Twenty-one protons

The David Guetta / Sia hit Titanium has been floating around the internet and our airwaves for about five years now; the first adio broadcasts are just now passing Alpha Centauri. Despite the popularity of the song, its lesser known prequel is hardly ever heard. This may be an accident of economics; not everything can be a hit, especially if it is odd. Or it might just be a sign of the times; unlike Titanium, the prequel came out before Youtube or streaming, and was released on a rather more old-fashioned musical medium known as the 45.  Or, to be more precise, the 44.9559…

You leave it out

And ekaboron holds the space

Nilsen cooked my oxide up

I’m purified, but molten salts are such a waste

Reduced fluorite makes me up

Mendeleev proved nothing removed

Fire assay fire assay

Trivalent, but not the same

As lanthanum or gallium.

You melt me down, strengthen Al

I am the Scandium

Lighter than the yttrium

I am the Scandium

Cut me with

Aluminum increase the strength

Rare earth, the lightest one

Take your choice of ore min’rals to soak me up

Phosphates and uranium.

Mendeleev proved nothing removed

Fire assay fire assay

Trivalent, but not the same

As lanthanum or gallium.

You melt me down, strengthen Al

I am the Scandium

Lighter than the yttrium

I am the Scandium

I am the Scandium

I am the Scandium

One chart, calcium

takes a proton on the run

Scandium oxide makes glass!

You melt me down, strengthen Al

I am the Scandium

Lighter than the Yttrium

I am the Scandium

You melt me down, strengthen Al

I am the Scandium

Lighter than the Yttrium

I am the Scandium

I am the Scandium!

Rare Earth Revelry #5: Fission and neutron capture products

With interest in the Fukishima reactor disaster decaying at an exponential rate, I thought it would be an opportune time to take the long view of this phenomenon, and consider what sort of isotopic anomalies are left behind billions of years after an uncontained uranium fission reactor. And the best place to find these, of course, is Gabon.

Gabon is not known for its nuclear ambitions or industrial fiascos. It is a sparsely populated sub-saharan country known mostly for tropical rainforest and oil reserves. But crucially, it is also the host of the Oklo natural reactors. These are uranium deposits which naturally achieved criticality about 2050 million years ago. At that time, natural uranium had a much higher proportion of the fissionable ²³⁵U than modern uranium does, because the ²³⁵U had not yet radioactively decayed. So at this time, all uranium had a similar isotopic ratio to the isotopically enriched uranium used to commercial reactors (~3%). In the Oklo reactors, this allowed parts of these uranium deposits to achieve criticality, and they bubbled away as natural boiling water reactors for tens to hundreds of thousands of years.

Because there was no containment system- it was just a rock- these fossilized reactors are great for tracking the movement of radioactive materials and their daughter products. These have been extensively studied, and the study we’ll focus on today is Hidaka and Kikuchi (2010).

There are two main types of tell-tale isotopes produced in nuclear reactions.

The first are the fission decay products. When a uranium atom splits, these are the fragments left over. Must fission decay products are neutron rich and highly radioactive, and they to quickly decay by beta decay to form stable isotopes of the same nominal mass. The mass of fission decay products can be quite variable, but the heavier ones, with a mass greater than 139 amu, end up decaying to form light rare earth elements such as cerium or neodymium. The ratio of isotopes produced by fission is different to the ratio of isotopes produced by stellar nucleosynthesis, so fissionogenic Nd or Ce be distinguished from naturally occurring Nd or Ce by its isotopic ratio.

Nuclear fission of uranium does not just produce fission products. It also produces free neutrons. The neutrons are what allow the process to continue. A ²³⁵U nucleus which absorbs a neutron will often fission, releasing additional neutrons, so when the ²³⁵U is concentrated enough for one neutron from each fission to cause another fission, the reaction is considered critical, and will be self-sustaining.

Of course, ²³⁵U is not the only nucleus which can absorb a neutron. A variety of nuclear reactions can absorb neutrons. One of the most common is neutron capture, where a neucleas simply captures the neutron, increasing its atomic mass by one and releasing the excess binding energy as a gamma ray. The ability of nuclei to capture neutrons is expressed as their neutron capture cross section, and the cross sections of various nucleii vary by many orders of magnitude.
Three of the nuclei with the largest cross sections are rare earth elements, ¹⁴⁹Sm, ¹⁵⁵Gd, and ¹⁵⁷Gd. These react with neutrons to form ¹⁵⁰Sm, ¹⁵⁶Gd, and ¹⁵⁸Gd, respectively, and these reaction products have much lower cross sections, so are unlikely to absorb further neutrons. As a result, the Sm and Gd isotopic ratio of pre-existing Gd and Sm in the reactor zone will change. Table two shows that in the reactor zone, the ¹⁴⁹/¹⁴⁷ ratio can be two orders of magnitude lower than the non-radiogenic value.

What the Hidaka and Kikuchi study does is to look at the isotopic ratios of Ce, Nd, and Sm (along with other non-REE isotopes) in individual mineral grains from various Oklo rocks using the SHRIMP. This allows them to determine where the isotopically distinct REE from the reactor zone have traveled, and which mineral phases and reactions have caused them to be immobilized.

What the study shows is that the isotopcially anomalous REE dispersed quite easily through the underlying sandstone, but were effectively adsorbed by the clay overlying the deposit, so that the black shales above the clay layer show lower degrees of REE migration from the reactor core. The study also shows that this migration most likely occurred 1200 million years after the reactor went critical, in response to heating from a nearby mafic dyke intrusion.

Hidaka, H., & Kikuchi, M. (2010). SHRIMP in situ isotopic analyses of REE, Pb and U in micro-minerals bearing fission products in the Oklo and Bangombé natural reactors: A review of a natural analogue study for the migration of fission products Precambrian Research, 183 (1), 158-165 DOI: 10.1016/j.precamres.2010.07.012

Rare Earth Revelry four: Yttrium aluminum garnet

OK, so this is not really geology. YAG is not a naturally occurring mineral, and yttrium is only a REE wannabe. But REE-doped YAG is used in a variety of technological applications that enable geochemical analyses, such as Nd-YAG lasers that form the core of the low-end laser ICPMS systems.

I’m posting because I have heard of YAG for years and years, but never figured out what YAG actually was until tonight, when I got sidetracked in wikipedia.

Garnet is a mineral structure. It is a complex cubic structure that has a formula X₃Y₂Z₃O₁₂. The X site has a distorted 8-fold coordination, the Y site has 6-fold octahedral coordination, and the Z site has tetrahedral coordination. This tetrahedral site is trpically occupied by silica tetrahedrons in the case of most natural garnets, such as almandine, the typical crustal garnet: Fe₃Al₂(SiO₄)₃

Figure 1: The garnet crystal structure.

Most natural garnet compositions consist of solid solutions between common +2 8-fold and +3 octahedral ions: Mg, Mn, or Ca substituting for Fe, and Fe³⁺ or Cr substituting for Al. However, coupled substitutions allow for more complicated compositional changes.

The most common coupled substitution is the formation of majorite garnet at high pressure. Pyrope is magnesium aluminum garnet: Mg₃Al₂(SiO₄)₃. At high pressure, however, Si can start substituting into the octahedral coordination. Despite this pressure increase, magnesium can also still squeeze into the octahedral site, so the substitution is Mg+Si -> Al₂. The resulting mineral, Mg₃MgSi(SiO₄)₃, has a formula unit that simplifies to Mg₄Si₄O₁₂, or MgSiO₃. This is the same formula unit as enstatite (magnesium orthopyroxene), and indeed the transition of enstatite to majorite is one of the main transition zone reactions that marks the change from upper mantle to lower mantle mineralogy.

Yttrium aluminum garnet also has a coupled substitution. Starting with almandine (Fe₃Al₂(SiO₄)₃, the Fe is replaced with Y, and the charge balance is maintained by replacing the Si with tetrahedrally coordinated Al. This gives a formula of Y₃Al₂(AlO₄)₃, or Y₃Al₅O₁₂. Substitution of various REE (most commonly Nd) for Y then gives the mineral its unique optical properties via mechanisms which I do not understand.

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

Rare earth revelry three: Bastnäsite

Bastnäsite is the principal ore mineral for the rare earth elements. Most of the rare earth elements in the Earth are dissolved into silicate minerals such as garnet in very low concentrations. In the continental crust, however, they can occasionally be enriched in accessory mineral phases and phosphates. Bastnäsite is none of these. Rather, it is a rare earth fluorocarbonate, (REE)CO3F*. The economic deposits chiefly occur in carbonatites, which are igneous rocks where the melt is mostly molten carbonate instead of silicate material. While the source and petrogenesis for carbonatites is (or at least was when I lived in the mantle) hotly debated, then basics are that they represent very small percentage melts of deep mantle rocks that have been metasomatized (enriched in trace elements in processes that are not necessarily well understood).


Figure 1. REE compositional spaces for various mantle melt types. Source unfortunately blocked.

As I mentioned last week**, the smaller the degree of melting, the more the LREE are enriched. And for deep mantle melting, HREE are retained in the source, so long as garnet is not completely melted out. So carbonatites, like all trace mantle melts, have strong enrichment of the light rare earths over the heavy ones. Those which end up hosting ore deposits generally retain this pattern.

I suspect that the reason that bastnäsite deposits are economic is that they tend to have high REE concentrations, and that extraction of the REE into oxides by decarbonation is easier than from phosphates.

So, while it is true that nucleosythetic processes that created all the elements made more of the light rare earths than the heavy ones, (as mentioned in the chondrite abundance puzzle and followup) this is only a secondary reason as to why the heavy rare earth elements are much less abundant. The main reason is that the geologic processes that create REE deposits preferentially enrich the light rare earth elements by several orders of magnitude.

*Most REE deposits also contain co-existing REE phosphates (generally monazite), and in these days of tight REE markets that mineral is also processed.

** er, month?

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

Rare Earth Revelry two: Incompatibility and rare earth patterns

As I explained in the last Rare Earth Revelry, the ionic radius of the Rare Earth Elements decreases in a systematic manner with atomic number (figure 1, revisited).

Figure 1. ionic radii of various REE and other elements (repeated from last time).

As you can see, the ionic radii for aluminum and magnesium are quite small. Silicon (not shown) is smaller still. So the magnesium/silicon/aluminum minerals that make up the bulk of the Earth’s mantle have difficulty fitting the large REE into their structures. However, the mantle also contains calcium minerals, and this allows some of the REE, particularly the heavier ones, to squeeze into a calcium site under some circumstances.

When the mantle melts, the melt has a much more fluid structure than the minerals, so it can accommodate the REE more easily. This means that during partial melting ,the REE generally partition into the melt relative to the residual minerals.

The ratio in which an element partitions into melt vs. residual crystals is known as its compatibility, and is represented by something called a D value. The D value is the ratio of the concentration of an element in the mineral relative to the concentration in the melt.

An element with a D value higher than one is compatible; it tends to remain in the mineral. An element with a D value less than one is incompatible. D values depend on the minerals present during melting, but under most circumstances, the REE are incompatible, and the light REE are more incompatible than the heavy REE.

The degree of the difference in compatibility between the light and heavy rare earth depends on the degree of melting, and the exact minerals present in the residue. Garnet residues, in particular, tend to hang on to the heavy rare earth elements while excluding the light ones.

Some examples of this can be shown in figure 2.

Figure 2. The REE pattern of a lunar anorthosite (blue line), compared to some terrestrial rocks. From the wustl meteorite pages

This figure, from the lunar crust, shows the REE pattern of feldspar crystallized from the lunar magma ocean (blue line). The REE pattern is almost flat. In contrast, garnet is present in the deep terrestrial mantle where the magmas that bring diamonds to the surface form. Figure 3 shows the rare earth element pattern for one of these magmas, called a kimberlite.

Figure 3. The REE pattern of a kimberlite. From Le Roex et al.

The relative incompatibility is important for understanding the composition and economics of rare earth element mines, as will be discussed next week.

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

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.

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

Chondrite normalization

Chris and Ron are mostly right. The ‘red’ line is actually 4 overlapping lines, representing 4 different published values for the REE composition of CI chondrites. The blue line is the REE plot for an ordinary chondrite.

Chondrites are the original condensates from the solar nebula- the hot cloud of gas from which the sun and planets formed. As this cloud of gas cooled, the refractory (hard-to-evaporate) elements condensed to form minerals, and the minerals stuck together. There are a number of different types of chondrites, based on texture, temperature, etc.

Geochemists like the REEs (lanthanides). The reason for this is that in natural systems, rare earth elements all have broadly similar chemistry. As a result, changes in relative REE concentrations are easier to interpret than changes in 14 randomly selected elements.

Because scientists are interested in the relative changes, they like to normalize to an initial condition. And that’s where chondrites come in. Chondrites are the solar system’s initial condition- they represent the leftover raw material from which the terrestrial planets were assembled. They trouble is, they aren’t all the same.

Ordinary chondrites are the most common type of meteorite. They consist mostly of silicates and metal, and for most refactory (low vapor pressure) elements they have a composition that is similar to the sun. The generally have various metamorphic textures that indicate variable amounts of post-formational reheating. For volatile elements, ordinary chondrites generally show various degrees of depletion- those elements evaporated as the meteorite headed up.

CI chondrites are a rare type of chondrite. They contain lots of organic matter and structural water, and have no history of reheating. As a result, they have solar composition for almost all non-gaseous elements.

The trouble is, CI chondrites are rare. Of the 36,000 meteorites that have been found and catalogued, we have 5 CI’s.

Because the REE (lanthanides) are all refactory, many earlier papers and studies normalize to ordinary chondrites- they work fine. But as better measurements of CI chondrites became available, and as their importance was realized, most folks started normalizing to CI compositions instead. And for the unwary, this can cause complications.

This is because CI chondrites contain a large amount of water, sulphur, and organic material. As a result, the absolute REE concentrations are somewhat diluted, compared to ordinary chondrites. That makes perfect sense, and is no big deal, AS LONG AS YOU SPECIFY WHICH CHONDRITE YOU USED for normalization.

Let’s see how the professionals did.

This link is a REE search of the figure, report a total of 4 different reported values for normalization. They are:
Anders & Grevese 1989 (2)
McDonough & Sun 1995
Wakita et al. 1971
Anders & Grevese 1989 x 1.36
Anders and Ebihara 1982

The 1.36 multiplication factor is used to calculate a volatile-free CI equivalent. In otherwords, if the CI had been a normal chondrite, that is what the concentration would be. The rationale behind this is explained in this lab’s website, and a decent compilation of early chondrite results is given.

Rimas et al. do not specify which type of chondrite they use for normalization. Floss et al. normalize to CI chondrite values, but their figure captions don’t say which Ci values they use. Both methods sections are blocked, so I can’t check to see if they say there.

Finally, here’s a table of all of the above, normalized to Sun & McDonough 1989:


References:
Anders E. and Ebihara M. (1982) "Solar-system abundances of the elements" Geochim. Cosmochim. Acta 46, 2363-2380.

Anders E. and Grevesse N. (1989) "Abundances of the elements: Meteoritic and solar" Geochim. Cosmochim. Acta 53, 197-214.

McDonough W. F. & Sun S-s. (1995) “The Composition of the Earth” Chemical Geology 120 223-253.

Sun S-s. & McDonough W. F. (1989) “Chemical and isotopic systematics of oceanic basalts: implications for mantle compositions and processes.” In: A. D. Saunders and M. J. Norry (editors). Magmatism in the ocean basins. Geological Society. London. 313-345.

Wakita H., Rey P., and Schmitt R. A. (1971) Elemental abundances of major, minor, and trace elements in Apollo 11 lunar rocks, soil and core samples. Proc. Apollo 11 Lunar Sci. Conf., 1685-1717.

See also:
Rare Earth Revelry
Week -1
Introduction
Week 1
Week 2
Week 3

Geochemopuzzle

A number of geobloggers have been posting cool looking outcrop or deskcrop pictures with the aim of letting us guess what they are. As a geochemist, I find this rather passé. Firstly, the more interesting a rock looks, the less likely it is to be useful for geochemical extrapolation. Geochemists prefer featureless, homogenized rocks, and if necessary, we will homogenize the pretty rocks by grinding them into powder. One we make a rock look boring, we generally then measure the chemical composition of that rock, and use our measurements to make vast extrapolations about the history of our rock, our planet, and even the formation of the solar system. And this all comes from squiggly graphs.

So, as an inaugural geochemical puzzler, I present the following.


These squiggles show the concentration of the Rare Earth Elements (known to non-geologists as lanthanides) in rocks. The aim of this contest is to guess what rocks they are from, and why they are important. Bonus points are awarded for telling us why taking these squiggles for granted can lead to all sorts of trouble. And anyone who can rattle off the references these are from just by looking at the graphs wins the title of geochemoblogpspherohero.

Diffusion and melt inclusions

I haven’t written much about actual lab science recently, and Dr. Carl Spandler, a former postdoc of ours at the ANU, recently published a funky nature paper using data from our laser lab, so here’s a summary of what they did. I should point out that I didn’t do any of the gruntwork for this research. I did, however, provide some minor assistance for some of the followup studies.

A summary of the paper, and the abstract are free. The full paper requires a subscription.

But first, a description of melt inclusions.

As a magma (or silicate melt) cools, it starts to crystallize. In the case of basalt, the first crystals to form are generally olivine. Sometimes, as these crystals are growing, they grow in a geometry that allows a blob of the melt from which they grow to become trapped as an inclusion inside of the crystal. If the crystal is then transported somewhere else, it keeps this entombed blob of melt.

Sometimes, melt inclusions are anomalous. An anomalous melt inclusion in any melt inclusion with a trace element composition different to what one would expect to find. One trace elemental component that is often anomalous is the relative abundances of the lanthanides, known to geologists as the rare earth elements. For those who can’t remember the names of these elements, a mnemonic is available here.

In the Earth’s mantle, rare earth elements all generally occur as trivalent oxides in melts, silicates, or (rarely) phosphates. Because they have the same charge, similar oxygen affinity, and gradually decreasing ionic radius, their behavior in geochemical systems relative to each other can be predicted fairly well. Thus, deviations in the relative abundances of REE’s compared to a particular reference value are used to infer all sorts of geological stories.

Many anomalous melt inclusions have anomalous REE patterns, and these have been used to constrain the origin of these melt inclusions. For example, the heavy REE have a small enough ionic radius to easily fit into the crystal structure of garnet, while the light REE do not. So if a rock containing garnet partially melts, and that melt is in equilibrium with some residual garnet, then the melt will have a very high La/Lu ratio, because some of the rock’s Lu will stay behind in the garnet, while none of the La will.

Because garnet is only stable in the mantle at high pressures, a “garnet signature” REE pattern can be used to infer a deep source of melting- and most basalts do not show significant garnet signatures. A melt inclusion with a garnet REE signature in a rock with no bulk garnet signature would be said to be anomalous.

Of course, in order to be geologically meaningful, the REE in a melt inclusion have to be effectively trapped by the crystal. If the temperatures and residence times are too large, then solid state diffusion might allow the melt inclusion to equilibrate with the melt outside of the crystal. While most melt inclusion people have previously assumed that the existence of melt inclusions requires them to not re-equilibrate, the purpose of the experiments presented in the Spandler et al. paper was to determine whether or not REE diffusion can occur in typical magmatic systems.

So, this is what they did:
1. Get a population of normal MORB melt inclusions that were unlikely to have any anomalous inclusions in them.
2. Determine the temperature at which these inclusions were trapped in the host olivine.
3. Determine the composition of the olivine that traps the inclusions.
4. Calculate what the composition of a basalt should be, in order to be in equilibrium with the olivine at the trapping temperature of the inclusions, under a fixed fO2 and atmospheric pressure.
5. synthesize a basalt of that composition. A synthetic basalt made from lab reagents will have no REE in it.
6. Dope the synthetic basalt with several hundred ppm of the following REE: Pr, Eu, Tb, Ho, Lu
Presumably these were chosen for the following reasons: AS odd-numbered elements, they have lower abundance, so the ration of synthetic to natural is greater for a fixed concentration. Also, the detection limits and counting stats for the mass spec are better, because all but Eu are monoisotopic clear mass numbers, so you can count all the ions, not just those from a minor isotope.
7. Heat the synthetic basalt up to the trapping temperature, toss in the intact olivines containing natural melt inclusions, and let them sit for varying time periods.
8. Quench, extract the olivines, polish them down to expose the melt inclusions, and see if any of the doped elements diffused into the melt inclusions.

Not only did they find that diffusion occurred, but they were able to determine what the diffusion coefficients were. And applying those coefficients to magmatic systems showed that REE will diffusively re-equilibrate on a timescale of years. Short-lived nuclide and geophysical constraints suggest it takes thousands to tens of thousands of years for melts to migrate from their mantle sources to the surface. Thus, anomalous melt inclusions must be trapped in a late stage of magma migration, as any melt inclusion captured early on would re-equilibrate long before it was erupted the surface.

While the paper was languishing in review, Carl described the results in a talk at Goldschmidt. It was that talk that caused Al Hoffman to blow his top, which was highly entertaining for us pudknockers in the back row. But melt inclusion research is an incredibly finicky and laborious line of study, so I can see how being shown that it can’t possibly mean what you think it means could be upsetting.

Anyway, that’s the lab denizen’s view of the study. It would be interesting to see what a skeptical petrologist makes of it.

C. Spandler, H. St C. O'Neill & V. S. Kamenetsky 2007. Survival times of anomalous melt inclusions from element diffusion in olivine and chromite. Nature 447, 303-306

Geologic Mnemomics

Before I get stuck in, I’d like to interrupt this blog to plug a women in science survey promoted here. It would be a great place to complain about the inappropriateness, in a gender-neutral discipline, of the lewd and irreverent memory tricks that are described below.

Back to mnemomics.
For a scientist, I have the world’s leakiest memory. So I get a kick out of these things. After all, if I read enough stupid sentences, then maybe, possibly, one or two of them will sink in.

So, starting with the big picture, there is the spectral sequence for stars.

OBAFGKM

For the past 50 years, this has been immortalized by various science fiction writers. I think Larry Niven is one of them, but please correct me if I'm wrong. Their classic line is, of course,

Oh, Be A Fine Girl, Kiss Me.

That was fine for the 50’s, but a Canadian academic has suggested that in this day and age, on most university campuses, the phrase should probably be supplanted with

Only Boys Accepting Feminism Get Kissed Meaningfully.

For my brain, the less tasteful phrases are generally more memorable. Which is why I appreciate the universally offensive:

Oedipus, Basically A Friendly Guy, Knew Mother.

Scaling down, there is a famous mnemonic for the 9 planets, but my perverse brain can only recall,

My Very Endearing Mother Just Shot Up Near Prison.

I might have misremembered that one.

But enough astronomy. On my first ever geology field trip, the departmental trip to Lake Champlain in fall of 1991, we had a very long van trip from Rhode Island. I remember only two things about that trip. The first is that Katie Stanbury was in my van. The second is that we, including everyone else whose identity now eludes me, made up a mnemonic for the geologic time scale. Starting with the Precambrian, and using the old-fashioned American system, we decided that

Polonius Called Ophelia Stupid Daily. Meanwhile, Peter Pan Told Jesus Christ To Quit.

Subdividing the Precambrian into the Archean and the Proterozoic allows one to lead with Angry Polonius...

Two years later, Gayle Gleason, one of the best geology teachers I’ve ever had, subdivided the Cenozoic by teaching us that:

Pigeon Egg Omelettes Make People Puke.

Of course, an ideal mnemonic would allow the memorization of the entire periodic table. The first three rows are easy:
Harpies Heal
Little Bed Bugs Can Not Ovulate For Neophytes
Naked Mongols Always Slide Past Scantily Clad Argonauts.
But once you hit the transition elements, the 18 elements per line gets rather complicated. Nobody will ever remember:

“Kilt-Clad Scandinavians Tickle Very Cruel Men; “Feel Cold Nipples!” Cursed Zen-Grappling Germans, As Swedes Bribed Kraken.

I tried to go on, but ran out of gas at Antimony, after realizing that I would have no chance at getting anything for Xenon, three elements later.

Fortunately, we don’t need to. Since the table is in fact a table, and not just a list, all we really need to do is remember which column things are in (usually discernable from their charge), and then just remember those columns. The only tricky bits are the Group VIIIa elements (Feel Cold Nipples, Rude Rhyming Paladins, Oswald Irked Patrick), and the rare earth elements. But the latter are easy, given a suitably crass Lanthanide mnemonic:

Lazy Cesar Praised Nude Prometheus, Smooching Europe’s Gaudiest Tubercular Dykes Hopelessly. Eros Tempted Yobbos Luridly.

Lame, I know. But what else can be done with Yb? At least the REE’s aren’t boring anymore.

If any of you folks have a favorite or useful geologic mnemonic, please post it in comments.

edit: The entire periodic table is here.

So close, and yet so far.

I thought I found an on-line periodic table with ionic radii today. It was really good, yet simple; you click on the element you want, and it gives all sorts of useful facts, including the radii. It even had bonus features, like biological activity and safety information. There was just one problem that I saw.

Samarium. I clicked Samarium just to check a standard, everyday, working class rare earth element. What do I find?
Ionic radius: unknown.
That’s odd. REE ionic radii are probably the best known and most often studied of the trace elements. What would it say this? I read on…

Samarium is most often found in the +2 state, like Europium.

D’oh!

Ladies and gentlemen, here is a tip. There in no point putting up a beautiful, comprehensive, and well-organized webpage if the information it contains is not correct.

Maybe the reason they stated that the Samarium ionic radius is unknown is that they were scouring the literature for the radius of Sm+2. And there aren’t likely to be a whole lot of papers out there containing that data. Eu can be +2. And Ce can be +4. But aside from those two, all Lanthanides are +3 in nature. Including Samarium. And the ionic radius is 1.098 Ǻ.*

* According to this website- http://www.scescape.net/~woods/elements/samarium.html
I don’t have journal access at home. Additionally, I like angstroms, so all of you picometer lovers can shove your SI units into an area of low solar luminosity.