Hafnium–tungsten dating
It is also useful in determining the formation times of the parent bodies of iron meteorites.
[3] The use of the hafnium-tungsten system as a chronometer for the early Solar system was suggested in the 1980s,[4] but did not come into widespread use until the mid-1990s when the development of multi-collector inductively coupled plasma mass spectrometry enabled the use of samples with low concentrations of tungsten.
Since hafnium-182 is an extinct radionuclide, hafnium–tungsten chronometry is performed by examining the abundance of tungsten-182 relative to other stable isotopes of tungsten, of which there are effectively five in total, including the extremely long-lived isotope tungsten-180, which has a half-life much longer than the current age of the universe.
[8] The abundance of tungsten-182 can be influenced by processes other than the decay of hafnium-182, but the existence of a large number of stable isotopes is very helpful for disentangling variations in tungsten-182 due to a different cause.
Variations in tungsten isotopes caused by r- and s-process nucleosynthetic contributions also result in correlated changes in the ratios 182W/184W and 183W/184W, which means that the 183W/184W ratio can be used to quantify how much of the tungsten-182 variation is due to nucleosynthetic contributions.
[10] Nonetheless, cosmic ray effects can be corrected for by examining other isotope systems such as platinum, osmium or the stable isotopes of hafnium, or simply by taking samples from the interior that have not been exposed to cosmic rays, though the latter requires large samples.
[11][12] Tungsten isotopic data is usually plotted in terms of ε182W and ε183W, which represent deviations in the ratios 182W/184W and 183W/184W in parts per 10,000 relative to terrestrial standards.
Undifferentiated chondritic meteorites have ε182W = −1.9±0.1 relative to Earth, which is extrapolated to give a value of −3.45±0.25 for the initial ε182W of the Solar system.
If this process took place relatively early in a planet's history, hafnium-182 would not have sufficient time to decay to tungsten-182.
Since hafnium is a lithophile element the (undecayed) hafnium-182 would remain in the mantle (i.e. the outer layers of the planet).
As such, by looking at how much tungsten-182 is present in the outer layers of a planet, relative to other isotopes of tungsten, the time of differentiation can be quantified.
A model age for the time of core formation can then be calculated using the equation[1]
accounts for any differences in the general abundance of hafnium between the sample and chondritic meteorites,
It is important to note that this equation assumes that core formation is instantaneous.
For iron meteorites hafnium-tungsten dating yields ages ranging from less than a million years after the formation of the first solids (calcium-aluminium-rich inclusions, usually called CAIs) to around 3 million years for different meteorite groups.
[18] While chondritic meteorites are not differentiated as a whole, hafnium-tungsten dating can still be useful for constraining formation ages by applying it to smaller melt regions in which metals and silicates have separated.
[19] Martian meteorites have been examined and indicate that Mars may have been fully formed within 10 million years of the formation of CAIs, which has been used to suggest that Mars is a primordial planetary embryo.
[20] For Earth, models of accretion and core formation are strongly dependent on how much giant impacts, like that presumed to have formed the Moon, re-mixed the core and mantle, yielding dates of between 30 and 100 million years after CAIs depending on assumptions.
