Understandable Earth Science

Posts tagged ‘scientific researcher’

How old is that rock? Part 3: ⁴⁰Ar/³⁹Ar dating

In the previous blog I described how we measure the amount of ⁴⁰K, ⁴⁰Ar and ³⁶Ar to calculate how old a sample is.

All good yeah? Unfortunately, not quite.

While there are some samples and situations where this K-Ar dating technique works really well, it isn’t perfect. The technique uses a few key assumptions that are not always true. These assumptions are:

  1. The ⁴⁰K and ⁴⁰Ar* are homogenously distributed in the sample, so it doesn’t matter that the K and Ar measurements are carried out on different aliquots (sub-sample) of the sample using different techniques.
  2. When we measure the Ar content, we manage to release ALL of the Ar from the sample – we need absolute concentrations of ⁴⁰K and ⁴⁰Ar because they are measured with different techniques on different aliquots.
  3. All of the ⁴⁰Ar in the sample is either from radioactive decay of ⁴⁰K (i.e. ⁴⁰Ar*) or from the atmosphere (⁴⁰Arₐ).

Assumption 1 shouldn’t cause too many problems for old rocks where the concentration of ⁴⁰Ar* is high and it is easy to analyse small samples (10s of milligrams). But the younger the rock, or the lower K-content of the rock, the less ⁴⁰Ar* there is and larger samples need to be analysed to release enough gas to measure. The larger the sample, the greater chance of sample inhomogeneity – and that means there is a bigger chance that the two aliquots analysed for K and Ar don’t quite match.

Assumption 2 can cause problems when analysing certain minerals, especially a mineral called sanidine. This is a kind of K-rich feldspar that forms at high temperatures and has a very disordered crystal lattice. This disordered crystal lattice makes it more difficult for Ar to diffuse out of the sample during analysis, and the high melting temperature makes it difficult to completely melt the sample to release the all of the gas. This means you might end up underestimating the amount of ⁴⁰Ar* and getting an age that is too young.

Assumption 3 can be a problem in various situations. Sticking with the simple volcanic eruption scenario from the last blog, if the magma chamber is quite new and forms in old continental crust, there might be a lot of ⁴⁰Ar in the magma that has come from radioactive decay of ⁴⁰K in the crust.  This might become trapped in a crystal we want to date, either just by being in equilibrium with the gas, or the crystal might trap tiny pockets of magma or hydrothermal fluid as it grows – we call these magmatic or fluid inclusions. This kind of ⁴⁰Ar did form by radioactive decay, but in a different system to the one we are trying date (it formed in the host crust, rather than in the crystals we want to date), so for ⁴⁰Ar/³⁹Ar dating we stop referring to this as ⁴⁰Ar* and start calling it excess-argon, or ⁴⁰Arₑ.

Fortunately, in the late 1960s, 2 scientists called Craig Merrihue and Grenville Turner discovered that if you put a K-bearing sample into a nuclear reactor and bombarded it with neutrons, some of the ³⁹K changed into ³⁹Ar. This meant that you could measure the ³⁹Ar on a noble gas mass spectrometer, at the same time as the usual ⁴⁰Ar and ³⁶Ar and calculate ⁴⁰K/⁴⁰Ar* from ³⁹Ar/⁴⁰Ar*.

With this new technique, the amount of ³⁹Ar produced depends on how much neutron irradiation the sample received, which is difficult to directly measure. To get around this, a sample of well-known age (an “age standard” or “fluence monitor”) is included in the irradiation and the Ar-isotopic composition of this standard is used, along with its age,  to calculate something called the J-value, which is a proxy for neutron dose. This J-value is then used to help calculate the age of our samples.

This new technique dealt with any problems associated with assumption 1 of the K-Ar technique. It also dealt with assumption 2, because it no longer mattered if you didn’t release all of the Ar from the sample – it is the ratio of the parent to daughter isotopes that is important, rather than the absolute concentrations.

Being able to measure both the parent and daughter isotope at the same time also opened up a whole new level of gas-release technique that helped to address any problems associated with assumption 3. Ar could be released from samples by stepwise heating (heat the sample a little bit and analyse the gas released, and then increase the temperature – repeat until there is no more gas left)- this helps in two ways. Firstly, any separate reservoirs of ⁴⁰Arₑ, like the fluid inclusions I mentioned above, tend to be released from crystals at lower temperatures than the ⁴⁰Ar* stored in the crystal lattice. That means that stepwise heating can identify different reservoirs of Ar in a sample, and we can use this information to identify which heating steps can be used to calculate an age.  Secondly, multiple measurements from the same sample (either stepped heating, or multiple analyses of single crystals) can be plotted on isotope correlation diagrams and these can be used to calculate mixing lines between different end-member isotopic compositions, making it possible to interpret complex data.

In the next blog I will explain how some of these diagrams and data-analysis techniques work. But I want to finish this post with a brief summary of the capabilities and challenges of ⁴⁰Ar/³⁹Ar dating.

  • The ⁴⁰Ar/³⁹Ar technique can potentially date rocks and minerals between a few thousand, and a few billion years old;
    • The first ⁴⁰Ar/³⁹Ar dates produced in the late 1960s and early 1970s were on meteorites and lunar rocks recovered from the Apollo missions, which are all between 3 and 4.5 billion years old. The technique is still routinely used to date old, extra-terrestrial material.
    • With the right samples, it is also possible to date relatively young rocks – back in 1997 the ⁴⁰Ar/³⁹Ar lab at the Berkeley Geochronology Centre successfully dated the 79AD eruption of Vesuvius that wiped out the town of Pompeii.
  • The easiest samples to work on contain a lot of K (typically between 5 and 15% K₂O), but with increased sensitivity of mass spectrometers, changes to the gas extraction systems and improvements in sample preparation, it is now possible to date samples that have a K-content of less than 0.5%, even for quite young samples. This means that the range of material that can be analysed to give an ⁴⁰Ar/³⁹Ar age is massive.
  • ⁴⁰Ar/³⁹Ar dating gives cooling ages, so it can date not just volcanic eruptions, but igneous intrusions and metamorphism. It can even been used to date fault movement and meteorite impacts.
  • ⁴⁰Ar/³⁹Ar dates need to be calibrated against a standard of known age. This means that the accuracy and precision of the ages it produces is always going to be limited by the accuracy and precision of the age of the standards. This is being addressed as part of an international project called EARTHTIME that aims to improve the precision of various dating techniques.

So, in short, the technique covers a massive date range and it can date a wide range of materials to give age information on lots of different kinds of geological events.

Me and Grenville Turner at my PhD graduation in 2005

I was fortunate enough to do my PhD in the Ar-dating lab at The University of Manchester, using the MS-1 – the mass spectrometer that was built by Grenville Turner and produced the first Ar-dates. Grenville retired the year I started at Manchester, and the lab is now run by Prof. Ray Burgess, who was my PhD supervisor.  But here is a cheesy photo of me and Grenville at my graduation in 2005.

Grenville recently wrote an article giving a bit of the history of the MS-1 mass spectrometer, which you can read here.

And here are a couple of other interesting articles about Grenville and this history of his research:

Todmorden news

Floreat Domus (pdf – go to p. 34)

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Who am I?

Who am I and why am I calling myself the Noble Gasbag?

I am currently (summer 2014) a postdoctoral researcher in the School of Geosciences at The University of Edinburgh, Scotland. That means I work as a scientific researcher, mainly on a specific project (more on that in a moment). I did my PhD at The University of Manchester, UK, and have worked on postdoctoral projects at The Open University (Milton Keynes), UK and Roskilde University, Denmark.

I am an Earth Scientist – that means I use scientific methods to investigate processes happening in The Earth. I could also be classed as a Geoscientist or a Geologist.

A lot of my work involves a group of elements called the Noble Gases (Group 8 on the periodic table –those elements in the far right hand column, historically these have also been called “the rare gases” or inert gases). So The Noble Gasbag seemed an appropriate name for a blog where I talk about my scientific work.

One of my main interests is a geological dating technique called ⁴⁰Ar/³⁹Ar dating (that is finding out how old rocks are, not wining and dining fellow geologists 😉 ). This involves measuring the noble gas, Argon (Ar).  I am most interested in using this to find out when volcanoes erupted, which means I have been lucky enough to visit some exciting parts of the world with some very pretty volcanoes, all in the name of work.

Göreme, Cappadocia, Turkey

Houses built into volcanic ash, Göreme, Cappadocia, Turkey

Inerie, Flores, Indonesia

Inerie volcano near Bajawa, Flores, Indonesia

I am also interested in learning more about some of the fundamental assumptions of the technique and how to refine and improve it so we can produce more accurate and precise dates of geological events. Part of this involves studying how atoms diffuse in geological materials.

To fully understand how atoms diffuse, we really need to understand the structure, chemical and thermodynamic properties of the geological materials being studied. That means I have spent quite a lot of time studying geological and mineralogical microtextures (small scale – microns to centimetres – structures and features in rocks and minerals) and helping to develop ways to study microtextures more easily. The blogs summarising those papers will have a lot of pretty pictures.

The project I am currently working on is a bit of a new area for me. I will be finding out if we can use noble gases to make sure that geological storage sites for CO₂ are secure and don’t leak (and if they do leak, who should take responsibility for fixing the problem). CO₂ capture and storage (CCS) is a way that the world can quickly reduce CO₂ emissions and minimise anthropogenic (human-induced) climate change, IF large-scale CCS projects start working soon. More on that later.

So, that is me, and a bit of info about what I am going to be “gassing” about.

The Noble Gasbag

Photo of me in the Soa Basin, Flores, Indonesia.
Photo by David McGahan

 

Welcome

Hello, and welcome to The Noble Gasbag.

This is a blog that is all about making science more accessible and understandable to non-experts.

Science is fascinating and the  results of scientific experiments are vital for making  all sorts of decisions, from whether a particular food is good for us, through to large scale issues, like how can we minimise human-induced climate change and keep life on Earth comfortable.

But, let’s face it. Scientific publications are generally not the most interesting things to read, especially to non-experts. There’s all sorts of boring , tedious, perhaps difficult to understand technical information and lots of assumptions about how much the reader knows about the subject. As a practicing scientific researcher, when reading scientific papers I regularly find myself yawning, not fully understanding parts and my mind wandering.

All this “boring” information is critical to the scientific process. It needs to be included in the paper because that is what allows other experts to use and test the results. Perhaps more importantly, it is essential for the peer review process. I will talk more about this in another blog post, but peer review is one of the things that make science credible and robust.

Peer review means that, if I am reading a paper where I am not an expert in the particular method being used, I don’t need to understand all the fine details and assumptions of the methodology. I don’t need to know how it works. I can just read the introduction of the paper, to understand why the work is relevant, and read the discussion and conclusions section to know what their experiments found out. I don’t necessarily need to understand how they reached those conclusions, because a peer review panel of experts has already examined the paper in detail and checked that it is scientifically robust, and if they have missed anything important, then other subject-specific experts who read the paper will comment on the problems. I, as a non-expert reader, am free to accept and trust the conclusions reached by the paper authors, and enjoy the extra knowledge that their work has given me.

One of the things I want to do with this blog is to provide easy-to-understand summaries of my own peer-reviewed articles. I don’t want you to have to wade through paragraphs of information about mass-spectrometer corrections factors to find out when a particular volcano erupted and why that is interesting.

However, to really understand the relevance of some of my papers, you do need a bit of background knowledge, so I also plan to write a few blog posts summarising some of this information in what I hope is an easy to understand way.

Finally, scientific knowledge is incredibly important for making political decisions that affect the entire world. There is a lot of public misunderstanding about some of this scientific knowledge, and indeed the scientific process itself. I will also be blogging about some of these issues to try and clarify information and also to highlight some of the important and difficult decisions that need to be made about the way we live.