3.4 Sm-Nd system
7 min read•august 21, 2024
The Samarium-Neodymium (Sm-Nd) system is a powerful tool in isotope geochemistry for dating rocks and understanding Earth's evolution. It uses the radioactive decay of 147Sm to 143Nd to determine ages and trace geological processes over billions of years.
This system provides crucial insights into crustal formation, , and planetary evolution. The long half-life of 147Sm makes it ideal for dating ancient rocks, while its resistance to disturbance during geological processes ensures reliable results.
Sm-Nd system overview
- Samarium-Neodymium (Sm-Nd) system serves as a powerful tool in isotope geochemistry for dating rocks and understanding Earth's evolution
- Utilizes the radioactive decay of 147Sm to 143Nd to determine ages and trace geological processes
- Provides insights into crustal formation, mantle differentiation, and planetary evolution over billions of years
Isotopes of Sm and Nd
Naturally occurring isotopes
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- Samarium consists of seven naturally occurring isotopes (144Sm, 147Sm, 148Sm, 149Sm, 150Sm, 152Sm, 154Sm)
- Neodymium comprises seven stable isotopes (142Nd, 143Nd, 144Nd, 145Nd, 146Nd, 148Nd, 150Nd)
- 147Sm decays to 143Nd through alpha decay, forming the basis of the Sm-Nd dating system
- Relative abundances of these isotopes vary slightly due to radioactive decay and fractionation processes
Radioactive decay process
- 147Sm undergoes alpha decay to produce 143Nd
- Decay equation
- (λ) of 147Sm equals 6.54 × 10^-12 yr^-1
- Alpha particle emission changes the atomic number from 62 (Sm) to 60 (Nd)
- Process occurs over billions of years due to the long half-life of 147Sm
Sm-Nd dating principles
Half-life of 147Sm
- 147Sm has a half-life of approximately 106 billion years
- Long half-life makes the Sm-Nd system suitable for dating very old rocks and minerals
- Allows for accurate age determinations of early Earth and planetary materials
- Decay rate remains constant regardless of temperature, pressure, or chemical environment
Isochron method
- Plots 143Nd/144Nd ratio against 147Sm/144Nd ratio for multiple cogenetic samples
- Slope of the isochron line determines the age of the rock or mineral suite
- Y-intercept provides the initial 143Nd/144Nd ratio at the time of formation
- Requires samples with varying Sm/Nd ratios but identical initial Nd isotopic compositions
- Equation for the isochron
Geochemical behavior of Sm-Nd
Fractionation during melting
- Sm and Nd behave similarly during most geological processes due to their similar ionic radii and charge
- Slight fractionation occurs during partial melting of the mantle
- Nd preferentially enters the melt phase compared to Sm
- Results in lower Sm/Nd ratios in crustal rocks compared to the mantle
- Fractionation factor between Sm and Nd typically ranges from 1.1 to 1.3
Compatibility in minerals
- Both Sm and Nd are incompatible elements in most rock-forming minerals
- Preferentially concentrate in the liquid phase during magmatic processes
- Garnet strongly partitions Sm relative to Nd, leading to higher Sm/Nd ratios
- Plagioclase slightly favors Nd over Sm, resulting in lower Sm/Nd ratios
- Clinopyroxene and amphibole show minimal fractionation between Sm and Nd
Sm-Nd in crustal evolution
CHUR model
- stands for Chondritic Uniform Reservoir
- Represents the bulk Earth composition based on chondritic meteorites
- Serves as a reference for comparing Nd isotopic compositions of rocks
- CHUR evolution line describes the change in 143Nd/144Nd ratio over time for bulk Earth
- Equation for CHUR
Depleted mantle model
- Represents the evolution of the upper mantle after extraction of continental crust
- Characterized by higher Sm/Nd ratios compared to CHUR
- Results in more radiogenic Nd isotopic compositions over time
- Used to calculate model ages for crustal rocks
- Depleted mantle evolution line lies above the CHUR line on Nd isotope diagrams
Epsilon Nd notation
Calculation and interpretation
- Expresses the deviation of a sample's 143Nd/144Nd ratio from CHUR
- Calculated using the formula
- Positive values indicate derivation from a depleted mantle source
- Negative εNd values suggest incorporation of older crustal material
- εNd = 0 represents a composition identical to CHUR
Temporal variations
- εNd values change over time due to radioactive decay and crustal evolution
- Present-day εNd values differ from initial εNd values at the time of rock formation
- Crustal rocks generally evolve towards more negative εNd values over time
- Mantle-derived rocks tend to develop more positive εNd values with age
- Plotting εNd vs. time reveals trends in crustal growth and recycling
Applications in geochronology
Igneous rock dating
- Determines crystallization ages of and minerals
- Particularly useful for dating mafic and ultramafic rocks poor in other datable minerals
- Applies to a wide range of igneous rock types (basalts, granites, pegmatites)
- Provides insights into magma source characteristics and crustal contamination
- Often combined with other isotope systems for cross-validation (U-Pb, Rb-Sr)
Metamorphic rock dating
- Dates metamorphic events by analyzing newly formed or recrystallized minerals
- Garnet commonly used due to its high closure temperature for the Sm-Nd system
- Helps constrain timing of high-grade metamorphism and crustal evolution
- Can reveal multiple metamorphic events in polymetamorphic terranes
- Useful for dating eclogites and other high-pressure
Sm-Nd in provenance studies
Sedimentary rock analysis
- Determines the source areas of sedimentary rocks and sediments
- Utilizes the fact that Sm-Nd ratios remain relatively unchanged during weathering and transport
- Compares Nd isotopic compositions of sediments to potential source rocks
- Helps reconstruct paleogeography and sediment transport pathways
- Useful in petroleum geology for understanding basin evolution and sediment routing
Crustal residence time
- Calculates the time since extraction of crustal material from the mantle
- Uses the to estimate Nd model ages
- Provides insights into the age and evolution of continental crust
- Helps distinguish between juvenile and recycled crustal components
- Equation for Nd model age
Analytical techniques
Mass spectrometry methods
- Thermal Ionization (TIMS) provides high-precision Nd isotope measurements
- Multicollector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) offers rapid analysis with good precision
- Laser Ablation ICP-MS allows for in-situ analysis of minerals with spatial resolution
- Secondary Ion Mass Spectrometry (SIMS) enables microanalysis of Sm and Nd isotopes in small sample volumes
- Isotope dilution techniques used to accurately determine Sm and Nd concentrations
Sample preparation
- Involves careful mineral separation and purification to avoid contamination
- Acid digestion of rock samples using HF, HNO3, and HCl
- Ion exchange chromatography to separate Sm and Nd from other elements
- Ultra-clean laboratory conditions required to minimize blank contributions
- Spike addition for isotope dilution analysis to determine elemental concentrations
Limitations and challenges
Analytical precision
- Precision limited by low abundance of 147Sm and slow decay rate
- Requires large sample sizes for high-precision measurements using TIMS
- Interferences from isobaric nuclides (142Ce, 144Sm) must be carefully corrected
- Matrix effects in ICP-MS can affect accuracy and precision of measurements
- Long counting times often necessary to achieve desired precision levels
Geological complexities
- Open-system behavior can disturb the Sm-Nd system, leading to inaccurate ages
- Metamorphism may cause partial resetting of the isotope system
- Mixing of different source components can complicate interpretation of Nd isotope data
- Inherited components in igneous rocks can affect the accuracy of crystallization ages
- Crustal contamination of mantle-derived magmas can obscure primary isotopic signatures
Sm-Nd vs other isotope systems
Rb-Sr system comparison
- Rb-Sr system has a shorter half-life (48.8 billion years) compared to Sm-Nd
- Rb and Sr more susceptible to disturbance during metamorphism and alteration
- Sm-Nd system generally more robust for dating older rocks and high-grade metamorphic events
- Rb-Sr better suited for dating low-temperature processes and some sedimentary rocks
- Combined use of Sm-Nd and Rb-Sr can provide complementary information on
Lu-Hf system comparison
- Lu-Hf system has a similar half-life (37.1 billion years) to Sm-Nd
- Both systems behave similarly during mantle melting and crustal processes
- Lu-Hf system more sensitive to garnet fractionation in the source region
- Hf isotopes can be measured in-situ on zircons, providing additional geochronological information
- Combining Sm-Nd and Lu-Hf data enhances understanding of mantle evolution and crustal growth
Case studies in Sm-Nd dating
Planetary materials
- Sm-Nd dating of lunar rocks constrains the age of the Moon and its magmatic history
- Analysis of Martian meteorites provides insights into the geological evolution of Mars
- Dating of chondritic meteorites helps determine the age of the solar system
- Sm-Nd systematics in differentiated meteorites reveal early planetary differentiation processes
- Studies of calcium-aluminum-rich inclusions (CAIs) constrain the earliest stages of solar system formation
Ancient crustal fragments
- Sm-Nd dating of Acasta Gneiss Complex in Canada confirms its age as one of the oldest known crustal rocks (>4.0 Ga)
- Analysis of Jack Hills zircons from Australia provides evidence for early crustal formation on Earth
- Sm-Nd studies of Archean greenstone belts reveal the nature of early crustal growth and mantle evolution
- Dating of ancient metamorphic terranes helps reconstruct the assembly and evolution of early continents
- Sm-Nd isotope mapping of cratons provides insights into the architecture and growth of continental nuclei
Key Terms to Review (21)
Age calculation: Age calculation refers to the method of determining the age of geological materials based on the decay of radioactive isotopes. This process utilizes the ratios of parent isotopes to daughter isotopes to provide precise numerical ages, helping geologists understand the timing of events in Earth's history.
Chemical Separation: Chemical separation refers to the process of isolating specific isotopes or elements from a mixture through various chemical methods. This technique is crucial for obtaining pure samples needed for precise isotopic analysis, such as in the Sm-Nd system, where understanding the isotopic composition can reveal information about geological processes and age dating.
Chur: Chur is a term that refers to the relationship between samarium (Sm) and neodymium (Nd) isotopes in geochemistry, particularly within the context of the Sm-Nd isotopic system. This relationship is crucial for understanding geological processes, as it provides insights into the age and evolution of rocks and minerals through radiogenic isotopic dating methods.
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 model: The depleted mantle model describes the composition and evolution of the Earth's mantle, specifically focusing on the idea that certain areas of the mantle have undergone extensive partial melting, resulting in a depletion of certain elements like incompatible trace elements. This model is essential for understanding the geochemical processes that shape mantle-derived rocks and is particularly significant in the context of isotopic systems used in geochemistry, like the Sm-Nd system.
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 isotopic composition: Initial isotopic composition refers to the ratio of isotopes of a given element in a sample at the moment of its formation or the time of interest, before any alterations occur due to decay or other processes. This concept is crucial in isotope geochemistry as it serves as a baseline for understanding how isotopes evolve over time, particularly in radiogenic systems like the Sm-Nd system.
Isochrons: Isochrons are lines on a graph that represent points of equal age, typically used in geochronology to date rocks and minerals. They are constructed using isotopic ratios from samples and can reveal the time of formation of geological materials when plotted on a graph, allowing for the understanding of geological processes over time.
Isotope ratio: An isotope ratio is the relative abundance of different isotopes of a particular element, expressed as a ratio. This measurement is crucial for understanding various processes, such as radioactive decay, where the ratio can provide insights into the age of geological formations or archaeological artifacts. Additionally, isotope ratios are integral to examining chemical reactions and tracing sources in mass spectrometry and accelerator mass spectrometry.
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.
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 spectrometry: Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions, enabling the identification and quantification of different isotopes in a sample. This technique is crucial in isotope geochemistry for analyzing stable and radioactive isotopes, understanding decay processes, and determining isotopic ratios in various materials.
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.
Neodymium-143: Neodymium-143 is a stable isotope of neodymium that plays a critical role in understanding geological processes and the evolution of the Earth's mantle. It is primarily used as an isotopic tracer in geochemistry, especially in studies related to mantle differentiation, crust-mantle interactions, and as a part of the samarium-neodymium (Sm-Nd) dating system to determine the ages of rocks and minerals.
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.
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.
Samarium-147: Samarium-147 is a radioactive isotope of samarium that decays to neodymium-143 through beta decay. It is a crucial tool in geochronology, particularly in the samarium-neodymium dating method, which allows scientists to determine the age of rocks and minerals by analyzing the parent-daughter relationships between samarium-147 and neodymium-143. This isotope plays an important role in understanding geological processes and the history of Earth's formation.
T. s. m. h. a. f. w. g. g. b. h. h. v. a. l. c.: The term refers to a specific geochemical technique used to investigate the isotopic compositions of samarium (Sm) and neodymium (Nd) in geological samples, particularly for understanding their age and evolution. This technique helps researchers decipher the history and processes of rock formation, migration of fluids, and crustal development, providing insight into Earth's geochemical cycles.
W. M. F. G. L. H. M. M. Schmid: W. M. F. G. L. H. M. M. Schmid refers to a specific geochemical framework developed by Wolfgang M. F. G. L. H. M. M. Schmid for understanding isotopic systems, particularly the Sm-Nd isotopic system, in geological processes and time scales. This framework emphasizes the role of isotopes in tracing geological history, including mantle processes and crustal development.
εnd: εnd, or epsilon Nd, is a notation used to express the isotopic composition of neodymium in a sample, relative to a standard reference material. This value provides crucial insights into the source and evolution of geological materials, reflecting processes such as crustal formation, mantle evolution, and the differentiation of planetary bodies.