With 2016 now upon us, I felt it would be appropriate to think about what a new year means for uranium geochronology. What can we expect from the year ahead? Without getting into any of the active research going on, I felt it would be useful to address simply what is physically happening.

On Earth, there is roughly 1×10^{17} kg of uranium.[3] The ratio of ^{238}U:^{235}U is about 137.8:1, and ^{238}U has a mass of roughly 238 g/mol (=0.238 kg/mol). Looking only at ^{238}U, that gives us

`1x10`

^{17}[kg]x(137.8/138.8)/0.238[kg/mol] = 4.17x10^{17} mol [^{238}U]

Radioactive decay is exponential, with the surviving proportion given by `e`

where λ is the decay constant (in units of 1/time) and t is time, or alternatively, ^{-λt}`e`

, where T^{-ln(2)/T1/2*t}_{1/2} is the half-life and t is time.

To find the proportion that decays, we subtract the surviving proportion from 1: `(1-e`

^{-λt})

Multiplying this proportion by the number of moles of ^{238}U will give us the moles of decay, and multiplying by the molar mass will give the mass lost to decay:

`(1-e`

^{-λt})*mol_{U}

Plugging in numbers, with `λ`

y_{238} = 1.54*10^{-10}^{-1}, `t`

= 1 y and the moles of ^{238}U from above, we get:

`(1-e`

^{-1.54*10-10})*4.17*10^{17} mol [^{238}U] = 6.4*10^{7} mol

That yields (with proper use of metric prefixes) roughly 64 Mmol U decay, or 15 Gg of U on Earth that will decay over the next year.

Although those numbers sound very large, they are much smaller than even the increase in US CO_{2} emissions from 2013 to 2014 (50 Tg, or 50,000 Gg); total US CO_{2} emissions in 2014 were estimated at 5.4 Pg (=5.4 million Gg).[US EIA]

As for what’s in store for geochronology as a field, I think there will be a lot of discussion and consideration regarding yet another analysis of the Bishop Tuff.[4] Dating samples which are <1 Ma (refresher on geologic time and conventions) using U/Pb can be tricky, and Ickert *et al.* get into some of the issues when trying to get extremely high-precision dates from zircons. The paper is not open access, but the authors can be contacted for a copy (@cwmagee and @srmulcahy are active on Twitter, too!).

***

[1] J. L. Crowley, B. Schoene, S. A. Bowring. “U-Pb dating of zircon in the Bishop Tuff at the millennial scale” *Geology* **2007**, 35, p. 1123-1126. DOI: 10.1130/G24017A.1

[2] K. J. Chamberlain, C. J. N. Wilson, J. L. Wooden, B. L. A. Charlier, T. R. Ireland. “New Perspectives on the Bishop Tuff from Zircon Textures, Ages, and Trace Elements” *Journal of Petrology* **2014**, 55, p. 395-426. DOI: 10.1093/petrology/egt072

[3] G. Fiorentini, M. Lissia, F. Mantovani, R. Vannucci. “Geo-Neutrinos: a short review” *Arxiv* **2004**. arXiv:hep-ph/0409152 and final DOI: 10.1016/j.nuclphysbps.2005.01.087

[4] R. B. Ickert, R. Mundil, C. W. Magee, Jr., S. R. Mulcahy. “The U-Th-Pb systematics of zircon from the Bishop Tuff: A case study in challenges to high-precision Pb/U geochronology at the millennial scale” *Geochimica et Cosmochimica Acta* **2015**, 168, p. 88-110. DOI: 10.1016/j.gca.2015.07.018