The Re-Os system is a powerful tool in isotope geochemistry, used for dating and tracing geological processes. It provides unique insights into Earth's mantle, crust, and ore deposits, complementing other radiogenic isotope systems used in geochemistry.
Rhenium and osmium have distinct properties that make them valuable for studying Earth's history. Their decay scheme, natural abundance, and geochemical behavior allow scientists to investigate a wide range of geological phenomena, from mantle evolution to ore formation and petroleum generation.
Fundamentals of Re-Os system
Re-Os system serves as a powerful tool in isotope geochemistry for dating and tracing geological processes
Provides unique insights into the formation and evolution of Earth's mantle, crust, and ore deposits
Complements other radiogenic isotope systems used in geochemistry
Rhenium and osmium properties
Top images from around the web for Rhenium and osmium properties
Solvent extraction used to isolate Os from other elements
Typically employs carbon tetrachloride or chloroform
Rhenium separated using ion exchange chromatography
Ultra-clean laboratory conditions required to minimize contamination
Spike addition for isotope dilution analysis to determine elemental concentrations
Mass spectrometry for Re-Os
Negative (N-TIMS) commonly used for Os isotope analysis
Provides high precision measurements of Os isotope ratios
Utilizes platinum filaments for sample loading
(ICP-MS) employed for Re analysis
Multi-collector ICP-MS allows simultaneous measurement of multiple isotopes
Laser ablation ICP-MS enables in-situ analysis of Re-Os in minerals
Provides spatial resolution for heterogeneous samples
Data reduction and interpretation
Correction for instrumental mass fractionation using standard reference materials
Blank correction to account for laboratory contamination
Isobaric interference corrections (especially for 187Os and 187Re)
Calculation of isotope ratios and elemental concentrations
Application of age equations or isochron methods for geochronological interpretations
Assessment of analytical uncertainties and propagation of errors
Re-Os in different geological reservoirs
Re-Os systematics vary significantly across different geological reservoirs
Provides insights into the chemical evolution and differentiation of Earth's major components
Helps trace the movement of material between different reservoirs over geological time
Mantle composition and heterogeneity
Primitive mantle characterized by chondritic Re/Os ratios and 187Os/188Os of ~0.13
Depleted mantle shows lower Re/Os ratios due to melt extraction events
Results in subchondritic 187Os/188Os ratios over time
Mantle plumes often exhibit distinct Os isotope signatures
May reflect contribution from recycled crustal materials or core-mantle interaction
Abyssal peridotites and ophiolites provide insights into upper mantle Os isotope composition
Crustal Re-Os signatures
Continental crust generally exhibits elevated Re/Os ratios compared to mantle
Results from incompatible behavior of Re during partial melting
Crustal rocks develop radiogenic Os isotope compositions over time
187Os/188Os ratios can exceed 1.0 in old continental crust
Sedimentary rocks show wide range of Os isotope compositions
Reflect mixing between crustal and mantle-derived components
Ore deposits often preserve initial Os isotope ratios of their source regions
Oceanic vs continental lithosphere
Oceanic lithosphere typically shows less radiogenic Os isotope compositions than continental lithosphere
Reflects younger age and less evolved nature of oceanic crust
Abyssal peridotites provide insights into the Os isotope composition of oceanic lithosphere
Often show evidence of melt depletion and subsequent enrichment processes
Continental lithospheric mantle can preserve ancient Os isotope signatures
Subcontinental lithospheric mantle xenoliths used to study long-term mantle evolution
Contrast between oceanic and continental lithosphere helps trace subduction and recycling processes
Re-Os isotope systematics
Re-Os isotope systematics provide powerful tools for understanding geological processes
Interpretation of Re-Os data requires consideration of various factors affecting the isotope system
Careful analysis of isotope ratios and model ages yields insights into rock formation and evolution
Initial Os ratios
Initial 187Os/188Os ratio (Os_i) reflects the isotopic composition at the time of rock formation
Calculated by subtracting the radiogenic Os component from the measured ratio
Provides information about the source of the rock or mineral
Mantle-derived rocks typically have low Os_i values (~0.13)
Crustal-derived materials often show elevated Os_i values
Used to distinguish between different magma sources and assess crustal contamination
Model ages vs isochron ages
Re-Os model ages calculated assuming a single-stage evolution from a known initial composition
Often referenced to chondritic uniform reservoir (CHUR) or primitive upper mantle (PUM)
Model age equation: TMA=λ1ln[(187Re/188Os)sample−(187Re/188Os)reference(187Os/188Os)sample−(187Os/188Os)reference+1]
Isochron ages determined from multiple cogenetic samples with varying Re/Os ratios
Slope of the isochron yields the age, intercept gives the initial Os ratio
More robust than model ages for complex geological systems
Mixing and assimilation effects
Mixing of materials with different Re-Os compositions can produce complex isotope signatures
Common in magmatic systems where crustal assimilation occurs
Binary mixing often results in hyperbolic mixing curves on isotope diagrams
Assimilation-fractional crystallization (AFC) processes can significantly alter Re-Os systematics
May lead to erroneous age interpretations if not properly accounted for
Careful examination of trace element patterns and other isotope systems helps identify mixing effects
Challenges and limitations
Re-Os system presents unique challenges in isotope geochemistry
Understanding limitations crucial for accurate interpretation of Re-Os data
Ongoing research aims to address and mitigate these challenges
Low abundance and analytical precision
Ultra-low concentrations of Re and Os in many geological materials
Requires highly sensitive analytical techniques
Increases susceptibility to contamination during sample preparation
Precision of Os isotope measurements limited by low abundance of 187Os
Typically requires large sample sizes for accurate analysis
Improvements in mass spectrometry techniques gradually enhancing analytical precision
Development of N-TIMS and MC-ICP-MS methods
Disturbance of Re-Os system
Re-Os system susceptible to disturbance by geological processes
Metamorphism can cause redistribution of Re and Os
Hydrothermal alteration may introduce or remove Re and Os from the system
Mobility of Re under oxidizing conditions can lead to open-system behavior
Particularly problematic in weathered or altered samples
Post-formation processes may reset or partially reset the Re-Os systematics
Complicates interpretation of ages and initial ratios
Interpretation of complex datasets
Heterogeneous distribution of Re and Os in many geological materials
Can result in scatter on isochron diagrams
Requires careful sample selection and characterization
Multiple geological events may be recorded in a single sample
Challenging to deconvolve different age components
Mixing of different reservoirs can produce complex isotope signatures
Requires integration with other geochemical and geological data for robust interpretation
Limited database of Re-Os compositions for some geological reservoirs
Ongoing research expanding our understanding of Re-Os systematics in various settings
Case studies and applications
Re-Os system applied to diverse geological problems across various settings
Case studies demonstrate the power and versatility of Re-Os isotope geochemistry
Applications continue to expand as analytical techniques improve
Platinum group element deposits
Re-Os dating of sulfides in Bushveld Complex, South Africa
Provided precise age constraints on the formation of world's largest PGE deposit
Yielded an age of 2054.4 ± 1.3 Ma for the Merensky Reef
Os isotope studies of Noril'sk-Talnakh Ni-Cu-PGE deposits, Russia
Revealed contribution of crustal contamination to ore formation
Helped constrain the source of metals and timing of mineralization
Organic-rich sedimentary rocks
Re-Os dating of black shales from the Exshaw Formation, Western Canada Sedimentary Basin
Yielded depositional age of 358.0 ± 3.4 Ma
Provided insights into Late Devonian paleogeography and ocean chemistry
Os isotope stratigraphy of Cenomanian-Turonian boundary sediments
Recorded global Os isotope excursion related to oceanic anoxic event (OAE2)
Helped constrain timing and duration of widespread ocean anoxia
Mantle xenoliths and ophiolites
Re-Os study of mantle xenoliths from the Kaapvaal craton, South Africa
Revealed ancient (>3 Ga) depletion events in the subcontinental lithospheric mantle
Provided evidence for long-term stability of cratonic keels
Os isotope analysis of chromitites from the Oman ophiolite
Indicated presence of ancient, recycled crustal material in the mantle source
Challenged models of ophiolite formation and mantle heterogeneity
Future directions in Re-Os research
Ongoing advancements in Re-Os isotope geochemistry open new avenues for research
Integration with other isotope systems and analytical techniques enhances applicability
Emerging applications continue to expand the utility of Re-Os systematics in geosciences
Improvements in analytical techniques
Development of high-sensitivity mass spectrometers for Re-Os analysis
Enables measurement of smaller sample sizes and lower concentrations
Refinement of in-situ analytical methods (laser ablation ICP-MS)
Allows for high spatial resolution studies of complex samples
Automation of chemical separation procedures
Increases sample throughput and reduces potential for contamination
Improved blank reduction techniques
Enhances precision for low-abundance samples
Integration with other isotope systems
Combined Re-Os and Lu-Hf studies in mantle geochemistry
Provides complementary information on mantle evolution and heterogeneity
Integration of Re-Os with U-Pb and Ar-Ar
Allows for multi-system dating of complex geological events
Coupling Re-Os with stable isotope systems (O, S)
Enhances understanding of fluid sources and ore-forming processes
Development of coupled chronometers (Re-Os-Pb)
Improves constraints on the timing of mineralizing events
Emerging applications in geosciences
Re-Os dating of diagenetic pyrite in sedimentary basins
Provides insights into timing of fluid flow and hydrocarbon migration
Application to climate change studies through analysis of marine sediments
Traces changes in weathering inputs and ocean circulation patterns
Re-Os fingerprinting of conflict minerals and precious metals
Aids in determining the provenance of economically important resources
Expanding use in planetary sciences and meteorite studies
Constrains early solar system processes and planetary differentiation
Key Terms to Review (18)
Age dating of molybdenite: Age dating of molybdenite refers to the process of determining the age of geological materials using the rhenium-osmium (Re-Os) isotopic system, specifically focusing on molybdenite (MoS2), a common mineral found in various ore deposits. This method is significant because it helps establish the timing of mineralization events and the formation of associated deposits, which is crucial for understanding geological history and processes.
Chalcophile elements: Chalcophile elements are a group of chemical elements that have a strong affinity for sulfur and tend to bond with it to form sulfide minerals. These elements are often found in ore deposits and are crucial in the study of geochemistry, especially when analyzing mineral formation processes and the behavior of elements in magmatic systems.
Crustal recycling: Crustal recycling refers to the process where continental crust is created, destroyed, and reshaped through geological processes such as subduction, erosion, and sedimentation. This dynamic cycle plays a critical role in the formation of continental crust and the recycling of elements, which are essential for understanding the geological history and evolution of the Earth’s surface.
Geochemical modeling: Geochemical modeling is a computational approach used to simulate and understand the chemical processes and interactions occurring in natural systems. It helps in predicting how elements behave under different conditions and allows scientists to visualize complex geochemical cycles, such as those involving isotopes and mineral interactions. This modeling is especially useful in studying the distribution of elements, reaction kinetics, and the evolution of geological formations over time.
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.
Half-life: Half-life is the time required for half of the radioactive atoms in a sample to decay into their stable daughter isotopes. This concept is essential for understanding the rate of radioactive decay, which links to various processes including radiometric dating and the behavior of isotopes over time.
Identification of mineral sources: The identification of mineral sources refers to the process of determining the origin and composition of minerals found in geological samples. This is crucial for understanding mineral resources, their formation processes, and their economic significance, particularly in the context of metal extraction and environmental studies.
Incompatible Elements: Incompatible elements are those that preferentially concentrate in the liquid phase during partial melting and are not easily incorporated into the solid phase of minerals. This characteristic is crucial in geochemistry, particularly in understanding the processes of magma formation and evolution, as well as the distribution of elements in different rock types.
Inductively Coupled Plasma Mass Spectrometry: Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is a powerful analytical technique used to detect and quantify trace elements and isotopes in various samples. It works by ionizing the sample with an inductively coupled plasma and then analyzing the ions with mass spectrometry, making it essential for determining isotopic ratios, understanding radiometric dating, and assessing environmental contamination.
Isotopic Equilibrium: Isotopic equilibrium refers to the state in which the isotopic composition of two or more substances reaches a balance, typically due to physical or chemical processes that allow isotopes to exchange or redistribute among the substances. This concept is crucial for understanding how isotopic signatures can reflect environmental conditions and processes like evaporation, condensation, and temperature changes.
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-dependent fractionation: Mass-dependent fractionation refers to the phenomenon where isotopes of the same element are separated based on their mass during physical or chemical processes, leading to variations in isotope ratios. This process is crucial for understanding the distribution of isotopes in natural systems, as it affects measurements and interpretations in areas such as biological and geological processes, including those related to isotope notation, kinetic effects, geochemical cycles, and phase changes.
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.
Ophiolite complexes: Ophiolite complexes are sections of the Earth's oceanic crust and the underlying upper mantle that have been uplifted and exposed above sea level, often associated with tectonic plate boundaries. These geological formations provide critical insights into the processes of oceanic crust formation, mantle composition, and the dynamics of plate tectonics.
Osmium-187: Osmium-187 is a stable isotope of osmium with 115 neutrons and 76 protons, notable for its role in geochronology and the Re-Os (rhenium-osmium) dating system. It is particularly significant in the study of mantle-derived rocks and meteorites, helping scientists understand the age and formation processes of various geological materials.
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.
Rhenium-187: Rhenium-187 is a radioactive isotope of rhenium that decays to form osmium-187, playing a crucial role in the Re-Os dating system. This isotope is particularly significant in geochronology, as it helps in understanding the age and evolution of geological materials, especially those related to mantle processes and crust formation.
Thermal ionization mass spectrometry: Thermal ionization mass spectrometry (TIMS) is a technique used to measure the isotopic composition of elements by heating a sample to high temperatures, causing atoms to ionize. This method allows for precise measurements of isotopic ratios, which are essential for understanding various geochemical processes, dating techniques, and the behavior of elements in different environments.