3.3 Rb-Sr system
7 min read•august 21, 2024
The Rb-Sr system is a key tool in isotope geochemistry for dating rocks and minerals. It uses the decay of to 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 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:
- λ represents the (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:
- 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 ( ~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
- 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 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
Key Terms to Review (16)
Age equation: The age equation is a mathematical formula used to calculate the age of a rock or mineral based on the ratio of parent and daughter isotopes in radiometric dating. It plays a crucial role in isotope geochemistry, particularly in understanding the Rb-Sr system, where rubidium-87 decays into strontium-87 over time, allowing scientists to determine the timing of geological events and the history of rock formation.
Closure temperature: Closure temperature is the temperature below which a mineral or a rock becomes a closed system to the diffusion of isotopes, meaning that no parent or daughter isotopes can escape or enter the mineral. This concept is crucial in geochronology as it helps to determine the age of geological materials by establishing when the isotopic clock starts. Different minerals have unique closure temperatures, affecting their utility in dating processes and providing insight into the thermal history of geological formations.
Decay Constant: The decay constant is a fundamental parameter that quantifies the rate at which a radioactive isotope decays over time. It is directly related to the half-life of a radioactive isotope and indicates how likely an unstable nucleus is to undergo decay in a given time period. Understanding the decay constant is crucial for comprehending various radioactive decay processes, the calculation of age in radiometric dating, and the relationships between parent and daughter isotopes.
Geochronology: Geochronology is the science of determining the age of rocks, fossils, and sediments through the study of their isotopes and radioactive decay processes. This field plays a critical role in understanding the timing of geological events, the history of the Earth, and the processes involved in crustal growth and recycling.
Igneous rocks: Igneous rocks are formed from the solidification of molten material called magma or lava. They are classified into two main types: intrusive, which crystallizes below the Earth's surface, and extrusive, which solidifies after erupting onto the surface. Understanding igneous rocks is crucial for various dating methods that rely on the minerals they contain and their formation processes.
Initial strontium ratio: The initial strontium ratio refers to the ratio of the isotopes strontium-87 ($$^{87}Sr$$) to strontium-86 ($$^{86}Sr$$) present in a sample at the time of its formation or crystallization. This ratio is crucial in isotope geochemistry as it helps scientists understand the geological history and processes that have influenced the sample since its formation.
Isochron dating: Isochron dating is a radiometric dating technique used to determine the age of rocks and minerals by analyzing the ratios of isotopes within them, particularly the parent and daughter isotopes. This method helps to create a graphical representation, or isochron, that illustrates the relationship between these isotopes over time, allowing for more accurate age estimates while accounting for any initial daughter isotope presence.
Isochron equation: The isochron equation is a mathematical expression used in radiometric dating to determine the age of a rock or mineral sample by analyzing the ratios of isotopes present in the sample. This equation is particularly important in the Rb-Sr dating system, where it relates the ratio of radiogenic strontium-87 ($^{87}$Sr) to stable strontium-86 ($^{86}$Sr) in a sample to the ratio of rubidium-87 ($^{87}$Rb) to strontium-86 at the time of formation, allowing for the calculation of time since crystallization.
Isotopic Fractionation: Isotopic fractionation is the process by which different isotopes of an element are separated or partitioned due to physical or chemical processes, leading to variations in their abundance. This phenomenon is crucial for understanding how isotopes behave in various geological and biological contexts, as it can influence measurements in atomic structure, isotope notation, and radiometric dating methods.
Metamorphic Rocks: Metamorphic rocks are types of rocks that have undergone transformation due to heat, pressure, and chemically active fluids. This process alters the mineralogy, texture, and sometimes chemical composition of the original rock, known as the parent rock or protolith. Metamorphic rocks play a crucial role in geochronology and isotope studies, particularly in understanding geological time and processes through various isotopic systems.
Open system behavior: Open system behavior refers to the interaction of a geological system with its surroundings, where mass and energy can enter or exit the system freely. This concept is essential in understanding the dynamics of isotopic systems, as it influences the distribution and concentration of isotopes in a sample, particularly in radiogenic isotopic systems like Rb-Sr, where rubidium can be added or strontium lost due to external processes.
Parent-daughter relationship: In isotope geochemistry, a parent-daughter relationship refers to the relationship between a radioactive isotope (parent) and its stable or unstable decay product (daughter). This concept is crucial for understanding radioactive decay processes, where the parent isotope transforms into one or more daughter isotopes over time. Recognizing this relationship allows scientists to date materials and study geological processes through various decay chains and isotopic systems.
Petrogenesis: Petrogenesis refers to the process of rock formation, particularly the origins and evolution of igneous and metamorphic rocks. Understanding petrogenesis involves examining the sources of magma, the conditions of crystallization, and the changes that rocks undergo during their formation. This concept is crucial for unraveling the history of crustal growth, recycling processes, and various mantle activities.
Radiometric dating: Radiometric dating is a method used to determine the age of rocks, minerals, and fossils by measuring the abundance of radioactive isotopes and their decay products. This technique relies on the principles of radioactive decay, half-lives, and parent-daughter relationships to establish a timeline for geological and archaeological events.
Rubidium-87: Rubidium-87 is a radioactive isotope of rubidium that has a half-life of approximately 48.8 billion years, making it useful for geochronology and dating geological samples. It is particularly significant in the rubidium-strontium (Rb-Sr) dating system, where it decays to strontium-87, providing a means to determine the age of rocks and minerals based on the ratio of Rb to Sr present.
Strontium-87: Strontium-87 is a stable isotope of strontium that plays a significant role in geochemical processes, particularly in understanding the evolution of the Earth's mantle and crust. This isotope is commonly used as a tracer in various geological studies, helping scientists unravel the complexities of mantle dynamics, the interactions between volcanic plumes and the lithosphere, and even dating techniques like isochron dating. Its importance extends to lunar geochemistry, where it aids in the analysis of extraterrestrial materials.