In situ x-ray diffraction study on pressure-induced structural changes in hydrous forsterite and enstatite melts
In situ x-ray diffraction study on pressure-induced structural changes in hydrous forsterite and enstatite melts
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原位 X 射线衍射研究水合镁橄榄石和顽辉石熔体压力引起的结构变化
DOI：
10.1016/j.epsl.2011.05.036
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发表时间：
2011
期刊：
影响因子：
--
通讯作者：
and T. Irifune
中科院分区：
文献类型：
--
作者：
A. Yamada;T. Inoue;S. Urakawa;K. Funakoshi;N. Funamori;T. Kikegawa;and T. Irifune
我们利用原位同步加速器 X 射线衍射研究了水合镁橄榄石和顽辉石熔体中压力引起的结构变化。在超液线温度下收集高达 6.9 GPa 的衍射数据。在低于 3 GPa 的压力下，第一个尖锐的衍射峰 (FSDP) 反映了硅酸盐熔体中由……单键单键单键单键……键组成的硅酸盐网络有序性，在两种熔体成分中显着向更高的 Q（散射矢量 [Å−1]）移动。这一观察结果表明，水对低于 3 GPa 的硅酸盐网络具有解聚作用，这意味着……单键单键单键单键……键联被羟基物质（Si-OH 单元）部分破坏。相反，尽管存在压缩，但峰值仍会移动到高于 3 GPa 的较低 Qat 压力，这表明硅酸盐网络有序性延长（即硅酸盐网络聚合）。这一观察结果表明，在 3 GPa 以上，水会通过新的游离羟基（例如 Mg-OH）对硅酸盐网络产生聚合作用，这是之前在水合硅酸盐玻璃结构研究中提出的。事实上，本研究中的结构变化在含水 Mg2SiO4 熔体中更为明显，这表明 MgO 组分对 3-5 GPa 下含水熔体结构的聚合具有重要影响。水合硅酸盐熔体中目前的结构变化、高压下的再聚合会影响粘度。这种相对高粘度的含水岩浆可能能够在100-180公里的深度停留（或降低上升速度），这可以增强先前地震观测中提出的地球软流圈中地震波速度的降低。
We investigated the pressure-induced structural changes in hydrous forsterite and enstatite melts using in situ synchrotron X-ray diffraction. Diffraction data was collected up to 6.9 GPa at superliquidus temperatures. At pressures below 3 GPa, the first sharp diffraction peak (FSDP), which reflects the silicate network ordering in the silicate melts that consist of …single bondSisingle bondOsingle bondSisingle bond… linkages, is shifted notably toward higherQ(scattering vector [Å− 1]) in both melt compositions. This observation indicates that water has a depolymerizing effect on the silicate network below 3 GPa, which means that …single bondSisingle bondOsingle bondSisingle bond… linkages are partially disrupted by hydroxyl species (Si-OH units). In contrast, the peaks move to lowerQat pressures above 3 GPa in spite of the compression, which indicates lengthening of the silicate network ordering (i.e., polymerization of silicate network). This observation indicates that water changes to have a polymerizing effect on the silicate network above 3 GPa by a new free hydroxyl group such as Mg-OH, which was previously proposed in the study on hydrous silicate glasses structure. In fact, the structural changes in the present study are more pronounced in the hydrous Mg2SiO4melt, suggesting that the MgO component has an important influence on the polymerization of hydrous melt structure at 3–5 GPa. The present structural change, re-polymerization at high pressure, in hydrous silicate melt can influence the viscosity. Such a relatively high-viscosity hydrous magma may be able to stay (or be decreased in the rising velocity) at a depth of 100–180 km, which can enhance the decrease in a seismological wave velocity in the Earth's asthenosphere as proposed in previous seismological observations.