Intimate link between ammonium loss of phengite and the deep Earth's water cycle
Intimate link between ammonium loss of phengite and the deep Earth's water cycle
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DOI：
10.1016/j.epsl.2019.02.022
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发表时间：
2019-05
影响因子：
5.3
通讯作者：
Wendi Liu;Yan Yang;V. Busigny;Q. Xia
中科院分区：
文献类型：
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作者：
Wendi Liu;Yan Yang;V. Busigny;Q. Xia
多硅白云母是一种高压白色云母，在俯冲环境下，在深达300 km的深度内是稳定的，是氮和氢向地球深部输送的重要载体。虽然氮和氢的再循环效率的俯冲带的测定是至关重要的挥发性元素的全球地球动力学模型的建立，到目前为止，没有实验数据已经评估了铵从多硅白云母的释放率和其潜在的影响多硅白云母脱水下降板。在这里，我们研究了含铵和无铵多硅白云母样品的铵损失和脱水的动力学，使用FTIR光谱。在从720 ° C至850° C的高温下退火不同的时间段之后，分析单个多硅白云母晶粒。结果表明，在相同温度下，铵的损失比脱水快一个数量级（例如，在800° C下，铵损失和脱水的扩散系数分别为1.5× 10− 15 m 2/s和1.0× 10− 16 m 2/s）。此外，含铵多硅白云母的脱水速度比不含铵多硅白云母快一个数量级（含铵和不含铵多硅白云母在800° C时的脱水扩散系数分别为1.0× 10− 16 m2/s和2.0× 10− 17 m2/s）。含铵多硅白云母的脱铵、脱水和无铵多硅白云母的脱水活化能分别为150、535和548 kJ/mol。这些结果表明，铵的释放不仅比多硅白云母的脱水更容易，而且可以促进脱水。铵在多硅白云母中的地球化学行为与前人对不同地温梯度俯冲带天然样品的观察结果一致。此外，由于铵释放可引发脱水，水可能在比理论多硅白云母破裂浅的深度轨迹处（即< 300 km）释放。因此，存在于含水矿物中的铵可能对俯冲带释放的水通量产生强烈的控制作用。
Phengite, a high-pressure white mica, is stable down to∼ 300 km depth in subduction environment and represents an important carrier of nitrogen and hydrogen to the deep Earth. Although the determination of nitrogen and hydrogen recycling efficiency to subduction zones is crucial for building global geodynamic models of volatile elements, no experimental data have so far evaluated the rate of ammonium release from phengite and its potential effect on phengite dehydration in a descending slab. Here, we investigated the kinetics of ammonium loss and dehydration of ammonium-bearing and ammonium-free phengite samples using FTIR spectroscopy. Single phengite grains were analyzed after annealing at high temperatures from 720 to 850° C for different periods of time. The results show that ammonium loss is one order of magnitude faster than dehydration at the same temperature (for instance, with diffusion coefficients of 1.5× 10− 15 m 2/s and 1.0× 10− 16 m 2/s at 800° C for ammonium loss and dehydration, respectively). Moreover, dehydration of ammonium-bearing phengite is one order of magnitude faster than that of ammonium-free phengite (with diffusion coefficients of 1.0× 10− 16 m 2/s and 2.0× 10− 17 m 2/s at 800° C for dehydration in ammonium-bearing and ammonium-free phengite, respectively). The activation energies of ammonium loss, dehydration in ammonium-bearing phengite and dehydration in ammonium-free phengite are around 150, 535, 548 kJ/mol, respectively. These results imply that ammonium release from phengite is not only easier than dehydration but also can promote dehydration. The geochemical behavior of ammonium in phengite is consistent with previous observations on natural samples from subduction zones of different geothermal gradients. Furthermore, since ammonium release can trigger dehydration, water may be released at depth locus shallower than theoretical phengite breakdown (ie< 300 km). Thus, ammonium present in hydrous minerals may exert a strong control on the water flux released in subduction zones.