Young rocks

Evy, the skepchick who is now geoblogging at georneys, recently expressed surprise at a paper which labeled rocks with an nine digit age as young. I see nothing wrong with this, however. Depending on how you define 'beginning', the Earth is between 4567 and 4460 million years old. using 4500 as a convenient round number, we can then take half that as a midpoint, and place the old rock / young rock divide at 2250 MA. Alternatively, we could lash out and use 1.5 billion year divisions to call rocks old (> 3.0 Ga), middle-aged (between 3 and 1.5) and young < 1.5 billion.

In Gondwanaland, the 1.2-1.0 Ma Grenville orogeny is much less prevalent than it is in laurentian rocks. It shows up here and there, but is not a major event. The middle proterozoic was in fact fairly quiet in Gondwana, with the Neoproterozoic-to-Cambrian Pan-African / Brasiliano / Ross orogens being the defining tectonic events that assembled the various pieces of Gondwanaland into the supercontinent that we all know and love. So for those of us interested in the pre-assumblage history of Gondwana, it makes perfect sense to describe anything after the midproterozoic as young. I even did so in my thesis, in this sketchmap of the geology of the Brazilian state of Bahia:

Rare Earth Revelry, week one

As I mentioned in the introduction, the Rare Earth Elements (known to chemists as lanthanides) are an esoteric yet commonly studied group of elements. The reason they are studied is that both their behavior as a group, and the more subtle change in behavior between the different rare earth elements can reveal information about the system in which they are observed.

The REE are refactory lithophile elements, meaning that for the most part they condensed at high temperature in the solar nebula as oxides*, and thus have similar behavior to calcium and aluminum during the planet-forming process.

In nature, the REE on earth generally form large, 3+ cations in a variety of complex oxides (figure 1). They are most commonly found as trace elements in silicates, but are readily concentrated in phosphate minerals. They rarely occur as carbonates.

Figure 1. ionic radii of the REE and selected other elements. Data from Shannon & Prewitt (1969), via the web. REE are light blue, comparison trivalent cations are dark blue. Divalent ions are green, with light green for Eu. Tetravalent cations are red, with pink for Ce. pdf available on request.

The ionic radius of the REE decreases with increasing atomic number, so that lutetium is about 20% smaller than lanthanum. The heavy rare earth elements (HREE) are similar to yttrium, but are still substantially larger than other common rock-forming trivalent elements (figure 1).

Although all REE are generally trivalent, two of them have other valence states that occur in nature. Europium can have a +2 valence under moderately reducing conditions, which makes it behave much like the element strontium (figure 1). Under oxidizing conditions found in surface processes on the modern Earth, cerium can be tetravalent, and Ce+4 has a size intermediate between zirconium and uranium.

In general, the large ionic radius makes the REE incompatible in most mantle mineral lattices (which are comprised mostly of Mg, Si, Al, and Ca). So mantle melts are enriched in REE relative to the residual mantle. The continental crust in enriched further still relative to the oceanic crust. However, the larger light rare earth elements are more incompatible than the more compact heavy rare earths. The degree of incompatibility is related to the minerals present in the mantle when it melts, so the pattern of REE in igneous rock at the surface can give us a clue as to what minerals are present deep in the inaccessible part of the Earth where the melting occurs.

Ref:
R. D. Shannon and C. T. Prewitt, Acta Cryst., 1969, B25, 925

Rare Earth Revelry
Week -1
Introduction
Week 1
Week 2
Week 3
Week 4

* We’re ignoring enstatite chondrites for now. I’ll come back to them another time.

Pie crust phase equilibria: update

Four years ago, I posted the basics on the phase equilibria of pie crust in this blog. A summation of that post, as well as an update, appears below:

With American Thanksgiving and Christmas rapidly approaching, the pie baking season is rapidly approaching. One of the most important, but least quantified, aspects of pie creation is the crustal composition. A simple ternary phase diagram for three-phase pie crust is presented below.

While the “traditional composition” point is plotted to scale, the positions and shapes of the curve are poorly constrained approximations. Lack of accurate thermodynamic data for the system precludes accurate prediction of these fields. It is the shapes and positions of the top two curves that is of paramount importance; anyone who reaches the butter-water two phase field should be banished from the kitchen.

As anyone with baking experience knows, the stability region for pie crust is a relatively small area on the wet side of the two phase flour + dough field. This field is generally approached by adding water to a flour/butter mixture, as is shown below.

Four years ago, I suggested the following approach:

However, if the approximated slopes shown above are correct, then a radical new approach to crustal formation might be advisable. By generating a flour-water mixture, and then adding butter, a wider range of valid crustal compositions should be achievable before exiting the edible portion of this phase diagram. This approach is shown below.

In hindsight, this was silly. There are two reasons. Firstly, accurately ganguing the initial water/flour mixture is difficult, as your starting composition is in the 2 phase flour water field. And secondly, this procedure generates a crust with the minimum possible butter, and butter is yummy. So this year, I will endeavor to explore the left hand side of the diagram. I suspect that the dough / slime boundary curves over a bi farther than is illustrated here.

Rare Earth Revelry: Week -1

Ladies and gentlemen, welcome to week -1 of the Rare Earth Revelry. That is not a dash folks, this is week negative one. Why start here? Because we are starting before the beginning. We are turning the clock back 5 billion years to discuss rare earth elements in stars that died before the Earth was formed.

Cosmologists tell us that the universe started out with no heavy elements in it; all the carbon, oxygen, silicon, and everything else was formed in stars, a byproduct of the energetic reactions that makes stars shine.

The exact details on nucleosynthesis, the manner in which the elements of the periodic table came to be, were worked out in theory in the middle to end of the 20th century. For elements heavier than iron, there are two main mechanisms, both of which involve neutron capture.

In large, elderly stars, after the hydrogen fuel of the main sequence is exhausted, the star turns to burning helium. There are numerous reactions between helium nuclei and those of heavier elements, and some of these (e.g. ²¹Ne + ⁴He -> ²⁴Mg + n) produce neutrons.

In the S process (S for stellar), heavy elements grow heaver by absorbing neutrons produced in such a manner. However, the flux of neutrons is fairly low, so that if neutron capture results in the formation of an unstable nucleus, that nucleus generally has time to beta decay into something more stable before the next neutron capture.

For the light rare earth elements, this process is illustrated in figure 1.

Figure 1. S process formation of Nd isotopes. X axis is number of neutrons in nucleus, Y axis is number of protons. Figure generated from NuDat 2.5

It generally takes a few million years for a star to chew through its helium, after which the process stops. Helium burning can be a fairly unstable process, so these stars can mix their core material up into their atmospheres, (called “dredge-up”) and then blow their atmospheres off into space, allowing the elements to escape.

The S process has a few drawbacks, specifically its inability to explain a number of heavy isotopes, as well as the elements uranium and thorium. If you add a neutron to ²⁰⁹Bi, to form ²¹⁰Bi, the ²¹⁰Bi decays into ²¹⁰Po, the isotope used to kill Viktor Litvinenko. ²¹⁰Po alpha decays back down to ²⁰⁶Pb, so you can’t get up to mass 232 or 238. Also, as shown in Figure 1, ¹⁴²Ce, ¹⁴⁸Nd, and ¹⁵⁰Nd cannot be produced by the S process.

The R-process (R for rapid) explains these elements. In the R process, an extremely high neutron flux means that nuclei absorb neutrons faster than they can decay. (figure 2)
Because there is no time for decay, the instability gap between polonium and thorium can be bridged, and actinides, as well as heavy isotopes of other elements, can be formed by this process.

Figure 2. R-process for Nd isotopic formation. Note that ¹⁴²Nd is not formed in this process.

In the case of neodymium, ¹⁴²Nd can only be formed by the S process, ¹⁴⁸Nd and ¹⁵⁰Nd can only be formed by the R process, and 143-146 can be formed from both.

The bulk solar (and meteorite, and planetary) isotopic composition can be 99% explained as a mix of S process and R process isotopes, suggesting that the gas cloud that collapsed to form our solar system contained the remnants of both supernovas and material expelled from old, evolved stars.

From the 1950’s onward, that was the theory, and it was a nice theory. But there is one more thing we need to know about the theory before letting it loose on real rocks.

As the S process requires a star to have entered the helium burning phase of its life, there is another compositional change that occurs. Stars like the sun have about twice as much osygen as carbon in them. But helium burning produces carbon, so in some helium burning stars, there is more carbon than oxygen. This is important, because if a carbon rich atmosphere gets ejected, the excess carbon means that instead of just forming carbon monoxide gas, condensates of carbide such as SiC can form.

In the late 1980’s, researchers working with primitive meteorites discovered that they contained trace amounts of tiny silicon carbide grains. It was hypothesized that these might be dust from carbon-rich helium burning stars, which avoided getting remelted in the primordial solar nebula. So, in the early 1990's, the Nd isotopes were measured. And what did they find?

All the Nd in the silicon carbide was S-process only. Just like the theory predicted.

Figure 3. S-process Nd in presolar SiC grains. Source.

Rare Earth Revelry
Week -1
Introduction
Week 1
Week 2
Week 3
Week 4

Further reading:
Zinner 1998 (fee)
Zinner et al. 1991 (free)
Guber et al. 1997 (fee)

Rare Earth Revelry

It has been an odd year for the lanthanides. After decades of sitting, quietly, in a row of their own at the bottom of the periodic table, the rare earth elements have been thrust into the limelight by numerous MSM articles.

As it turns out the REE have been a quiet favorite of geochemists for decades. They are used for everything from determining how stars make chemical elements to keeping the compact fluorescent lights on. And they are the bread and butter of numerous geochemical studies. For example, the latest issue of Geology reports rare earth concentrations in 4 of its 23 articles- and this is a general earth science publication, not a geochemistry specialist journal. Rare Earths are used to in stellar nucleosynthesis, missiles, planetary formation, phosphors, continental evolution, magnets, geochronology, neutron capture, volcanism, lasers, and gas light mantles. There is a lot to know, but I will try to start at the beginning. Their names. If you can't remember the list lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, then I suggest you use a mnemonic.

Next up, stellar nucleosynthesis, and rare earths in stars that died before the sun was born.

Rare Earth Revelry
Week -1
Introduction
Week 1
Week 2
Week 3
Week 4

Google bails on the greenback

Google AdSense to me
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Reference checking drinking game

Sip :
et al
et. al
et. al.
Inconsistent & vs. and.
Gratuitous references to common knowledge which you tracked down as a form of procrastination (e.g. Petri 1887).
Obvious misplaced fields in electronic database citation collection.
Commas instead of semicolons, or similar format errors.
Incnst. Abbrv.

Chug:
Mixed formats in bibliography.
Autoformatting that undoes manual corrections.
Crummy old Xerox copies which cut off the journal information.
Bibliography references orphaned by aggressive cuts.
References missing from the bibliography because they are only listed in figure captions, which is a different file.
Subtle misplaced fields in electronic database citation collection (e.g. page and volume numbers switched).
Correcting a reference database error in the proof, without fixing the database.
References written in non-latin alphabets.


Drain the bottle:
Referenced papers you’ve never actually read.
My reference formatter is set up perfectly, I have no need to look at the output.
Software updates delete all the corrections you performed to your reference list.
Actually reading the referenced paper only to find that it actually says the opposite of what you inferred based on the way that other people referenced it.