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 S r = 87 S r 0 + 87 R b ( e λ t − 1 ) ^{87}Sr = ^{87}Sr_0 + ^{87}Rb(e^{\lambda t} - 1) 87 S r = 87 S r 0 + 87 R b ( e λ 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 = 1 λ ln ( s l o p e + 1 ) t = \frac{1}{\lambda} \ln(slope + 1) t = λ 1 ln ( s l o p e + 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
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
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