Ask me anything
Nov. 6th, 2013 01:23 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
kaberettI understand I'm not always great at explaining what I'm up to here, so -- if you have questions about how things are going for me or what I'm doing or how exactly my life fits together, please feel encouraged to ask. <3
(Today on my way home from work I saw: (1) someone busking with a tuba that belched flames every time they played a note; (2) a cavalcade of police motorcycles, blues flashing, who as far as I can tell were learning how to halt a busy junction in the rush hour. It was kind of endearing.)
(Today on my way home from work I saw: (1) someone busking with a tuba that belched flames every time they played a note; (2) a cavalcade of police motorcycles, blues flashing, who as far as I can tell were learning how to halt a busy junction in the rush hour. It was kind of endearing.)
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Date: 2013-11-06 02:39 am (UTC)(no subject)
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Date: 2013-11-06 10:30 am (UTC)(no subject)
Date: 2013-11-06 11:12 am (UTC)-- no, wait, let's try that again. I'm possibly pitching this a bit high - I've completely lost track of what level of understanding non-specialists have of plate tectonics - so if you have clarifying questions please feel encouraged to ask ♥
There are (currently understood to be) three major kinds of volcanism on Earth. Type the first is associated with destructive margins, where one tectonic plate is subducting beneath an other - think the Pacific Ring of Fire, including Japan and the West Coast of the US. Type the second: constructive margins, where tectonic plates are being created -- mid-ocean ridges (the Mid-Atlantic Ridge, e.g., which you can see beautifully in Google Earth).
Type the third is "weird shit" (normally referred to as Ocean Island Basalts) - it's intra-plate, rather than anywhere near a margin, and understanding how you get magmatism through the thick overlying plate. Hawai'i is pretty much the classic example - the Hawai'i-Emperor seamount chain shows a very clear age progression (oldest furthest away from current active volcanoes!), consistent chemical behaviour, etc - and is the basis of the theory of mantle plumes - the idea that the mantle is convecting, so some of the time you'll get hot upwelling streams (like when you boil a pot of water and get an obvious rising column).
But not everywhere looks like a plume; in addition to a higher temperature, you can also get increased magma production if you're melting a bit of the mantle that has a lower melting point than usual (because it's a slightly different composition). Working out which is going on is a major ongoing debate.
What I specifically am going to be doing is looking at the ratios of stable isotopes in OIB samples - for a particular set of elements, which occur naturally in several different forms depending on their number of neutrons (think radiocarbon dating versus the kind of carbon we're mostly made of, except I'm going to be using metals) to try to work out:
- where the stuff that's melting came from (is it mantle that's been around for ever/melted multiple times, or can we "see" evidence that sediment that's gone down in subduction zones has been mixed in?)
- why it's melting (T vs composition), in at least some cases
- what this means for how the mantle convects (does it all go in one big lump? is it layered? how do subducted plates get stirred back in? how efficiently is the mantle stirred?)
... which suddenly got super-technical, sorry, but -- yeah, six hours' sleep; again, please do ask if you have questions <3
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Date: 2013-11-06 05:13 pm (UTC)Way cool. Er, way hot. Oh, you know what I mean. :)
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Date: 2013-11-06 09:58 pm (UTC)(no subject)
Date: 2013-11-06 06:18 pm (UTC)(no subject)
Date: 2013-11-06 10:42 pm (UTC)So! I mentioned isotopes above. They're the tool I'm going to use.
To take several steps back... atoms are made up of three types of particle: protons and neutrons (in the nucleus) and electrons (whizzing around outside, ish). What defines whether an atom is hydrogen or helium or oxygen or carbon is how many protons it has - something with 6 protons is always carbon. However, the number of neutrons can also vary - so you can get carbon with 6 neutrons (carbon-12, the vast majority), carbon with 7 neutrons (carbon-13, which is also stable but present in much smaller quantities) and carbon with 8 neutrons (carbon-14, the radioactive carbon isotope used in radiocarbon dating).
If you look at all the carbon on the planet, there's an overall ratio of carbon-12 to carbon-13 (I'm ignoring the 14 for these purposes but the same argument holds with a few tweaks to take into account its radioactivity). However, when you look at individual reservoirs of carbon - for example, a tree or coal or limestone - you tend to find that the ratio of carbon-12 to carbon-13 is all over the place from your expected value, and not consistent between reservoirs (so even if you've got your best guess at what the ratio should be right, you're still seeing significant variations from that ratio).
So: something has (or somethings have!) to be happening to shift the number, and as it turns out they are, and this is called isotope fractionation. Specifically, the examples I'm giving - and the species (i.e. elements/oxides/isotopes) I'll be working with - are all stable isotope fractionation (i.e. no radioactive decay is involved). If you'd like to know about how this all works with radioactive species playing along then by all means ask, but I'm not going to include it in this comment for the sake of keeping it at least a bit short ;)
So! The particular element I'm going to be looking at in the first instance is thallium, which is a heavy metal element (not hugely abundant!). There's barely any thallium in the mantle, and we know that its isotopic ratio is significantly affected during marine sediment formation - different kinds of marine sediment have different thallium ratios. However, the isotope ratio shouldn't be affected by processes like melting.
When sediment is subducted it is expected to melt pretty efficiently (that is, most of it will melt). This means that you're quite right, we don't actually see bits of mud and shells ;) BUT via the magic of going "huh, this lava has a lot of thallium in it... in a very precise isotopic ratio", we're able to say "the fact that this lava with this composition was produced means that we must have had a source material that meled to create it, and that source material must have been at least X% of this particular type of sediment."
Obviously using just one set of elemental isotopes leaves a lot of room for interpretation, and that's part of the problem with existing work using radiogenics (radioactive isotope systems) - it's great as far as it goes, but there are some cases it simply can't distinguish between. However, the ability to detect small differences in isotope ratios of heavy elements at low concentration is relatively recent, so the hope is that by using these new techniques and newly-available stable systems, we'll be able to build up a better picture of what must have happened.
... which possibly got massively technical at the end? I'm sorry, I can't tell, I'm underslept (but enjoying myself immensely! This is good practice). If you'd like more on any of those bits, please please ask (because chances are someone else would too :D).
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Date: 2013-11-06 11:29 pm (UTC)(no subject)
Date: 2013-11-07 12:29 pm (UTC)It involves lots of really tedious labwork but I should get some initial results before Christmas, and that will be exciting :-) Plus tedious labwork is frequently just right for my level of brain!
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Date: 2013-11-07 12:32 pm (UTC)(no subject)
Date: 2013-11-07 12:51 am (UTC)(no subject)
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Date: 2013-11-07 08:15 pm (UTC)Also, had bit of a lightbulb moment - radioactive is just another way of saying unstable, isn't it! O_O I mean, it seems so obvious now that I put that together, but !!!!! I am going to enjoy the bright sizzling noises that my brain is making even though I also feel a little silly ;p because CONNECTING THINGS UP IS YAY!
Okay, and, I have more questions! :D
1. How arbitrary is the decision to use thallium, or is it a thing that is somewhat uniquely suited for looking at this?
2. Sooo this isotope fractionation thing. How does it work? What are the things-that-happen that make the numbers shift?
[I am really enjoying having the chance to be all 'academicgeekery brain ENGAGE!' again, it is so much fun, thank you :)]
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Date: 2013-11-09 01:48 pm (UTC)And yes, connecting things up is awesome - you're spot-on with radioactive/unstable!
1. There are several reasons we've gone for thallium! One is that it's only in the past few years that it's really become possible to detect variations in isotope ratio for such heavy elements, and nobody has yet (at all seriously) applied this to the problem that we're looking at. Another is that we know it's basically nonexistent in the ambient mantle, whereas there's a relatively enormous amount in marine sediments, so we're pretty sure *any* thallium we see in an erupted magma has to have originated as sediment, and therefore lets us work out some stuff about (1) what the magma source was and (2) mantle convection dynamics.
(2) Lots of things cause the numbers to shift! Unfortunately some of it involves quantum maths in a way that makes me pull faces. I ended up explaining this to That One Gentleman the other night, but I got to wave my hands around for that, so I will see if I can manage without the physical handwaving... ;)
What I care about is mass-dependent stable isotope fractionation. Every chemical species has a range of energy levels available to it - it can be in its ground state or lowest energy level, or it can be many different kinds of "excited" (for a molecule, think "bonds wiggling about faster" - it takes energy to wave your arms about). Where exactly the ground state is - what a molecule's minimum possible energy is - is dependent on mass (it's not zero!). The heavier isotope has a lower minimum energy, Because Of Quantum.
There are two main processes I'm interested in: equilibrium fractionation and kinetic fractionation. The next bit is easier to understand with diagrams -- here's an example of a potential energy map, which shows how you have your starting materials (reagents) at one energy level; they pass through a transition state (energy maximum); and end up as product (lower energy state).
Let's take equilibrium first: for this, you want to end up with the lowest overall energy of the system. This tends to favour the heavier isotope ending up in the lower-energy product (where the lighter isotope is more likely to end up in unreacted starting material). Equilibrium processes tend to be slow - it takes them a while to work everything out - and to dominate at lower temperatures.
At higher temperatures or in unidirectional reactions (i.e. the products are removed once made, so they can't react to form the starting material again), kinetic fractionation is the dominant process. (This is the case when you're e.g. evaporating water from the ocean - the evaporated water becomes part of atmospheric humidity and gets more-or-less taken out of contact with the surface of the ocean). In this situation, what you care about is the energy gap between starting material (reagents) and transition state - a smaller gap makes it easier to get over the hump. And atoms of and species containing the lighter isotope have a slightly higher ground-state energy, so more of them will make it over the transition-state energy-maximum to make it into product.
With thallium in particular, there's an additional weird effect going on - the Nuclear Field Shift effect - due to the fact that it's quite heavy (~205 atomic mass units), to the point where the nucleus can no longer be treated as a point charge. Instead, the atomic nucleus is big enough that it starts to interact with the orbits of the innermost electrons, which has knock-on effects for the rest of the electrons, which in turn alters how different isotopes (which show differing degrees of this effect) undergo chemical reactions - which leads to much bigger fractionations than would be expected for an element this big if this effect didn't exist. (Because most isotopes have only 1 amu difference in mass - and while this is a Big Deal proportionally when you're comparing hydrogen-1 with hydrogen-2, when you're looking at thallium-205 versus thallium-206 the relative difference in mass is much smaller - a fraction of a percent, rather than 200%, so you don't expect to see such big mass-dependent effects.)
Erm. I think I have run out of steam again, but feed me another question and I'll give you more answer... ;)
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Date: 2013-11-06 07:10 pm (UTC)(no subject)
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Date: 2013-11-06 10:05 pm (UTC)Also, super interesting, so thanks for sharing!
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Date: 2013-11-06 10:14 pm (UTC)(no subject)
Date: 2013-11-07 12:51 am (UTC)