3.4 Sm-Nd system

The Samarium-Neodymium (Sm-Nd) system is a powerful tool in isotope geochemistry for dating rocks and understanding Earth's evolution. It uses the radioactive decay of 147Sm to 143Nd to determine ages and trace geological processes over billions of years.

This system provides crucial insights into crustal formation, mantle differentiation, and planetary evolution. The long half-life of 147Sm makes it ideal for dating ancient rocks, while its resistance to disturbance during geological processes ensures reliable results.

Sm-Nd system overview

• Samarium-Neodymium (Sm-Nd) system serves as a powerful tool in isotope geochemistry for dating rocks and understanding Earth's evolution
• Utilizes the radioactive decay of 147Sm to 143Nd to determine ages and trace geological processes
• Provides insights into crustal formation, mantle differentiation, and planetary evolution over billions of years

Isotopes of Sm and Nd

Naturally occurring isotopes

Top images from around the web for Naturally occurring isotopes

• 

  Frontiers | The Neodymium Stable Isotope Composition of the Oceanic Crust: Reconciling the ... View original

  Is this image relevant?

• 

  Frontiers | The Neodymium Stable Isotope Composition of the Oceanic Crust: Reconciling the ... View original

  Is this image relevant?

• 

  Frontiers | The Neodymium Stable Isotope Composition of the Oceanic Crust: Reconciling the ... View original

  Is this image relevant?

• 

  Frontiers | The Neodymium Stable Isotope Composition of the Oceanic Crust: Reconciling the ... View original

  Is this image relevant?

• 

  Frontiers | The Neodymium Stable Isotope Composition of the Oceanic Crust: Reconciling the ... View original

  Is this image relevant?

1 of 3

Top images from around the web for Naturally occurring isotopes

• 

  Frontiers | The Neodymium Stable Isotope Composition of the Oceanic Crust: Reconciling the ... View original

  Is this image relevant?

• 

  Frontiers | The Neodymium Stable Isotope Composition of the Oceanic Crust: Reconciling the ... View original

  Is this image relevant?

• 

  Frontiers | The Neodymium Stable Isotope Composition of the Oceanic Crust: Reconciling the ... View original

  Is this image relevant?

• 

  Frontiers | The Neodymium Stable Isotope Composition of the Oceanic Crust: Reconciling the ... View original

  Is this image relevant?

• 

  Frontiers | The Neodymium Stable Isotope Composition of the Oceanic Crust: Reconciling the ... View original

  Is this image relevant?

1 of 3

• Samarium consists of seven naturally occurring isotopes (144Sm, 147Sm, 148Sm, 149Sm, 150Sm, 152Sm, 154Sm)
• Neodymium comprises seven stable isotopes (142Nd, 143Nd, 144Nd, 145Nd, 146Nd, 148Nd, 150Nd)
• 147Sm decays to 143Nd through alpha decay, forming the basis of the Sm-Nd dating system
• Relative abundances of these isotopes vary slightly due to radioactive decay and fractionation processes

Radioactive decay process

• 147Sm undergoes alpha decay to produce 143Nd
• Decay equation $^{143}Nd = ^{143}Nd_0 + ^{147}Sm(e^{\lambda t} - 1)$
• Decay constant (λ) of 147Sm equals 6.54 × 10^-12 yr^-1
• Alpha particle emission changes the atomic number from 62 (Sm) to 60 (Nd)
• Process occurs over billions of years due to the long half-life of 147Sm

Sm-Nd dating principles

Half-life of 147Sm

• 147Sm has a half-life of approximately 106 billion years
• Long half-life makes the Sm-Nd system suitable for dating very old rocks and minerals
• Allows for accurate age determinations of early Earth and planetary materials
• Decay rate remains constant regardless of temperature, pressure, or chemical environment

Isochron method

• Plots 143Nd/144Nd ratio against 147Sm/144Nd ratio for multiple cogenetic samples
• Slope of the isochron line determines the age of the rock or mineral suite
• Y-intercept provides the initial 143Nd/144Nd ratio at the time of formation
• Requires samples with varying Sm/Nd ratios but identical initial Nd isotopic compositions
• Equation for the isochron $(\frac{^{143}Nd}{^{144}Nd})_m = (\frac{^{143}Nd}{^{144}Nd})_i + (\frac{^{147}Sm}{^{144}Nd})_m(e^{\lambda t} - 1)$

Geochemical behavior of Sm-Nd

Fractionation during melting

• Sm and Nd behave similarly during most geological processes due to their similar ionic radii and charge
• Slight fractionation occurs during partial melting of the mantle
• Nd preferentially enters the melt phase compared to Sm
• Results in lower Sm/Nd ratios in crustal rocks compared to the mantle
• Fractionation factor between Sm and Nd typically ranges from 1.1 to 1.3

Compatibility in minerals

• Both Sm and Nd are incompatible elements in most rock-forming minerals
• Preferentially concentrate in the liquid phase during magmatic processes
• Garnet strongly partitions Sm relative to Nd, leading to higher Sm/Nd ratios
• Plagioclase slightly favors Nd over Sm, resulting in lower Sm/Nd ratios
• Clinopyroxene and amphibole show minimal fractionation between Sm and Nd

Sm-Nd in crustal evolution

CHUR model

• CHUR stands for Chondritic Uniform Reservoir
• Represents the bulk Earth composition based on chondritic meteorites
• Serves as a reference for comparing Nd isotopic compositions of rocks
• CHUR evolution line describes the change in 143Nd/144Nd ratio over time for bulk Earth
• Equation for CHUR $(\frac{^{143}Nd}{^{144}Nd})_{CHUR} = 0.512638 - 0.1967 \times (\frac{^{147}Sm}{^{144}Nd})_{CHUR} \times (e^{\lambda t} - 1)$

Depleted mantle model

• Represents the evolution of the upper mantle after extraction of continental crust
• Characterized by higher Sm/Nd ratios compared to CHUR
• Results in more radiogenic Nd isotopic compositions over time
• Used to calculate model ages for crustal rocks
• Depleted mantle evolution line lies above the CHUR line on Nd isotope diagrams

Epsilon Nd notation

Calculation and interpretation

• Expresses the deviation of a sample's 143Nd/144Nd ratio from CHUR
• Calculated using the formula $\epsilon Nd = [(\frac{^{143}Nd}{^{144}Nd})_{sample} / (\frac{^{143}Nd}{^{144}Nd})_{CHUR} - 1] \times 10^4$
• Positive εNd values indicate derivation from a depleted mantle source
• Negative εNd values suggest incorporation of older crustal material
• εNd = 0 represents a composition identical to CHUR

Temporal variations

• εNd values change over time due to radioactive decay and crustal evolution
• Present-day εNd values differ from initial εNd values at the time of rock formation
• Crustal rocks generally evolve towards more negative εNd values over time
• Mantle-derived rocks tend to develop more positive εNd values with age
• Plotting εNd vs. time reveals trends in crustal growth and recycling

Applications in geochronology

Igneous rock dating

• Determines crystallization ages of igneous rocks and minerals
• Particularly useful for dating mafic and ultramafic rocks poor in other datable minerals
• Applies to a wide range of igneous rock types (basalts, granites, pegmatites)
• Provides insights into magma source characteristics and crustal contamination
• Often combined with other isotope systems for cross-validation (U-Pb, Rb-Sr)

Metamorphic rock dating

• Dates metamorphic events by analyzing newly formed or recrystallized minerals
• Garnet commonly used due to its high closure temperature for the Sm-Nd system
• Helps constrain timing of high-grade metamorphism and crustal evolution
• Can reveal multiple metamorphic events in polymetamorphic terranes
• Useful for dating eclogites and other high-pressure metamorphic rocks

Sm-Nd in provenance studies

Sedimentary rock analysis

• Determines the source areas of sedimentary rocks and sediments
• Utilizes the fact that Sm-Nd ratios remain relatively unchanged during weathering and transport
• Compares Nd isotopic compositions of sediments to potential source rocks
• Helps reconstruct paleogeography and sediment transport pathways
• Useful in petroleum geology for understanding basin evolution and sediment routing

Crustal residence time

• Calculates the time since extraction of crustal material from the mantle
• Uses the depleted mantle model to estimate Nd model ages
• Provides insights into the age and evolution of continental crust
• Helps distinguish between juvenile and recycled crustal components
• Equation for Nd model age $T_{DM} = \frac{1}{\lambda} \ln[1 + \frac{(\frac{^{143}Nd}{^{144}Nd})_{sample} - (\frac{^{143}Nd}{^{144}Nd})_{DM}}{(\frac{^{147}Sm}{^{144}Nd})_{sample} - (\frac{^{147}Sm}{^{144}Nd})_{DM}}]$

Analytical techniques

Mass spectrometry methods

• Thermal Ionization Mass Spectrometry (TIMS) provides high-precision Nd isotope measurements
• Multicollector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) offers rapid analysis with good precision
• Laser Ablation ICP-MS allows for in-situ analysis of minerals with spatial resolution
• Secondary Ion Mass Spectrometry (SIMS) enables microanalysis of Sm and Nd isotopes in small sample volumes
• Isotope dilution techniques used to accurately determine Sm and Nd concentrations

Sample preparation

• Involves careful mineral separation and purification to avoid contamination
• Acid digestion of rock samples using HF, HNO3, and HCl
• Ion exchange chromatography to separate Sm and Nd from other elements
• Ultra-clean laboratory conditions required to minimize blank contributions
• Spike addition for isotope dilution analysis to determine elemental concentrations

Limitations and challenges

Analytical precision

• Precision limited by low abundance of 147Sm and slow decay rate
• Requires large sample sizes for high-precision measurements using TIMS
• Interferences from isobaric nuclides (142Ce, 144Sm) must be carefully corrected
• Matrix effects in ICP-MS can affect accuracy and precision of measurements
• Long counting times often necessary to achieve desired precision levels

Geological complexities

• Open-system behavior can disturb the Sm-Nd system, leading to inaccurate ages
• Metamorphism may cause partial resetting of the isotope system
• Mixing of different source components can complicate interpretation of Nd isotope data
• Inherited components in igneous rocks can affect the accuracy of crystallization ages
• Crustal contamination of mantle-derived magmas can obscure primary isotopic signatures

Sm-Nd vs other isotope systems

Rb-Sr system comparison

• Rb-Sr system has a shorter half-life (48.8 billion years) compared to Sm-Nd
• Rb and Sr more susceptible to disturbance during metamorphism and alteration
• Sm-Nd system generally more robust for dating older rocks and high-grade metamorphic events
• Rb-Sr better suited for dating low-temperature processes and some sedimentary rocks
• Combined use of Sm-Nd and Rb-Sr can provide complementary information on petrogenesis

Lu-Hf system comparison

• Lu-Hf system has a similar half-life (37.1 billion years) to Sm-Nd
• Both systems behave similarly during mantle melting and crustal processes
• Lu-Hf system more sensitive to garnet fractionation in the source region
• Hf isotopes can be measured in-situ on zircons, providing additional geochronological information
• Combining Sm-Nd and Lu-Hf data enhances understanding of mantle evolution and crustal growth

Case studies in Sm-Nd dating

Planetary materials

• Sm-Nd dating of lunar rocks constrains the age of the Moon and its magmatic history
• Analysis of Martian meteorites provides insights into the geological evolution of Mars
• Dating of chondritic meteorites helps determine the age of the solar system
• Sm-Nd systematics in differentiated meteorites reveal early planetary differentiation processes
• Studies of calcium-aluminum-rich inclusions (CAIs) constrain the earliest stages of solar system formation

Ancient crustal fragments

• Sm-Nd dating of Acasta Gneiss Complex in Canada confirms its age as one of the oldest known crustal rocks (>4.0 Ga)
• Analysis of Jack Hills zircons from Australia provides evidence for early crustal formation on Earth
• Sm-Nd studies of Archean greenstone belts reveal the nature of early crustal growth and mantle evolution
• Dating of ancient metamorphic terranes helps reconstruct the assembly and evolution of early continents
• Sm-Nd isotope mapping of cratons provides insights into the architecture and growth of continental nuclei