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H_2O循环，必须了解H_2O在地幔矿物中的溶解度。一些上地幔矿物具有较高的H2O储存能力，但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，硅矿物在俯冲板玄武质碎片主机H2O在下地幔。激光加热金刚石压砧室（LH-DAC）实验报告了在下地幔上部（称为中地幔）的条件下，高压二氧化硅矿物（即stishovite和CaCl 2结构的SiO2）中H2O的溶解度很大。然而，LH-DAC研究的结果是矛盾的。因此，我们将研究H2O的溶解度stishovite和CaCl 2结构的SiO2中地幔条件下，使用我们先进的多砧技术。由于已知Al会增加硅镁石的H2O溶解度，并且被认为是硅镁石的主要次级阳离子，因此本项目将研究含Al和不含Al的体系。该项目包括两个分项目。分项目一是利用多砧压机在DESY PETRA-III的光束线P61 B上进行现场X射线衍射研究。首先，我们将研究在减压过程中和减压后，高压二氧化硅是否发生H2O损失。接下来，我们将在室温和高压下校准体积与H2O含量之间的关系。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100 K的步长将温度升高到2000 K。在每个温度步骤中，将样品骤冷至环境温度一次，并根据上文确定的体积-H2O-含量关系确定H2O含量。同时，仔细观察（120）-（210）、（211）-（121）、（130）-（310）和（031）-（301）峰的分裂，以确定方石英和CaCl 2结构的SiO2之间的相变。子项目二是在14 - 35 GPa压力和1300 - 2300 K温度下合成无铝和含铝高压二氧化硅单晶。合成的单晶将用于使用傅里叶变换红外光谱和二次离子质谱法测定H2O含量。在单晶生长之前，最大Al 2 O3溶解度将被确定为含Al系统中的压力和温度的函数。

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.