Geoscience Reference
In-Depth Information
10
Seismic Observations of Mantle
Discontinuities and Their Mineralogical
and Dynamical Interpretation
ARWEN DEUSS, JENNIFER ANDREWS,
AND ELIZABETH DAY
Bullard Labs, University of Cambridge, Madingley Road, Cambridge, United Kingdom
Summary
discontinuity. Additional discontinuities are seen
in the upper and lower mantle. The Lehmann dis-
continuity at 220 km depth appears to be related
to a change in deformation mechanism from dis-
location to diffusion creep. The X-discontinuity
at 330 km depth and lower mantle reflectors at
depths of 800, 1200 and 1800 km have no definite
mineralogical explanation yet, but some of the
deeper lower mantle reflectors seem to be related
to subduction zone areas.
A range of seismic techniques has been used
successfully to observe the transition zone dis-
continuities at 410, 520 and 660 km depth. In
general, the 410 km discontinuity is seen in most
long period data types, but becomes more diffi-
cult to observe in short period data. This might
be due to the influence of water which broadens
the olivine to wadsleyite phase transition. Melt-
ing above 410 km depth has also been invoked to
explain a low velocity layer which is sometimes
seen above this discontinuity. One outstanding
issue is that the influence of water is expected
to be strong enough only in cold regions (i.e.
subduction zones), but the 410 km discontinu-
ity is also invisible in short period data in other
regions. The 520 and 660 km discontinuity show
much more complexity, which is due to com-
bined garnet and olivine phase transitions leading
to seismic observations of multiple discontinu-
ities. The garnet phase transitions also change
the traditional expectation of a thin transition
zone in hot regions and a thick transition in cold
regions. It is currently unclear why the 660 km
discontinuity is seen in short period P
P
precur-
sors, suggesting it is sharp, while it disappears for
long period PP precursors, suggesting it is a broad
10.1 Introduction
Global seismology has made significant progress
in mapping the deep interior of the Earth, eluci-
dating the internal structure and dynamics of our
planet. Such studies paint a picture of the Earth's
mantle with high velocity regions, interpreted as
subducted slabs, and low velocity regions, some
of which may be interpreted as rising plumes.
Two complementary approaches have been
used to make these pictures. Combining long
period seismic normal modes and surface waves
with shorter period body wave observations has
resulted in tomographic models showing the
existence of smoothly varying large scale hetero-
geneity (e.g. Ritsema
et al.
, 2011, and chapter 11,
this volume, below). In addition to smooth
structure, the Earth's mantle is delineated by
sharp changes in seismic properties. These sharp
Corresponding author, afd28@cam.ac.uk
