Geoverse

My colleague from Boston has carried out a successful defomation experiment this week. The result is shown in the figure below. The deformation occurred at 1300C and 300 MPa confining argon pressure on titanium and calcium bearing synthetic olivine (fine-grained). Compressive differential stress (upwards force) was up to 315 MPa / 33 kN in the dislocation creep field. The transition to this form of power law creep deformation occurs at around 200 MPa and higher depending on the type of olivine material that is deformed. At lower stresses diffusion creep is dominant. In the figure you can see a typical barrel shaped specimen that was originally longer and a straight cylinder in shape. The higher compression direction compared to the uniform confining pressure comes from the sides. The specimen will be sectioned and imaged to determine the microstructure using the scanning electron microscope (SEM). The remainer will be precision ground to 10 mm in diameter and put back in the apparatus for another deformation experiment.

The most important result from the experiment will be a new flow law that predicts how this type of material deforms under high temperature and pressure comparable to upper mantle conditions in the Earth (100-400 km deep). However, whether the data trend can be extrapolated to geological time scales and coarser grain size remains debatable.

Fig. 1 Deformed fine-grained olivine (original length 20.8 mm) sandwiched between two alumina loading pistons.

Phwoooaaar! I could hardly believe what I was hearing on Friday arvo after I held a pleasant presentation of my research at the Utrecht University. My post-doc advisor there told me that I won the Paterson Fellowship grant!!! If you backtrack to number 3 in the previous post, that is what I’ll be doing. I will have a budget up to AUD 7000. Very very cool! I’ve never been to the US, so it’s definitely going to be most interesting. This also means that I will have an opportunity to visit the Netherlands again next year in September. So for the people there,… I’ll catch ya later alligator, in a while crocodile, give a hug ladybug, dont forget to write! 🙂

Time to put more science on this blog, specifically related to my research. 🙂

Part of my research is about looking at crystal defects, more specifically line defects called dislocations. These phenomena are missing half planes of atoms in a crystal structure.

In an ideal world there is a certain spacial configuration of specific atoms (like carbon in diamond) that is energetically the most stable with minimal entropy (or chaos). This was assumed to be all the way up to the early 1930s until certain individuals, such as Egon Orowan started looking at crystal plasticity, the ability for a material to deform without breaking. Deformation experiments were carried out on metals and alloys to assess their strength and breaking point. Oddly, the calculations did not match the experimental results. It turned out that materials were a lot weaker, by several orders of magnitude, than was presumed. Around 1932 Orowan and Taylor simultaneously proposed the existence of missing atoms in the crystal structure to explain the observed weakness in materials. And thus was born the theory of dislocation glide to accomodate plastic deformation. Below you can see two dislocations in an otherwise perfect atomic lattice. These are the so called end-members. Normally dislocations form a mix between the two.

On the left is shown a line dislocation (pointing inwards below the red atom) with the blue atoms representing an extra half plane above the dislocation. The yellow atoms highlight the glide plane over which the dislocation line moves. On the right is shown a screw dislocation. Again the dislocation line points into the screen at the red atom. Think of this type of dislocation as a tear in a sheet of paper.

Not much later on, with the invention of the electron microscope, dislocations were proven to be real and have been observed in any material conveivable including upper mantle rocks which is what I am interested in.

A region in the Earth at depths between 80km and 400km there is one dominant mineral present called olivine. It is otherwise known as Peridot. This region in the Earth also experiences anything from brittle deformation at shallow depths, where Earth quakes occur, to plastic deformation.

(Don’t be fooled to think that there is a huge magma ocean deep in the Earth just because volcanoes spew out molten rocks. Volcanoes only exist in very specific settings, mostly at plate margins. The presence of subducted water at those locations greatly enhances the melting point of rocks.)

Olivine (Fe,Mg)2SiO4 otherwise known as peridot. This specimen comes from a mine in Pakistan and is about 2 cm long.

So in order to understand how the Earth works, i.e. mountain building, formation and destruction of oceanic crust, earth quakes and stress states in the Earth’s lithosphere we need to know about the strength of the rocks in the Earth. Knowing that dislocations in materials greatly reduce its strength means that they can essentially not be ignored and have to be studied.

This is where I come in. 🙂 Not only is it the above that interests me, but also the applications. For years now have geophysicists imaged the inside of the Earth by recording sound waves (seismic waves) as they are reflected and refracted in the Earth and returned to the geophones connected to a seismometer. The seismic waves can come from either Earthquakes or explosions.

The Earth is highly anisotropic in nature, therefore the understanding of a material’s behaviour to the propagation of seismic waves needs to be carried out in the lab. My study attemps to systematically prove or disprove that deformed rocks in the Earth, containing these dislocations, can slow down and attenuate (dampen) these seismic waves.

Other factors that play a role are partial melts in rocks, grain size, water and of course temperature. That makes this study not easy!! 😀

Below is shown an electron microscope image of decorated dislocations (by oxidation) in a synthetic (home made) deformed olivine aggregate.

Electron microscope image of dislocations (short white lines) in several grains of synthetic (home made) deformed olivine. Using image manipulation software I can work out the dislocation density.

So there you have it. If anything is unclear, please leave a comment. I’d only be too happy to clarify. 🙂

If you remember i’ve been to the Wagga conference on condensed matter and materials. There I submitted a manuscript of my preliminary research. Now, not surprisingly the organisation can’t get their act in gear to get the peer-reviewing done of all the submitted papers. So guess what? I’ve been asked if I could peer-review two papers!! 🙂 Ones are at least a bit related to what I do though since it’s not really an Earth Sciences conference. 😉 I feel pretty excited about doing this. Now I’m a real scientist! I’m also a bit nervous because..well..i’ve got no experience doing this sort of thing what so ever. Anyway I have 3 weeks apparently. I’m sure my supervisor could give me some helpful tips. Currently he is away to a conference in Japan and will be back next week Tuesday. I was going to do a hotpress experiment in the meantime but it looks like the bloody furnace is broken again. This time an open circuit in one of the furnace windings. Real nuisance because we just recalibrated a stable hotzone at 1300C with the furnace. We really need a technician in the lab. The last one left us 6 months ago for a better job. There hasn’t been enough funding for a replacement but maybe we can get a part-time tech.

In other news im about to submit my masters research to the journal Geophysical Research Letters. I’m holding off just a few more weeks because I want my former supervisor, who hasn’t helped me a bit, to reply with a few last comments and feel guilty as hell*.

I’m also in the process of writing a manuscript for GRL about my current research. Once I get this finalized I can make it a part of my mid-term assessment report. I’ll have a chapter on that and a chapter on specimen preparation, preliminary results from the forced seismic wave attenuation experiments / future prospects and feasibility of the phd project. Heaps to do, if only the bloody machines would co-operate.

Oh btw, Happy Easter everyone!!!! 😀

EDIT * It worked. Emails like those are not left unanswered. I always find it more rewarding generally to cause guilt in someone who is abusing me rather than to cause anger. It’s more of a challenge too.