Geoscience Reference
In-Depth Information
Partial melting is frequently invoked to explain
a weak (a low seismic wave velocity, and low
viscosity) layer in Earth (e.g., Lambert & Wyllie,
1970; Kawakatsu
et al
., 2009). However, the influ-
ence of partial melting on rheological properties
is rather modest particularly when deformation
occurs by dislocation creep. Laboratory studies
showing this point are reviewed by Kohlstedt
(2002) and Kohlstedt and Zimmerman (1996).
Recently Takei and Holtzman (2009a,b,c) pre-
sented a new analysis of deformation of a partially
molten material and suggested that partial melt-
ing reduces viscosity by an order of magnitude in
the diffusion creep regime. They predicted that
the amount of enhanced deformation is larger
than the results reported by Kohlstedt (2002)
and Kohlstedt and Zimmerman (1996) but the
reason for this discrepancy is not well under-
stood. Takei-Holtzman's analysis includes more
detailed treatment of the stress distribution in
a deforming partially molten material and the
diffusion path and the stress distribution are dif-
ferent from a simpler model by Kohlstedt (2002)
(see Chapter 3, this volume, above). In any case,
the enhancement of strain-rate in their model
is only modest for diffusion creep (a factor of
∼
upper mantle (Karato, 2010b). Therefore mixing
is likely to be inefficient at least in the upper
mantle. Other mechanisms of homogenization
need to be considered. Mixing of materials with
different composition is often studied assuming
the homogeneous viscosity (e.g., Christensen &
Hofmann, 1994; van Keken
et al
., 2002). Influence
of viscosity contrasts needs to be included in
more realistic studies. Karato (2012) proposed
that the asthenosphere is made of the residual of
partial melting at 410 km and homogeneous and
modestly depleted composition is due to partial
melting rather than mixing.
4.6.3 Transition zone
(a) Is the 410 km and/or 660 km discontinuity a
rheological barrier for mantle convection?
Some
geodynamic observations strongly suggest that
mantle viscosity increases substantially at around
the transition zone. Viscosity below that depth is
higher than that above (the depth at which vis-
cosity increases is not well constrained). Seismo-
logical observations show the presence of seismic
anisotropy in the transition zone (Trampert &
van Heijst, 2002; Visser
et al
., 2008) suggesting
that dislocation creep dominates in this region.
However, currently available mineral physics ob-
servations do not provide robust explanation for
this rheological layering. This is due mostly to
the very limited quantitative experimental data
on minerals in these regions.
However, some new experimental observations
are becoming available. According to Nishihara
et al
. (2008) and Kawazoe
et al
. (2010), the creep
strength of wadsleyite in the power-law creep
regime is comparable to that of olivine compared
at the similar pressure and temperature (andwater
content). Water content in the transition zone is
generally higher than that of the upper mantle and
varies significantly from one region to another
(Karato, 2011b). It is likely that the 410 km is
not a strong rheological barrier for convection but
the viscosity contrast at the 410 km discontinuity
will depend on the water content.
Not much can be said about the rheological con-
trast at the 660 km discontinuity. Firstly, there is
5-10), and in case of dislocation creep, the en-
hancement of deformation will be much less. The
presence of strong seismic anisotropy suggests
that dislocation creep dominates in the astheno-
sphere, and therefore partial melting unlikely has
a large direct effect on rheological properties.
Rather, the main role of partial melting is its
indirect effect through the redistribution of water
(Karato, 1986).
The asthenosphere is also characterized by the
homogeneous and modestly depleted chemical
composition. In a model of chemical evolution
proposed by Hofmann (1988), he assumed that
highly depleted components and undepleted
components have been well mixed to explain
the homogeneous and modestly depleted nature
of the asthenosphere. However, mixing is dif-
ficult if two components have largely different
viscosity (Manga, 1996). Depleted and unde-
pleted components have largely different water
contents and hence different viscosity in the
