generation. Here we present evidence that suggests Bermuda sampled a previously unknown mantle domain, characterized by silica-undersaturated melts that are substantially enriched in incompatible elements and volatiles, and a unique, extreme isotopic signature. To our knowledge, Bermuda records the most radiogenic Pb-206/Pb-204 isotopes that have been documented in an ocean basin (with Pb-206/Pb-204 ratios of 19.9-21.7) using high-precision methods. Together with low Pb-207/Pb-204 ratios (15.5-15.6) and relatively invariant Sr, Nd, and Hf isotopes, the data suggest that this source must be less than 650 million years old. We therefore interpret the Bermuda source as a previously unknown, transient mantle reservoir that resulted from the recycling and storage of incompatible elements and volatiles6-8 in the transition zone (between the upper and lower mantle), aided by the fractionation of lead in a mineral that is stable only in this boundary layer, such as K-hollandite(9,10). We suggest that recent recycling into the transition zone, related to subduction events during the formation of Pangea, is the reason why this reservoir has only been found in the Atlantic Ocean. Our geodynamic models suggest that this boundary layer was sampled by disturbances related to mantle flow. Seismic studies and diamond inclusions(6,7) have shown that recycled materials can be stored in the transition zone(11). For the first time, to our knowledge, we show geochemical evidence that this storage is key to the generation of extreme isotopic domains that were previously thought to be related only to deep recycling.