Better know a 'tope volume 3: 19F

This week we are happy to bring you Fightin' Fluorine! Fluorine is monoisotopic meaning that there is only one naturally occurring isotope on Earth. It is the stable isotope ¹⁹F. Because fluorine is monoisotopic, isotopic differences are nonexistent and it is not useful in the study of stable nor radiogenic isotopes.

That is all.

Better know a 'tope volume 2: 238U

Hi Kids. Welcome to the second installment of the sporadically published "better know a 'tope" series. Today we continue our journey with ²³⁸U; a long lived, radioactive isotope. Uranium is a high atomic number (92), naturally occurring element in the Actinide series of the inner transition metals. Interestingly, uranium is directly below neodymium (our element from volume 1) in the periodic table indicating that they have similar valence electrons in the f-orbitals. Uranium is commonly associated in the public eye with nuclear reactors and weapons. This is due to the high contents of radioactive isotopes, but most nuclear applications rely on ²³⁵U rather than ²³⁸U. No elements with atomic numbers higher than lead (82) have naturally occurring stable isotopes but that is a story for another day. ²³⁸U is the heaviest naturally occurring isotope on earth, with the exception of trace amounts of ²⁴⁴Pu (plutonium) which, having a half life of ~80 Myr (1/50 the age of the earth), is almost undetectable and was not discovered until 1971. Uranium is therefore considered by some to be the heaviest naturally occurring element for most (but not all) geologic applications

²³⁸U has a half life of 4.468 billion years, coincidentally close to the age of the earth. This makes it particularly useful for studying early earth processes and dating very old rocks. ²³⁸U decays by alpha decay to ²³⁴Th and on through a series of alpha and beta decays ultimately to ²⁰⁶Pb. This chain forms one third of the basis of U-series dating which is particularly useful for dating young rock, one application of which has been employed by members of our department for studying magma evolution and volcano building (see attached). One of the most useful tools in uranium geochronology is the fact that uranium has two relatively long lived radioactive isotopes (²³⁸U t1/2≈4.4 Gyr; ²³⁵U t1/2≈700 Myr). By measuring and considering the paired decay of these two isotopes to ²⁰⁶Pb and ²⁰⁷Pb respectively it is possible to accurately determine ages of billions of years. This is the basis of much zircon geochronology as well as the first definitive paper on the age of the earth. The seminal paper by Clair Patterson was the first study to calculate the "correct" age of the earth radiometrically. The age determined of 4.55 Ga is very close to the now commonly accepted age of 4.567 Ga. For a little inspiration and history: this work, which is world famous and basically spawned Pb-Pb geochronology was Patterson's Ph.D. thesis, and the results were first presented at a small conference in Wisconsin while he was a graduate student at the University of Chicago.

In addition to geologic applications, ²³⁸U is important in some weapons such as armor piercing rounds and a few civilian applications. Depleted uranium is uranium which consists mostly of ²³⁸U, the ²³⁵U having been separated out to make enriched uranium (the enrichment or depletion referring to the relative amount of ²³⁵U compared to naturally occurring uranium). Depleted uranium has a very high density which leads to its use as a counterweight in aircrafts, as a shield to gamma radiation for x-ray cameras and armor piercing ammunition.

For further reading see:

Patterson, C.C., Tilton, G.R., Inghram, M.G., 1955. Age of the earth. Science, 121(3134): 69-75.

Singer, B.S., Smith, K.E., Jicha, B.R., Beard, B.L., Johnson, C.M., Rogers, N.W., 2011. Tracking Open-system Differentiation during Growth of Santa Maria Volcano, Guatemala. Journal of Petrology, 52(12): 2335-2363.

Better know a 'tope volume 1: 143Nd

Greetings class! Today we start out our series in "better know a 'tope" with the stable nuclide ¹⁴³Nd. Neodymium has atomic number 60 and has gotten a lot of attention in recent years as a key component in powerful magnets and previously in use in lasers. One of the most common solid state lasers is the neodymium-yttrium aluminum garnet or Nd:YAG laser. Neodymium magnets are among the most powerful magnets known and are used in things such as motors (in hybrid cars) and in wind turbines to generate electricity.

The stable isotope ¹⁴³Nd is the daughter product by alpha decay of ¹⁴⁷Sm; this is the basis for Sm-Nd geochronology. The decay of ¹⁴⁷Sm to ¹⁴³Nd has a half life of ~106 billion years. This makes the system potentially useful for dating minerals across a wide range of ages back to earth formation. Sm-Nd geochronology has many challenges including the relatively low concentrations of rare earths in many minerals, low parent-daughter ratios, and (similar to many systems) potential parent daughter fractionation after the growth of the mineral. Despite these challenges, Sm-Nd geochronology lends itself quite well to a few minerals, in particular the dating of garnets. Garnets tend to have very high Sm/Nd ratios compared to most other crustal minerals, leading to well constrained isochrons.

In addition to mineral specific geochronology, ¹⁴³Nd is very important in determining Nd-model ages and mantle evolution over time. In samples that are well constrained, knowing the current ^143/144Nd ratio as well as the Sm/Nd ratio and the age, an initial ^143/144Nd ratio can be calculated. Based on the age and the initial ratio calculated, information can be derived relating to when the original magma from which that rock is derived was segregated from the mantle because Sm and Nd have slightly different compatibilities.

For further reading on neodymium and some applications to garnet geochronology see

DePaolo, D.J., 1988. Neodymium isotope geochemistry; an introduction. Minerals and Rocks, 20. Springer-Verlag, Berlin-Heidelberg, Federal Republic of Germany (DEU), 187 pp.

Pollington, A.D., Baxter, E.F., 2010. High resolution Sm-Nd garnet geochronology reveals the uneven pace of tectonometamorphic processes. Earth and Planetary Science Letters, 293(1-2): 63-71.

Pollington, A.D., Baxter, E.F., 2011. High precision microsampling and preparation of zoned garnet porphyroblasts for Sm-Nd geochronology. Chemical Geology, 281(3-4): 270-282.

Introduction

Hi everybody!

As many of you know I love 'topes. I love thinking about them, I love measuring them, I love interpreting them, I love talking about them, I love how they can be used for pretty much everything. I love calling them 'topes instead of isotopes.

I have talked for a while about making a series inspired by Colbert's "better know a district" called "better know a 'tope." So this will be my space for sharing my love of 'topes.

I think everyone could stand to learn a little about isotopes, and to that end I will be writing from time to time about a given isotope. I will select one and give some information about that isotope (and element if you're lucky). I also take requests. Sometimes we may have guest writers. We'll see how things go.

Some of the papers I have permission to link to and some you will have to look up for yourselves. Enjoy!

Some of my favorite sources about isotopes are:

Dickin, A.P., 2005. Radiogenic isotope geology. Cambridge University Press, Cambridge, United Kingdom.

Faure, G., 1986. Principles of isotope geology. John Wiley & Sons, New York, United States of America.

Valley, J.W., Cole, D.R. (eds.), 2001. Stable isotope geochemistry. Reviews in Mineralogy and Geochemistry (RiMG) vol. 43.