1. GEOCHRONOLOGY HONOURS 2007 Lecture 05 The Samarium-Neodymium System

2. Behaviour of Rb and Sr in Rocks and Minerals
Rb behaves like K  micas and alkali feldspar
Sr behaves like Ca  plagioclase and apatite (but not clinopyroxene)
Rock Type Rb ppm K ppm Sr ppm Ca ppm
Ultrabasic ,000
Basaltic , ,000
High Ca granite , ,300
Low Ca granite , ,100
Syenite , ,000
Shale , ,100
Sandstone , ,100
Carbonate 3 2, ,300
Deep sea carbonate 10 2, ,400
Deep sea clay , ,000

3. Behaviour of Sm and Nd in Rocks and Minerals
Both Sm and Nd are LREE
Because Sm and Nd have very similar chemical properties that are not fractionated very much by igneous processes such as fractional crystallisation.
Useful for looking at metamorphic processes not igneous processes
Rock / Min Sm ppm Nd ppm Sm/Nd
Olivine
Garnet
Apatite
Monazite 15,000 88,
MORB Thol
Rhyolite
Eclogite
Granulite
Sandstone
Chondrites

4. Rb-Sr vs Sm-Nd Sm-Nd Rb-Sr Mafic and Ultramafic igneous rocks
Metamorphic Events
Rocks that have lost Rb-Sr
Rb-Sr
Acidic and Intermediate igneous rocks
Rocks enriched in rubidium and depleted in strontiu,

5. Application of Sm-Nd Similar chemical properties of Sm and Nd
Sm very long half life (106 Byr)
Means that large variations in Sm/Nd ratios in natural rocks are rare
Therefore difficulty in obtaining a wide range of Sm/Nd ratios from a single rock body
Combined with greater technical demands of Nd-isotope work has limited applications

6. Rb-Sr VS Sm-Nd Isochrons

7. Applicability of Sm-Nd
Generally applied to problems where Rb-Sr not appropriate
Very old rocks with likely disturbance of the Rb/Sr ratio
Rocks with very low Rb/Sr ratios, ie achondrites
Mineral pairs that concentrate Sm or Nd

8. Applicability of Sm-Nd
Mineral isochrons for Sm-Nd can often work quite successfully because variations in partition coefficients causes moderately large variations in Sm-Nd ratios unlike whole rock systems
Garnet and Cpx have mirror image partition coefficients which therefore give rise to large variations in Sm/Nd ratios.
Common occurrence of garnet + cpx is in eclogites where Sm-Nd has been used extensively to date the timing of metamorphism
Sm-Nd as REE are relatively immobile and may therefore not fully re-equilibrate during metamorphism

9. Sm-Nd Isotope System
Sm has seven naturally occurring isotopes – 144, 147, 148, 149, 150, 152, 154 as well as 146 which is an intermediate between 150Gd and 142Nd, and 151 which is extinct because its half life is only 93 years.
Of these 147Sm, 148Sm and 149Sm are radioactive but only 147Sm has a half life that impacts on the abundance of 143Nd.
Nd also has seven naturally occurring isotopes – 142, 143, 144, 145, 146, 148, Nd is extinct because the half life is only days. 143Nd is produced by alpha decay of 147Sm

10. Sm/Nd Decay Equation The decay equation for Sm/Nd is
143Nd/144Nd = (143Nd/144Nd)i + 147Sm/144Nd(elt – 1)
The decay equation is ratio-ed to 144Nd because the number of atoms of 144Nd in a unit weight of rock or mineral remains unchanged so long as the system in which it resides remains closed to Nd exchange.

11. Problems with the applicability of Sm-Nd
Garnet + Omphacite Eclogite

12. Mineral Transformations
Transformation of igneous augite to metamorphic omphacite
Relatively minor cation exchange
(Ca,Mg,Fe,Al)2(Si,Al)2O6 -> (Na,Ca)(Mg,Fe,Al)Si2O6
Monoclinic -> Monoclinic
Often does not completely re-equilibrate
Transformation of plagioclase to garnet
Major chemical exchange and structural re-organisation
CaAl2Si2O8 -> Ca3Al2Si3O12
Triclinic -> Isotropic
Likely to completely reset Sm-Nd systematics and give the metamorphic age

13. Sm-Nd Remobilisation and Re-equilibration

14. Inclusions in Mineral Phases
Garnet Inclusions
Monazite in particular a problem because it also concentrates Sm and Nd
Other inclusions that concentrate heavy REE’s

15. eNd(t) = ((143Nd/144Nd)sample (t)/(143Nd/144Nd)CHUR(t) – 1) x 104
Epsilon Notation
Archean plutons have initial 143Nd/144Nd ratios that are very similar to that of the Chondritic Uniform Reservoir (CHUR) predicted from meterorites.
Because of the similar chemical behaviour of Sm and Nd, departures in 143Nd/144Nd isotopic ratios from the CHUR evolution line are very small in comparison to the slope of the line.
Therefore Epsilon notation for Sm/Nd system is:
eNd(t) = ((143Nd/144Nd)sample (t)/(143Nd/144Nd)CHUR(t) – 1) x 104