Seismic Observations of Mantle Discontinuities and Their Mineralogical and Dynamical Interpretation - Physics and Chemistry of the Deep Earth - page 304

(wadsleyite) (Akaogi et al ., 1989; Katsura & Ito,
1989). This phase transition occurs over a narrow
depth interval of 5 km in a dry mantle (Frost,
2003). There are no other phase transitions in
pyroxene or garnet appearing at similar pressures,
which agrees with the seismic observations of
a simple peak (Figure 10.6). Receiver functions
are suitable to study discontinuities at a range
of frequencies and can be used to investigate the
sharpness of the 410 km discontinuity in seismol-
ogy. By filtering receiver function data at a range
of periods, it was found that the '410' and '660'
amplitudes are similar at long period, but that
the 410 amplitudes become very weak at short
period, leaving the 660 amplitudes unchanged
(van der Meijde et al ., 2003). These results
can be interpreted by the 410 km discontinuity

being spread over a wider transition interval
than the 660 km discontinuity. This agrees with
reflections from the '410' always being visible in
long period PP precursors, but only occasionally
in short P P precursors.
The precursor and receiver function observa-
tions of the '410' indicate that the phase transi-
tion around 410 km depth is not sharp and the
two-phase α + β olivine region occurs over depth
intervals of about 20 to 25 km in certain regions.
This is much broader than the predicted 5 km
thickness of the '410' in a drymantle (Frost, 2003).
Water or melt could be a major factor influencing
the width of the phase transition, widening it by
up to 35 km (Wood, 1995; Frost & Dolejs, 2007;
Karato, 2011). The broadening of the 410-km dis-
continuity has mainly been interpreted as being

Depth (km)

400 450 500 550 600 650 700

2200

2000

1800

Ol Wd + Mj

Mg − Pv +

1600

Rw + Mj +

Mw

Ca − Pv

Rw +

1400

Mj + Il

1200

Rw +
Mj

1000

12 14 16 18 20 22 24 26

Pressure(GPa)

Fig. 10.6 Schematic cartoon summarizing the phase transitions occurring in the mantle transition zone as a
function of pressure and temperature, using data from Hirose (2002); Frost (2003); Frost and Dolejs (2007); Saikia
et al . (2008). Thick solid denote the olivine phase transitions, thick dashed lines are the garnet phase transitions.
Grey areas denote the main transitions responsible for the 410, 520 and 660 km discontinuities. Thin solid lines
denote additional phase transitions in ilmenite. The olivine to wadsleite phase transition around 410 km depth
broadens as a function of water content, but only for low temperatures. Abbreviations are: Ol olivine, Wd
wadsleyite, Mj majorite garnet, Rw ringwoodite, Il ilmenite, Ca-Pv Calcium perovskite, Mg-Pv Magnesium
perovskite, Mw magnesiow ustite.