Seismic Discontinuity
نویسنده
چکیده
The 41 0-kilometer seismic discontinuity is generally considered to be caused by a phase transformation of the main constituent of the upper mantle, olivine, ot-(Mg,Fe)2SiO4, to P-(Mg,Fe)2SiO4. Recent data show that H20 dissolves in olivine and other nominally anhydrous mantle minerals and that the partitioning of H20 between olivine and P-(Mg,Fe)2Si04 is about 1:10. Such behavior strongly affects the region over which the cx to phase transformation occurs and hence the seismic discontinuity that results. The observed width of the discontinuity constrains the maximum H20 content of upper mantle olivine to about 200 parts per million by weight. cies in the Mg sublattices. New experiments (13-15) indicate a solubility of up to 3% H20 (by weight) in the 13 phase and a partitioning of H2) between the phase and olivine of greater than 10: 1 in favor of the former (15). The hypothetical endmember has the formula Mg7Si4O14(OH)2, and in a mantle saturated with (Mg,Fe)2SiO4 and the (Mg,Fe)SiO3 component of pyroxene, its formation may be represented by the equilibrium
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