SOME GEOLOGICALLY IMPORTANT ELEMENTS - Chemical Fundamentals of Geology and Environmental Geoscience - page 176

causes HREEs to be retained in the residual garnet-
bearing solid, leaving the melt formed relatively
depleted in HREEs relative to LREEs. The rare earth
pattern of such a basalt would show a steep negative
slope.
The lanthanides have acquired great technological
importance in recent decades, for example in lasers
and in the high-field permanent magnets used in
wind-turbine generators and electric vehicles. Global
demand for REEs is therefore rising faster than supply,
and REEs are at the top of the world's 'supply risk list'
of technologically vital elements whose future avail-
ability is uncertain. Since the 1990s China has become
the world's main supplier of REEs.
The trace element yttrium (Y) also forms a 3+ ion.
The ionic radius of Y 3+ is the same as Ho 3+ , and in geo-
logical materials yttrium is always closely associated
with the HREEs.

100

50

20

10

5

La Ce Pr Nd Eu Gd

Rare earth element (in order of atomic number)

Sm Tb Ho Er Tm Yb Lu

Figure 9.10 Chondrite-normalized rare earth element
diagram for two lunar basalts. Note that the vertical
(enrichment) scale is logarithmic. REE patterns are prepared
by plotting the ratio for each element versus element
position in the REE series, then joining up the array of
points with straight lines. (Source: Data from Taylor (1982)). Actinides

source region differed little from chondrite, at least in
terms of REE abundances. The upper pattern is also
flat in overall terms but differs in two important ways:
REE concentrations are generally about ten times
higher, but there is a pronounced deficiency in the abun-
dance of the element europium (Eu), a feature known
as a 'negative Eu anomaly'. This basalt evidently rep-
resents magma that has undergone partial crystalliz-
ation. Most REEs are incompatible in basalt minerals,
and crystallization of olivine, clinopyroxene, plagio-
clase, and so on, must therefore concentrate REE in the
decreasing amount of remaining melt. The exception is
europium which, unique among the REEs, has a 2+
oxidation state (Figure 9.9). Eu 2+ has a radius similar to
Ca 2+ (Box 9.1), and can substitute for Ca in plagioclase
(but not in clinopyroxene, whose more compact struc-
ture excludes it). When plagioclase is crystallizing
under relatively low f O 2 (reducing conditions), there-
fore, Eu behaves as a compatible element and becomes-
depleted in the melt as plagioclase crystallization
removes it. Thus a negative Eu anomaly is a valuable
geochemical tracer of plagioclase crystallization dur-
ing magma fractionation.
REEs are useful for detecting the influence of other
minerals on igneous processes. For example, the pres-
ence of garnet in a basalt source region during melting

The second row of the f-block consists solely of radio-
active elements, known collectively as the actinides
after the element actinium (Ac) that precedes them in
the Periodic Table (cf. the lanthanides). Thorium (Th)
and uranium (U) have long-lived isotopes 232 Th, 235 U
and 238 U (see Table 10.1), so these elements occur as
trace elements in nature. All three long-lived isotopes
are α-emitters, whose decay generates chains of
shorter-lived α- and β-emitting radionuclides that lead
eventually to different stable isotopes of lead (see
Figure 3.3.1 and Table 10.1). For the Earth scientist,
there are two important consequences. Firstly, collision
of these multiple α-particles with surrounding nuclei
captures their kinetic energy within the host rock 7 and
raises its temperature. Decay of Th and U (and to a
lesser extent 40 K) within the Earth thus makes an
important contribution to the Earth's heat budget and
the heat flow we observe at the surface. Secondly, as the
decay constants for these three naturally occurring
radioisotopes are accurately known (Table 10.1), they
provide an important means of dating the formation of

The high charge (2+) of α-particles causes them to interact
strongly with surrounding atoms, limiting the distance to
which they can penetrate matter: a sheet of paper is sufficient
to shield against α-radiation.

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