Geoscience Reference
In-Depth Information
12
Mantle Mixing: Processes
and Modeling
PETER E. VAN KEKEN
Department of Earth and Environmental Sciences,
University of Michigan, Ann Arbor, MI, USA
Summary
tectonics and mantle convection cause relatively
efficient stirring of heterogeneity, melting at
plate boundaries also continuously introduce
new chemical heterogeneity. Interdisciplinary
studies of the Earth's mantle are necessary to im-
prove our understanding the relative importance
of the many processes that generate, maintain
and sample this heterogeneity. The interplay
between mantle convection and geochemical
differentiation has become an important field of
study under the generic umbrella of ''chemical
geodynamics.'' Various reviews in the last few
decades are still relevant in discussing how the
Earth achieved and retained its heterogeneity
(All egre, 1982; Zindler & Hart, 1986; van Keken
et al
., 2002; Hofmann, 2003; Helffrich, 2006). In
this chapter I will provide an update to these
reviews with a focus specifically on relevant
geophysical and geochemical research in the
last decade. This review will be followed by a
discussion about how geodynamical modeling
can be used effectively to test critical hypotheses.
As an illustration of the use of modeling to
understand the generation and preservation of
heterogeneity I will use some of the PhD work by
JP Brandenburg with the most significant chapter
published as Brandenburg
et al
. (2008). In this
work we used a simple but thermodynamically
consistent approximation to plate tectonics and
The Earth's mantle is heterogeneous at multiple
scales as seen by geochemical and geophysical
studies. The introduction of heterogeneity is
caused by early Earth differentiation events
and ongoing differentiation due to melting at
the surface and possible deeper in the interior.
Mantle convection partly erases the introduced
heterogeneity, but the strongly variable nature
of the Earth's mantle allows for the preservation
of isotopic and structural heterogeneity over
billions of years. As the heterogeneity is sam-
pled by mid-oceanic ridges and mantle plumes
the melting events can additionally filter the
heterogeneity. Dynamical models can be used to
investigate the mixing efficiency in models with
realistic Earthlike convective vigor and to test
fundamental hypotheses on how heterogeneity in
the Earth's mantle is introduced and preserved.
12.1
Introduction
Geophysical and geochemical studies of the
Earth's mantle show a complex heterogeneity at
multiple scales. This heterogeneity is due to a
complex interplay between processes that include
early Earth differentiation and ongoing intro-
duction of chemical differentiation. While plate
