Water Solubility In Mg₃Al₂Si₃O₁₂ Garnet Under Upper Mantle Conditions

Garnet,X₃Y₂Z₃O₁₂(where X²⁺=Mg,Ca,Fe,Mn;Y³⁺=Al,Cr,Fe;Z⁴⁺=Si,Ge),is one of the most plentiful rock-forming minerals in the Earth’s crust and mantle.As the primary nominally anhydrous minerals（NAMs）of the upper mantle,garnet is capable of containing some amounts of hydrogen inside the crystals in the form of hydroxyl(OH^-1).The storage capacity of water in garnet significantly affect the physical and chemical properties of garnet and thereby the mineralogy of Earth’s crust and upper mantle.Mg₃A_l2Si₃O₁₂ pyrope is an important end member in the garnet family and exists in a wide depth from crust to the upper region of lower mantle.Therefore,the knowledge of water storage in Mg₃Al₂Si₃O₁₂ pyrope is of great significance for understand the water cycle in the Earth’s interior.However,its water storage is still controversial from earlier studies due to analytical uncertainties from different synthesis and analytical methods.Therefore,it is necessary to carry out a series experiments to clarify the real water content in garnet under high pressure and high temperature conditions of upper mantle.In order to create high pressure and high temperature conditions of Earth’s interior,we firstly develop high pressure techniques using hard tungsten carbide（WC）anvils in a Walker-type large volume press（LVP）.The phase transition from semiconductor to metal of Zn Te,Zn S and Ga As were used to calibrate the behaviors of pressure generation of HXR WC anvils and three types of cell assemblies（10/5,10/4,and 8/3,the edge length of octahedral/the truncated edge length of WC anvils）.We obtained the relationship between the external oil pressure of the press and the actual pressure inside the pressure chamber of doped Mg O octahedral using HXR anvils.We found that pressure can reach 18.3GPa under a press load of 280 bar using the 8/3 cell assembly,which is significantly higher than that generated by 10/4and 10/5 assemblies using the same HXR WC anvils.The efficiency of pressure generation for 8/3 assemblies is thereby higher than that of 10/4 and 10/5 assemblies.For the same HXR WC anvils,pressure medium with the highest hardness possesses a higher efficiency pressure generation than the softer pressure mediums.We have synthesized high quality single crystals of Mg₃Al₂Si₃O₁₂ garnet in the presence of hydrous melts at pressures of 5-12GPa and temperatures of1000-1200°C for keeping 24-40 hours.We well examined the quality of these garnet crystals using scanning electron microscope（SEM）and Raman spectroscopy.Water solubility in these garnets was characterized by Fourier transform infrared spectrum（FTIR）.We found that the water content in the synthetic garnet at 1000°C decreased from 205ppm wt.H₂O at 5GPa to 71ppm wt.H₂O at 8GPa.Then,it increased to the maximum value of 646ppmwt.H₂O at 12GPa.At the same pressure,the water content decreased with increasing temperatures.The induce water into the crystal structure of garnet is probably due to the replacement of silicon atoms by hydrogen atoms in the structure of garnet.Although the water content of these synthetic pyrope is generally lower than that of olivine and pyroxene,it can take some amounts of water into the Earth’s upper mantle.