Tag Archives: zircon

Geoscientist’s Toolkit: Pressure Vessels

Pressure vessels ("bombs") used for high-temperature dissolution of zircon.  Image credit: Bill Mitchell (CC-BY).
Pressure vessels used for high-temperature dissolution of zircon. Image credit: Bill Mitchell (CC-BY).

When scientists are measuring the uranium and lead in a rock—specifically in the mineral zircon, found in many igneous rocks—to determine its age (U/Pb geochronology), they need to dissolve the zircon. Zircon is a very stable mineral, so to dissolve zircon, the mineral grains are subjected to acids at high temperatures (~200 °C) and pressures. Thick steel pressure vessels are needed to contain an inner teflon vessel when it heats up and the liquid inside boils.

In the picture above, there are two pressure vessels. On the right of the red marker, a smaller vessel is used when the zircons from one rock sample are being partially dissolved to remove the exterior surface (chemical abrasion). To the left of the red marker, the large vessel is used for the final dissolution, when zircon grains are on teflon racks with individual teflon capsules.

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This Year in Uranium Decay

Pumice from the Bishop Tuff (~767 ka).  Zircons in this pumice are rich (relatively) in uranium, with up to 0.5% U.[1,2]  Image credit: Bill Mitchell (CC-BY).
Pumice from the Bishop Tuff (~767 ka). Zircons in this pumice are rich (relatively) in uranium, with up to 0.5% U.[1,2] Image credit: Bill Mitchell (CC-BY).

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 1×1017 kg of uranium.[3] The ratio of 238U:235U is about 137.8:1, and 238U has a mass of roughly 238 g/mol (=0.238 kg/mol). Looking only at 238U, that gives us
1x1017[kg]x(137.8/138.8)/0.238[kg/mol] = 4.17x1017 mol [238U]

Radioactive decay is exponential, with the surviving proportion given by e-λt where λ is the decay constant (in units of 1/time) and t is time, or alternatively, e-ln(2)/T1/2*t, where T1/2 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 238U will give us the moles of decay, and multiplying by the molar mass will give the mass lost to decay:

(1-e-λt)*molU

Plugging in numbers, with λ238 = 1.54*10-10 y-1, t = 1 y and the moles of 238U from above, we get:

(1-e-1.54*10-10)*4.17*1017 mol [238U] = 6.4*107 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 CO2 emissions from 2013 to 2014 (50 Tg, or 50,000 Gg); total US CO2 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!).

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[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

Split Rock Anorthosite

Looking SW from Split Rock Point (a large anorthosite block).  Note the gentle dip of the rocks toward Lake Superior.  Image credit: Bill Mitchell (CC-BY).
Looking SW from Split Rock Point (a large anorthosite block). Note the gentle dip of the rocks toward Lake Superior. Image credit: Bill Mitchell (CC-BY).

This summer, I took a field trip up to Split Rock State Park in northern Minnesota, along the north shore of Lake Superior. While I wrote a little bit about the trip, there is still a bit more to be said and shown.

Part of what makes Split Rock interesting, besides a picturesque lighthouse which I didn’t take many pictures of, is the large blocks of anorthosite. Anorthosite is a rock formed primarily of the mineral anorthite, which is a calcium-rich feldspar, and the mineral zircon—used in U/Pb dating—can be found in anorthosite as well.[1] Its appearance is generally light grey or whitish, and has relatively coarse grains (mm to cm).

Anorthosite is an intrusive igneous rock formed through the crystallization and accumulation of anorthite within a magma body. It is abundant on the Moon, and lunar anorthosites are believed to have accumulated on top of a magma ocean early in lunar history. A relatively dense magma will act as a heavy liquid, and cause the less dense anorthite to float, separating the original magma from the crystallized anorthite. These types of crystallization processes, where the magma becomes separated from crystals it produces, are called fractional crystallization, and can cause the resulting magma to be enriched in some elements or components (such as SiO2). Even with massive basalt flows, fractional crystallization can cause an occasional rhyolite flow as well, but I’ll leave discussion of the rhyolites of the North Shore for another day.

Pictured above is the view from Corundum Point, a large block of anorthosite at Split Rock State Park. Below is a close-up view of some of the anorthosite, as well as a benchmark which has been placed in the anorthosite block [Thanks to Jessica Ball (@tuff_cookie) for giving me the idea of photographing the benchmark]. Despite being far from the ocean, Minnesota is home to National Ocean Survey benchmarks.

Anorthosite with survey point, Split Rock State Park, MN.  Image credit: Bill Mitchell (CC-BY).
Anorthosite with survey point, Split Rock State Park, MN. Image credit: Bill Mitchell (CC-BY).

The name Corundum Point suggests the presence of corundum—a mineral used in abrasives—and it comes from a mining operation on the site in the early 1900s. However, the point is actually anorthosite, which was much less useful for abrasives. Between the incorrect mineral identification and a fire which burned down the crushing house, the operation was eventually shuttered.

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[1] Mark D. Schmitz, Samuel A Bowring, Trevor R Ireland, “Evaluation of Duluth Complex anorthositic series (AS3) zircon as a U-Pb geochronological standard: new high-precision isotope dilution thermal ionization mass spectrometry results” Geochimica et Cosmochimica Acta (2003), 67, p. 3665–3672. DOI: 10.1016/S0016-7037(03)00200-X

Update Upon further study, it appears that the naming convention of Split Rock State Park is to call this point Corundum Point. However, Google Maps displays this point as Split Rock Point, with Corundum Point a few hundred meters to the northeast. Regardless of the arbitrary common name, the benchmarks are on the point to the southwest.