Geoscience Reference
In-Depth Information
Chapter 25
Crystallization of the mantle
Rocks, like everything else, are subject
to change and so also are our views
on them.
Melt trapping and melt transport can create
distinctive chemical and isotopic components.
Chemical heterogeneity can be formed internal
to the mantle.
Franz Y. Loewinson-Lessing
Components vs. reservoirs revisited
The Earth is cooling and crystallizing. The man-
tle has evolved considerably from the magma
ocean era to the plate tectonic era. Part of the
evolution is due to igneous processes, and part
is due to plate tectonic processes. The visible
rocks are the end, or present, product of man-
tle evolution. If these were our only source of
information, we could come up with a fairly
simple scheme of magma genesis, perhaps involv-
ing single-stage melting of a homogenous, even
primitive, mantle. We could design simple one-
and two-layer mantle models, such as were pop-
ular in the last century. It now seems unlikely
that we will find the 'Rosetta Stone,' a rock or
a meteorite fragment that represents 'original
Earth' or even the parent or grandparent of other
rocks. Rocks and magmas represent products of
complex multistage processes, and they are mix-
tures of components with various melting his-
tories. As we delve deeper into the Earth and
further back in time, we depend more and more
on isotopes and on modeling of planetary accre-
tion and mantle processes. Melts and rocks are
averages of various components and processes.
Sampling theory and the manipulation of aver-
ages are involved in this modeling. In addition
to recycling, and an intrinsically heterogenous
mantle, there are igneous processes that cause
chemical heterogeneity interior to the mantle.
A variety of studies have lent support to the con-
cept of a
chemically inhomogenous man-
tle
. The mantle contains a variety of
components
that differ in major elements, intrinsic density,
melting point, large-ion-lithophile (LIL)-contents
and isotope ratios; they have maintained their
separate identities for at least 10
9
years. These
components
have been termed
reservoirs
but only
very loose bounds can be placed on the sizes of
these so-called
reservoirs
. They could be grain-size,
or slab-size, depending on the ability of chem-
ical species to migrate from one to another. A
component
can have dimensions of tens of kilo-
meters, a typical scale for subducted or delam-
inated assemblages. The melting process gath-
ers together these components, and partial melts
therefrom. Some components or reservoirs have
high values of Rb/Sr, Nd/Sm, U/Pb and
3
He/U.
These components are
enriched
, but for historic
reasons they are sometimes called
primitive
,
unde-
gassed
or
more-primitive
(than MORB).
Lithology of the upper mantle
Since the seismic velocities and anisotropy of the
shallow mantle are consistent with an olivine-
rich aggregate, and since most mantle xenoliths
are olivine-rich, it has been natural to assume
