Geology Reference
In-Depth Information
produced in the atmosphere by cosmic ray spallation of N
and O nuclei, from where it is washed out by rain into
the oceans and becomes incorporated into sediment. Its
detection in young island-arc volcanic rocks is important
evidence for the subduction of significant amounts of
oceanic sediment down subduction zones, since its short
half-life rules out any other source within the Earth.
• Recent work suggests that the isotopic compositions
of
transition metals
like Fe, Cu, Zn (Figure 10.15) and
Mo have much to tell us about the history of the
Earth's oceans.
•
Cosmogenic radioisotope
systems include
14
C, which
provides the foundation for radiocarbon dating, and
10
Be, which allows estimation of the role of sediment
subduction in island arc magma genesis.
Review
Further reading
Although some elements are mono-isotopic in nature
(see exercise 11.1), most consist of two or more isotopes
(Box 10.1). High-precision mass spectrometry
(Box 10.3) measurements of isotope abundance ratios
of selected elements provide opportunities to quantify
a host of geological processes:
Allegre, C. (2008)
Isotope Geology
. Cambridge: Cambridge
University Press.
Dickin, A.P. (2005)
Radiogenic Isotope Geology
, 2nd edition.
Cambridge: Cambridge University Press.
Faure, G. and Mensing, T.M. (2005)
Isotopes -Principles and
Applications
, 3rd edition. New York: John Wiley and Sons,
Ltd.
Lenton, T. and Watson, A. (2011)
Revolutions that Made the
Earth
. Oxford: Oxford University Press.
White, W.M. (2013)
Geochemistry
. Chichester: Wiley-
Blackwell.
•
Radiogenic isotope
systems like K-Ar (including
40
Ar/
39
Ar - Box 10.2), Rb-Sr, Sm-Nd and U-Th-Pb give
us tools for determining the
ages
of rocks, rock assoc-
iations and meteorites (Box 10.2, Figures 10.5 and 10.7).
• In addition to geochronology, radiogenic isotope
systems - illustrated in the text by Rb-Sr and Sm-
Nd, although Lu-Hf, Re-Os and U-Th-Pb are
important too (Table 10.1) - shed light on the
origins
of igneous magmas (Figure 10.8), on
mantle hetero-
geneity
(Figure 10.8)
,
on
crustal evolution
, and on
sed-
iment provenance
.
• Measurements of δ
18
O (Equation 10.8) in rocks and
minerals allow us to estimate the
temperature of crys-
tallization
of relevant minerals, such as marine car-
bonates (Figure 10.11). δ
18
O and δD together provide
important quantitative information on
past climates
(Figure 10.12). δ
18
O and δD measurements also pro-
vide insights into the terrestrial
water cycle
(Figure 10.10).
• Other
stable isotope
systems provide avenues for
quantifying other geological and Earth-surface
phenomena (Table 10.3). δ
13
C and δ
34
S data docu-
ment the dramatic changes in the composition of
the Earth's atmosphere through geological time
(Figures 10.13 and 10.14), notably the 'Great
Oxidation Event' marking the first appearance of a
significant level of oxygen in the global atmosphere
at the close of the Archaean, and its rise to higher
concentrations with the appearance of land plants in
the Neoproterozoic (see Figure 11.8).
Exercises
10.1
(a) By examining Figure 10.1.1 and the Periodic
Table, list the elements that are monoisotopic
(i.e. have only one stable isotope) in Nature.
(b) Which chemical elements have no stable
isotopes?
10.2
A Mesozoic limestone from an oceanic drill
core is found to have an
87
Sr/
86
Sr ratio of
0.707980 ± 0.000022. Determine the age of the
sediment, and estimate the precision of the age
determination.
10.3
The table below gives Sm-Nd isotopic data for the
separated minerals and whole-rock specimen for a
gabbro from the Stillwater Intrusion in Montana.
Plot a mineral isochron for these samples and, using
the constants given in Table 10.1, determine the age
and initial Nd isotope ratio of the intrusion.
Sample
147
Sm/
144
Nd
143
Nd/
144
Nd
STL-100 whole rock
0.20034
0.511814
STL-100 plagioclase
0.09627
0.509965
STL-100 orthopyroxene
0.28428
0.513317
STL-100 clinopyroxene
0.24589
0.512628
Data from DePaolo and Wasserburg (1979).
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