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source sediments will remain largely intact during the erosion, transport, deposition,
and diagensis of the sediment allowing them to serve as highly effective tracers. In
the case of Sr and Nd, their isotopes have proven to be particularly effective at deter-
mining sediment provenance over large areas (ranging up to and including the global
scale for the tracking of dust). In contrast, Pb isotopes have not been extensively used
to track sediment per se, but are primarily used to track Earth materials contaminated
by Pb and other related trace metals/metalloids from a particular source.
4.2 Sr and Nd Isotopic Systems
4.2.1 Tracing Sediments and Other Geological Materials
The abundances of Sr and Nd isotopes vary significantly in geological materials,
primarily as a function of lithology and age of the rocks. These variations have been
extensively used by the geological community to determine the source and disper-
sal patterns of sediment within oceanic, coastal, atmospheric and riverine systems
(Table
4.1
) (Nelson and DePaolo
1988
; McLennan
1989
; Awwiller
1994
; Calanchi
et al.
1996
). Sr possesses four naturally occurring isotopes, of which
87
Sr is radi-
ogenic, being produced from the radioactive decay of
87
Rb (Table
4.1
). Since isotopic
measurements aremost easily and preciselymade as isotopic ratios (Banner
2004
), Sr
isotopic data are generally presented by dividing radiogenic
87
Sr by
86
Sr (
87
Sr/
86
Sr),
the latter a stable, non-radiogenic isotope. The relative mass difference between the
isotopes (
87
Sr and
86
Sr in this case) is relatively small
1%). Thus, mass-dependent
fractionation is limited, and that which does occur during analysis is typically cor-
rected for. Thus, fractionation during sediment dispersal from a source as well as
during sample analysis can generally be considered negligible (Banner
2004
).
The
87
Sr/
86
Sr ratiosmeasuredwithin oceanic basalts fall within a relatively narrow
range of values (from 0
(
∼
.
7020 to 0
.
7070) and within an even more narrowly defined
range for mid-oceanic basalts (0
7045) (Allègre
2008
) (Fig.
4.1
). In marked
contrast, the
87
Sr/
86
Sr ratios of granites and gneisses, which make up the bulk of
.
7022-0
.
Table 4.1
Decay schemes and data for selected radiogenic isotopes (modified from Banner
2004
)
Radioactive
Radiogenic
Decay
Half-Life
ʻ
Decay Constant
parent
daughter
mechanism
(billions of years)
87
Rb
87
Sr
10
−
11
Beta
48.8
1
.
42
×
147
Sm
143
Nd
10
−
12
Alpha
106
6
.
54
×
238
U
a
234
U;
230
Th;
206
Pb
10
−
10
.
×
Alpha &Beta
4.468
1
551
235
U
a
231
Pa;
207
Pb
10
−
10
Alpha &Beta
0.704
9
.
8485
×
232
Th
208
Pb
10
−
11
Alpha &Beta
14.010
4
.
9475
×
138
La
138
Ce
10
−
12
.
×
Beta branched
310
2
24
10
−
11
a
Decays to produce a radioactive daughter which eventually decays to stable radiogenic isotope
176
Lu
176
Hf
Beta
35.7
1
.
94
×
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