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Silicates Get Denser When You Go Deeper

Photonics Spectra
Aug 2008
Lynn M. Savage

Far below the Earth’s surface, where pressure and temperature escalate ferociously at the border between the mantle and the core, lies the history of primordial chemical differentiation. Molten silicates, such as MgSiO3 melts, likely comprised a large proportion of the magma ocean on the early Earth. However, gaining an understanding of changes in the silicate’s density, its viscosity and the partitioning between its crystalline and molten states means trying to replicate the mineral’s native conditions above ground. However, there aren’t many laboratory instruments capable of gleaning information under high temperature and pressure.

A team of researchers with members from South Korea, Japan, Taiwan and the US has solved the problem by using high-pressure x-ray Raman spectroscopy, testing MgSiO3 glass at up to 39 GPa. The investigators acquired data using beam lines at the SPring-8 synchrotron in Sayo, Japan, and at the Geo Soil Enviro Consortium for Advanced Radiation Sources in Argonne, Ill.

They found that the MgSiO3 glass, which functions as a precursor to magnesium-silicate melts, produces molecules composed of oxygen set upon triplet arrangements of silicon when the pressure in the sample chamber is set to >20 GPa. The presence of these so-called oxygen triclusters is thought to enhance the partitioning coefficients of magnesium-silicate melts and possibly to inhibit the solubility of noble gases in the melts. The high pressure also reduced the amount of nonbridging oxygen — oxygen attached to only one silicon atom — in the material.

According to the researchers, increasing the pressure to more than 20 GPa would result in greater density, higher viscosity and lower element diffusivity of the MgSiO3 melt, as well as in enhanced crystal-melt partitioning.

PNAS, June 10, 2008, pp. 7925-7929.

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