H2O solubilities in Al-free and Al-bearing high-pressure silica polymorphs as a function of pressure and temperature: H2O storage in the lower mantle

要了解深部H2O循环，有必要知道地幔矿物中H2O的溶解度。一些上地幔矿物具有较高的H2O贮量，但在下地幔橄榄岩矿物中H2O的溶解度可以忽略不计。因此，我们推测俯冲板块玄武岩碎屑中的硅质矿物在下地幔中含有H2O。激光加热金刚石砧腔(LHDAC)实验表明，在下地幔上部，即中地幔的条件下，高压二氧化硅矿物，即辉石和CaCl2结构的二氧化硅中，H2O有很大的溶解度。然而，黄体生成素-脱氢表雄酮的研究结果是相反的。因此，在中地幔条件下，我们将利用我们先进的多压头技术研究水在辉石和CaCl2结构的SiO_2中的溶解度。由于铝能增加辉石的H2O溶解度，并且被认为是辉石的主要二次阳离子，因此本项目将对含铝和不含铝体系进行研究。该项目由两个子项目组成。子项目I是在DESY利用多压头压力机在PETRA-III的光束线P61B上进行的现场X射线衍射研究。首先，我们将调查高压二氧化硅在减压过程中和减压后是否会发生H2O损失。接下来，我们将计算常温高压下体积与H2O含量的关系。然后我们将在10、20、30、40和50 Gpa的压力下测量高压二氧化硅的体积，并以100K的步长将温度提高到2000K。在每个温度步骤，样品将一次淬火到环境温度，H2O含量根据上面确定的体积-H2O-含量关系确定。同时，将仔细观察(120)-(210)、(211)-(121)、(130)-(310)和(031)-(301)峰的分裂，以确定辉石和CaCl2结构的SiO_2之间的相变。在子项目II中，将在14-35 Gpa的压力和1300-2300K的温度下合成无铝高压二氧化硅和含铝高压二氧化硅单晶。合成的单晶将用于傅立叶变换红外光谱和二次离子质谱仪测定水含量。在单晶生长之前，Al_2O_3的最大溶解度将被确定为含铝系统中压力和温度的函数。

To understand the deep H2O cycle, it is necessary to know the H2O solubility in the mantle min-erals. Some upper-mantle minerals have high H2O storage capacity, but the H2O solubility in lower-mantle peridotite minerals is negligible. Hence, we speculate that silica minerals in basaltic fragments of subducted slabs host H2O in the lower mantle. Laser-heated diamond-anvil-cell (LH-DAC) experiments reported large H2O solubility in high-pressure silica minerals, namely, stishovite and CaCl2-structured SiO2, under conditions of the upper part of the lower mantle, which is referred to as the mid-mantle. However, the results of the LH-DAC studies were contra-dictory. Therefore, we will investigate the H2O solubility in stishovite and CaCl2-structured SiO2 under mid-mantle conditions using our advanced multianvil technology. Since Al is known to increase H2O solubility of stishovite and is considered a major secondary cation for stishovite, both Al-bearing and Al-free systems will be investigated in this project. This project consists of two sub-projects. Sub-project I is an in situ X-ray diffraction study using the multi-anvil press on beamline P61B of PETRA-III at DESY. First, we will investigate whether H2O loss occurs from high-pressure silica during and after decompression. Next, we will cali-brate the relationship between volume and H2O-content at ambient temperature and high pres-sure. We will then measure the volumes of high-pressure silica at pressures of 10, 20, 30, 40 and 50 GPa and increasing temperatures to 2000 K in 100 K steps. At each temperature step, the sample will be once quenched to ambient temperature, and the H2O content is determined from the volume – H2O-content relationship determined above. At the same time, splits of (120)-(210), (211)-(121), and (130)-(310), and (031)-(301) peaks will carefully be observed to determine the phase transition between stishovite and CaCl2-structured SiO2. In the Sub-project II, single crystals of Al-free and Al-bearing high-pressure silica will be synthe-sized at pressures of 14 - 35 GPa and temperatures of 1300 - 2300 K. The synthesized single crystals will be used to determine the H2O content using Fourier transform infrared spectrosco-py and secondary ion mass spectrometry. Prior to the single-crystal growth, the maximum Al2O3 solubility will be determined as a function of the pressure and temperature in the Al-bearing system.