3.7 Lu-Hf system
8 min read•august 21, 2024
The Lu-Hf system is a powerful tool in isotope geochemistry, used for dating rocks and understanding Earth's evolution. It relies on the radioactive decay of lutetium-176 to hafnium-176, with a half-life of 37.1 billion years.
This system provides insights into crustal growth, , and meteorite formation. By analyzing Lu-Hf isotope ratios in minerals and rocks, geologists can uncover valuable information about Earth's geochemical processes and planetary formation events.
Fundamentals of Lu-Hf system
- Lu-Hf system provides valuable insights into Earth's geochemical processes and evolution
- Widely used in isotope geochemistry for dating rocks and understanding planetary formation
Lutetium and hafnium properties
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- Lutetium (Lu) belongs to the lanthanide series with atomic number 71
- Hafnium (Hf) sits in group 4 of the periodic table with atomic number 72
- Both elements exhibit similar ionic radii but different chemical behaviors
- Lu tends to concentrate in minerals like garnet and clinopyroxene
- Hf commonly substitutes for zirconium in crystals
Radioactive decay process
- 176Lu decays to 176Hf through beta decay
- Decay equation
- Process involves emission of an electron and an antineutrino
- Decay rate remains constant regardless of temperature or pressure
- Allows for accurate dating of geological materials
Half-life and decay constant
- Half-life of 176Lu measures 37.1 billion years
- (λ) calculated as
- Long half-life makes Lu-Hf system suitable for dating ancient rocks
- Decay constant used in age calculations and isotope evolution models
- Slow decay rate allows for high precision in geochronological studies
Isotopic composition
- Isotopic composition of Lu and Hf crucial for understanding geochemical processes
- Variations in isotope ratios provide information on source materials and geological history
Lu isotopes
- Lutetium has two naturally occurring isotopes: and 176Lu
- 175Lu stable isotope with 97.41% natural abundance
- 176Lu radioactive isotope with 2.59% natural abundance
- used in geochronological calculations
- Isotopic composition can vary slightly due to fractionation processes
Hf isotopes
- Hafnium has six naturally occurring isotopes: 174Hf, 176Hf, 177Hf, 178Hf, 179Hf, and 180Hf
- 176Hf both radiogenic (from 176Lu decay) and primordial in origin
- 177Hf commonly used as a reference isotope in ratio measurements
- key parameter in Lu-Hf dating and geochemical tracing
Natural abundance ratios
- 176Lu/177Hf ratio in chondrites approximately 0.0332
- 176Hf/177Hf ratio in chondrites about 0.282785
- Variations in these ratios used to determine age and petrogenetic history
- Ratios differ between reservoirs (crust, mantle, meteorites)
- Understanding natural abundances essential for accurate isotope measurements
Lu-Hf dating method
- Lu-Hf dating method allows determination of rock formation ages
- Particularly useful for dating igneous and metamorphic rocks
Principles of isochron dating
- based on linear relationship between 176Lu/177Hf and 176Hf/177Hf ratios
- Slope of proportional to age of the rock
- Y-intercept represents initial 176Hf/177Hf ratio
- Requires analysis of multiple cogenetic samples with varying Lu/Hf ratios
- Equation for Lu-Hf isochron:
Sample preparation techniques
- Careful mineral separation to isolate Lu and Hf-bearing phases
- Crushing and sieving to obtain appropriate grain sizes
- Heavy liquid separation to concentrate target minerals (garnet, zircon)
- Magnetic separation to further purify mineral fractions
- Acid washing to remove surface contamination
Analytical procedures
- Dissolution of samples in strong acids (HF, HNO3, HCl)
- Chemical separation of Lu and Hf using ion exchange chromatography
- to measure isotope ratios
- Internal standardization and external calibration for accurate results
- Data reduction and error propagation to calculate final ages and uncertainties
Applications in geochronology
- Lu-Hf system widely applied in various fields of
- Provides insights into Earth's early history and ongoing geological processes
Crustal evolution studies
- Lu-Hf isotopes track crustal growth and recycling over time
- Hf isotope compositions in zircons reveal crustal formation events
- Allows reconstruction of continental growth rates throughout Earth's history
- Helps identify periods of major crust formation (Archean, Proterozoic)
- Useful for understanding tectonic processes and plate reconstructions
Mantle differentiation
- Lu-Hf system traces mantle melting and differentiation events
- Hf isotopes in basalts provide information on mantle source compositions
- Allows identification of different mantle reservoirs (depleted, enriched)
- Helps constrain timing and extent of major mantle melting episodes
- Useful for understanding mantle dynamics and convection patterns
Meteorite dating
- Lu-Hf dating applied to various types of meteorites (chondrites, achondrites)
- Provides constraints on the timing of solar system formation
- Allows dating of early planetary differentiation events
- Helps establish chronology of asteroid and planetesimal formation
- Useful for understanding the thermal and chemical evolution of planetesimals
Lu-Hf vs Sm-Nd systems
- Lu-Hf and Sm-Nd systems both used in isotope geochemistry
- Comparison of these systems provides valuable insights into geological processes
Similarities and differences
- Both systems based on radioactive decay of parent to daughter isotope
- Lu-Hf system has longer half-life (37.1 Ga) compared to Sm-Nd (106 Ga)
- Lu-Hf more sensitive to fractionation during partial melting and crystallization
- Hf more incompatible than Lu, while Sm and Nd have similar compatibilities
- Lu-Hf system often provides higher precision ages for certain rock types
Complementary applications
- Combined Lu-Hf and Sm-Nd studies provide robust age constraints
- Dual-dating approach helps identify disturbances in isotopic systems
- Lu-Hf better for dating high-Lu/Hf phases (garnet, zircon)
- Sm-Nd useful for whole-rock dating and tracing crustal residence times
- Integration of both systems improves understanding of petrogenetic processes
Fractionation processes
- Fractionation of Lu and Hf during geological processes affects isotopic compositions
- Understanding fractionation crucial for accurate interpretation of Lu-Hf data
Lu-Hf behavior during melting
- Lu more compatible than Hf during mantle melting
- Partial melting increases Lu/Hf ratio in residual mantle
- Melts have lower Lu/Hf ratios compared to their source
- Degree of melting affects extent of Lu-Hf fractionation
- Fractional crystallization can further modify Lu/Hf ratios in evolving magmas
Partitioning in minerals
- Lu and Hf exhibit different partitioning behaviors in various minerals
- Garnet strongly incorporates Lu relative to Hf
- Zircon preferentially incorporates Hf over Lu
- Clinopyroxene and amphibole show moderate Lu/Hf fractionation
- Mineral partitioning affects Lu-Hf systematics in igneous and metamorphic rocks
Geochemical reservoirs
- Earth composed of distinct geochemical reservoirs with varying Lu-Hf compositions
- Understanding reservoir compositions crucial for interpreting isotopic data
Chondritic uniform reservoir (CHUR)
- CHUR represents bulk Earth composition based on chondritic meteorites
- Used as a reference for calculating values
- Present-day CHUR 176Hf/177Hf ratio approximately 0.282785
- CHUR evolution line represents undifferentiated mantle composition over time
- Deviations from CHUR indicate differentiation or mixing processes
Depleted mantle
- formed by extraction of continental crust
- Characterized by higher 176Hf/177Hf ratios compared to CHUR
- Represents source of most mid-ocean ridge basalts (MORB)
- Depleted mantle evolution line used to calculate model ages
- Provides insights into mantle differentiation and crust formation processes
Continental crust
- Continental crust generally has lower 176Hf/177Hf ratios than mantle
- Old continental crust characterized by highly negative epsilon Hf values
- Juvenile crust shows epsilon Hf values close to depleted mantle
- Crustal Lu-Hf compositions reflect age and petrogenetic history
- Useful for tracing crustal recycling and mantle-crust interactions
Analytical techniques
- Various analytical methods employed for Lu-Hf isotope measurements
- Choice of technique depends on sample type, required precision, and research goals
Thermal ionization mass spectrometry
- provides high-precision isotope ratio measurements
- Samples loaded onto metal filaments and thermally ionized
- Allows for precise measurement of 176Hf/177Hf ratios
- Requires chemical separation of Lu and Hf prior to analysis
- Suitable for whole-rock and mineral separate analyses
Laser ablation ICP-MS
- LA-ICP-MS enables in situ analysis of minerals (zircon, garnet)
- Laser ablates small spots on sample surface
- Allows for spatial resolution and analysis of zoning patterns
- Rapid analysis of large numbers of grains
- Useful for detrital zircon studies and provenance analysis
MC-ICP-MS methods
- Multi-collector ICP-MS provides high-precision isotope measurements
- Allows simultaneous measurement of multiple isotopes
- Higher sample throughput compared to TIMS
- Requires careful correction for isobaric interferences (176Yb on 176Hf)
- Suitable for both solution and laser ablation analyses
Data interpretation
- Proper interpretation of Lu-Hf data crucial for understanding geological processes
- Various notations and calculations used to present and analyze results
Epsilon Hf notation
- Epsilon Hf (εHf) expresses deviation from CHUR in parts per 10,000
- Calculated using formula:
- Positive εHf values indicate depleted mantle source
- Negative εHf values suggest crustal contamination or enriched sources
- Useful for comparing Hf isotope compositions across different geological settings
Model age calculations
- Hf model ages estimate time of separation from a reference reservoir
- Depleted mantle model age (TDM) assumes derivation from depleted mantle
- Two-stage model ages account for crustal residence time
- Calculated using measured 176Hf/177Hf and 176Lu/177Hf ratios
- Provides insights into crustal formation and reworking processes
Hf isotope evolution diagrams
- Plot 176Hf/177Hf or εHf against time to show isotopic evolution
- Evolution lines for different reservoirs (CHUR, depleted mantle, crust)
- Allows visualization of isotopic changes over geological time
- Useful for identifying mixing trends and source components
- Helps constrain timing of major geological events
Limitations and challenges
- Lu-Hf system, while powerful, faces several limitations and challenges
- Understanding these issues crucial for accurate data interpretation
Analytical precision issues
- Precise measurement of 176Lu/177Hf ratios challenging due to low Lu abundance
- Isobaric interference of 176Yb on 176Hf requires careful correction
- Matrix effects can influence isotope ratio measurements
- Interlaboratory calibration important for data comparison
- Improvements in mass spectrometry continuously enhancing precision
Sample contamination risks
- Lu and Hf concentrations often low, making samples susceptible to contamination
- Laboratory blanks must be carefully monitored and minimized
- Sample preparation procedures critical to avoid cross-contamination
- Weathering and alteration can disturb Lu-Hf systematics in natural samples
- Careful sample selection and screening essential for reliable results
Interpretation complexities
- Multiple geological processes can produce similar isotopic signatures
- Mixing of different reservoirs complicates straightforward interpretations
- Metamorphism and metasomatism may reset or disturb Lu-Hf systematics
- Inherited components in igneous rocks can skew age determinations
- Integration with other isotope systems and geochemical data often necessary
Case studies
- Examination of specific applications of Lu-Hf system in various geological contexts
- Demonstrates versatility and power of Lu-Hf isotope geochemistry
Lu-Hf in igneous petrology
- Lu-Hf isotopes used to trace magma sources and differentiation processes
- Zircon Hf isotopes reveal magma mixing and crustal assimilation
- Allows identification of juvenile vs. reworked crustal components in granitoids
- Helps constrain timing and extent of large igneous province formation
- Useful for understanding arc magmatism and subduction zone processes
Sedimentary provenance analysis
- Detrital zircon Hf isotopes trace sediment sources and transport pathways
- Combination with U-Pb ages provides powerful provenance tool
- Allows reconstruction of paleogeography and tectonic configurations
- Helps identify major crustal formation events in source regions
- Useful for understanding basin evolution and sedimentary recycling
Metamorphic rock dating
- Lu-Hf system applied to date metamorphic events and P-T-t paths
- Garnet Lu-Hf dating provides insights into metamorphic crystallization
- Allows dating of high-grade metamorphic events in lower crust
- Helps constrain rates of metamorphic processes and exhumation
- Useful for understanding tectonic and orogenic processes in deep crust
Key Terms to Review (25)
175Lu: 175Lu is a stable isotope of the element lutetium, with an atomic mass of approximately 175 atomic mass units. It plays a significant role in the Lu-Hf (lutetium-hafnium) isotopic system, which is used in geochronology and to trace geological processes. The ratio of 175Lu to its daughter isotope, 176Hf, helps geologists understand the age and evolution of rocks and minerals, making it an essential tool in isotope geochemistry.
176hf/177hf ratio: The 176hf/177hf ratio is the ratio of two isotopes of hafnium, specifically 176Hf and 177Hf. This ratio plays a critical role in understanding geological processes and the Lu-Hf isotopic system, which is used to date rocks and minerals and to trace their origins and evolution over time.
176Lu/175Lu ratio: The 176Lu/175Lu ratio is a crucial isotopic measurement used in the Lu-Hf geochronology system to determine the age of geological materials. This ratio indicates the relative abundance of the two isotopes of lutetium, where 176Lu is radioactive and decays to 176Hf over time, while 175Lu is stable. Understanding this ratio helps geoscientists date rocks and minerals, providing insights into their formation and the processes that shaped them.
Apatite: Apatite is a group of phosphate minerals that share a similar crystal structure and chemical composition, typically containing calcium phosphate along with other elements. This mineral is significant in geology and geochemistry, as it serves as a crucial source of phosphorus and can be used in radiometric dating methods, particularly in the context of the Lu-Hf system.
Chondritic uniform reservoir (chur): The chondritic uniform reservoir (CHUR) is a reference model used in isotope geochemistry to represent the average isotopic composition of chondritic meteorites, which are believed to reflect the primordial materials that formed the solar system. This model provides a baseline for understanding the isotopic ratios of elements such as neodymium and hafnium in various geological and extraterrestrial samples, facilitating comparisons between their isotopic signatures and those of chondrites.
Concordia Diagram: A concordia diagram is a graphical representation used in geochronology to illustrate the relationship between isotopes of a parent-daughter pair, particularly useful in age dating of minerals. It plots the ratios of isotopes on a two-dimensional graph, allowing geologists to visualize whether the samples have remained closed systems or undergone alteration. This diagram is essential for interpreting isotopic data from various decay systems and helps in assessing the reliability of age estimates.
Crustal evolution: Crustal evolution refers to the process by which the Earth's crust has changed and developed over geological time. This includes the formation, alteration, and recycling of crustal materials through tectonic activities, magmatism, and sedimentation, all of which are crucial in understanding the geological history of the planet. The insights gained from studying crustal evolution provide a framework for interpreting the composition and distribution of elements and isotopes in various 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.
Depleted mantle: The depleted mantle refers to a portion of the Earth's mantle that has undergone significant extraction of certain elements, especially incompatible elements like lithium, rubidium, and potassium, leaving it enriched in compatible elements such as magnesium and iron. This depletion occurs due to processes like partial melting, which leads to the formation of magmas that extract these elements from the mantle, resulting in a composition distinct from the more primitive, undepleted mantle material.
Epsilon hf: Epsilon hf is a measure used in geochemistry to describe the isotopic composition of hafnium (Hf) in relation to the isotopic composition of the Earth’s mantle. It is expressed as a deviation from a reference value, usually $$^{176}Hf/^{177}Hf$$ ratios, and provides insight into the sources and evolution of Hf in geological processes. This term is crucial in understanding the Lu-Hf dating system, which helps geologists determine the age of rocks and the processes that shaped them.
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.
Hf-176: hf-176 (Hafnium-176) is a stable isotope of hafnium that plays a crucial role in the Lu-Hf dating system, particularly in geochronology and understanding the evolution of terrestrial and extraterrestrial materials. This isotope is integral to determining the ages of rocks and minerals, helping to provide insights into geological processes and the history of the Earth and other planetary bodies.
Hf-177: hf-177 (Hafnium-177) is a stable isotope of hafnium, an element with atomic number 72. It is particularly important in the context of the Lu-Hf geochronology system, which is used to date geological materials and understand the processes involved in the formation of the Earth and other planetary bodies.
Isochron line: An isochron line is a graphical representation used in radiometric dating that depicts the age of a group of samples based on their isotopic compositions. This line reflects the relationship between the ratios of parent and daughter isotopes in a set of coeval samples, allowing for the determination of their formation age while compensating for potential alterations or disturbances in isotope ratios over time.
Isochron method: The isochron method is a radiometric dating technique used to determine the age of rocks and minerals by analyzing the ratio of parent and daughter isotopes within a sample. This method relies on the assumption that the system remained closed to parent and daughter isotopes since the time of formation, allowing for accurate age determinations through the construction of isochron plots. It is particularly useful in systems like Lu-Hf and U-Th-Pb, where multiple isotopes can provide cross-verification of ages.
Isotope dilution: Isotope dilution is a method used in isotope geochemistry to determine the concentration of an element in a sample by adding a known quantity of isotopically enriched material. This technique allows for precise measurements by comparing the ratios of isotopes in the sample before and after dilution. It's particularly useful in systems like the Lu-Hf system, where accurate isotopic ratios are critical for dating and tracing geological processes.
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.
Lu-176: Lu-176 is a radioactive isotope of lutetium, which has a half-life of approximately 38 billion years. This long half-life makes it an important tool in geochronology, particularly in the Lu-Hf (Lutetium-Hafnium) isotopic system used to date geological samples and to study the evolution of planetary bodies.
Mantle differentiation: Mantle differentiation refers to the process through which the Earth's mantle separates into distinct layers or reservoirs based on variations in chemical composition and physical properties. This process is crucial for understanding how elements are redistributed in the Earth's interior, influencing the formation of different mantle isotope reservoirs and affecting isotopic systems that help trace the history of the Earth’s formation and evolution.
Mass spectrometric analysis: Mass spectrometric analysis is a powerful analytical technique used to measure the mass-to-charge ratio of ions, enabling the identification and quantification of chemical species in a sample. This method is crucial for understanding isotopic compositions, particularly in the context of radiogenic and stable isotopes, which play a vital role in isotope geochemistry and dating applications.
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.
Quadrupole mass spectrometer: A quadrupole mass spectrometer is an analytical instrument used to measure the mass-to-charge ratio of ions, consisting of four parallel rods that create an oscillating electric field to filter ions based on their stability. This design allows for precise mass analysis and is especially useful in isotope geochemistry for determining isotopic compositions, including in systems like Lu-Hf.
Radiogenic Isotopes: Radiogenic isotopes are isotopes that are formed through the radioactive decay of parent isotopes. They provide crucial information about geological processes, age dating, and the evolution of the Earth’s crust and mantle over time.
TIMS: Thermal Ionization Mass Spectrometry (TIMS) is an analytical technique used to determine the isotopic composition of elements, particularly useful for radiometric dating and tracing geological processes. This method utilizes thermal ionization to convert sample atoms into ions, which are then separated and detected based on their mass-to-charge ratio. TIMS is particularly significant in isotope geochemistry as it provides high precision and accuracy in measuring isotopes like Lutetium (Lu) and Hafnium (Hf) in the Lu-Hf dating system.
Zircon: Zircon is a mineral that is widely used in geochronology due to its ability to preserve information about the age of geological formations. Its resilience to weathering and high temperatures makes it an ideal candidate for radiometric dating, especially in systems like the Lu-Hf and U-Th-Pb methods, which help determine the timing of geological events and the evolution of Earth's crust.