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Rb-Sr Isotope System

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Geochemical Thermodynamics Trace Element Geochemistry→→Mantle Geochemistry Sm-Nd Isotope System

Core Idea

The Rb-Sr system is based on the beta decay of 87Rb to 87Sr (half-life 48.8 Gyr). Because Rb (an alkali metal) and Sr (an alkaline earth) have different geochemical behaviors, igneous processes fractionate Rb/Sr ratios: Rb is incompatible (concentrates in melts and K-bearing minerals like micas and K-feldspar), while Sr is compatible in plagioclase and apatite. The isochron method dates rocks by plotting 87Sr/86Sr versus 87Rb/86Sr for co-genetic samples: the slope gives the age and the y-intercept gives the initial 87Sr/86Sr ratio. Initial Sr ratios are themselves powerful petrogenetic tracers -- mantle-derived rocks have low initial 87Sr/86Sr (~0.703-0.704) while rocks incorporating ancient continental crust have higher values (>0.710), reflecting long-term Rb/Sr fractionation.

Explainer

The Rb-Sr system was one of the first radiometric dating methods applied to rocks and remains important for both geochronology and petrogenetic tracing. Its power derives from the strong geochemical fractionation of Rb from Sr during igneous and metamorphic processes, which creates the range of parent/daughter ratios needed for isochron dating.

The isochron equation is: (87Sr/86Sr)_measured = (87Sr/86Sr)_initial + (87Rb/86Sr) * (e^lambda*t - 1). In a plot of 87Sr/86Sr versus 87Rb/86Sr, co-genetic samples that shared the same initial ratio define a line (the isochron) whose slope is (e^lambda*t - 1). With lambda known (1.42 x 10^-11 yr^-1), the age t is calculated from the slope. The y-intercept gives the initial 87Sr/86Sr ratio, which is a petrogenetic fingerprint of the source region.

Mineral isochrons use different minerals from a single rock (biotite has high Rb/Sr, plagioclase has low Rb/Sr, whole-rock is intermediate). Whole-rock isochrons use co-genetic rocks (from the same magmatic suite) with varying bulk Rb/Sr. Mineral isochrons are more easily reset by thermal events (because inter-mineral diffusion distances are short) and thus date cooling or metamorphism. Whole-rock isochrons are more robust because resetting requires homogenization at the whole-rock scale.

As a petrogenetic tracer, 87Sr/86Sr initial ratios discriminate mantle versus crustal sources. Mid-ocean ridge basalts have 87Sr/86Sr of 0.7025, reflecting the depleted mantle. Ocean island basalts range from 0.703 to 0.706, reflecting various mantle source enrichments. Continental crust, with higher Rb/Sr ratios and billions of years of 87Rb decay, has evolved to 87Sr/86Sr of 0.710-0.740. Granites, andesites, and other crustally influenced magmas plot between these endmembers, and the 87Sr/86Sr ratio quantifies the extent of crustal involvement in their petrogenesis. Paired with Nd isotopes, Sr isotopes provide the definitive discrimination between mantle and crustal components.

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

Longest path: 186 steps · 1065 total prerequisite topics

Prerequisites (2)

Geochemical Thermodynamicssoft Trace Element Geochemistrysoft

Leads To (2)

Mantle Geochemistrysoft Sm-Nd Isotope Systemsoft