Magma "silent" as thought

Conclusions about the chemical composition of the Earth's interior are possible only up to a depth of 20 kilometers

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Magma from the Earth's interior contains information about the chemical composition of the Earth's mantle. But contrary to the previous view, the molten rock by no means provides data about the Earth's interior to a depth of 200 kilometers. Because only two to twenty kilometers deep can pull back conclusions, as geologists report in "Nature".

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Magma is the end product of a long series of chemical processes and consists of molten rock and crystals. Due to the inclusion of fissures in these crystals, geologists conclude that the Earth's core is chemically composed. A new study, carried out at the Australian National University (ANU), now shakes the previous assumptions: The chemistry of the inclusions does not reflect the prevailing conditions up to about 200 kilometers depth - but only up to about 2 to 20 kilometers depth.

The magma with the melt inclusions is formed in the earth's mantle, which is around 3, 000 kilometers thick, and moves from the depths upwards towards the earth's surface, where it finally stores in so-called magma chambers 2 to 20 kilometers underground and develops chemically. When growing olivine crystals, the most common silicate species contained in mantle magma, a tiny melt droplet is trapped in the crystal lattice. So far, scientists have assumed that this melt inclusion is hermetically sealed off from the environment after its formation and thus reflects the chemical state of a layer of earth at a clearly defined depth.

However, laboratory experiments by Carl Spandler of the Australian National University showed that the melt inclusions "communicate" with the environment after diffusion via diffusion, said Thomas Pettke from the University of Bern. Their chemical composition could even change within days, much faster than expected. Thus, the inclusions no longer count as "absolute low signal, " the researchers conclude. "Specifically, these results will strongly influence our understanding of chemical development and processes in the Earth's interior. For example, they give new clues about where volcanoes are fed. " display

Searching for traces with the laser beam

Spandler analyzed the diffusion behavior of the melt inclusions with the new analytical technique of laser ablation: a laser beam burns a spot on the crystal sample with the highest energy. Evaporation releases an aerosol of the material, which is immediately channeled through a gas stream into the so-called plasma mass spectrometer. "Ion analysis enables us to analyze even the smallest amounts - one milligram per ton - of almost all existing elements, " says Pettke. With this technique, almost all solids can be chemically analyzed. The research focus of Spandler and Pettke is the analysis of inclusions of minerals: "Basic research, which - as we see - can be of great importance, " says Pettke. According to the researchers, some "golden truths in Earth science now need to be rethought."

(University of Bern, 21.05.2007 - AHE)