Rhenium-Osmium Systematics – Measuring the trillionths? – Rhenium

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Rhenium-Osmium Systematics – Measuring the trillionths? – Rhenium

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rhenium.osmium.systematics

Slides for the talk in my Isotope Geology class

On Github sdaityari / rhenium.osmium.systematics

Rhenium-Osmium Systematics

Measuring the trillionths?

Shaumik Daityari / @ds_mikhttp://sdaityari.github.io/rhenium.osmium.systematics

Rhenium

  • Symbol "Re"
  • Atomic number 75
  • Heavy (specific gravity - 21.02)
  • Silvery-White
  • Alloys used in filaments of instruments, catalysts in chemical and petroleum industries
  • Average concentration of 1 ppb
  • Oxidation states from -1 to +7
  • Two stable isotopes (185, 187)
  • One mineral - Rheniite
Rheniite, Source: Wikipedia

Osmium

  • Symbol "Os"
  • Atomic number 76
  • Hard, Brittle
  • Heavy (specific gravity - 22.59)
  • Bluish-White
  • Alloys are highly durable and extremely hard
  • Fountain pen tips and electrical contacts
  • Oxidation states from -2 to +8
  • Seven naturally occurring isotopes(184, 186, 187, 188, 189, 190, 192)

Where are they located?

Source: Wikipedia

Rhenium and Osmium

  • Practically sister elements
  • Rare only in Earth's crust
  • Siderophiles - Iron loving
  • Dissolved in molten Iron - present in the core
  • Minor quantities left in crust and mantle - less than 1 ppb
  • Rhenium is also a chalcophile - forms sulfides (Rheniite)
  • Instruments of high precision (NTIMS)

Sample Preparation

  • Osmium and Rhenium separated
  • Form negatively charged oxides
  • ReO3-, ReO4-
  • OsO3-, OsO4-

Thermal Ionization Mass Spectrometry

  • Also called surface ionization
  • Material coated on a heated metal surface
  • Material evaporates into ions due to the heat
  • Negative or Positive depends on the charge of the ions
  • Governed by the Saha-Langmuir equation

Saha Langmuir Equation

  • β - Ionization Efficiency
  • EI - First Ionization Potential
  • EA - Electron Affinity
  • g - partition functions*
  • Φ - Work function of filament
  • k - Boltzmann constant
* weighted average of the fraction of neutral, positive or negative atoms in each energy state

A Mass Spectrometer

Credits: DĄBEK and HALAS (2007)

Challenges

  • Low concentrations of Re-Os in the crust and mantle
  • Os studies first carried out using NTIMS in 1989, precise measurements in 1991
  • Debate on the Re-Os decay constant

The Rhenium-Osmium Clock

  • 187Rhenium is slightly radioactive
  • 187Rhenium converts into 187Osmium when a neutron decays to a proton (β decay)
  • Half life - 42.3 billion years (~ 10 x age of Earth)
  • Osmium is compatible (no preference in absence of iron)
  • Rhenium is highly incompatible
  • Any rock that undergoes melting in the mantle stops its Re-Os clock

The Rhenium Osmium systematics

The Isochron

Applications

Extraterrestrial Occurrence (1) - Crater size

  • Meteorites add osmium to the sedimentary record
  • Meteorite osmium hasn't gone through 4 billion years of mantle evolution
  • Larger pulses of osmium in the record of seafloor sediments point to asteroid impacts
  • Could be used to find out how big the crater was
  • Francois Paquay, 2009
  • Challenge - determine what fraction vaporized and what fraction dissolved in seawater
  • Within 50% variation of known impact sizes
Source: Paquay, 2007

Extraterrestrial Occurrence (2) - Age of meteorite cores

  • Meteorite cores contain iron
  • Re and Os are siderophiles
  • Age of the meteorite core can be determined
  • J. J. Shen (1996)
Source: Shen, 1996

Absolute Dating of Mineralization Events

  • Rb-Sr, Ar-Ar, Sm-Nd, U-Pb dating can not be directly performed on sulfides
  • Proxies like gangue, wall rock or host rocks are dated
  • Rhenium-osmium dating performed on molybdenite, a rhenium-enriched sulfide
  • Suzuki in 1996, Ruiz in 1999, David Selby in 2000s
Source: Selby, 2014

Absolute Dating of Mineralization Events (cotd..)

  • U-Pb dates give similar age to that of Re-Os dates, Ar-Ar dates are 4-7% younger
  • Molybdenite and Gold deposition happened nearly at the same time as the crystallization of the pluton in Fort Knox, Alaska
  • One explanation - Ar-Ar blocking temperature is lower
  • This study raises questions about interaction of molybdenite with hydrothermal fluids

References

  • Dąbek, Józef, and Stanislaw Halas. "PHYSICAL FOUNDATIONS OF RHENIUM-OSMIUM METHOD–A REVIEW." Geochronometria 27.1 (2007): 23-26.
  • Paquay, François S., et al. "Absence of geochemical evidence for an impact event at the Bølling–Allerød/Younger Dryas transition." Proceedings of the National Academy of Sciences 106.51 (2009): 21505-21510.
  • Selby, David, et al. "Absolute timing of sulfide and gold mineralization: A comparison of Re-Os molybdenite and Ar-Ar mica methods from the Tintina Gold Belt, Alaska." Geology 30.9 (2002): 791-794.
  • Shen, J. J., D. A. Papanastassiou, and G. J. Wasserburg. "Precise Re Os determinations and systematics of iron meteorites." Geochimica et Cosmochimica Acta 60.15 (1996): 2887-2900.
  • Walczyk, Thomas, Erhard H. Hebeda, and Klaus G. Heumann. "Osmium isotope ratio measurements by negative thermal ionization mass spectrometry (NTI-MS)." Fresenius' journal of analytical chemistry 341.9 (1991): 537-541.

THE END

Any questions?

Source: APOKALIPS