S. Karato et H. Jung, WATER, PARTIAL MELTING AND THE ORIGIN OF THE SEISMIC LOW-VELOCITY ANDHIGH ATTENUATION ZONE IN THE UPPER-MANTLE, Earth and planetary science letters, 157(3-4), 1998, pp. 193-207
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.