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

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?

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

A Mass Spectrometer

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

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)

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

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