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

无铝和含铝高压二氧化硅多晶型物中的水溶解度与压力和温度的函数关系：下地幔中的水储存

• 批准号: 518398967
• 负责人: Professor Dr. Tomoo Katsura
• 金额: --
• 依托单位: Bayerisches Forschungsinstitut für Experimentelle Geochemie und Geophysik (BGI)
• 依托单位国家: 德国
• 项目类别: Research Grants
• 资助国家: 德国
• 项目状态: 未结题
• 来源: https://gepris.dfg.de/gepris/projekt/518398967?language=en
• 关键词: H2O; solubilities; Al; free; bearing

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.

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。