Volcanic arcs fed by rapid pulsed fluid flow through subducting slabs
Volcanic arcs fed by rapid pulsed fluid flow through subducting slabs
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DOI：
10.1038/ngeo1482
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发表时间：
2012-07-01
影响因子：
18.3
通讯作者：
Seitz, Hans-Michael
中科院分区：
文献类型：
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作者：
John, Timm;Gussone, Nikolaus;Seitz, Hans-Michael
在俯冲带，与海水相互作用的海洋岩石圈返回地幔，在下降过程中升温，并通过含水矿物的脱挥发分释放流体。俯冲带上方形成岩浆供给火山的模型需要将这些流体大规模输送到上覆的地幔楔中(1-3)。流体流动似乎也与俯冲板片的地震活动有关。然而，这种流体流动的空间和时间尺度仍然很大程度上未知，建议的时间尺度范围为数万年至数万年(3-5)。在这里，我们使用 Li-Ca-Sr 同位素系统来考虑流体来源，并定量限制俯冲海洋岩石圈内大约 70 公里深度的俯冲带流体释放的持续时间，这些岩石圈现已在中国天山挖掘出来。使用锂扩散模型，我们发现流体流道附近的围岩孔隙度比背景水平增加了十倍。我们表明，脱挥发分释放的流体沿着主要管道以脉冲形式穿过板块，持续时间约为 200 年。因此，尽管整个板片脱水过程在数百万年和广泛的压力和温度范围内连续进行，但我们得出的结论是，俯冲板片脱水产生的流体在短暂的通道化流体流动事件中流动。
At subduction zones, oceanic lithosphere that has interacted with sea water is returned to the mantle, heats up during descent and releases fluids by devolatilization of hydrous minerals. Models for the formation of magmas feeding volcanoes above subduction zones require largescale transport of these fluids into overlying mantle wedges(1-3). Fluid flow also seems to be linked to seismicity in subducting slabs. However, the spatial and temporal scales of this fluid flow remain largely unknown, with suggested timescales ranging from tens to tens of thousands of years(3-5). Here we use the Li-Ca-Sr isotope systems to consider fluid sources and quantitatively constrain the duration of subduction-zone fluid release at similar to 70 km depth within subducting oceanic lithosphere, now exhumed in the Chinese Tianshan Mountains. Using lithium-diffusion modelling, we find that the wall-rock porosity adjacent to the flowpath of the fluids increased ten times above the background level. We show that fluids released by devolatilization travelled through the slab along major conduits in pulses with durations of about similar to 200 years. Thus, although the overall slab dehydration process is continuous over millions of years and over a wide range of pressures and temperatures, we conclude that the fluids produced by dehydration in subducting slabs are mobilized in short-lived, channelized fluid-flow events.