Geoscience Reference
In-Depth Information
The crust is extremely enriched in many of
the so-called incompatible elements, particularly
the
large ionic-radius lithophile
(LIL) or
high-field
strength
(HFS) elements that do not readily fit into
the lattices of the major mantle minerals, olivine
(ol) and orthopyroxene (opx). These are also called
the
crustal elements
, and they distinguish
enriched
magmas
from
depleted magmas
. The crust is not
particularly enriched in elements of moderate
charge having ionic radii between the radii of Ca
and Al ions. This suggests that the mantle has
retained elements that can be accommodated in
the garnet (gt) and clinopyroxene (cpx) structures.
In other words, some of the so-called LIL elements
are actually compatible in gt and cpx. The crust is
also not excessively enriched in lithium, sodium,
lead, bismuth and helium.
Table 17.1
Radioactive nuclides and their
decay products
Radioactive
Decay
Half-life
Parent
Product
(billion years)
238
U
206
Pb
4.468
232
Th
208
Pb
14.01
176
Lu
176
Hf
35.7
147
Sm
143
Nd
106.0
87
Rb
87
Sr
48.8
235
U
207
Pb
0.7038
40
K
40
Ar,
40
Ca
1.250
129
I
129
Xe
0.016
26
AI
26
Mg
8.8
×
10
−
4
refractory, have similar geochemical characteris-
tics and are probably in the Earth in chondritic
ratios, or at least, in their original ratios. The
neodymium isotopes can therefore be used to
infer ages of mantle components or reservoirs
and to discuss whether these are enriched or
depleted, in terms of Nd/Sm, relative to chon-
dritic or undifferentiated material. The Rb/Sr and
Nd/Sm ratios are changed when melt is removed
or added or if sediment, crust or seawater is
added. With time, the isotope ratios of such com-
ponents diverge.
The isotope ratios of the crust and differ-
ent magmas show that mantle differentiation
is ancient and that remixing and homogeniza-
tion is secondary in importance to separation
and isolation, at least until the magma cham-
ber and eruption stages. Magma mixing is an
efficient way to obtain uniform isotopic ratios,
such as occur in MORB. Although isotopes cannot
tell us where the components are, or their bulk
chemistry, their long-term isolation and lack of
homogenization plus the temporal and spatial
proximity of their products suggests that, on
average, they evolved at different depths or in
large blobs that differ in lithology. This suggests
that the different components differ in intrinsic
density and melting point and therefore in bulk
chemistry and mineralogy. Melts, and partially
molten blobs, however, can be buoyant relative
to the shallow mantle even if the parent blob is
dense or neutrally buoyant.
Isotopes as fingerprints
Box models
Radiogenic isotopes are useful for understanding
the chemical evolution of planetary bodies. They
can also be used to fingerprint different sources
of magma. In addition, they can constrain timing
of events. Isotopes are less useful in constrain-
ing the locations or depths of mantle compo-
nents or reservoirs. Just about every radiogenic,
nucleogenic or cosmogenic isotope has been used
at one time or another to argue for a deep mantle
or lower mantle source, or even a core source, for
ocean island and continental flood basalts and
carbonatites, but isotopes cannot be used in this
way. Isotope ratios have also been used to argue
that some basalts are derived from unfraction-
ated or undegassed reservoirs, and that reservoir
boundaries coincide with seismological bound-
aries (implying that major elements and physical
properties correlate with isotopes).
Some mantle rocks and magmas have high
concentrations of incompatible elements and
have isotope ratios that reflect long-term enrich-
ment of an appropriate incompatible-element
parent. The crust may somehow be involved
in the evolution of these magmas, either by
crustal contamination prior to or during erup-
tion, by recycling of continent-derived sediments
or
by
delamination
of
the
lower
continental
