[S-IT20] 地球深部科学

[SIT20-P07] Compressibility of single crystal zircon up to 19 GPa: implication to the partitioning coefficient of trace elements

*米谷 珠萌¹、新名 良介¹、樋口 光²、深澤 倫子²、河口 沙織³ (1.明治大学大学院理工学研究科物理学専攻、2.明治大学大学院理工学研究科応用化学専攻、3.高輝度光科学研究センター)

キーワード：ジルコン、単結晶XRD測定、高圧、分子動力学計算、微量元素、分配係数

Zircon is a widely studied accessory mineral that helps us understand the evolution of the Earth. The unique nature of the zircon is due to its chemical/physical strength and the characteristic crystallographic zirconium site, which is compatible with rare Earth elements and actinoids. According to the lattice strain model, the trace element partitioning coefficients can be predicted by the zircon’s compressibility of the cation site. However, the crystal structure of the zircon has been precisely determined at less than ~5 GPa by single crystal X-ray diffraction measurements, while zircon is believed to be stable up to around 10 GP. Here, we have precisely determined the crystal structure of the zircon under high-pressure conditions up to 19 GPa, covering the whole stability field of the zircon by using a diamond anvil cell. We also theoretically calculated the unit cell parameters of zircon at high-pressure by molecular dynamics simulations. The unit cell parameters and the bond length showed abnormal trends above the high-pressure stability limit of the zircon. The previous discrepancy in the compressibility is likely due to the non-hydrostatic condition and instability of the sample. We determined the compressibility of the Zr-O bond and ZrO₈ polyhedral based on the determined crystal structure. The averaged length of the Zr-O bond showed consistent modulus with the partitioning coefficients predicted by the lattice strain model. On the contrary, the model with Poisson solid assumption is inconsistent with the partitioning coefficients. The averaged lengths of the cation-anion bonds under high pressure can help in understanding the partitioning coefficient between minerals and melt.