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Sm-Nd Isotope System

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Trace Element Geochemistry Rb-Sr Isotope System→→Crustal Evolution and Geochemistry Mantle Geochemistry

Core Idea

The Sm-Nd system is based on the alpha decay of 147Sm to 143Nd (half-life 106 Gyr). Because both Sm and Nd are light rare earth elements with very similar geochemical behavior, Sm/Nd fractionation during geological processes is small but systematic: partial melting preferentially concentrates the lighter Nd over the slightly heavier Sm in the melt, giving the crust lower Sm/Nd (and therefore lower time-integrated 143Nd/144Nd) than the residual mantle. Epsilon-Nd notation expresses 143Nd/144Nd as deviations from a chondritic reference (CHUR): positive epsilon-Nd indicates a depleted-mantle source; negative epsilon-Nd indicates enriched or crustal source. The Sm-Nd system is resistant to metamorphic resetting and provides robust constraints on crust-mantle differentiation, mantle source heterogeneity, and crustal residence ages.

Explainer

The Sm-Nd system is the definitive tracer of crust-mantle differentiation because it directly records the fractionation between two elements that are separated only during major silicate melting events. Unlike Rb-Sr (where Rb and Sr have very different chemistry and are easily disturbed), Sm and Nd behave almost identically in all processes except partial melting of silicates.

The epsilon-Nd notation makes the system intuitive. CHUR (Chondritic Uniform Reservoir) represents the undifferentiated bulk Earth composition. Epsilon-Nd = [(143Nd/144Nd-sample)/(143Nd/144Nd-CHUR) - 1] x 10⁴. Positive values mean the source has evolved with higher Sm/Nd than CHUR (depleted mantle, from which crust has been extracted). Negative values mean the source has evolved with lower Sm/Nd (continental crust or enriched mantle). The depleted mantle today has epsilon-Nd of approximately +8 to +12; old continental crust ranges from -10 to -40.

Epsilon-Nd vs 87Sr/86Sr plots (mantle arrays) are one of the most powerful discrimination tools in igneous petrology. Depleted mantle plots at high epsilon-Nd, low 87Sr/86Sr; continental crust plots at low epsilon-Nd, high 87Sr/86Sr. Mantle sources enriched by subducted sediment, metasomatism, or recycled oceanic crust plot in characteristic positions on this diagram, enabling identification of mantle source components in ocean island basalts, arc volcanics, and continental magmas.

Nd model ages (T-DM and T-CHUR) calculate when a sample's Nd isotopic composition intersects the depleted mantle or chondritic evolution curves, respectively. These are interpreted as crustal extraction ages -- the time when new continental crust was generated from the mantle. Global compilations of Nd model ages reveal episodic crustal growth, with major peaks at ~2.7, 1.9, and 1.1 Ga corresponding to supercontinent assembly events. This makes the Sm-Nd system a fundamental tool for understanding the growth and evolution of continental crust through Earth history.

Practice Questions 3 questions

Prerequisite Chain

Understanding Zero → The Number Zero → Counting to Five → Counting to 10 → Counting to 20 → Counting a Set of Objects Up to 20 → Cardinality: The Last Number Counted → Matching Numerals to Quantities → Subitizing Small Quantities → Addition Within 10 → Number Bonds to 10 → Addition Within 20 → Doubles and Near Doubles → Doubles Facts Within 10 → Near Doubles Facts Within 20 → Mental Math Strategies for Addition → Mental Math: Adding and Subtracting Tens → Addition Within 100 → Repeated Addition as Multiplication → Multiplication as Equal Groups → Multiplication: Arrays → Basic Multiplication Facts (0s, 1s, 2s, 5s, 10s) → Multiplication Facts Within 100 → Division as Equal Sharing → Division as Grouping (Measurement Division) → Division: Grouping (Repeated Subtraction) Model → Division: Fair Sharing Model → Division as Equal Sharing → Division as Grouping → Basic Division Facts → Division Facts Within 100 → Multiplication and Division Fact Families → Relationship Between Multiplication and Division → Division Facts as Inverse of Multiplication → Remainders and Quotients in Division → Division Word Problems → Multi-Step Word Problems → Solving Multi-Step Word Problems → Multiplication Word Problems → Division Word Problems → Introduction to Long Division → Factors and Multiples → Prime and Composite Numbers → Equivalent Fractions → Relating Fractions and Decimals → Decimal Place Value → Integers and the Number Line → Comparing and Ordering Integers → Absolute Value → Adding Integers → Subtracting Integers → Multiplying Integers → Dividing Integers → Unit Rates → Proportions → Percent Concept → Converting Between Fractions, Decimals, and Percents → Operations with Rational Numbers → Two-Step Equations → Solving Multi-Step Equations → Equations with Variables on Both Sides → Angle Pairs: Complementary, Supplementary, and Vertical → Parallel Lines and Transversals → Corresponding Angles → Alternate Interior Angles → Triangle Angle Sum Theorem → Exterior Angle Theorem → Triangle Inequality Theorem → Similar Triangles: AA Similarity → Similar Triangles: SSS and SAS Similarity → Proportions in Similar Triangles → Right Triangle Trigonometry Introduction → Sine, Cosine, and Tangent Ratios → Trigonometric Ratios Review → Radian Measure → Converting Between Degrees and Radians → The Unit Circle → Graphing Sine and Cosine → Graphing Tangent and Reciprocal Trigonometric Functions → Derivatives of Trigonometric Functions → Antiderivatives → Indefinite Integrals → Basic Integration Rules → Riemann Sums → Definite Integral Definition → Double Integrals: Definition and Setup → Iterated Integrals and Fubini's Theorem → Double Integrals over Rectangular Regions → Double Integrals over General Regions → Applications of Double Integrals: Area, Mass, and Moments → Triple Integrals in Cartesian Coordinates → Triple Integrals in Cylindrical and Spherical Coordinates → Change of Variables and the Jacobian Determinant → Applications of Triple Integrals: Volume and Mass → Vector Fields and Their Representations → Line Integrals of Vector Fields → Work and Circulation → Line Integrals of Scalar and Vector Functions → Fundamental Theorem for Line Integrals → Conservative Vector Fields → Conservative Vector Fields and Potential Functions → Curl and Divergence of Vector Fields → Curl and Divergence → Divergence Theorem → Electric Flux and Divergence Theorem → Gauss's Law: Integral Form and Meaning → Solving Problems with Gauss's Law → Conductors in Electrostatic Equilibrium → Capacitance and Capacitors → Dielectrics → Dielectric Constant and Relative Permittivity → Electric Field Inside Dielectric Materials → Dielectric Materials and Polarization → Dielectric Susceptibility and Permittivity → Energy Density in Electric Fields → Electric Current and Current Density → Electrical Resistance and Resistivity → Ohm's Law and Circuit Elements → Electromotive Force (EMF) and Batteries → Kirchhoff's Circuit Laws: Voltage and Current → DC Circuit Network Analysis Methods → Transient Response in RC Circuits → RC Circuits → LC and RLC Circuits → AC Circuits: Fundamentals → Impedance and Reactance → AC Power and Resonance → Electromagnetic Waves → Postulates of Special Relativity → Time Dilation → Length Contraction → Lorentz Transformation → Relativistic Velocity Addition → Relativistic Momentum and Energy → Mass-Energy Equivalence and E=mc² → Photons as Particles with Energy and Momentum → Planck-Einstein Relation: Energy and Frequency → Photoelectric Effect → The Photon: Light as Quanta → Compton Scattering → Wave-Particle Duality → de Broglie Wavelength → The Schrödinger Equation → State Vectors and Wavefunctions → Quantum Superposition → Quantum Entanglement → Bell Theorem and Bell Inequalities → Postulates of Quantum Mechanics → Observables and Quantum Operators → Commutators and Commutation Relations → Quantum Angular Momentum → Quantum Mechanical Treatment of Hydrogen → Solving the Schrödinger Equation for Hydrogen Atom → Quantum Numbers → Electron Configuration → Periodic Trends → Covalent Bonding → Electronegativity and Bond Polarity → Ionic Bonding → Lewis Structures → VSEPR Theory and Molecular Geometry → Molecular Geometry and Electron Pair Geometry → Molecular Polarity and Dipole Moments → Intermolecular Forces → States of Matter and Phase Changes: Melting, Boiling, and Sublimation → Gas Laws and the Ideal Gas Equation → Gas Stoichiometry and Volume-Volume Calculations → Thermochemistry and Enthalpy → Heat Capacity and Calorimetry → Entropy and Molecular Disorder → Spontaneity and ΔG → Entropy and Gibbs Free Energy → Chemical Equilibrium → Statistical Mechanics: Ensembles and the Boltzmann Distribution → Partition Function: Definition and Properties → The Canonical Partition Function and Thermodynamic Derivation → Free Energy and Thermodynamic Relations from Partition Functions → Legendre Transformations and Thermodynamic Potentials → Chemical Potential and Partial Molar Properties → Phase Equilibrium and Coexistence Conditions → Clausius-Clapeyron Equation → Phase Diagrams and Clausius-Clapeyron Equation → Chemical Potential and Thermodynamic Equilibrium → Geochemical Thermodynamics → Trace Element Geochemistry → Rb-Sr Isotope System → Sm-Nd Isotope System

Longest path: 187 steps · 1066 total prerequisite topics

Prerequisites (2)

Trace Element Geochemistryhard Rb-Sr Isotope Systemsoft

Leads To (2)

Crustal Evolution and Geochemistryhard Mantle Geochemistryhard