Radiogenic Isotopes - Application of Geochemical Tracers to Fluvial Sediment - page 88

Fig. 4.1 Frequency diagram showing the distribution of 87 Sr/ 86 Sr ratios in mid-oceanic-ridge-
basalts (MORBs) and granites and gneisses of the lower and upper continental crust (from Allègre
2008 ; Isotope Geology, Fig. 6.1, pp 222; copyright ©2008 Claude J Alle'gre. Reprinted with the
permission of Cambridge University Press)

the continents, are highly variable, ranging from about 0 850

(Allègre 2008 ). The observed difference between the 87 Sr/ 86 Sr ratios of basalts and
granites is dependent, in part, on the age of the rock units. Oceanic basalts exhibit an
average age of about 80Ma, the oldest being about 200 Ma (Allègre 2008 ). The age
of the continental crust is highly variable as it consists of multiple tectonic segments
sutured together in complex ways, but the age of the rock units can measure in the
billions of years. Thus, the time over which 87 Rb in continental rocks has had to
decay to 87 Sr, while variable, is typically much longer than for basalts, producing
87 Sr/ 86 Sr ratios in continental materials that are generally higher than those observed
in basalt. The 87 Sr/ 86 Sr ratios also depend on the mineralogy and the initial Rb/Sr
ratios within the rock units. As an alkaline earth element, the chemical characteristics
of Sr are similar to that of Ca, and it often replaces Ca (as well as K) in rock forming
minerals. Rb, on the other hand, is an alkali element and exhibits chemical properties
similar to K. It is therefore more abundant as a replacement for K in K-rich minerals.
Minerals that possess high K/Ca ratios are therefore likely to also possess higher
Rb/Sr ratios, and through time will develop more radiogenic 87 Sr values (and vice
versa). Put differently, for rocks containing minerals of similar age, those with high
Rb and lowSr concentrations (e.g., biotite, muscovite) will be characterized by higher
87 Sr/ 86 Sr ratios than minerals possessing relative low Rb and high Sr contents (e.g.,
plagioclase, apatite) (Tripathy et al. 2011 ).
Nd, a light rare earth element, has seven naturally occurring isotopes. Two of
these isotopes ( 142 Nd and 143 Nd) are radiogenic, produced by the radioactive decay
of 146 Sm and 147 Sm, respectively. However, 146 Sm is an extinct radionuclide as it
possesses a short half-life. Thus, tracer studies focus on 143 Nd/ 144 Nd. Unlike the
isotopic abundances of Sr, isotopic Nd values are expressed in units of

. 705 to more than 0 .

ʵ Nd which
portrays the deviation of 143 Nd/ 144 Nd in the sample relative to the 143 Nd/ 144 Nd in
a standard. The use of makes it easier to report and interpret the data since the

differences in 143 Nd/ 144 Nd values are very small. In this case, the utilized standard

ʵ