Water and Iron effect on the P-T-x coordinates of the 410-km discontinuity in the Earth upper mantle

摘要

We performed multi-anvil experiments in the system MgO-SiO_2 ± H_20 at 13.0-13.7 GPa and 1,025-1,300°C and in the system MgO-FeO-SiO_2 ± H_2O, under reducing conditions, at 11.0-12.7 GPa and l,200°C, to depict the effect of H_2O on the P-T-x coordinates of the 410-km discontinuity, i.e. the olivine-wadsleyite phase boundary. The charges were investigated with Electron Microprobe (EMP), Raman Spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), Secondary Ion Mass Spectrometry (SIMS) and Electron Energy Loss Spectroscopy (EELS). We observe in the MgO-SiO_2-H_2O system at l,200°C a 0.6 GPa shift of the phase boundary to lower pressure compared to dry conditions, due to the stronger water fractionation into wadsleyite (wad) rather than in olivine (ol). In the MgO-FeO-SiO_2-H_2O system, we reproduced the triple point, i.e. observed coexisting hydrous ol, wad and ringwoodite (ring). SIMS H quantifications provided partitioning coefficients for water: D_(wad/ol)~(water) ~ 3.7(5) and D_(ring/ol)~(water) ~ 1.5(2) and D_(wad/ring)~(water) ~ 2.5(5). For a bulk composition of x_(Fe) = 0.1, our data indicate only a slight difference in the width of the loop of the two phase field ol-wad under hydrous conditions compared to dry conditions, i.e. no broadening with respect to composition but a shift to lower pressures. For bulk compositions of x_(Fe) > 0.2, i.e. in regions where wad-ring and ol-ring coexist, we observe, however, an unexpected broadening of the loops with a shift to higher iron contents. In total, the stability field of hydrous wad expands in both directions, to lower and higher pressures. Fe~(3+) concentrations as determined by EELS are very low and are expected to play no role in the broadening of the loops.