3.3 Rb-Sr system

The Rb-Sr system is a key tool in isotope geochemistry for dating rocks and minerals. It uses the decay of rubidium-87 to strontium-87 to measure geological time, providing insights into Earth's crust and mantle evolution.

Understanding the geochemical behavior of Rb and Sr in different rock types and minerals is crucial for accurate dating. The isochron method allows determination of ages and initial Sr ratios, making Rb-Sr dating widely applicable in geochronology and planetary science.

Fundamentals of Rb-Sr system

• Rb-Sr system forms a cornerstone of isotope geochemistry used to determine ages of rocks and minerals
• Utilizes the radioactive decay of rubidium-87 to strontium-87 to measure geological time
• Provides insights into the formation and evolution of Earth's crust and mantle

Rubidium and strontium isotopes

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• Rubidium exists as two naturally occurring isotopes: 85Rb (stable) and 87Rb (radioactive)
• Strontium has four stable isotopes: 84Sr, 86Sr, 87Sr, and 88Sr
• 87Sr increases over time due to decay of 87Rb
• Relative abundances of Sr isotopes vary in nature due to radioactive decay and fractionation processes

Radioactive decay process

• 87Rb decays to 87Sr through beta decay
• Decay equation: $^{87}Sr = ^{87}Sr_0 + ^{87}Rb(e^{\lambda t} - 1)$
• λ represents the decay constant (1.42 × 10^-11 year^-1)
• t denotes the time since system closure
• 87Sr_0 indicates initial 87Sr content

Half-life of 87Rb

• Half-life of 87Rb equals approximately 48.8 billion years
• Long half-life makes Rb-Sr system suitable for dating very old rocks (Precambrian)
• Allows measurement of geological processes spanning billions of years
• Comparable to the age of the Earth (4.54 billion years)

Geochemical behavior

• Rb-Sr system behavior reflects the chemical properties of rubidium and strontium in geological environments
• Understanding geochemical behavior crucial for accurate interpretation of Rb-Sr dating results
• Variations in Rb/Sr ratios among different rock types and minerals form basis for dating applications

Rb and Sr in igneous rocks

• Rubidium behaves as a large ion lithophile element (LILE) in magmatic systems
• Strontium substitutes for calcium in many rock-forming minerals
• Rb concentrates in late-stage magmatic differentiates (granites)
• Sr enriched in early-formed minerals (plagioclase)
• Rb/Sr ratios increase during magmatic differentiation

Rb and Sr in sedimentary rocks

• Rb often associated with clay minerals and micas in sedimentary rocks
• Sr commonly found in carbonate minerals and detrital feldspars
• Weathering processes can fractionate Rb from Sr
• Marine sediments typically have lower Rb/Sr ratios than terrestrial sediments
• Diagenesis may affect Rb-Sr systematics in sedimentary rocks

Rb/Sr ratios in minerals

• Biotite and muscovite micas contain high Rb/Sr ratios
• Feldspars (especially K-feldspar) have intermediate Rb/Sr ratios
• Pyroxenes and amphiboles generally have low Rb/Sr ratios
• Apatite and calcite typically have very low Rb/Sr ratios
• Mineral Rb/Sr ratios crucial for constructing isochrons and determining ages

Isochron method

• Isochron method provides a powerful tool for determining ages and initial Sr isotope ratios
• Relies on the linear relationship between 87Rb/86Sr and 87Sr/86Sr ratios in cogenetic samples
• Allows for correction of initial 87Sr/86Sr ratio without assuming its value

Principles of isochron dating

• Assumes all samples formed at the same time with identical initial 87Sr/86Sr ratios
• Requires a suite of samples with varying Rb/Sr ratios
• Closed system behavior since formation (no gain or loss of Rb or Sr)
• Samples must be cogenetic (formed from the same source at the same time)
• Utilizes the decay equation in a graphical form

Rb-Sr isochron diagram

• X-axis represents 87Rb/86Sr ratio
• Y-axis represents 87Sr/86Sr ratio
• Slope of isochron line proportional to age of the system
• Y-intercept gives initial 87Sr/86Sr ratio
• Goodness of fit (MSWD) indicates reliability of the isochron

Age calculation techniques

• Slope of isochron used to calculate age: $t = \frac{1}{\lambda} \ln(slope + 1)$
• Least squares regression often employed to fit isochron line
• Monte Carlo simulations used to estimate uncertainties in age and initial ratio
• York regression accounts for errors in both x and y variables
• Isoplot software commonly used for isochron calculations and plotting

Applications in geochronology

• Rb-Sr dating widely applied in various geological settings and rock types
• Provides crucial information on timing of geological events and processes
• Complements other isotopic dating methods (U-Pb, K-Ar) for comprehensive geochronology

Dating igneous rocks

• Determines crystallization ages of plutonic and volcanic rocks
• Whole-rock isochrons used for fine-grained volcanic rocks
• Mineral isochrons (feldspar, mica) employed for coarse-grained plutonic rocks
• Useful for dating felsic rocks with high Rb/Sr ratios
• Can date ancient igneous events in Precambrian terranes

Metamorphic rock dating

• Dates timing of metamorphic events and cooling history
• Reset of Rb-Sr system during high-grade metamorphism allows dating of metamorphism
• Mineral isochrons (garnet, mica) used to constrain P-T-t paths
• Cooling ages obtained from Rb-Sr in micas (closure temperature ~300-500°C)
• Helps unravel complex metamorphic histories in orogenic belts

Sedimentary rock provenance

• Initial 87Sr/86Sr ratios used as tracers for sediment sources
• Rb-Sr dating of authigenic minerals constrains timing of diagenesis
• Detrital mica ages provide information on sediment provenance
• Useful in reconstructing paleogeography and tectonic settings
• Helps identify source terranes in sedimentary basins

Analytical techniques

• Precise and accurate measurement of Rb and Sr isotopes crucial for reliable age determinations
• Advances in mass spectrometry have greatly improved precision and sensitivity of Rb-Sr analyses
• Careful sample preparation and data reduction essential for high-quality results

Mass spectrometry for Rb-Sr

• Thermal ionization mass spectrometry (TIMS) traditionally used for high-precision analyses
• Multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) gaining popularity
• TIMS offers highest precision for Sr isotope ratios (±0.002% 2σ)
• MC-ICP-MS allows for rapid analyses and smaller sample sizes
• Laser ablation MC-ICP-MS enables in situ microanalysis of minerals

Sample preparation methods

• Whole-rock powders prepared by crushing and grinding
• Mineral separates obtained through magnetic and density separation techniques
• Chemical separation of Rb and Sr using ion exchange chromatography
• Spike addition for isotope dilution analysis of Rb and Sr concentrations
• Ultra-clean laboratory conditions required to minimize contamination

Error analysis in Rb-Sr dating

• Analytical uncertainties in isotope ratio measurements propagated through age calculations
• Decay constant uncertainty contributes to systematic errors in absolute ages
• Isochron statistics (MSWD) used to assess data quality and geological scatter
• Monte Carlo simulations employed to estimate realistic age uncertainties
• Interlaboratory comparisons and standard analyses ensure data accuracy and precision

Limitations and challenges

• Understanding limitations of Rb-Sr dating essential for proper interpretation of results
• Various geological processes can disturb Rb-Sr systematics and lead to erroneous ages
• Careful sample selection and geological context crucial for meaningful age determinations

Closed system assumptions

• Rb-Sr system must remain closed since formation for accurate dating
• Open system behavior can result from weathering, alteration, or metamorphism
• Partial resetting of Rb-Sr system may yield meaningless "mixed" ages
• Careful petrographic and geochemical screening necessary to identify disturbed samples
• Multiple chronometers (U-Pb, Ar-Ar) can help verify closed system behavior

Effects of metamorphism

• High-grade metamorphism can reset Rb-Sr systematics in whole rocks and minerals
• Partial resetting may occur during low-grade metamorphism or fluid alteration
• Metamorphic overprinting can produce complex age patterns in polymetamorphic terranes
• Rb-Sr ages in metamorphic rocks may reflect cooling rather than peak metamorphism
• Careful interpretation required to distinguish between protolith and metamorphic ages

Rb-Sr vs other dating methods

• U-Pb zircon dating generally more precise for igneous rock crystallization ages
• K-Ar and Ar-Ar dating offer advantages for dating volcanic rocks and low-Rb systems
• Rb-Sr system more susceptible to disturbance than U-Pb system
• Rb-Sr dating valuable for rocks lacking suitable minerals for other methods
• Multi-chronometer approach provides most robust geochronological constraints

Rb-Sr in planetary science

• Rb-Sr dating plays crucial role in understanding formation and evolution of solar system bodies
• Provides insights into early solar system processes and planetary differentiation
• Complements other isotopic systems (U-Pb, Sm-Nd) in cosmochemistry studies

Lunar rock dating

• Rb-Sr dating of lunar rocks constrains timing of lunar crust formation
• Ages of lunar basalts reveal history of mare volcanism
• Initial 87Sr/86Sr ratios provide information on lunar mantle evolution
• Rb-Sr systematics in lunar samples affected by impact metamorphism
• Combination with other chronometers (U-Pb, Sm-Nd) gives comprehensive lunar chronology

Meteorite age determination

• Rb-Sr dating of chondrites constrains timing of solar system formation
• Ages of achondrites reveal differentiation history of asteroidal parent bodies
• Initial 87Sr/86Sr ratios in meteorites used to study early solar system heterogeneity
• Rb-Sr systematics in some meteorites disturbed by shock metamorphism or terrestrial weathering
• Precise Rb-Sr dating crucial for understanding early solar system chronology

Early solar system chronology

• Rb-Sr dating of calcium-aluminum-rich inclusions (CAIs) in chondrites
• Constrains timing of earliest solid formation in solar nebula
• Initial 87Sr/86Sr ratio of solar system determined from primitive meteorites
• Rb-Sr systematics in planetary differentiation processes
• Comparison with short-lived radionuclide systems (26Al-26Mg, 53Mn-53Cr) for early solar system events

Recent advances

• Ongoing technological and methodological improvements enhance capabilities of Rb-Sr dating
• New applications expand utility of Rb-Sr system in various geological and planetary science contexts
• Integration with other isotopic systems and analytical techniques provides more comprehensive geochronological information

High-precision Rb-Sr dating

• Development of new generation TIMS instruments with improved ion detection
• Achieves precision comparable to U-Pb zircon dating for some applications
• Enhanced precision allows resolution of short-lived geological events
• Improved spike calibration and measurement protocols reduce systematic errors
• Application to dating of young volcanic rocks and quaternary sediments

In situ Rb-Sr analysis

• Laser ablation MC-ICP-MS enables microanalysis of Rb-Sr systematics
• Allows dating of individual mineral grains and zones within crystals
• Useful for studying complex metamorphic and igneous histories
• Combines geochronology with textural and compositional information
• Challenges include correcting for isobaric interferences and matrix effects

Rb-Sr thermochronology

• Utilizes diffusion behavior of Sr in minerals to constrain thermal histories
• Combines with other thermochronometers (Ar-Ar, fission track) for multi-system approach
• Rb-Sr in phlogopite and biotite used to date cooling in ultramafic rocks
• Application to geothermal systems and hydrothermal ore deposits
• Modeling of Sr diffusion in minerals allows reconstruction of time-temperature paths