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caused by water in mantle minerals at this depth.
These minerals can contain a substantial amount
of water: as much as 1000 ppm by weight of water
in olivine and about 20 000 ppm by weight of
water in wadsleyite. The estimated water con-
tent in olivine for a transition interval of 20 to
25 km is around 500 ppm by weight (Wood, 1995).
Frost and Dolejs (2007) show that the effect of
water on widening the transition interval is only
strong at low temperatures (see Figure 10.6). Large
amounts of water and low temperatures would be
expected in regions of subduction, so it is not
unlikely that there will be significant amounts of
water in the transition in some places. However,
the very small amplitude of the '410' in P
P
pre-
cursors is also seen away from subduction zones
(Xu
et al
., 2003), suggesting that its wide transi-
tion interval may not be limited to subduction
zones only and could be more widespread.
Meier
et al
. (2009) used a novel technique of
inverting surface wave measurements for topog-
raphy on the 410 km discontinuity and found very
poor agreement between their results and the
results from SS precursors. They also interpret
the discrepancy as being caused by the existence
of water in the transition zone, but surprisingly
they find that the largest amounts of water exist
away from subduction zones, in disagreement
with Frost and Dolejs (2007) who show that the
effect of water can only be observed in regions
with low temperatures. This may be related to the
water filter model of Bercovici and Karato (2003)
and may be tested by investigating the varying
sharpness or width of the 410 km discontinuity
in different regions.
The existence of melt has been invoked in seis-
mic studies which observe a low velocity zone
above the 410 km discontinuity (Revenaugh &
Sipkin, 1994; Schaeffer & Bostock, 2010). The
suggestion by Bercovici and Karato (2003) that
the 410 km discontinuity acts as a water filter
might lead to widespread melt appearing above
410 km depth. Song
et al
. (2004) found evidence
for a low velocity zone using S wave tripli-
cations. More recently, receiver functions have
also been used. Sdp receiver functions showed
low velocity zones mainly in regions of ancient
mantle upwelling (Vinnik & Farra, 2007), while
Pds receiver functions found wide spread occur-
rence of a low velocity zone in agreement with
the water filter hypothesis (Tauzin
et al
., 2010).
SS precursors have also shown hints of multiple
reflectors around 410 km depth, which might be
related to partial melt (Schmerr &Garnero, 2007).
Even though the 410 km discontinuity is easy to
see in most data types, there are still significant
contradictions, especially when looking at the
sharpness of the discontinuity and its interpreta-
tion in terms of water content of the transition
zone (Karato, 2011).
10.3.2 520 km discontinuity
In addition to the olivine phase transitions which
explain the 410- and 660-km discontinuities, a
third discontinuity was predicted in olivine phase
transitions from the
β
(wadsleyite) to the
γ
phase
(ringwoodite), well before a discontinuity was
confirmed at this depth in seismology (Rigden
et al
., 1991). Regional observations of reflections
from the '520' had been seen occasionally in array
studies (Helmberger & Engen, 1974; Fukao, 1977;
Hales
et al
., 1980), but it was only confirmed as
occurring in many different regions by Shearer
(1996). The 520 km discontinuity is most consis-
tently seen using SS precursors (see, for example,
Figure 10.5d), and because in this data type it is
more often seen than not, it is usually called a
global discontinuity.
Gu
et al
. (1998) made stacks for different tec-
tonic regions and found that the 520 km discon-
tinuity does not exist in shield areas, and is only
strong in oceanic areas. However, a few years later
it was demonstrated that in a number of regions
two discontinuities at 500 and 560 km depth can
be found, an effect which can be interpreted as
'splitting' of the 520 km discontinuity (Deuss &
Woodhouse, 2001). As shield regions contain a
mixture of single and double reflections, averag-
ing over all shield regions would lead to a zero
observation. An example of splitting is shown
in Figure 10.5c, where double peaks are seen in
the SS precursors and the Pds receiver functions
around 500 and 560 km depth. The PP precursors
