Abstract Olivine is the most abundant mineral in the Earth’s upper mantle and subducting slabs. Studying the structural evolution and equation of state of olivine at high-pressure is of fundamental importance in constraining the composition and structure of these regions. Hydrogen can be incorporated into olivine and significantly influence its physical and chemical properties. Previous infrared and Raman spectroscopic studies indicated that local structural changes occur in Mg-rich hydrous olivine (Fo ≥ 95; 4883–9000 ppmw water) at high-pressure. Since water contents of natural olivine are commonly <1000 ppmw, it is inevitable to investigate the effects of such water contents on the equation of state (EoS) and structure of olivine at high-pressure. Here we synthesized a low water content hydrous olivine (Fo95; 1538 ppmw water) at low SiO2 activity and identified that the incorporated hydrogens are predominantly associated with the Si sites. We performed high-pressure single-crystal X‑ray diffraction experiments on this olivine to 29.9 GPa. A third-order Birch-Murnaghan equation of state (BM3 EoS) was fit to the pressure-volume data, yielding the following EoS parameters: VT0 = 290.182(1) Å3, KT0 = 130.8(9) GPa, and K T 0 ′ = 4.16 ( 8 ) . $K_{\mathrm{T} 0}^{\prime}=4.16(8).$The KT0 is consistent with those of anhydrous Mg-rich olivine, which indicates that such low water content has negligible effects on the bulk modulus of olivine. Furthermore, we carried out the structural refinement of this hydrous olivine as a function of pressure to 29.9 GPa. The results indicate that, similar to the anhydrous olivine, the compression of the M1-O and M2-O bonds are comparable, which are larger than that of the Si-O bonds. The compression of M1-O and M2-O bonds of this hydrous olivine are comparable with those of anhydrous olivine, while the Si-O1 and Si-O2 bonds in the hydrous olivine are more compressible than those in the anhydrous olivine. Therefore, this study suggests that low water content has negligible effects on the EoS of olivine, though the incorporation of water softens the Si-O1 and Si-O2 bond.

中文翻译：

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低含水量橄榄石晶体结构研究至 29.9 GPa：高压单晶 X 射线衍射研究

摘要 橄榄石是地球上地幔和俯冲板块中含量最丰富的矿物。研究高压下橄榄石的结构演化和状态方程对于限制这些区域的组成和结构具有重要意义。氢可以掺入橄榄石中并显着影响其物理和化学性质。先前的红外和拉曼光谱研究表明，在高压下，富含镁的含水橄榄石（Fo ≥ 95；4883–9000 ppmw 水）会发生局部结构变化。由于天然橄榄石的含水量通常<1000 ppmw，因此有必要研究这种含水量对橄榄石在高压下的状态方程（EoS）和结构的影响。在这里，我们合成了一种低含水量的含水橄榄石（Fo95；1538 ppmw 水）在低 SiO2 活性下，并确定掺入的氢主要与 Si 位点相关。我们对该橄榄石进行了高压单晶 X 射线衍射实验，压力达到 29.9 GPa。三阶 Birch-Murnaghan 状态方程 (BM3 EoS) 拟合压力-体积数据，得出以下 EoS 参数：VT0 = 290.182(1) Å3，KT0 = 130.8(9) GPa，KT 0 ′ = 4.16（8）。$K_{\mathrm{T} 0}^{\prime}=4.16(8).$KT0 与无水富镁橄榄石的 KT0 一致，表明如此低的含水量对体积模量的影响可以忽略不计黄绿。此外，我们将这种含水橄榄石的结构改进作为压力的函数，达到 29.9 GPa。结果表明，与无水橄榄石相似，M1-O 和 M2-O 键的压缩相当，比 Si-O 键的压缩大。这种含水橄榄石的 M1-O 和 M2-O 键的压缩性与无水橄榄石相当，而含水橄榄石中的 Si-O1 和 Si-O2 键比无水橄榄石中的更可压缩。因此，这项研究表明，尽管水的加入软化了 Si-O1 和 Si-O2 键，但低水含量对橄榄石的 EoS 的影响可以忽略不计。