Diamond Growth And Characteristics In The Metal-silicate-H₂O-C System At Hpht Conditions And Their Geological Implications

Natural diamond is regarded as a"window"to investigate the composition and reaction mechanism inside the deep earth because it carries rich mantle materials.At present,the discovered components of natural diamond inclusions are mainly silicates,H₂O,CO₂,metals and alloys.These inclusions may significantly affect the formation process of natural diamond crystals.However,it is difficult to quantitatively evaluate the specific effects of these components on the formation process of diamond by traditional geological analysis methods.Since the first synthetic diamond was produced by high-temperature and high-pressure（HPHT）method in 1954,it has become an important direction in high-pressure geoscience and physics to explore the formation environment of natural diamonds through direct HPHT experiments.It is possible to investigate the impacts of silicates,H₂O,and metals on the formation process of diamond through HPHT experiments.A series of diamond crystals were successfully grown at⁵.5 GPa and 1355-1385℃in the metal-C system with different contents of(Mg_0.9,Fe_0.1)₂Si O₄,Si O₂,Mg Si O₃,Fe₃Al₂（Si O₄）₃ and Mg₂Si₃O₈·5H₂O added by using a SPD 6×1200 large volume cubic high-pressure apparatus.The detailed phases of the additives in the system at HPHT conditions were analyzed by XRD,SEM-EDS Mapping and Raman spectra.The crystallized(Mg_0.9,Fe_0.1)₂Si O₄,Si O₂ and amorphous Mg Si O₃ were found to transform into olivine,coesite and enstatite（partially containing coesite and periclase）in the Fe Ni-C system at the carbon source block after HPHT experiments,respectively.The addition of Fe-rich almandine(Fe_2.2Mg_0.5Ca_0.2Mn_0.1)Al₂(Si_0.9Al_0.1O₄)₃ to the Ni Mn Co-C system was found to react with the catalyst,and finally exist in spessartine and a small amount of calcite.The Mg₂Si₃O₈·5H₂O is found to decompose and react with Fe Ni catalyst to form forsterite,enstatite,coesite,fayalite,ferrosilite,and H₂O under HPHT conditions.While in the Ni Mn Co-C system,Mg₂Si₃O₈·5H₂O was found to react with catalysts and form tephroite,forsterite,coesite and H₂O.With the increase of silicates in the carbon source,the diamond crystals are all cubic-octahedron,and the graphite and residual pits on the surface gradually disappear with the increase of the pressure transfer efficiency.As the Mg₂Si₃O₈·5H₂O content in the carbon source of Fe Ni-C system increases to 1.5 wt.%,the morphology of diamond crystals evolves from cubic-octahedron to skeletal cubic-octahedron,rhombic dodecahedron with dendrites,and finally to{110}dendrites.Simultaneously,the dendritic growth texture on the（111）surface gradually changed into growth pits and the{110}related features of trigonal pyramids,skeletal structure,and finally the（111）face disappeared completely.The systematic changes of diamond crystals are considered to be caused by the decomposition of H₂O under HPHT conditions.In addition,when the content of Mg₂Si₃O₈·5H₂O added in the Ni Mn Co-C system is further increased to 3.0 wt.%and 5.0 wt.%,the morphology and surface characterization of diamond crystals are also significantly affected by the impurity absorption effect,the development of{111}sector disappears,{110}dendrites are gradually replaced by the growing-out pyramids and{100}antiskeletal dendrites,and the{100}growth sprials is significantly developed in this process.When 2.0 wt.%to 10.0 wt.%Fe-rich almandine is added to the carbon source and catalyst of the Ni Mn Co-C system,the total content of nitrogen impurities(N_total)in the C center and N⁺center gradually increases from⁶9 ppm and¹05 ppm to¹09 ppm and¹53 ppm,respectively.The increased N_total is related to the decrease of nitrogen solubility in the catalyst caused by the increased oxygen fugacity by the addition of Fe-rich almandine.For comparison,when Mg-rich and Fe-poor(Mg_0.9,Fe_0.1)₂Si O₄,Si O₂and Mg Si O₃ are added to the carbon source of Fe Ni-C system,the nitrogen impurities of diamond fluctuate in the range of[175 ppm,230 ppm],which may be caused by the small effect of the addition of Mg-rich and Fe-poor silicate on the oxygen fugacity of the synthetic system.We attribute this insignificant change of N_total to the fact that the addition of silicate did not significantly change the oxygen fugacity in the synthetic system.With the content of Mg₂Si₃O₈·5H₂O in the Fe Ni-C system increasing from 0.0wt.%to 1.5 wt.%,the content of C center nitrogen impurity in diamond gradually decreased from²10 ppm to⁶0 ppm.When adding 0.0 wt.%to 5.0 wt.%Mg₂Si₃O₈·5H₂O to the Ni Mn Co-C system,the N_total of diamond grown in[111]orientation gradually decreases from¹17 ppm to³6 ppm,while the N_total of diamond grown in[100]orientation gradually decreases from¹00 ppm to²1 ppm.We attribute the reduction of N_total in diamonds grown in the above metal-silicate-H₂O-C system to the addition of H₂O under HPHT conditions.The decrease of N_total in diamonds grown in the above metal-silicate-H₂O-C system was accompanied by the enhancement of CH₂,CH₃ group and C-H,C-O,and C=O bonds.The intensity of these groups also exhibits an edge step density related orientational dependence of C_（100）>C_（111）.The decomposed hydrogen and oxygen atoms are speculated to bond with carbon atoms and distribute on the growing diamond structure.The formation of C-H,C-OH and C=O bonds will terminate the extension of the three-dimensional network of C-C bonds.These defects will accumulate along the[111]direction and form{110}related characteristics.These chemical bonds also compete with the nitrogen in the system during entering into the diamond lattice and cause the decreased N_total in diamond crystals.By comparing the color,morphology,surface characteristics,and impurities between synthetic diamond and natural Ia type diamond,fibrous diamond and ophiolite type diamond crystals,our work can provide experimental basis for the formation process and environment of natural diamond crystals.