Enikő Bali

Geologist, Current residence: Reykjavík, Iceland (Home: XV. district (Rákospalota), Budapest, Hungary)

Rationale behind high pressure and temperature experiments

Of course there is always a rationale behind our experiments. We do not just put all kinds of materials into our capsules or in our diamond cell to check what happens with them at high pressure and temperature.

We always need to have some direct measurements that need to be explained and investigated. Geology by know is far more than determining the composition and structure of minerals and rocks. We would like to know why these compositions or structures developed. We want to know the conditions in which they formed. We want to describe those processes that lead to their development. And finally we would like to be able to predict processes through these results that might have effect on our every-day life. These can be earthquakes, volcanism, formation of economic grade ore deposits, and so on.

One can ask, how I know that those “green pebbles” are coming from the upper mantle. One can ask how I know what the composition of the Earth is at 1000 km depth. Well, most of the pioneering observations and experiments have been done between the 50´s and 70´s. Let us see briefly what the observation were that led to our present knowledge on the structure and composition of the Earth.

What was possible to measure?

1)       The compositions:

of ancient meteorites that have similar age to the Earth can be measured. It is assumed that these meteorites have similar composition to the early undifferentiated Earth before the development of crust, mantle and core (though the exact analogue is not known). The composition of the rocks on the Earth´s surface can also be analyzed. Their volume can be estimated by geological mapping. Based on the volume (--> mass) of different surface rocks and their composition the composition of the crust can be calculated. Than, the crust´s composition can be compared to that of the ancient meteorites. Those elements that are missing from the crust or are in small quantities relative to meteorites might be resided in the deep interior of the Earth or escaped to Space. Basically, what could be done at this point was, to check how the physical properties and chemical composition of these “missing components” vary with pressure and temperature. Having the right starting material in these experiments allowed the researchers to fit these "experimentally produced  properties" to some of the directly measured physical properties of the Earth.

2)      Propagation of seismic waves:

On one hand they show us the major discontinuities in the Earth, as their velocity abruptly changes at certain depths. This change can happen due to a change in composition (for example at the core-mantle boundary) or due to the transformation of crystal structure of the major mineral phases in the Earth´s mantle.  

On the other hand the propagation velocity itself gives us an indication on the chemical composition and physical conditions of the materials. For example the transversal (S) waves “disappear” between 5100 and 2900 km depth which suggests that the material there should be a fluid. Moreover, the propagation velocity of seismic waves is proportional to the density of the material they are passing through. Density is related to volume and mass, thus knowing the volume and density of the given unit of the Earth, the mean atomic number could have been estimated. For example the material of the core should have a mean atomic number of 56, whereas the mantle should be “lighter”. This suggests us that the core should be dominantly composed of Iron (iron-rich alloy) whereas the mantle is built up by the mixture of Si-Mg-Fe-Al-Ca and O.

3)      The Earth has a magnetic field:

and that can also be measured. Its existence and properties are in accordance with the existence of convecting molten Fe-alloy in the outer core.


What did we get to know?

Based on these observations and high pressure and temperature experiments we know that the core should be a Fe-Ni alloy containing some minor elements and 10% of light component. There is still a debate which this component can be (O, Si, Mg, C, S etc.) most probably it is not one component. We also know that the major discontinuities in the Earth´s mantle are caused by the structural transformation of minerals. Namely olivine that is the most common mineral of upper mantle transforms to wadsleyite at 410 km depth and this wadsleyite component transforms to ringwoodite at 520 km. At 660 km depth ringwoodite transforms to Mg-silicate perovskite and Mg-wustite drawing the last large discontinuity in the mantle.

Of course the development of seismic and other geophysical measurement methods give us further work to clarify minor discontinuities and heterogeneities in the Earth, but by now the Earth´s structure and composition is quite well known.

If you would like to know more about the Earth´s composition I suggest to read this paper.  It is detailed enough but easy to read and contains the most important references that helps you to dig deeper in the subject.

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