Geoscience Reference
In-Depth Information
There is no clear understanding what is causing
these reflections. Only a few phase transitions
have been discovered in minerals at such high
pressure and temperature, but none of these min-
erals exists in large amounts in the lower man-
tle. One possibility would be a phase transition
in quartz from stishovite to CaCl
2
type, which
appears around a depth of 1700 km for an typical
mantle geotherm with a large Clapeyron slope al-
lowing depth variations of up to 200 km (Nomura
et al
., 2010). It is possible that significant amounts
of quartz may be transported into the lower man-
tle by subduction if the subducted oceanic crust is
silica-rich, so this would explain the reflections
at 1500-1800 km depth in subduction zone ar-
eas. The shallower discontinuities may be caused
by the phase transition from tetragonal to cubic
structure in Ca perovskite (Komabayashi
et al
.,
2007). However, we find seismological scatterers
at almost any depth in many different regions.
These observations may suggest that we need a
significant amount of chemical heterogeneity in
the lower mantle and that the mantle is not as
well mixed as is generally thought.
depth which may also be related to subduction
zones (Simmons & Gurrola, 2000; Andrews and
Deuss, 2008). These multiple discontinuities can
be explained as phase transitions form garnet to
ilmenite, which mainly appear at low tempera-
ture (Figure 10.6). Alternatively, these reflectors
may be related to subducted MORB and harzbur-
gite creating compositional heterogeneity in the
mantle transition zone.
Lower mantle discontinuities have predom-
inantly been found in regions with subducted
slabs, ranging from 800 up to 1800 km depth.
Interestingly, both SdP receiver functions (Vin-
nik
et al
., 2001) and SS precursors (Deuss &
Woodhouse, 2002) find reflections from 1150
and 1250 km depth in the Indonesian subduction
zone. These data types have completely different
source-receiver geometry, so it is encouraging
that the same lower mantle reflectors can be seen.
Whitcomb and Anderson (1970) and LeStunff
et al
. (1995) observed lower mantle reflectors
under Africa away from any subduction zones
using P
P
precursors. However, these observa-
tions have been questioned (Xu
et al
., 2003)
as potential misidentifications of other mantle
phases, which is a particular problem of using P
P
precursors. As these deep reflectors are mainly
seen in subduction zones, it seems likely that
they may be related to the subduction of slabs
into the lower mantle causing compositional
heterogeneity at greater depths.
10.6 Geodynamical Interpretation
10.6.1 Subduction zones
Subduction zones have been studied extensively
using both regional and global studies. Detailed
studies have been performed using a combina-
tion of reflected and refracted waves from deep
earthquakes (see Figure 10.3c). Generally it is
found that the 410 km discontinuity is shallower
and the 660 km discontinuity is deeper in agree-
ment with olivine phase transition Clapeyron
slopes in low temperature slabs. A thick transi-
tion zone has indeed be seen in subduction zone
regions using both SS precursors and Pds receiver
functions (Figure 10.7). The phase diagram for a
pyrolite mantle composition (Figure 10.6) shows
that at lower temperatures, the influence of gar-
net is small and that 660 km depth is dominated
by the post spinel transition in olivine which has
a negative Clapeyron slope. In some regions, mul-
tiple peaks have been observed around 660 km
10.6.2 Mantle plumes
It has been much more difficult to image the
transition zone discontinuities in mantle plume
regions. As most mantle plumes are in ocean
regions, where there are only limited seismic sta-
tions, it is difficult to find data that covers mantle
plume locations. The most successful attempts at
imaging mantle plumes come from Pds receiver
function studies for stations on ocean islands,
some of which happen to be mantle plume lo-
cations such as Hawaii (Li
et al
., 2000; Tauzin
et al
., 2008). Alternatively, SS precursors have
been used as their sensitivity is in the mid-point
between the source and receiver, leading to good
