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
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Rhenium (Re) atomic number 75 belongs to the transition metal group
Osmium (Os) atomic number 76 classified as a platinum group element
Both elements exhibit high melting points (Re: 3180°C, Os: 3033°C)
Rhenium displays a hexagonal close-packed crystal structure
Osmium possesses the highest density of any naturally occurring element (22.59 g/cm³)
Decay scheme of Re-Os
187 R e ^{187}Re 187 R e decays to 187 O s ^{187}Os 187 O s through beta decay
Half-life of 187 R e ^{187}Re 187 R e approximately 41.6 billion years
Decay constant (λ) for 187 R e ^{187}Re 187 R e equals 1.666 × 10^-11 year^-1
Decay equation: 187 O s = 187 O s i + 187 R e ( e λ t − 1 ) ^{187}Os = ^{187}Os_i + ^{187}Re(e^{λt} - 1) 187 O s = 187 O s i + 187 R e ( e λ t − 1 )
Allows for dating of very old geological materials (billions of years)
Natural abundance and distribution
Rhenium average crustal abundance approximately 1 part per billion (ppb)
Osmium average crustal abundance about 50 parts per trillion (ppt)
Both elements concentrated in the Earth's core due to siderophile nature
Enriched in certain ore deposits (molybdenite, chromite)
Found in trace amounts in organic-rich sedimentary rocks and mantle-derived materials
Geochemical behavior of Re-Os
Re-Os system provides unique insights into mantle processes and crustal evolution
Behavior of Re and Os differs significantly from lithophile elements used in other isotope systems
Understanding their geochemical characteristics crucial for interpreting Re-Os data in various geological settings
Siderophile and chalcophile affinities
Rhenium exhibits strong siderophile (iron-loving) behavior
Concentrates in metallic phases during planetary differentiation
Partitions into the Earth's core during early formation
Osmium displays both siderophile and chalcophile (sulfur-loving) tendencies
Forms strong bonds with sulfur in sulfide minerals
Concentrates in base metal sulfides and platinum group minerals
These affinities result in fractionation between Re and Os during geological processes
Compatibility in mantle minerals
Rhenium moderately incompatible in most mantle minerals
Preferentially partitions into melts during partial melting
Enriched in basaltic magmas relative to mantle source
Osmium highly compatible in mantle minerals
Retained in residual mantle during partial melting
Concentrated in refractory mantle phases (olivine, chromite)
Compatibility contrast leads to Re/Os fractionation during mantle melting events
Fractionation during melting processes
Partial melting of the mantle causes significant Re-Os fractionation
Rhenium preferentially enters the melt phase
Osmium remains in the residual solid mantle
Results in elevated Re/Os ratios in crustal rocks compared to mantle
Leads to distinct isotopic evolution paths for mantle and crustal reservoirs
Fractionation degree depends on melting conditions (temperature, pressure, oxygen fugacity)
Applications in geochronology
Re-Os system provides unique geochronological applications in isotope geochemistry
Allows dating of materials and processes not easily accessible with other isotope systems
Particularly useful for understanding the timing of ore formation and petroleum generation
Dating of ore deposits
Re-Os system effective for dating sulfide-rich ore deposits
Applies to porphyry copper deposits, massive sulfide deposits
Molybdenite (MoS₂) ideal mineral for Re-Os dating due to high Re content and low initial Os
Isochron method used to determine the age of mineralization events
Provides insights into the timing of hydrothermal fluid circulation and metal deposition
Age determination of petroleum
Re-Os system applied to date the timing of petroleum generation
Rhenium and osmium incorporated into organic matter during deposition
Subsequent thermal maturation leads to hydrocarbon generation
Asphaltene fraction of crude oil analyzed for Re-Os isotopic composition
Yields information on the age of source rocks and timing of oil formation
Helps constrain basin thermal history and petroleum system evolution
Mantle evolution studies
Re-Os systematics in mantle-derived rocks provide insights into Earth's early history
Allows investigation of core formation and mantle differentiation processes
Osmium isotope ratios in mantle xenoliths reflect long-term evolution of the mantle
Platinum group element alloys in ophiolites preserve ancient mantle Os isotope signatures
Contributes to understanding the chemical heterogeneity of the Earth's mantle over time
Analytical techniques for Re-Os
Precise measurement of Re-Os isotopes requires specialized analytical techniques
Advances in mass spectrometry have greatly improved the accuracy and precision of Re-Os analyses
Careful sample preparation and data reduction essential for reliable results
Sample preparation methods
Chemical separation of Re and Os from rock or mineral samples
Involves acid digestion techniques (Carius tube, high-pressure asher)
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 thermal ionization mass spectrometry (N-TIMS) commonly used for Os isotope analysis
Provides high precision measurements of Os isotope ratios
Utilizes platinum filaments for sample loading
Inductively coupled plasma mass spectrometry (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 187 O s ^{187}Os 187 O s and 187 R e ^{187}Re 187 R e )
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 187 O s / 188 O s ^{187}Os/^{188}Os 187 O s / 188 O s of ~0.13
Depleted mantle shows lower Re/Os ratios due to melt extraction events
Results in subchondritic 187 O s / 188 O s ^{187}Os/^{188}Os 187 O s / 188 O s 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
187 O s / 188 O s ^{187}Os/^{188}Os 187 O s / 188 O s 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 187 O s / 188 O s ^{187}Os/^{188}Os 187 O s / 188 O s 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: T M A = 1 λ l n [ ( 187 O s / 188 O s ) s a m p l e − ( 187 O s / 188 O s ) r e f e r e n c e ( 187 R e / 188 O s ) s a m p l e − ( 187 R e / 188 O s ) r e f e r e n c e + 1 ] T_{MA} = \frac{1}{\lambda} ln[\frac{(^{187}Os/^{188}Os)_{sample} - (^{187}Os/^{188}Os)_{reference}}{(^{187}Re/^{188}Os)_{sample} - (^{187}Re/^{188}Os)_{reference}} + 1] T M A = λ 1 l n [ ( 187 R e / 188 O s ) s am pl e − ( 187 R e / 188 O s ) re f ere n ce ( 187 O s / 188 O s ) s am pl e − ( 187 O s / 188 O s ) re f ere n ce + 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 187 O s ^{187}Os 187 O s
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 geochronology
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