How to be a geochronologist - Part 1

I thought I would do a post explaining what I've actually been up to in Canada in terms of work, as I'm aware it appears that I do very little geology. I'm going to start with some of the lab work I've been up to, and what happens to the rocks after I collect them in the field, and what answers I'm trying to get out of them. But don't worry, I won't bore you guys with the in depth account of my research proposal and why I'm doing this particularly project (although my proposal is a good read!). 

Basically a previous study of these rocks had found some pretty interesting geochemical characteristics in the rocks (the ones I collected from California in December) that we weren't expecting, specifically to do with the ages we were getting from the analyses. The parts of the rocks that we typically get ages for rocks from these days are excruciatingly tiny minerals (definitely <1mm) like zircon and monazite. These minerals are surprisingly tough little blighters that form in molten rocks that pop up in the earth's crust, and are then found in rocks like my granites. During their lifetime they are very good at recording the chemistry of the magma around them, as well as picking up signatures of other things that happen to them, like if they get attacked by fluids deep underground or up at the surface. Using fancy equipment we can measure the chemistry of these minerals and figure out what happened to the minerals, and most importantly, when it happened.


As I said, these minerals are tiny. You can't see them in the rock with your eyes. You may be able to see them in the microscope if they're big and you're a hawk. So how do we get them out of the rock and in a state where we can measure them? Well...

You get your rocks, of which there may be a couple kilograms of chunks like this,

Using this array of metal slabs, nuts, washers, ethanol and tools,

Dolled up in a dust mask, safety glasses and ear protectors, you submit your rocks to a number of medieval torture methods...

Splitting (put the rock in between these two teeth and squeeze until it breaks into smaller chunks, if the US Postal Service haven't adequately broken up your samples for you),

Crushing (use big metal plates that move towards and away from each other to crush the rocks into chunks a couple of centimetres big),

Milling (use two rotating disks to grind the rocks down into a sandy powder),

Using the terrifyingly named "Pulverizer",

Quartering (to split your sand sample into 4 representative portions,

And then sieving to get rid of the coarser material and the finer dust (that volunteer job working with aggregates did come in handy afterall!).

After all this dusty manual labour, which involves a lot of heavy duty duct tape and brown paper, you are then ready to do some science! The zircon and monazite grains we're after are all about the same size as the rest of the sand grains. But how do we then separate out the <5% of grains that we need from everything else? Well helpfully zircon and monazite are largely denser than most of the other stuff in the sand. So using the "heavy liquid technique" we mix our sample with a really dense chemical called methylene iodide and thanks its density...

The lighter stuff (quartz, feldspars and micas, that we don't want) floats to the top (where all the grey crap is),

And the heavier stuff (zircon, monazite and garnet if it's there) sinks to the bottom, where we can tap it off and move onto the next step,

All the while rocking the lab coat, safety glasses and latex gloves look, that at least makes you look more like a scientist than a construction worker.

You can then separate out the good stuff even more by using the Frantz magnetic separator. By this point we've already removed the actual magnetic stuff with a big hand magnet, but all the stuff that's left is paramagnetic and dimagnetic which means they can still interact with magnetic fields but in different ways. This big ol' electromagnet takes advantage of that, and by varying the strength of the magnetic field and/or the slope of the chute you can separate out the zircon and monazite, and anything else that's still knocking about.

You set the kit up with a bit of arts and crafts skill. The sample (a couple grams worth of tiny minerals in my case) goes in the funnel to top right,

And passes through the electromagnet and splits down the shoot into two fractions, which we then collect in our weighing paper boxes.

From here the process involves physically picking the grains and putting them on a mount, which we can then put in one of our analytical machines to get our geochemical and geochronological data. Now, may I remind you again that these grains are very small! It takes a bit of practice and a lot of concentration to be able to use the really fine tweezers to pick up the grains and move them. Especially as monazite tends to shatter into a million pieces if you squeeze it too hard. To help keep focused and to prevent being spooked by any sudden noises I tend to blast carefully constructed playlists of classic rock that I can sing along to. This approach has largely succeeded, and my singing is very effective at keeping people out of the lab!

Big fancy microscope,

With a wee dish containing one of the fractions separated out before, mixed with some ethanol to make it easier to pick grains up.

Camera attached to the microscope helps with identification. The golden brown grains are typically monazite, whereas zircon, found in one of the other fractions, is completely clear and typically with pointy pyramid ends.

Once located and picked up with the tweezers grains are moved, whilst trying your utmost not to drop them, to another microscope,

Where they are placed carefully onto super strong sticky back tape on the mount,

And after a picking a few hundred garins begins to look something a little like this. That little red bar is 1mm in size. I told you these things were small!

After the grains are on the mount they are coated in epoxy and polished down to make the final mount. This is the stage i am currently up to; I'm waiting for some new epoxy to arrive and then before you know it we'll be shooting lasers at the grains and getting data out of them! Well that's the plan anyways. I will write about the next stage of the process when it happens and I have some pretty pictures to accompany. Until next time y'all...