WATER, PARTIAL MELTING AND THE ORIGIN OF THE SEISMIC LOW-VELOCITY ANDHIGH ATTENUATION ZONE IN THE UPPER-MANTLE

The common belief that the seismic low velocity and high attenuation z one (the asthenosphere) is caused by the presence of a small amount of melt is not supported by recent mineral physics and seismological obs ervations. A review of recent mineral physics observations suggests th at water significantly reduces seismic wave velocities through anelast ic relaxation and hence, at a small melt fraction expected in most of the Earth's upper mantle, partial melting will increase seismic wave-v elocities through the removal of water from minerals such as olivine. Therefore the asthenosphere, in this model, is a layer where no signif icant partial melting occurs and hence a high water content is retaine d. We apply this model to calculate seismic wave Velocities and attenu ation in the upper mantle with a range of water contents. The seismic structures calculated from this model depend on geotherm, the mode of partial melting (batch or fractional melting) and the geometry of upwe lling flow (passive flow or dynamic upwelling). The sharp velocity cha nge around 60-80 km (the Gutenberg discontinuity) can be attributed to a sharp change in water content due to partial melting, if the temper ature there is relatively high as implied by the plate model and if me lting occurs as fractional melting but not by batch melting. However, the significant increase in seismic wave Velocity with age in young oc eanic upper mantle suggests rapid cooling as predicted by a cooling ha lf-space model. Thus, the present model suggests fast cooling in the e arly stage but slow cooling in the later stage of evolution of the oce anic upper mantle, the latter being caused presumably by some addition al heat in the old oceanic upper mantle. The seismic structures of typ ical oceanic upper mantle with a fast spreading rate (e.g., the Pacifi c) is consistent with passive spreading, whereas the greater depth of the G-discontinuity and the weaker seismic anisotropy in back-are regi ons (e.g., the Philippine Sea) suggest dynamic upwelling caused presum ably by a higher degree of melting due to a larger amount of water. (C ) 1998 Elsevier Science B.V. All rights reserved.