Abstract The widespread low seismic velocity zone (LVZ) in the shallow oceanic mantle has long been debated in terms of mantle melting. At LVZ depths, volatiles (CO2 and H2O) are present in minute amounts, which implies mantle incipient melting down to below 1000 °C with the production of minute amounts of volatile-rich melt, well below 1 vol.%. However, melt compositions and distributions in the incipient melting regime have only been inferred from experiments departing from actual mantle conditions. Here, we experimentally reproduce incipient melting by re-equilibrating a naturally CO2- and H2O-bearing mantle rock at mantle temperatures and pressure. By using cutting-edge microscopy characterizations, we evidence that minute amounts of volatile-rich melts fully interconnect in mantle rocks down to lithospheric temperatures, enabling thus the modification of geophysical signals from the mantle. These findings and the correspondence of the domain of local, sharp drops in shear wave velocity (Vs) with the domain of (CO2+H2O)-melting in the LVZ strongly supports that these geophysical anomalies relate to mantle melting. Geophysical surveys image in situ the very low and highly heterogeneous distribution of melt in the mantle generated by the very low and highly heterogeneous distribution of volatiles probed by surficial geochemical surveys. The global-scale geophysical signature of the LVZ appears mainly unaffected because the average background melt fraction is very low, estimated at ∼0.03-0.05 vol.% melt. However, enhanced geophysical signals arise from sporadic, localized areas where melt fraction is increased, such as the ∼0.2 vol.% melt estimated for detecting sharp Vs drops using SS precursors. In-depth deciphering of the dynamics of melt and volatiles in the LVZ calls for investigations on the seismic velocity, permeability and rheology of partially molten mantle rocks covering the diversity of mantle melt compositions, fractions and temperatures.

中文翻译：

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解开海洋低速区的部分熔体分布

摘要 浅层大洋地幔中广泛存在的低地震速度带（LVZ）长期以来一直在地幔熔化方面存在争议。在 LVZ 深处，挥发物（CO2 和 H2O）以微量存在，这意味着地幔初期熔化至 1000 °C 以下，产生微量的富含挥发物的熔体，远低于 1 vol.%。然而，初期熔融状态的熔体组成和分布只能从脱离实际地幔条件的实验中推断出来。在这里，我们通过在地幔温度和压力下重新平衡天然含有 CO2 和 H2O 的地幔岩石，实验性地再现了初期熔化。通过使用尖端的显微镜表征，我们证明微量的富含挥发物的熔体在地幔岩石中完全相互连接，直到岩石圈温度，从而能够修改来自地幔的地球物理信号。这些发现以及剪切波速度 (Vs) 局部急剧下降域与 LVZ 中 (CO2+H2O) 熔化域的对应关系强烈支持这些地球物理异常与地幔熔化有关。地球物理勘测现场成像了地幔中极低且高度不均匀的熔体分布，这是由地表地球化学调查探测到的挥发物的极低且高度不均匀分布所产生的。LVZ 的全球尺度地球物理特征似乎主要不受影响，因为平均背景熔体分数非常低，估计约为 0.03-0.05 vol.% 熔体。然而，增强的地球物理信号来自于熔体分数增加的零星局部区域，例如~0.2 vol. % 熔体估计用于检测使用 SS 前体的急剧 Vs 下降。深入解读 LVZ 中熔体和挥发物的动力学需要研究部分熔融地幔岩石的地震速度、渗透率和流变学，涵盖地幔熔体成分、分数和温度的多样性。