Radiogenic Isotopes in Weathering and Hydrology – after Blum and Erel 20031
There are a small group of elements that display variations in their isotopic
composition resulting from radioactive decay within minerals over geological timescales.
These isotopic variations provide natural fingerprints of rock-water interactions and have
been widely utilized in studies of weathering and hydrology. The isotopic systems that have
been applied in such studies are dictated by the limited number of radioactive parent nuclides
with half-lives and isotopic abundances resulting in measurable differences in daughter
isotope ratios among common rocks and minerals. Prior to their application to studies of
weathering and hydrology each of these isotopic systems was utilized in geochronology and
petrology. As in the case of their original introduction into geochronology and petrology,
those isotopic systems with the highest concentrations of daughter isotopes in common rocks
and minerals and those systems with the largest observed isotopic variations were introduced
first and have made the largest impact on our understanding of weathering and hydrologic
processes. Although radiogenic isotopes have helped elucidate many important aspects of
weathering and hydrology, it is important to note that in almost every case that will be
discussed in this chapter, our fundamental understanding of these topics came from studies of
variations in the concentrations of major cations and anions.
The first applications of radiogenic isotopes to weathering processes were based on
studies that sought to understand the effects of chemical weathering on the geochronology of
whole rock samples and geochronologically important minerals (Goldich and Gast, 1966;
Dasch, 1969; Blaxland, 1974; Clauer, 1979, 1981; Clauer et al, 1982); as well as the
observation that radiogenic isotopes are sometimes preferentially released compared to
nonradiogenic isotopes of the same element during acid leaching of rocks (Hart and Tilton,
1966; Silver et al. 1984; Erel et al., 1991). A major finding of these investigations was that
weathering often results in anomalously young Rb-Sr isochron ages, and discordant Pb-Pb
ages. Rubidium is generally retained relative to Sr in whole rock samples, and in some cases
radiogenic Sr and Pb are lost preferentially to common Sr and Pb from weathered minerals.
The most widely utilized of these isotopic systems is Rb-Sr, followed by U-Pb. The
K-Ar system is not directly applicable to most studies of rock-water interaction because Ar is
a noble gas, and upon release during mineral weathering mixes with atmospheric Ar, limiting
its usefulness as a tracer in most weathering applications. Ar and other noble gas isotopes
have, however, found important applications in hydrology. Three other isotopic systems
commonly used in geochronology and petrology include Sm-Nd, Lu-Hf and Re-Os. These
parent and daughter elements are in very low abundance and concentrated in trace mineral
1
Blum J. D. and Erel Y. (2003) Radiogenic isotopes in weathering and hydrology. In Surface and
Ground Water, Weathering, Erosion and Soils, (ed. J. I. Drever). Elsevier Science. Vol. 5 in Treatise
on Geochemistry, Editors K. K. Turekian and H. D. Holland.
phases. Sm-Nd, Lu-Hf and Re-Os have been used in a few weathering studies but have not
yet been utilized extensively in investigations of weathering and hydrology.
The decay of 87Rb to 87Sr has a halflife of 48.8 byr and this radioactive decay results
in natural variability in the 87Sr/86Sr ratio in Rb-bearing minerals (e.g., Blum, 1995). The trace
elements Rb and Sr are geochemically similar to the major elements K and Ca, respectively.
Therefore, minerals with high K/Ca ratios develop high 87Sr/86Sr ratios over geologic
timescales. Once released into the hydrosphere, Sr retains its isotopic composition without
significant fractionation by geochemical or biological processes, and is therefore a good tracer
for sources and cycling of Ca. The decay of 235U to 207Pb, 238U to 206Pb and 232Th to 208Pb
have halflives of 0.704, 4.47, and 14.0 byr, respectively, and result in variations in the 207Pb/
204
Pb, 206Pb/ 204Pb, and 208Pb/ 204Pb ratios (e.g., Blum, 1995). 234U has a halflife of 0.25 myr
and the ratio 234U/238U approaches a constant secular equilibrium value in rocks and minerals
if undisturbed for ~1 myr. Differences in this ratio are often observed in solutions following
rock-water interaction and have been used in studies of weathering and hydrology. U and Th
tend to be highly concentrated in the trace accessory minerals such as zircon, monazite,
apatite and sphene, which therefore develop high 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb
ratios. Once released into the hydrosphere, Pb retains its isotopic composition without
significant geochemical or biological fractionation and tends to generally follow the
chemistry of Fe in soils and hydrologic systems (Erel and Morgan 1992). The use of the U-Th
disequilibrium series as a dating tool falls outside the scope of this summary. The decay of
147
Sm to 143Nd, 176Lu to 176Hf and 187Re to 187Os have halflives of 106, 35.7 and 42.3 byr,
respectively, and result in natural variability in the 144Nd/143Nd, 176Hf/177Hf and 187Os/188Os
ratios (e.g., Blum, 1995). Nd is a rare earth element, Hf is a transition metal with chemical
similarities to Zr, and Os is a platinum group element. The chemical behaviors of these
elements in the hydrosphere are largely determined by their chemical affinities.
Radiogenic isotopes have proven to be an important and powerful tool in
investigations of many aspects of weathering and hydrology. The general absence of isotope
fractionation of heavy radiogenic isotopes in nature gives these tracers many advantages over
major and trace element ratios. Well over 200 articles have been written in this topical area,
and it is now evident that this methodology provides important scientific insights, and will
increasingly become a routine tool in studies of weathering and hydrology. Strontium isotopes
have unquestionably become the most commonly used radiogenic isotope tracer because of
the large variability in isotopic composition and the interest in tracing sources and cycling of
the analog element calcium. The most notable applications are: 1) use in identifying the
dissolution of trace Ca-bearing phases in experiments, soils, and along groundwater
flowpaths, 2) use in differentiating atmospheric from weathering sources of calcium to
ecosystems, and 3) use in differentiating distinct subsurface waters that have interacted with
contrasting aquifer materials. The mechanisms controlling differences in strontium isotope
ratios in the environment and the hydrogeochemical behavior of strontium have become
reasonably well understood and as a result, strontium will be increasing used in a routine
manner in weathering and hydrologic studies.
Lead isotopes have found limited applications in the study of the weathering of U and
Th–rich accessory phases in laboratory experiments and in soils. The preferential release of
234
U compared to 238U has been more widely used as a tracer of weathering reactions, as a
tracer of the geochemical behavior of uranium, and as a tracer of groundwater sources and
mixing. Neodymium has proven useful in a few studies as a tracer of the weathering release
of trace phases such as apatite, as well as of inputs of atmospheric dust to soils. Osmium has
been used in only one weathering study to date and was useful in inferring rates of dissolution
of magnetite in crystalline rocks. In general, the use of neodymium, hafnium and osmium
isotopes in weathering and hydrology is in its infancy, and much additional research will be
needed to gain a thorough understanding of the behavior of these systems and to ascertain
their usefulness in more routine investigations. We expect that major breakthroughs in the use
of radiogenic isotopes in weathering and hydrology will increasingly rely on the combined
use of several isotopic systems together, which yield contrasting information and insight into
any given scientific application.
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