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I. Chemical properties and elemental abundances of Sm and Nd II. Rare earth patterns I. Chemical properties and elemental abundances of Sm and Nd

II. Rare earth patterns

III. Decay scheme and isotopic abundances

IV. Evolution/isochron diagrams

V. Chondritic assumption, CHUR, and earth evolution
VI.

Nd and model ages
VII.
Nd - Sr (
87
Sr/
86
Sr) diagrams and mixing lines

Sm and Nd chemical properties and elemental abundances

Sm and Nd are rare earth (or Lanthanide) elements, of which there are 15 (14 naturally
occurring), from Z=57 (La) to Z=71 (Lu).


60Nd, 62Sm

The rare earths are classified in a group because they have similar electronic structure.
The all have 2 6s electrons and 1 5d electron. They have varying numbers of 4f electrons
(0 to 14). Under most geologic conditions, they all have a valence of +3, except for Ce
which can also be +4 under oxidizing conditions and Eu, which generally is +2 under
reducing conditions. Because of their similar electronic structure, the rare earths have
similar chemical properties when in the same (+3) oxidation state. In this oxidation state,
the major property that distinguishes their chemical behaviors is ionic radius. Ionic
radius progressively decreases with increasing Z (Lanthanide Contraction).

57La
+3
= 1.15 Å; 71Lu
+3
=0.93 Å

60Nd
+3
= 1.08 Å; 62Sm
+3
=1.04 Å

All rare earths are LILs (other LILs include: Sr
+2
=1.3Å, Rb
+1
=1.7Å, K
+1
=1.6Å,
Na
+1
=1.1Å, Ba
+2
=1.5Å, Cs
+1
=1.8Å, Th
+4
=1.1Å, U
+4
=1.1Å, Hf
+4
=0.9Å); by comparison
(Si
+4
=0.34Å (IV), Al
+3
=0.47Å (IV), Mg
+2
=0.8Å (VI), Fe
+2
=0.8Å (VI)).

Typical concentrations (see Table 12.1 in book):
Nd: ppb to ppm, 1 ppm common.
Sm: Nd concentration/3

(both typically trace elements; except

in
monazite,
apatite)

A diagram that summarizes rare earth element concentrations is a rare earth pattern.
The concentration of each rare earth element, normalized to chondrites, is plotted as a
function of Z. Normalization to some solar system material is useful as this smoothes out
the even-odd abundance effect. However, normalization to chondrites may have more
significance as chondrites are thought to have bulk earth rare earth concentrations
(chondritic assumption). If this is true, then chondrite-normalized rare earth patterns tell
you how rare earth concentrations in a particular sample compare to those in the average
earth.
Imagine that you have some bulk earth material, which you partially melt (see figure and
Hanson paper). In general, the melt will have higher rare earth concentrations then the
residue. This effect will be more pronounced for the lighter rare earths. The melts rare
earth pattern will have negative slope (light rare earth enriched). The residues rare earth
pattern will have a positive slope (light rare earth depleted). The melt is considered
enriched because it has high concentrations of the rare earths and LILs. The residue is
considered depleted because it has low concentrations of rare earths and LILs. The
enrichment or depletion refers to incompatible elements, those that tend to fractionate
into melts (as opposed to compatible elements - those that fractionate into residual
solids). Note the position of Nd and Sm in the figure. Nd is the lighter rare earth so
following the discussion above, the melt has higher Sm and Nd concentrations and lower
Sm/Nd then the residue.

Abundances of Sm and Nd isotopes

Sm
Nd
144 3.1%

142 27.1%
147 15.0%

143 12.2%
148 11.2%

144 23.9%
149 13.8%

145 8.3%
150 7.4%

146 17.2%
152 26.7%

148 5.7%
154 22.8%

150 5.6%

147 143
Sm
Nd +
62 60


=6.54 x 10
12
/y, t1/2=106 billion years

Evolution (Isochron) and Development Diagrams

These are completely analogous to Rb-Sr system with D,
87
Sr, and
143
Nd analogous; Dx,
86
Sr, and
144
Nd analogous, and P,
87
Rb, and
147
Sm analogous (see figures).

One of the main differences between the Rb-Sr and Sm-Nd systems is the degree to
which they may be disturbed in the presence of fluids (see figure).
Chondritic assumption



The earths average
147
Sm/
144
Nd and
143
Nd/
144
Nd is the same as chondrites.
This is supported by:
(1) Sm and Nd have similar chemical properties and dont fractionate from each other
easily.
(2) Solar Sm/Nd ratios are similar to chondritic ratios
(3) Terrestrial rocks plot within an envelope around the chondritic value in an Sm/Nd
development diagram (see figure).

CHUR is the Chondritic Uniform Reservoir, which has the
147
Sm/
144
Nd of chondrites
(and presumably the bulk earth) and the
143
Nd/
144
Nd of chondrites (and presumably the
bulk earth) as a function of time. It is an evolving reservoir.

Sm-Nd development diagrams and earth evolution

Given the chondritic assumption, one can now relate the isotopic characteristics of major
portions of the earth to bulk earth values (see development diagram, figure 1 in Depaolo,
1981). The most striking finding of this comparison is the complementary relationship
between the continental crust and the source of mid-ocean ridge basalts (MORB source).
i.e., in terms of Sm/Nd isotopic systematics, the continental crust is to the MORB source
as melt is to residue, with model dependent average extraction times of about 1.8 Æ.

Rb-Sr development diagrams and earth evolution

The chondritic assumption does not hold for Rb/Sr because Rb is much more volatile
than Sr. While the Rb/Sr ratio in chondrites can be measured, the bulk earth Rb/Sr ratio
was not known prior to development of the Sm/Nd system. If one plots the Nd versus Sr
isotopic compositions of modern mantle-derived rocks, they plot along a line called the
Mantle Array. The Mantle Array crosses the bulk earth Nd isotopic composition at an
87
Sr/
86
Sr ratio of 0.7045. This is taken to be the bulk earth Sr isotopic composition. The
initial Sr isotopic composition (about 0.699) has been known for many years from
analyses of meteorites with phases with relatively low Rb/Sr. Using these two numbers,
the bulk earth Rb/Sr ratio can be calculated. These numbers define an evolving reservoir
UR for uniform reservoir. UR is the Rb/Sr equivalent to CHUR (see figure). Given
UR, one can examinie earth evolution in terms of Rb/Sr. Given the good correlation
between Sr and Nd isotopic compositions of terrestrial materials, it is not surprising that
the Rb/Sr system also records the complementarity of the continents and the MORB
source. In addition it is very clear that the Rb/Sr ratio of the bulk earth is much lower
than the chondritic and solar values, presumably due to volatilization of Rb during
accretion of the earth (see figure 1 in Depaolo, 1981). -notation


o
Nd={[(
143
Nd/
144
Nd)sam/(
143
Nd/
144
Nd)
o
CHUR]-1}10
4
T
Nd={[(
143
Nd/
144
Nd)sam/(
143
Nd/
144
Nd)
T
CHUR]-1}10
4


-where the superscript
o
refers to present day values and the superscript T refers to values
at some time in the past (often the initial values).
Nd is therefore the deviation in parts
per 10,000 of the
143
Nd/
144
Nd ratio of a material from the CHUR value at a given point in
time.

Sometimes the I is used in the following fashion:

(
143
Nd/
144
Nd)
T
CHUR = I
T
CHUR

(
143
Nd/
144
Nd)
o
CHUR = I
o
CHUR

Development diagrams with the variable on the ordinate being
Nd instead of
143
Nd/
144
Nd can be constructed (see figure). The slope of a line in this kind of
development diagram is a function of
147
Sm/
144
Ndsam,
147
Sm/
144
NdCHUR, and .
Because
is very small, the slope is very close to being constant. To understand the
quantities that affect the slope, we need to define two quantities: Q and f, where Q is a
function of quantities that are not specific to a particular sample and f is a function that
includes quantities that are specific to a particular sample:

Q={
(
147
Sm/
144
NdCHUR) (10
4
)}/(
143
Nd/
144
Nd)
o
CHUR =25.13/Æ

f=[(
147
Sm/
144
Nd)sam/(
147
Sm/
144
Nd)CHUR]-1 (the fractional deviation of the sample
parent/daughter ratio from CHUR), (
147
Sm/
144
Nd)CHUR=0.1967,

in principle f can range from -1 to .

It can be shown with a tremendous amount of algebra, the definitions of
, Q, f, the
isochron equation,