Transition Metals in the Transition Zone: Crystal Chemistry of Minor Element Substitution in Wadsleyite

Abstract:

As the most abundant solid phase at depths of 410 to 525 km, wadsleyite constitutes a large geochemical reservoir in the Earth. In order to better understand minor element substitution and cation ordering in wadsleyite, we have synthesized wadsleyite coexisting with pyroxene with 2 to 3 weight percent each of TiO₂, Cr₂O₃, V₂O₃, CoO, NiO, and ZnO under hydrous conditions in separate experiments at 1300ºC and 15GPa. We have refined the crystal structures of these wadsleyites, analyzed the compositions by electron microprobe, and estimated M3 vacancy contents from b/a cell-parameter ratios. Trivalent cations, Fe, Cr and V, show a strong preference for M3 over M1 and M2 and significant substitution up to 2.8% at the tetrahedral site. Divalent Ni, Co, and Zn show site preferences similar to those of Fe²⁺ with M1≈ M3 > M2 > T. This site preference appears to be due crystal field stabilization energies rather than cation radius effects. Trivalent Cr, and V have much greater solubilities in wadsleyite than in olivine and so may be enriched in a melt or an accessory phase if hydrous melting occurs on upward convection across the wadsleyite-olivine boundary and may be useful as indicators of high pressure origin.