School of Engineering and Science - Jacobs University

deviation of the 143 Nd/ 144 Nd ratio measured in a sample relative to the 143 Nd/ 144 Nd ratio
in a chondritic uniform reservoir (CHUR) in parts per 10 4 :
Є Nd (t)
=
⎡(
⎢
⎢⎣
143
Nd/
I
144
t
CHUR
Nd)
i
⎤
− 1⎥
⎥⎦
x 10
4
where ( 143 Nd/ 144 Nd) i is the initial ratio in the sample (i.e., at the time it formed, t), I t CHUR
is the CHUR 143 Nd/ 144 Nd ratio at the time the sample formed, and CHUR is considered
to represent bulk silicate Earth. Since continental crust is characterized by low Sm/Nd
ratios that produce relatively low amounts of radiogenic Nd, it displays negative Є Nd (t)
values. Conversely, mafic oceanic crust derived from a depleted upper mantle is
characterized by positive Є Nd (t) values, as MORB has relatively high Sm/Nd ratios and
excess radiogenic Nd. As the REY in modern seawater are primarily sourced from the
weathering of continental crust, modern seawater possesses negative Є Nd (0) values
between -1 and -20, though extreme values in this range are restricted to shallow waters
(Goldstein and Hemming, 2003). Figure 11 shows average Є Nd (0) values for different
world oceans, as well as simple models for the Nd isotopic evolution in continental
crust and a depleted mantle over the past 4.0 Ga.
The significant Nd isotopic variations between ocean basins indicates that the
marine residence time of Nd (τ Nd ) is less than the mixing time of the oceans (~1000
years, e.g., Broecker and Peng, 1982), and estimates of τ Nd in oxic seawater typically
range from 300-1500 years (Jeandel et al., 1995; Tachikawa et al., 2003). This is
consistent with the particle reactive nature of the REE and the efficient Nd scavenging
processes operating in modern oceans, as REE-rich hydrothermal fluid with highly
positive Є Nd (0) has a negligible effect on 143 Nd/ 144 Nd ratios in world seawater (Fig. 11).
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