Chemical Behavior Of Minor Metal Elements In Minerals Of The Mantle Transition Zone And Its Geodynamic Implications

Transition metals Ni,Co,Zn,Ti,Cr and V are all important minor constituent elements that are potential indicators of geochemical processes in the Earth's mantle.According to the previous experimental studies,transition metals Ni,Cr,Ti partition preferentially into wadsleyite relative to olivine indicating that these transtition metals have higher solubilities in wadsleyite than in olivine.For ringwoodite and coexisting wadsleyite,Ni is more likely to be incorporated by ringwoodite.Although the transition metal partitioning between these mantle phases has already been documented,the factors which control transition metal fractionation still need to be constrained.In order to better understand the cation ordering in wadsleyite and minor metal element partitioning between wadsleyite and other coexisting phases in the mantle transition zone.Three groups of experimental studies have been conducted.(1)We have synthesized wadsleyites coexisting with pyroxenes with 2–3 wt% of either TiO2,Cr2O3,V2O3,CoO,NiO,or ZnO under hydrous conditions in separate experiments at 1300 °C and 15 GPa.We have refined the crystal structures of these wadsleyites by single-crystal X-ray diffraction,analyzed the compositions by electron microprobe,and estimated M3 vacancy concentration from b/a cell-parameter ratios.According to the crystal structure refinements,Cr and V show strong preferences for M3 over M1 and M2 sites and significant substitution up to 2.9 at% at the tetrahedral site(T site).Ni,Co,and Zn show site preferences similar to those of Fe with M1? M3 > M2 > T.The avoidance of Ni,Co,and Fe for the M2 site in both wadsleyite and olivine appears to be partially controlled by crystal field stabilization energy(CFSE).The estimated CFSE values of Ni2+,Co2+,and Zn2+ at three distinct octahedral sites in wadsleyite show a positive correlation with octahedral occupancy ratios [M2/(M1+M3)].Ti substitutes primarily into the M3 octahedron,rather than M1,M2,or T sites.Ti,Cr,and V each have greater solubility in wadsleyite than in olivine.Therefore these transition metal cations may be enriched in a melt or an accessory phase if hydrous melting occurs on upward convection across the wadsleyite-olivine boundary.(2)We have conducted single crystal X-ray diffraction analyses for refining the crystal structure of pyroxenes which coexist with wadsleyites.For the octahedral sites in the clinoenstatite,according to the results,Ni2+ and Co2+ show strong site preferences for M1 over M2 whereas Zn2+ shows a weak cation ordering in both octahedral and tetrahedral sites.The positive correlation between occupancy ratios(M2/M1)and the estimated CFSE of divalent cations in octahedron for Ni2+,Co2+,and Zn2+ in clinoenstatite can be observed.This correlation indicates that the crystal field effect is an important factor that controls the cation ordering at octahedral sites in both wadsleyite and coexisting clinoenstatite.The transition metal partitioning between clinoenstatite and wadsleyite appears to be partially controlled by the CFSE in both octahedral and tetrahedral sites.For Ni2+,Co2+,and Zn2+,the ratios of mean octahedral site occupancies between wadsleyite and clinoenstatite display a negative correlation with their CFSE,implying that divalent transition metal cation which has larger negative CFSE value is more likely to occupy octahedral sites in wadsleyite than those in clinoenstatite when partitioning.The similar trend can also be observed for divalent cation partitioning between tetrahedral sites in these two phases.(3)We have synthesized crystal samples at 18 and 19 GPa and 1400oC with heating time up to 27 h under hydrous condition,using a KLB-1 starting material.We measured the chemical composition by electron micro-probe,collected Raman spectra and determined crystal structure by single-crystal X-ray diffraction,showing that phase E can coexist with ringwoodite,wadsleyite,pyroxene,and majorite in hydrous pyrolite compositions at temperatures to at least 1400 oC.Raman spectra of phase E samples at ambient conditions are characterized by several broad bands between 80 cm–1 and 1100 cm–1,of which the most intense ones occur at about 240 cm-1,370 cm–1,680 cm–1,and 900 cm–1.In the O-H stretching region(from 2500 to 3700 cm-1),we observe two broad bands which occur at about 3610 cm-1 and 3420 cm-1.Electron microprobe analyses show that Phase E can incorporate significant amount of Fe and Al(up to 6.8 and 1.47 wt % for FeO and Al2O3,respectively),and the Phase E samples which coexist with ringwoodite have higher average Al2O3 content(1.4 wt %)than those(0.7 wt %)coexist with wadsleyite.This phenomenon implies that Al3+ is able to stabilize the Phase E structure under higher pressure.The long heating time indicates that Phase E is a stable component of the mantle and not a metastable phase under very hydrous conditions.The X-ray diffraction analyses show that the space group of Phase E is R-3m,a = 2.9650(13)?;c = 13.870(4)?;V = 105.60(4)?.As previously observed,the structure is significantly disordered.The M1 site is about 72% occupied,whereas the M2 which is also octahedral but near the Si position,is only about 2% occupied.Si is in a tetrahedral site with unusually long Si-O distances(average 1.78 ?).The layer of M2 and Si sites is likely to be weak and the compression is likely to be anisotropic.The weak layer can lead to crystal preferred orientation during deformation and result in shear anisotropy in the subducting slab within middle transtition zone.