Geoscience Reference
In-Depth Information
In ma c and ultrama c rocks, where the concentrations
of U and Th are extremely low, these elements tend to
occur as trace components in common rock-forming min-
erals. In more felsic rocks, accessory minerals such as
zircon are more common, which further contributes to
the increased abundance of radioelements. High-silica
rocks formed in the final stages of igneous differentiation,
such as pegmatites, have high U and Th contents com-
pared with K, owing to the concentration of incompatible
elements in the late-stage melt. Note that data from felsic
and ultrafelsic rocks are displaced from the linear trends
defined by less silicic rock types. Leucocratic peraluminous
alkalic granitoids may contain 5
Decay
series
Potential causes
of disequilibrium
Half-
lives
238 U
4.468 x 10 9 yr
234 Th
24.1 days
1.18 min
234 Pa
Selective leaching
relative to 238 U
234 U
2.48 x 10 5 yr
Selective leaching
relative to other isotopes
in the decay series
Soluble -
mobilised in groundwater
230 Th
7.52 x 10 4 yr
1602 yr
226 Ra
20 ppm U, enough to
constitute an economic resource if the intrusion is suffi-
ciently large.
Carbonatite and other highly alkaline rocks are anomal-
ous in terms of their radioelement content, being enriched
in both U and Th. Some examples have very high Th/U
ratios, but this is not always the case. Kimberlites are also
enriched in U and Th compared with most other igneous
rock types.
Variations in the ratio Th/U have been variously sug-
gested as a means of discriminating a range of petrological
and geological parameters such as: igneous suites; degree of
differentiation; tectonic provinces; and the conditions
under which the rocks formed. However, many factors
control U and Th content, and this kind of analysis should
probably be restricted to a local scale. Furthermore, much
of the work on which it is based relies on laboratory
geochemical analyses. The greater uncertainties associated
with estimating radioelement concentrations from field
measurements of
-
Gaseous -
highly mobile
3.825 days
222 Rn
3.05 min
218 Po
26.8 min, 2 s
214 Pb
218 At
214 Bi
19.7 min
214 Po
210 Ti
164 μ s, 1.32 min
210 Pb
~22 yr
210 Bi
5.02 days
210
206
Po
Ti
138.3 days, 4.19 min
206
Pb
Stable
Figure 4.15 The 238 U decay series showing half-lives and potential
mechanisms for disequilibrium within the series.
radioactivity will
also hinder
its
application.
likelihood of equilibrium. In general, equilibrium is only a
safe assumption in unaltered rocks older than about 1.5
million years.
4.6.3 Potassium, uranium and thorium in altered
and metamorphosed rocks
The effect of metamorphism on the distribution of K,
U and Th is somewhat uncertain. The generally higher
levels of K, U and Th in upper crustal rocks, such as
granites and granodiorites, compared with lower crustal
rocks, such as granulites, may indicate that metamorphic
processes transport these elements upwards, either as solu-
tions or melts (Haack, 1983 ) . There is evidence for deple-
tion in U, and to a lesser extent Th, in granulite-facies
rocks relative to lower-grade equivalents. This is due to
fluid loss and increased mobility of these elements at high
temperature and pressure, enhanced by U often being
4.6.2 Potassium, uranium and thorium in
igneous rocks
There is generally a well-developed sympathetic variation
in the three radioelements in igneous rocks, especially
between U and Th. Silica content has a signi cant in u-
ence on the overall abundances of K, U and Th ( Fig. 4.12a ).
The radioelement content of ultrama c and ma c rocks is
very low, but increases in intermediate and, more so, in
felsic rocks. The increase is due to the greater abundance of
feldspars and micas.
 
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