Saturday, October 26, 2013

Carbonado, the diamond that looks like pizza.

I like to horrify current and recent students by telling them that I spent a month of my PhD figuring out what sort of film to use for best capturing the colors produced by the cathodoluminescence centres in carbonado diamond.  Of course, by the time it came to submit my thesis, I then had to spend days scanning all those slides.  Although, I still have a poster somewhere that is made from big glossy 8x10 prints, all glued to cardboard backing.

Now if you'll excuse me, I need to chase some kids off my psilphytopsid lawn...

Wednesday, October 23, 2013

Argon-Argon Dating: The Simple Version

  Sciency Thoughts has a post up on recent high-precision 39Ar/40Ar dating of the Toba supervolcano in Indonesia.  Unfortunately he seems a bit confused about the technique.

Argon has three naturally occurring isotopes: 36Ar, 38Ar, and 40Ar. Potassium also has three isotopes, 39K, 40K, and 41K.  One of these isotopes, 40K, is radioactive, with a half life of about 1248 million years, and one of its stable decay products is 40Ar.

 In the universe, and in Jupiter and the Sun locally, 36Ar is the most abundant argon isotope, followed by 38Ar.  In the cosmic scheme of things, 40Ar is so rare that we don’t even know what its overall abundance is.

However, Earth is a rocky planet.  It was not able to hold onto much gas during its formation, so there is very little 36Ar and 38Ar here.  Earth has lots of potassium though, so almost all the Ar in the atmosphere is 40Ar, which is the decay product of 40K.

In a potassium-bearing mineral, the 40K decays into 40Ar, so you can measure the ratio of these two isotopes to figure out how old the mineral is.

The problem is that it is technically very difficult to measure a potassium argon ration accurately, because one is a reactive solid, and the other is an inert gas.  They require different sorts of ion sources, different mass spectrometers, and there are all sorts of chemical effects that complicate the measurement.

If you want an accurate ratio, it is much easier to measure isotopes of the same element.

So for 39Ar-40Ar dating, what happens is that the mineral of interest is put into a nuclear reactor and bombarded by neutrons.  Some of the 39K (the most abundant stable potassium isotope) absorbs a neutron, ejects a proton, and transmutes into radioactive 39Ar.  39Ar has a half-life of a few hundred years, and is virtually non-existent in nature.   So as long as you know your nuclear 39K to 39Ar conversion ration well, this method allows you to use the 39Ar as a proxy for 39K.  The handy thing is that because it is argon, not potassium, it behaves chemically just like the other naturally occurring argon isotopes, so you can measure it in a gas source mass spectrometer much more accurately than you can measure the chemically different 39K and 40Ar.

The initial 39K-40K ratio doesn’t very much in nature, and is taken as constant (I think- I’ve never actually done Ar-Ar). But the take-home point is that 39Ar-40Ar dating is not its own decay system.  It is the 40K-40Ar decay system, but using a nuclear reactor to change some of the potassium into an unstable argon isotope to make the nuts and bolts of measuring it easier. 

A few brief words on sexual harassment in academia

 It appears to be sexual harassment revelations week here in the science blogosphere, so I figured I’d share a brief story.

In the year 2000, when I was a PhD student, I talked to the student counseling unit about making a formal complaint about sexual harassment by a senior member of staff.

They made it clear to me that taking this course of action would result in revocation of my student visa and deportation from Australia.

I chickened out and kept my mouth shut.

I was fortunate enough to be in a position where I was able to put my head down, write up, and finish my degree by making this choice.  Since that time, I have learned of other international students at other universities who did the right thing, and were deported for reporting. 

I tell myself that had I gone through with reporting, I would have been disappeared long before having the opportunity to make an official statement (way back before blogs, shipping someone halfway around the world was an effective way of shutting them up). And I thought that the incidents which I wished to report were not severe or well documented enough to bring to the police.  But while this is true, here I am, 13 years later, still awake at one in the morning second-guessing myself.

The recent round of revelations has focused heavily on the perpetrators of sexual harassment. Which is good.  But reporting wrongdoing is much more difficult than it should be, due to  the institutional coercion that universities use to protect their reputations at the expense of their students.

Sunday, October 20, 2013

Not much going on here

I realize that for the last few years, this blog has been sputtering along with a low level of volume and quality. I can offer neither explanation nor remediation at this stage.  But I have listed a few of my old favorites below, for anyone looking for straightforward explanations of how geology explains, among other things, sex, drugs, and Rock and Roll.

Phase Equilibria of Pie Crust
Thermodynamics of Hot Chicks
Cetacean Liposuction
Relativistic Obesity
Geologic Lifespan of Jon Bon Jovi
Stars get Lonely Too
Testing the Earthquake-Modesty Connection
Dear Hypothesis

Harry Connolly new series Kickstarter

In "The best book you've never read", I mentioned that Author Harry Connolly, fantastic Urban fantasy "Circle of Enemies", which was an amazing novel that hardly anyone has managed to get a hold of.  It turns out that the Author is launching his next series on Kickstarter.  The appeal finishes tomorrow, but anyone interested in getting in on this can still do so, if you read this blog post in the next 12 hours.

Friday, October 11, 2013

Jailbait zircons

As the company SHRIMP driver, I do a fair bit of demonstration analyses for potential customers.  One thing that has become increasingly common over the past two years is demonstration of the ability to successfully date jailbait zircons.  For those of you unfamiliar with the term, a jailbait zircon is a zircon so young that dating it while making all the usual assumptions will get you into all sorts of trouble. 

The chief problem is that for deep geologic time, we assume that the 238U to 206Pb decay is a simple process. In actuality, there are eight alpha decays and more beta decays than I can remember in this process, but most of the intermediate daughter products are short-lived relative to the age of the analyst, much less the Earth.

However, if you are dating a phase that is much, much younger than the Earth, then these intermediate decay products can become important.  Corrections need to be made relating to whether or not now-extinct intermediate species were incorporated into the target mineral more or less efficiently than uranium. 

For minerals which are a few hundred thousand years old, or younger, you can abandon the uranium-lead system entirely, and use uranium-thorium dating instead.  This simply looks at how close to secular equilibrium 230Th and 234U have grown after their initial incorporation into the target mineral in a unequilibrated ratio.  The linked wikipedia explanation is good (at least qualitatively).  Check it out.

Of course, even for targets old enough for uranium-lead dating, in addition to the theoretical problems above, there is the practical problem of measuring a statistically significant amount of very low levels of radiogenic lead, while somehow keeping common Pb contamination to an absurdly low level.  Because one of the nasty things about the disequilibrium species is that they disrupt many of the assumptions that are needed to accurately and precisely correct for common lead.  Which means that if you can’t keep the blank down, you’re screwed. 

Thursday, October 10, 2013

Who actually shuts down in a shutdown

So the government has been shut down for over a week now.  What does this mean?  NASA is shut down.  If you want to learn about the International Space Station, you'll need to learn Russian. The USGS is mostly shut down, except for hazards programs that are on skeleton staffs. On the other hand, the spy agencies are operating as normal. Taxes are still being collected. And you can still register as a congressional lobbyist. So all the unpleasant aspects of government are still business as usual. I guess that's what essential means. The main people suffering are those off work, and small business owners and employees and their suppliers.  See the Riprarian Rap to see how non-federal employees get screwed because contracts and grants get delayed or cancelled.

Sunday, October 06, 2013

How the shutdown plays in SE Asia

I have been flying home from Korea via SE asia the last couple of days, and reading a few local newspapers.  What is happening here is that while the US Government remains shut down, the premier and PM of China have been engaging on a major goodwill tour, offering things like a billion dollars towards a new monorail project in Indonesia, or 40 billion dollars in increased trade with Malaysia over the next four years.  In contrast, the US has cancelled the Presidents trip, sending John Kerry in his place.
The local take on this, from the Persian Gulf to Japan and everywhere in between, is that it makes America look weak and unreliable, and that it is a serious blow to the new "Asian Pivot" strategy. China's rivals are worried that this inability of America to project soft power will tip the regional balance too far in China's favor.  Whether the bickering factions in Washington are too myopic to see or too shortsighted to care is not entirely clear.

Saturday, October 05, 2013

The wood between the worlds

I’m typing this blog post on the airport wifi, while sitting in a lounge chair with full AC power.  Behind me, the waterfalls of the koi pond water gently tinkles, while to my left, the butterfly garden gently sleeps in the pre-dawn darkness.  I’m in Singapore airport, the long-haul air travel version of Narnia’s wood between the worlds.  

Singapore airport is everything LAX isn’t.  Spacious, relaxing, and accommodating to visitors, be they on the ground for half an hour or half a day.  As a long-haul transfer point, they know that people will need to stop somewhere, so they try to make themselves as attractive as possible to travelers.  It works, more than 50 million people came through here last year, most bound for somewhere else.

Friday, October 04, 2013

Never complain about your stove again

Geologist have it pretty easy, in terms of lab safety.  Compared to chemists and biologists, we have to deal with a relatively low number of lethal chemicals, and our habits confirm this.  It is not coincidence the people call us rock-lickers.  But there are still some reagents which are genuinely dangerous, and command respect.

For most rock knockers, the chief among these is hydrofluoric acid, or HF.  HF is a volatile (evaporates easily) acid which is notorious for being a contact poison.  You don’t have to drink it for you to kill you, as it will diffuse through skin, and attack muscle tissue and bone inside your body.  If the muscle it attacks is your heart, then you die.  As a result, geologists are taught from a young age to observe strict safety protocols with HF: gloves, face shields, aprons, appropriate supervision and fume cupboards are all part of the drill.

But not all fluorine health effects are as dramatic.  excess fluorine consumption can often cause dental fluorinosis, a condition in which excess fluorine is deposited in the teeth, discoloring them.  In more severe cases, fluorine deposition in the bones can lead to osteofluorosis, which can cause disfigurement, deformity, and chronic pain.

One area in which osteofluorosis is distressingly common is Guizhou, China.  Over the past decade, this disease here was linked to the combustion of high Fluorine coal.  Studies showed tha the clay that was intermixed wit hthe coal was high in F, and a steady stream of recommendations has come along describing how this must be getting aerosolized in smoke ,and adhering to food, particularly corn and chilies hung up in houses to dry.

But something didn’t add up.  People were educated to wash their vegetables, to not breathe coal smoke, and still the disease persisted.  Finally, recent studies showed that the F was not adhering to the food products.  TOF SIMS showed that  it appeared inside uncut chilies, and sometimes was associated with silica- particulate matter which should not be able to penetrate food and is biologically inactive.

This was the key to a renewed investigation into the coal.  Which, as it turns out, was not just rich in fluorine, but also rich in pyrite- fools gold.  And all of a sudden, everything fell into place.

When burned, pyrite reacts exothermically with oxygen and water to form iron oxide and sulfuric acid:

2FeS2 + 8.5O2 + 4H2O -> Fe2O3 + 4H2SO4.

Sulfuric acid is not great to breathe, but it doesn’t cause fluorine poisoning.  It will, however, react with fluorite (the most common fluorine mineral like this:
H2SO4 + CaF2 -> CaSO4 + 2 HF

And there is the chemical that terrifies geochemists even in controlled lab spaces, HF, being generated in the household stove. It, in turn reacts with coal ash to form the toxic gas SiF4, which permeates plant and animal tissues and deposits silicon inside of vegetables. In short, domestic cooking stoves are generating incredibly toxic F-bearing gases inside the home. Not even your brother-in-law’s cooking is as hazardous as this.  This was the coolest talk from the session I was fortunate enough to chair this afternoon; there is a paper here.