The long-term exchange of volatile species (such as water, carbon, nitrogen and the noble gases) between deep Earth and surface reservoirs controls the habitability of the Earth’s surface. The present-day volatile budget of the mantle reflects the integrated history of outgassing and retention of primordial volatiles delivered to the planet during accretion, volatile species generated by radiogenic ingrowth and volatiles transported into the mantle from surface reservoirs over time. Variations in the distribution of volatiles between deep Earth and surface reservoirs affect the viscosity, cooling rate and convective stress state of the solid Earth. Accordingly, constraints on the flux of surface volatiles transported into the deep Earth improve our understanding of mantle convection and plate tectonics. However, the history of surface volatile regassing into the mantle is not known. Here we use mantle xenon isotope systematics to constrain the age of initiation of volatile regassing into the deep Earth. Given evidence of prolonged evolution of the xenon isotopic composition of the atmosphere1,2, we find that substantial recycling of atmospheric xenon into the deep Earth could not have occurred before 2.5 billion years ago. Xenon concentrations in downwellings remained low relative to ambient convecting mantle concentrations throughout the Archaean era, and the mantle shifted from a net degassing to a net regassing regime after 2.5 billion years ago. Because xenon is carried into the Earth’s interior in hydrous mineral phases3–5, our results indicate that downwellings were drier in the Archaean era relative to the present. Progressive drying of the Archean mantle would allow slower convection and decreased heat transport out of the mantle, suggesting non-monotonic thermal evolution of the Earth’s interior.Mantle xenon isotope systematics reveals that no substantial recycling of atmospheric xenon into the deep Earth occurred before 2.5 billion years ago, indicating that downwellings were drier in the Archaean era than today.

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氙同位素对地幔挥发物再循环历史的限制

地球深层和地表水库之间挥发性物质（如水、碳、氮和惰性气体）的长期交换控制着地球表面的宜居性。当今地幔的挥发性收支反映了在吸积过程中释放到地球的原始挥发物的除气和保留、放射性向内生长产生的挥发性物质以及随着时间的推移从地表储层输送到地幔中的挥发物的综合历史。深部地球和地表储层之间挥发物分布的变化会影响固体地球的粘度、冷却速度和对流应力状态。因此，对输送到地球深处的表面挥发物通量的限制提高了我们对地幔对流和板块构造的理解。然而，地幔中地表挥发物重新加气的历史尚不清楚。在这里，我们使用地幔氙同位素系统学来限制挥发性再气体进入地球深处的开始年龄。鉴于大气中氙同位素组成长期演化的证据1,2，我们发现大气氙在 25 亿年前不可能大量再循环到地球深处。在整个太古代时期，相对于环境对流地幔浓度而言，下降流中的氙浓度仍然很低，并且地幔在 25 亿年前从净脱气转变为净再气态。由于氙以含水矿物相 3-5 的形式进入地球内部，因此我们的结果表明，与现在相比，太古代时代的下流更为干燥。