3.8 Re-Os system

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

• $^{187}Re$ decays to $^{187}Os$ through beta decay
• Half-life of $^{187}Re$ approximately 41.6 billion years
• Decay constant (λ) for $^{187}Re$ equals 1.666 × 10^-11 year^-1
• Decay equation: $^{187}Os = ^{187}Os_i + ^{187}Re(e^{λt} - 1)$
• 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 $^{187}Os$ and $^{187}Re$)
• 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 $^{187}Os/^{188}Os$ of ~0.13
• Depleted mantle shows lower Re/Os ratios due to melt extraction events
  • Results in subchondritic $^{187}Os/^{188}Os$ 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
  • $^{187}Os/^{188}Os$ 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 $^{187}Os/^{188}Os$ 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: $T_{MA} = \frac{1}{\lambda} ln[\frac{(^{187}Os/^{188}Os)_{sample} - (^{187}Os/^{188}Os)_{reference}}{(^{187}Re/^{188}Os)_{sample} - (^{187}Re/^{188}Os)_{reference}} + 1]$
• 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 $^{187}Os$
  • 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