This document discusses the radioactive decay of samarium-147 to neodymium-143 and its use in radiometric dating of rocks and meteorites. It provides background on samarium and neodymium as rare earth elements, their concentrations in common minerals and rocks, and how their ratio changes over time. The document also describes how the samarium-neodymium dating method is used to determine the age of the Earth based on analysis of meteorites and how it can be applied to date terrestrial and lunar rocks.

2. CONTENTS
 INTRODUCTION
 RADIOMETRIC DATING
 NEODYMIUM
 SAMARIUM
 GEOCHEMISTRY
 AGE DETERMINATION
 CHONDRTIES&ACHONDRITIES
 CONCLUSION
 REFERENCE

3. INTRODUCTION
 Samarium and neodymium are the rare earth
elements that occur in many rock forming
silicates ,phosphate and carbonate minerals.
 One of the samarium isotope( Sm)is radioactive
and
decays by alpha emission to a stable isotope of(
Nd). Although the half life of sm is very long
 This decay scheme is useful for dating
terrestrial rock, stony meteorites and lunar
rocks.
147
62
143
60

4. RADIOMETRIC DATING
 Radiometric dating is a technique used to date
materials such as rocks, usually based on a
comparison between the observed abundance of a
naturally occurring radioactive isotope and its
decay products.
 Modern dating methods
Samarium-neodymium dating method
Uranium-lead dating method
Potassium-argon dating method
Rubidium-strontium dating method
Uranium-thorium dating method

5. NEODYMIUM
 Neodymium is a rare earth element (REE) that occurs
in many silicate, phosphate, and carbonate minerals
by substitution for major ions.
 Naturally occurring neodymium (Nd) is composed of
5 stable isotopes, 142Nd, 143Nd, 145Nd, 146Nd and 148Nd,
with 142Nd being the most abundant (27.2% natural
abundance)
 Among 33 radioisotopes of Neodymium the most
stable being naturally occurring isotopes 144Nd
 All of the remaining radioactive isotopes have half-
lives that are less than 11 days, and the majority of
these have half-lives that are less than 70 seconds.

6. SAMARIUM
 Naturally occurring samarium (Sm) is
composed of five stable isotopes, 144Sm,
149Sm, 150Sm, 152Sm and 154Sm, and two
extremely long-lived radioisotopes, 147Sm
(1.06×1011y) and 148Sm (7×1015y), with
152Sm being the most abundant (26.75%
natural abundance).

7. GEOCHEMISTRY
 Neodymium and samarium are rare earth
elements. The rare earth elements generally
form ions with a charge of +3 whose radii
decrease with increasing atomic number 1.15 Å
in lanthanum to 0.93 Å in lutetium.
 The REE occur in high concentrations in several
economically important minerals such as
monazite & cerite.
 The REE also occur as trace elements in
common rock forming minerals in which they
replace major ions.

8. CONT…
 Sm & Nd belongs to the light REEs. Their ionic
radii are 1.08 Å for Nd &1.04 Å for Sm.
 The atomic Sm/Nd ratio in the solar system is
0.31. The ratio the Sm to Nd concentration in
terrestrial rocks and mineral varies only from
about 0.1 to 0.5.
 The concentration of both Sm& Nd in the
common rock forming silicate minerals increase
in the sequence in which they crystallize from
magma according to Bowens reaction series.

9. minerals Sm(ppm) Nd(ppm) Sm/Nd
Olivine 0.07 0.36 0.19
Pyroxene 3.34 9.09 0.367
Amphibole 6.03 17.3 0.347
Biotite 37 171.5 0.215
Plagioclase 0.541 1.85 0.292
Volcanic rocks
rocks Sm(ppm) Nd(ppm) Sm/Nd
Komatite 1.14 3.59 0.317
Alkali basalt 8.07 41.5 0.194
Andesites 3.90 20.6 0.189
Rhyolite 4.65 21.6 0.215
minerals
Average concentration of Sm & Nd in common terrestrial
rocks and minerals

10. rocks Sm(ppm) Nd(ppm) Sm/Nd
Gabbro 1.78 7.53 0.236
Granite 8.22 43.5 0.188
Syenite 9.5 86 0.11
Rocks Sm(ppm) Nd(ppm) Sm/Nd
Shale 10.4 49.8 0.209
Sand stone 8.93 39.4 0.227
Limestone 2.03 8.75 0.232
Greywacke 5.03 25.5 0.19
Plutonic rock
Sedimentary rocks

11.  The concentration of both elements in igneous rocks
increase with increasing degree of differentiation but
their Sm/Nd ratio decrease.
Sm/Nd ratio
 Mid oceanic ridge basalt-0.32
 Granite-0.19
 Alkalic igneous rock-0.10 to 0.20.

12. AGE DETERMINATIONS
 Samarium has five naturally occurring isotope whereas
neodymium has seven stable isotopes.
 The two elements are joined in a parent-daughter
relationship by the alpha decay of 147Sm to 143Nd and of
extinct 146Sm to stable 142 Nd.
 62Sm 60Nd+ α +Q,
T1/2=1.06×10 α
 62Sm 60Nd+ α +Q,
T1/2=1.0×10
Where Q is the decay energy and T1/2 is
the half
life.
147 143
10
146 142
8

13. ISOTOPIC EVOLUTION OF NEODYMIUM
 The 143Nd/ 144Nd ratio of the has increased with
time because of the decay of Sm to 143Nd.
 The age of the earth and the primordial 143Nd/
144Nd ratio are known from analysis of stony
meteorites.
147

14. CHONDRITES & ACHONDRITES
 Many stony meteorites have been dated by the Sm-Nd
method by analyzing mineral fraction separated from
individual specimens.
 The isotopic evolution of Nd in the earth is represented by a
model that assumes that the Sm/Nd ratio of the earth is
equal to that of chondritic meteorites. Jacobsen and
wasserburg analyzed five chondrites and achondrites
determined that these meteorites have an average Sm/ Nd
ratio of 0.1967 & used this result calculate the 143Nd/ 144Nd
ratio of chondritic reservoir at the present time assuming
that the age of the earth is 4.6 billion yrs..
147 144

15. CHUR , EPSILON &MODAL DATES
 The isotopic evolution of Nd in the earth is described in
terms of a model called CHUR (chondritic uniform
reservoir). This modal assumes that terrestrial Nd has
evolved in a uniform reservoir whose Sm/Nd ratio is equal to
that of chondritic meteorites.

16. CONT…….
 The Sm/Nd ratio of CHUR is lower than that of the solar
nebula and that the age of the is taken to be 4.6 b yrs.
 Partial melting of CHUR give rise to magma having lower
Sm/Nd ratios than CHUR.
 The igneous rocks that form from this magma therefore
have lower present day 143Nd/ 144Nd ratios than CHUR,
whereas the residual solids have a higher Sm/Nd ratio as a
result the present day 143Nd/ 144Nd ratio of the rocks formed
from the silicate liquid is than that of CHUR.

17. CONCLUSION
 Samarium and neodymium are rare earth elements joined in
a parent daughter relationship by the alpha decay of Sm to
stable Nd.
 This decay scheme is useful for dating terrestrial rock, stony
meteorites and lunar rocks.
 Both elements are widely distributed in common minerals
and rocks but their concentrations are generally less than 10
ppm.
 The Sm-Nd system is more valuable than the Rb-Sr system
for dating ancient volcanics because of the greater stability
of Sm/Nd compared with Rb/Sr during alteration processes.

18. REFERENCE
 Gunter Faure (1986), principles of isotope
geology, John Wiley & sons, p.p-200 to 213.
 C.S. Pichamuthu(1985), Archaean geology,
Oxford& ibh publishing co, p.p-139 to145.
 WEB:
www.google.com
www.wikipedia.com
www.geology.com