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How magma is formed
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The depths of the earth in diamond anvil cells
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By artificially creating in the laboratory the conditions that exist several tens of kilometres down in the Earth, scientists aim to improve their understanding of the “major recycling” stages of the elements linked to volcanic phenomena. The group at the ’Institute of Mineralogy and Physics of Condensed Matter (Paris VI) has thus used the diamond anvil cell technology on the DIFFABS beamline. Ready to up the pressure and temperature?...
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 Subduction and volcanic arcs
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Volcanic phenomena are, with earthquakes, the most impressive manifestations of the existence of tectonic plates. Approximately a dozen major plates form the outer layer of our planet: the crust and part of the upper mantle, i.e. the lithosphere (Figure 1). Oceanic or continental, they rest on another “layer”: the asthenosphere, which is solid but ductile (can be deformed without breaking). The plates move in relation to each other at speeds of a few centimetres per year.
When two plates collide in an oceanic context, it is the denser “oceanic” plate that pushes under the “continental” plate, and drops several hundred kilometres. This is known as subduction. With depth temperature and pressure increase and the subducted material is progressively dehydrated and transformed under these confined conditions. The migration of the fluids resulting from this dehydration is thought to allow melting of some of the minerals of the lithosphere, creating a magma that rises to the earth’s surface by infiltrating through the plate remaining at the surface. This creates a chain of volcanoes directly above the subducted plate, parallel to the border between the two plates, called the volcanic arc.
The “subduction factory” is therefore where elements are recycled between different terrestrial reservoirs: from the crust to the atmosphere via the mantle.
How this factory functions was central to the experiments carried out on the DIFFABS beamline in July 2008. | 
Figure 1 : Diagram of a transverse cut through the Earth
with the different successive layers that compose it.
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Specific magma
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Numerous questions still remain concerning the source of the magma from these volcanic arcs (arc magmas). The composition of arc magmas is specific and characteristic: when compared to reference magmas (basalts derived from the mid-oceanic ridge), they contain, on the one hand, higher levels of elements such as lead, uranium or thorium and on the other, lower levels of other elements (e.g. zirconium or titanium).
It is thought that this distinctive feature of arc magmas is linked to the recycling of chemical elements that make up the earth’s crust beneath the ocean by means of recycled fluids, in the presence of halogens such as chlorine. This fluid vector would therefore be the enriching agent, explaining this geochemical feature of arc magmas.
However, scientists still do not know much about the nature of these fluids and their role during the process of partial melting of minerals in the mantle. Aqueous or saline fluid, silicate liquid or supercritical fluid1 with solvent properties that cause these distinguishing features?
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A few microlitres of magma under synchrotron beamlines
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It is unfortunately impossible to experiment in situ, in the Earth’s mantle... However the temperature and pressure conditions that prevail can be reproduced artificially with the aid of specific instruments: diamond anvil cells (DAC). The sample is enclosed in a confined space in a metallic plate between two diamonds and heated by resistive ovens to reconstitute the pressure and temperature conditions required. This old experimental technology continually developed and improved by scientists can be combined with chemical characterisation techniques available on synchrotrons. Due to the transparency of the diamonds, it is possible to identify and measure the composition of a sample less than a hundredth of a micrometre long confined within the cell.
It was this combination of optimised tools that Hélène Bureau of the Institute of Mineralogy and Physics of Condensed Matter (Paris V) came to use the DIFFABS beamline in July 2008 (Figure 2), in collaboration with Pascal Munsch and Guilhem Simon from the same institute, Eddy Foy from the Pierre Süe Laboratory (CEA Saclay), as well as Andrea Somogyi and Stefan Kubsky at SOLEIL.
Temperatures of 800 to 900°C were applied and also pressures of up to 2 GPa (2 billion Pascals – atmospheric pressure is about 100,000 Pa, i.e. 20,000 times less), corresponding to the conditions found in the first hundred or so kilometres below ground.
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Figure 2 : Experimental set-up
on the DIFFABS beamline
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The importance of silicate fluid…
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| The experimental objective was to identify the likely nature of the fluid responsible for these distinctive arc magmas. To do this, the silicate liquid/water balance was reconstituted in the DAC, and the distribution of uranium and thorium between the different phases (saline fluid /silicate liquid, for example) was studied by X-ray micro fluorescence spectroscopy.
The transfers of Th and U were quantified in different fluid phases at different pressures and temperatures during the study: aqueous, silicate, but also supercritical, the result of the total miscibility of the latter two and in the presence or absence of halogen.
By achieving equilibrium between aqueous and silicate liquid phases, (Figure 3) the scientists were able to show that the transfers of U and Th changed in relation to the presence or absence of halogen elements (here bromine, used as an analogue of chlorine). The results confirmed that the silicate fluid was the most fundamentally enriched in U and Th in the presence of halogen.
It would seem, therefore, as suspected for other elements, that the enrichment of arc magmas with U and Th is the result of contamination of the original mantle by a silicate fluid rich in water and salt and not by a saline aqueous fluid, as has long been suggested.
Figure 3 : View in situ of sample chamber at an aqueous fluid- silicate liquid equilibrium | 
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Below ground but also in the air
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Halogens have a significant influence on the stratosphere2 and in particular on the ozone layer. The volcanic emission of gases such as hydrogen chloride (HCl) can provoke the localized destruction of the ozone layer. It is in the magma chamber under the volcano that the gases emitted during eruptions are formed. Their composition, as a result of the aqueous saline fluid / silicate liquid equilibrium, was at the heart of Dr Bureau’s research. Chlorine destroys the ozone layer (“CFCs” are often cited), but it also applies to bromine, which is in fact far more aggressive than chlorine.
Therefore, in a second experimental phase, scientists used the same experiments to characterise magma degassing in situ. Once chemical equilibrium has been reached, it is possible to soak the DAC and achieve rapid decompression similar to that experienced by magma on its rapid ascent to the earth’s surface. Decompression results in the process of magma degassing, i.e. the coming out of solution of volatile substances (H2O, Br in this experiment) dissolved in the silicate liquid.
It is thus very easy to understand the benefits of studying the impact of volcanic eruptions and notably bromine and other halogen cycles in order to improve our understanding of these stratospheric processes.
Analyses by synchrotron radiation allow bromine levels both before and after decompression at the centre of both phases to be determined and thus to quantify potential degassing, which cannot be measured directly in the volcanic conduits and volcanic plumes.
To complete the study, this experimental process may be applied again, in the future, on DIFFABS, using other elements.
1Supercritical: state when matter is submitted to high pressure or temperature. One refers to a supercritical fluid when this fluid is heated above its critical temperature and when it is compressed above its critical pressure. The physical properties of a supercritical fluid (density, viscosity, thermal diffusivity) are intermediate between those of liquids and of gases. Under supercritical conditions, a solvent’s ability to dissolve is considerably enhanced.
2Stratosphere: the atmosphere, gaseous envelope surrounding the Earth, composed of several layers. Moving away from Earth, one finds, in successive order: the troposphere, about fifteen kilometres thick, containing the air that we breathe; the asthenosphere, about thirty kilometres thick, which includes the ozone layer; the mesosphere and the thermosphere.
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