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X-rays reveal inner structure of the Earth's ancient magma ocean

Scientists have, for the first time, peered into molten magma at conditions of the deep Earth mantle using an x-ray source. The analysis at Desy's light source at the Petra III facility in Hamburg, Germany revealed that molten basalt changes its structure when exposed to pressure of up to 60 gigapascals (GPa), corresponding to a depth of about 1,400km below the surface.

Chrystèle Sanloup and a team of researchers from the University of Edinburgh in the UK have reported that at such extreme conditions, the magma changes into a stiffer and denser form. The findings support the concept that the early Earth's mantle harboured two magma oceans separated by a crystalline layer. Today, these presumed oceans have crystallised, but molten magma still exists in local patches and possibly as thin layers in the mantle.

‘Silicate liquids like basaltic magma play a key role at all stages of deep Earth evolution, ranging from core and crust formation billions of years ago to volcanic activity today,’ said Sanloup.

To investigate the behaviour of magma in the deep mantle, the researchers squeezed small pieces of basalt within a diamond anvil cell and applied up to 600,000 times the standard atmospheric pressure. ‘To investigate basaltic magma as it still exists in local patches within the Earth's mantle, we first had to melt the samples,’ explained Zuzana Konôpková from Desy, who supported the experiments at the Extreme Conditions Beamline (ECB), P02 at the Petra III facility.

The team used two strong infrared lasers that each concentrated a power of up to 40W onto an area just 20mm, which is about 2,000 times the power density of the surface of the sun. A clever alignment of the laser optics allowed the team to shoot the heating lasers right through the diamond anvils. With this setup, the basalt samples could be heated to 3,000°C in a few seconds, until they were completely molten.

To avoid overheating of the diamond anvil cell which would have skewed the x-ray measurements, the heating laser was only switched on for a few seconds before and during the x-ray diffraction patterns were taken. The short data collection times that were needed for this type of melting experiment were possible due to the high x-ray brightness at the ECB. ‘For the first time, we could study structural changes in molten magma over such a wide range of pressure,’ said Konôpková.

The powerful x-rays show that the so-called coordination number of silicon, the most abundant chemical element in magmas, in the melt increases from four to six under high pressure. This shows that the silicon ions rearrange into a configuration where each has six oxygen neighbours instead of the usual four at ambient conditions. As a result, the basalt density increases from about 2.7g/cm2 at low pressure to almost 5g/cm2 at 60GPa.

‘An important question was how this coordination number change happens in the molten state, and how that affects the physical and chemical properties,’ explained Sanloup. ‘The results show that the coordination number changes from four to six gradually from 10GPa to 35GPa in magmas, and once completed, magmas are much stiffer, that is much less compressible.’ In contrast, in mantle silicate crystals, the coordination number change occurs abruptly at 25GPa, which defines the boundary between the upper and lower mantle.

This behaviour allows for the possibility of layered magma oceans in the early Earth's interior. ‘At low pressure, magmas are much more compressible than their crystalline counterparts, while they are almost as stiff above 35GPa,’ explained Sanloup. ‘This implies that early in the history of the Earth when it started crystallising, magmas may have been negatively buoyant at the bottom of both upper and lower mantle, resulting in the existence of two magma oceans, separated by a crystalline layer. This has been proposed earlier by other scientists.’

At the high pressure of the lower Earth mantle, the magma becomes so dense that rocks do not sink into it anymore but float on top. This way, a crystallised boundary between an upper and a basal magma ocean could have formed within the young Earth. The existence of two separate magma oceans had been postulated to reconcile geochronological estimates for the duration of the magma ocean era with cooling models for molten magma.

While the geochronological estimates yield a duration of a few ten million years for the magma ocean era, cooling models show that a single magma ocean would have cooled much quicker, within just one million years. A crystalline layer would have isolated the lower magma ocean thermally and significantly delayed its cooling down. Today, there are still remnants of the basal magma ocean in the form of melt pockets detected atop the Earth's core by seismology.

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