The Effect of H_2O on the 410-Kilometer Seismic Discontinuity

The 410-kilometer seismic discontinuity is generally considered to be caused by a phase transformation of the main constituent of the upper mantle, olivine, α-(Mg,Fe)_2SiO_4, to β-(Mg,Fe)_2SiO_4. Recent data show that H_2O dissolves in olivine and other nominally anhydrous mantle minerals and that the partitioning of H_2O between olivine and β-(Mg,Fe)_2SiO_4 is about 1:10. Such behavior strongly affects the region over which the α to β phase transformation occurs and hence the seismic discontinuity that results. The observed width of the discontinuity constrains the maximum H_2O content of upper mantle olivine to about 200 parts per million by weight.