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Trace Element Geochemistry

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Geochemical Thermodynamics→→Mantle Geochemistry Partition Coefficients +4 more

Core Idea

Trace elements (concentrations <0.1 wt%) obey Henry's Law in dilute solutions within minerals and melts, meaning their behavior is governed by partition coefficients rather than stoichiometric constraints. Elements are classified as compatible (preferentially entering the solid phase during melting, e.g., Ni, Cr, Co) or incompatible (preferentially entering the melt, e.g., Rb, Ba, Th, U, Nb, La). During partial melting, incompatible elements are strongly concentrated in small melt fractions, while compatible elements remain in the residue. This partitioning creates systematic abundance patterns in igneous rocks that encode information about the degree of melting, source composition, crystal fractionation history, and tectonic setting. Normalized multi-element diagrams (spider diagrams) and REE plots are the standard visualization tools.

Explainer

Trace elements are the fingerprints of geological processes. While major elements (Si, Al, Fe, Mg, Ca, Na, K) define the rock type (basalt vs. granite), trace elements resolve the processes that produced it: how much the mantle melted, what minerals crystallized from the magma, whether subducted sediment or fluids were involved, and what was left behind in the source.

The partition coefficient D (concentration in mineral / concentration in melt) is the fundamental parameter. For olivine, D_Ni ~10-30 (nickel is strongly compatible), while D_La ~0.001 (lanthanum is strongly incompatible). During partial melting, an element with D << 1 concentrates almost entirely in the melt, especially at small melt fractions. An element with D >> 1 remains locked in the residual solid. The batch melting equation, C_melt = C_source / (D + F(1-D)), quantifies this for any element and any melt fraction F.

Multi-element diagrams normalize trace element concentrations to a reference (primitive mantle, chondrite, or N-MORB) and plot them in order of decreasing incompatibility. The resulting pattern contains enormous information. A smooth, downward-sloping pattern indicates derivation from a depleted mantle source by moderate to large degrees of melting (like N-MORB). A steep, enriched pattern indicates small-degree melting of an enriched source (like ocean island basalts). Negative anomalies at specific elements (Nb, Ti depletion in arc basalts; Eu anomalies in plagioclase-fractionated rocks) diagnose specific mineral controls.

The power of trace element geochemistry lies in the fact that different elements respond differently to the same process: a single melting event simultaneously enriches Ba 100-fold while barely affecting Yb. This differential behavior creates patterns that constrain not just whether melting occurred, but the degree, depth, and residual mineralogy of the melting event -- information that major elements alone cannot provide.

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

Longest path: 185 steps · 1064 total prerequisite topics

Prerequisites (1)

Geochemical Thermodynamicshard

Leads To (6)

Mantle Geochemistryhard Partition Coefficientshard REE Patterns in Geochemistryhard Rb-Sr Isotope Systemsoft Sm-Nd Isotope Systemhard U-Pb Geochronologysoft