Structural change in molten basalt at deep mantle conditions


Silicate liquids play a key part at all stages of deep Earth evolution, ranging from core and crust formation billions of years ago to present-day volcanic activity. Quantitative models of these processes require knowledge of the structural changes and compression mechanisms that take place in liquid silicates at the high pressures and temperatures in the Earth’s interior. However, obtaining such knowledge has long been impeded by the challenging nature of the experiments. In recent years, structural and density information for silica glass was obtained at record pressures of up to 100 GPa (ref. 1), a major step towards obtaining data on the molten state. Here we report the structure of molten basalt up to 60 GPa by means of in situ X-ray diffraction. The coordination of silicon increases from four under ambient conditions to six at 35 GPa, similar to what has been reported in silica glass1,2,3. The compressibility of the melt after the completion of the coordination change is lower than at lower pressure, implying that only a high-order equation of state can accurately describe the density evolution of silicate melts over the pressure range of the whole mantle. The transition pressure coincides with a marked change in the pressure-evolution of nickel partitioning between molten iron and molten silicates, indicating that melt compressibility controls siderophile-element partitioning.

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Figure 1: Structure of molten basalt at high pressures.
Figure 2: Pressure evolution of Si–O coordination number and dSi−O in molten basalt.
Figure 3: Density of molten basalt as a function of pressure.
Figure 4: Pressure evolution of nickel partitioning coefficient between metal and silicate melts.


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The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant numbers 312284 and 259649 (European Research Council starting grant to C.S.). The laser heating system on beamline P02.2 is funded by the BMBF (the German Federal Ministry of Education and Research, project number 05K10RFA). We acknowledge G. Prouteau for providing the starting basalt glass, and PETRAIII for provision of synchrotron radiation facilities.

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C.S. devised the project, and wrote the paper with input from J.W.E.D., Z.K. and W.v.W. Also, C.S., J.W.E.D., Z.K., P.D.-S., D.M.M., N.R. and W.v.W. participated in data acquisition. Z.K. and W.M. designed the laser-heating system used during the experiments.

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Correspondence to Chrystèle Sanloup.

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Extended Data Table 1 Chemical composition of the starting sample and recovered samples

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Sanloup, C., Drewitt, J., Konôpková, Z. et al. Structural change in molten basalt at deep mantle conditions. Nature 503, 104–107 (2013).

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