The geochemical quantification of double beta decay

In 1933, Enrico Fermi proposed a theory of beta decay using the neutrino particle suggested by Wolfgang Pauli three years earlier. During this time period, the promising young physicist Maria Goeppert married an American guy named Mayer and started 15 years of two body-related unemployment in the States (In contrast, the gap between the publication of her book, “Elementary Theory of Nuclear Shell Structure” and the awarding of the Nobel prize for that work was only 13 years).

During the great depression, Goeppert-Mayer decided to up the intellectual profile of housewives by suggesting, in 1935, the possibility of double beta decay. Where an intervening unstable isotope prevented sequential single beta decay, double beta decay would allow transformation to a lower energy Z-2 nucleus by simultaneous double electron-antineutrino emission. Most of the double decay candidates are neutron-rich R-process elements such as ⁹⁶Zr, ¹⁰⁰Mo, or ¹³⁰Te. The predicted decay product of ¹³⁰Te is ¹³⁰Xe, a noble gas. In the late 40’s, an isotopic study of telluride minerals was performed, and a ¹³⁰Xe excess in Xenon from telluride ores was reported by Inghram & Reynolds (1950).

Subsequent noble gas results verified the existence of ¹²⁸Te – ¹²⁸Xe and ⁸²Se – ⁸²Kr systems, all before the first direct detection of a double beta decay was established in Elliott et al. (1987)- almost 40 years after the initial geochemical result, and 15 years after Goeppert-Mayer's death.

Unfortunately, the ability of Tellurium and Selenium minerals to quantitatively retain noble gasses is poor, as these minerals have low closure temperatures, and are easily deformed. They are also difficult to date directly. But as every hard-up geochemist knows, if you’re desperate for a date and you don’t know where to turn, it never hurts to look for a zircon.

The dating of zircon using the uranium-lead decay scheme is arguably the most popular and rigorous geochronological method currently available. And as it just so happens, ⁹⁶Zr is expected to decay into ⁹⁶Mo. Since zircon usually doesn’t have much initial Mo, It should be possibly to detect a ⁹⁶Mo excess, and use the U/Pb age from the same mineral to calculate a decay constant for ⁹⁶Zr (Technically, this method determines the ²³⁸U/⁹⁶Zr decay constant ratio).

This has been done a few times (e.g. here and here), but the process is complicated by the spontaneous fission of ²³⁸U. The fission products of this decay include most heavy isotopes of Mo, so the fissionogenic Mo excess and the double beta ⁹⁶Mo excess have to be deconvolved. The result is that the precision on ⁹⁶Zr double beta decay is fairly poor, with Wieser & De Laeter (2001) reporting a value of 9.4 ± 3.2 x 10¹⁸.

However, there is another promising double beta decay isotope. ¹⁰⁰Mo decays into ¹⁰⁰Ru. And molybdenite generally contains Re, so that the mineral can be dated using the Re/Os single beta decay scheme. And moly contains fuck-all uranium. It looks like the NSF thinks this is a promising technique as well, as they’ve awarded a $225,000 grant to a leading Re/Os lab for support for this and other experiments. And on the other side of the Pacific, Hidaka et al. 2004 have reported a result of 2.1 ± 0.3 x 10¹⁸.

In the meantime, the direct counting mob have continued to count decays. According to Barabash (2006), their current best determination of the ¹⁰⁰Mo halflife is 7.1 ± 0.4 x 10¹⁸. It will be interesting to see if the Denver Re/Os crowd can do better, and if either group can explain why the direct counting gang have halflives that are approximately a factor of 2 higher than the geochemists (counters have ⁹⁶Zr as 2.0±0.3x10¹⁹- also double the geochemical determination). Barabash 2006 does not address this discrepancy, or even reference the more recent geochemical results.

I’m also curious about the physicist’s budget. After all, I have a sneaking suspicion that the direct counting experiments cost a little bit more than a quarter million dollars.

References:
Barabash (2006) (How is one supposed to reference arXiv entries?)
Elliott S R Hahn A A and Moe M K 1987 Phys. Rev. Lett. 59 2020
Goeppert-Mayer M 1935 Phys. Rev. 48 512
Hidaka H Ly C V Siziki K 2004. Physical Review C, 70, id. 025501
Inghram M G and Reynold J H 1950 Phys. Rev. 78 822
Wieser M E De Laeter J R 2001 Phys. Rev. C 64, 024308

The preferred paleontologist replies

At the beginning of this month, I ran a poll asking folks which paleontologist they would prefer working with: Josh Smith, the alleged sexual predator, Spencer Lucas, the alleged plagiarist, or Marcus Ross, the self-avowed young Earth paleontologist. Dr. Ross won the poll handily, and as it turns out, he is a reader of the Lounge. He sent me the following email, followed permission to post:

Dear Dr. Lemming,
Thanks for including me in your recent paleo poll, I thought the idea was hilarious. I’ve enjoyed checking your blog in the year or so since the NYT article came out. I have to say, I really enjoyed your blog post on the topic, as you took a far (far!) more measured and thoughtful approach to the issue.
Though we will disagree on issues of Earth history, thanks for being civil, and for finding ways to have fun with the news of the day. And I’m very pleased that the readers of your blog would pick me as a preferred co-worker. Dubious award or not, I now know that there exist entities in the Great Chain of Being below “trained parrot” young-Earthers (a la PZ Meyer’s description of me).
Cheers,
Marcus

Anyway, I’d like to thank Dr. Ross for having the grace and character to react so well to my sometimes callous sense of humor. I am still waiting for concession speeches from Drs. Smith and Lucas, but I suspect that they are respectively busy ogling perspective students or forming judicial committees which will impartially decree that 7 is larger than 32.

Comments are currently open, but please stay civil. I don’t want to come back from the field to find that the organic slime of the internet has spontaneously evolved into a fauna known colloquially as the troll.

Economic geology from first principles

I have to say that I kinda fell into the resources industry by accident. I was offered a job unexpectedly, it sounded like and interesting opportunity, so I said yes. Most of my formal geologic training is in big picture academic geology and associated analytical science, in which concepts and techniques are derived from first principles.

Industrial geology isn’t really like that, but that doesn’t mean that I can’t try. So I will. The law of supply and demand suggests that the price of a mineral should be related to the demand for it, and its abundance. Demand is beyond the prevue of geology, so we will ignore this half of the law for now. Doing this suggests that the price of a mineral should be related to the abundance of that element.


Above is a figure that plots the market price for various elements (US$ per mole) vs their crustal abundance (atomic ppm). If demand was irrelevant and the Earth’s crust was homogeneous, then these elements ought to fall on some sort of negatively sloped trend. As seen from the graph, maybe a third or so of the elements do, but the bulk fall somewhere to the left of the trend.

There are two ways to interpret this. The first is that there is reduced demand for these elements. The second is that the Earth’s crust is not homogeneous. Geology allows us to address this second option.

If the crust was homogeneous, we exploration geologists would be out of work. Mines aren’t just stuck in any old place, they are put where it is most economical to extract a particular resource. In general, this corresponds to am area where geologic processes have concentrated one or more elements of interest. So it is possible that the elements to the left of the trend are elements that are concentrated by geologic processes more easily than other elements.

Have another look at the figure. The elements labeled in yellow are chalcophiles. These are elements that will preferentially form sulphides instead of silicates in the presence of sulphur. This provides an enrichment mechanism not available to elements that only occur as oxides, so that the local concentration of these elements can be orders of magnitude higher than their average crustal abundance.

So if you were curious about why it is that economic geologists are always nattering on about sulphur, this is the reason. Even if the average crustal abundance of Pb is a few ppm, sulphide precipitation can concentrate it to 10% or more, which would place it to the right of Al on this figure.

The outback tours Osaka

In the better late than never category of blog announcements, I wanted to point out for my Japanese readers that a exhibition of the paintings of Utopia artist Emily Kame Kngwarreye is currently on tour at the National Museum of Art in Osaka. Utopia is in the same general non-specific part of the NT as some of our ground, so it's the least I can do to plug the local talent.

p.s. in completely unrelated news, there is still no winner for Where on (Google) Earth? #117.

Convection

Dry.


Wet.


How do you like to remove heat from boundary layers?

New field season

The boots are polished, the bags are packed, the roosters are growing in the pre-dawn gloom. Soon the sky will lighten and a cab will appear to take me to the airport. By noon local time, I'll be in the red center. Field season 2008 is about to begin.

Good luck with WoGE. If you haven't solved it by Easter or so, I might drop a clue...

Where on (Google) Earth? #117

This time, I've pulled a new trick out of my hat. I have altered the color balance of the image, so that you can't just scan the continents at a wide zoom looking for something that has the right average tone.

Y'all will have to do some geologizing instead. And if any of the sedimentologists can explain how this sort of drainage pattern evolves, I'd love to know.


Schott Rule is in effect: Wait one hour per win before posting. The winners list is in the WoGE kmz file.

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

94.6%

The above photo shows the Earth, Moon, Venus, and Mercury. As planetary pie aficionados know, these bodies together comprise 94.6% of the inner solar system. Only Mars and the asteroids are missing. Looking at the full size image (click it) will show that the mare are clearly visible in the Earth glow.

On the Moon, the Earth is almost full in the sky, and it has 16 times the area and 3 times the surface brightness as the full moon does. So the Earthlight lights up the dark side enough for us to be able to see the lunar landscape.

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.

Paleontology Promotion Day

Yesterday’s post made the strong implication that paleontologists were somehow more ethically challenged than most other scientists. I don’t think that’s fair. One of the things that pisses me off about this affair is that I know of similar cases in various subfield of hard rock geology, where junior researchers and students were taken advantage of by superiors or collaborators. None of those people had the documentation or the determination to press their cases, but it means that I find it especially disappointing when a group of young shafted researchers does manage to put a case together, only to be denied a fair and impartial venue to hear the case.

So, in fairness to all the good guys who study bones, shells, pollen, leaves, molecules, etc. I’d like y’all to plug your favorite paleontologist. Somebody who reflects well on the tribe of scientists. Someone who you think is a positive role model for those millions of six year olds who are in love with dinosaurs and archaeocyathids.

Pick your preferred paleontologist

Paleontologists, the scientists who study and learn ethics from fossils, are a pretty dodgy bunch of characters. But every year, one of them manages to distinguish himself from his peers in a particularly depressing fashion.

The 2006 paleontologist of the year was Joshua Smith, who finally got fired after sexually harassing and assaulting his students for at least 3 years running.

The 2007 Paleontologist of the year was Marcus Ross. A newly minted paleontology PhD at the time, he came out as a fundamentalist Christian who doesn’t actually believe in the science he practices.

And our 2008 winner is Spencer Lucas. The director of the New Mexico Museum of Natural History and Science has been accused of claim-jumping and plagiarizing the work of other researchers. At this time, two of Spencer Lucas’ best mates are currently sitting in judgment to decide if their longtime buddy is the honorable scientist they’ve known all these years, or a reconstruction-stealing scumbag.

This has inspired me to run the Lounge’s first ever poll:

Which paleontologist do you prefer? (see right sidebar)

Marcus Ross has admitted that he looks at science as a game or a puzzle to solve abstractly, while believing it can’t possibly be true. He believes that all of creation is 15,000 times younger than his samples. However, all the evidence suggests that he always played by the rules- nobody has ever accused him of doing dodgy science.

We have no idea what Spencer Lucas’s religious persuasion is, if indeed he has one. We don’t even know if he’s guilty. It sure looks like he stole somebody else’s work, but he has gamed the rules of the gentleman’s club so that his longtime collaborators form the board of inquiry. As of the time of this writing, they are still deliberating.

I’m leaving Smith off the menu, because his offenses are against people, not science, and one or more of his victims reads this blog.

So, which scientist would you prefer to work with?

(update: Lucas got off- surprise surprise. But if there are Paleontologists who work for good, feel free to mention them here.)

Ninja miners and blood diamonds

Last night I went to a presentation on the ninja miners of Mongolia. Ninja miners are East Asia’s garimpeiros- illegal, non mechanized miners who dig up everything from gold to gravel as a livelihood. I knew a bit about how garimpeiros worked in Brazil from my time there as a PhD student, but I had no idea of the scale of the enterprise in Asia.

There is occasional western focus on the crime and environmental damage caused by illegal gold and diamond mining, the most infamous of which is the whole “blood diamond” kerfuffle of the late 90’s and early 21st century.

Blood diamonds, which according to De Beers are any diamonds mined by someone other than them, were of course all the rage in the waning years of the Clinton presidency. And they are what made me decide to get out of the diamond field. While I am not necessarily adverse to morally dubious geology, the thing that really struck me about the conflict diamond activists was that they were generally not interested in solving the conflicts; instead their goal was to make sure the diamonds they bought were clean, so that they could sleep at night without being reminded of the horrors of West and central African civil wars. Wear Canadian diamonds, and you can wash your hands of the atrocities. And what sickened me about the research community was the way that academics were happy to use this opportunity to fund their pet projects, but with a conflict diamond spin.

Of course, there is no scientific method on Earth that will distinguish between an Angolan diamond found in 2001 and one found in 2003. But the former is a conflict diamond, while the latter is a reconstruction diamond. And the reason for the difference has nothing at all to do with kimberlites, or defect structures, or resorption features. The conflict was solved with 15 bullets and some help from Mossad, after the CIA decided that Savimbi was a liability, not an asset. All the inclusion composition statistics in the world wouldn’t have had any effect on that decision. And yet, during the conflict diamonds debates, there were precious few vocal advocates of solving the conflicts instead of the diamonds.

Illegal gold mining has never gained the notoriety necessary to spark a Leonardo DiCaprio flick, but it is still notorious due to pollution issues. The basic methodology is this:
1. Concentrate heavy sands in a wooden pan.
2. Add a blob of mercury to the pan, and swirl, allowing the Hg to dissolve any gold that is present.
3. Pour the mercury onto the blade of a shovel.
4. Use a blowtorch to evaporate the mercury, and collect the gold residue.

If only mercury were an environmentally benign health supplement, this would be a fantastic method.

So, I’ve known about these sorts of issues for a while, but what I learned last night was that toiling in the shadows of the glamour diggers for gold and diamonds were huge numbers of Asians, like the ninja miners, who work in low value commodities such as sand, gravel, and coal. This sort of mining is especially prevalent in Bangladesh, India, and Indonesia. By some counts, there are more than 10 million people worldwide earning a living from the blades of their shovels.

Not surprisingly, there are some academics who study this phenomenon, and refer to ninja miners, garimpeiros, and other low tech resource workers by the evocative name of “Artisinal and small-scale miners”. They even have a web site. And while digging up rocks for a living is a hard and dangerous way to live, for many people it provides a cash stream that subsistence farming does not, which explains why the numbers are increasing.

Here in the developed world, the trend is in the opposite direction. As mining companies merge, operations are getting bigger, and staffs are getting smaller. Just last month, BHP announced that it will be designing its new mines to be worked entirely by remote control, with nobody on site and the mining operations and vehicles running either by computer control, or remotely from the air-conditioned suburbs of Perth. While the Pilbara doesn’t have a large native population, I wonder how this trend would affect workers in more populous areas. Is a person better or worse off risking their life in a mine, or sitting at home with a welfare check funded by robotic royalties? And more to the point, where will that decision be made, and by whom?