Structural change in molten basalt at deep mantle conditions

• Published in: Nature (London) Vol. 503; no. 7474; pp. 104 - 107
• Main Authors: Sanloup, Chrystèle, Drewitt, James W. E., Konôpková, Zuzana, Dalladay-Simpson, Philip, Morton, Donna M., Rai, Nachiketa, van Westrenen, Wim, Morgenroth, Wolfgang
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 07-11-2013; Nature Publishing Group
• Subjects: 639/766/119/1002; 704/2151/209; 704/2151/2809; Basalt; Diffraction; Earth; Earth Sciences; Humanities and Social Sciences; Lasers; letter; Mantle; Methods; Mineralogy; multidisciplinary; Nickel; Science; Sciences of the Universe; Silica; Silicates; Structure; Thermal properties; Values; X-ray diffraction; Europe
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature12668

The structure of molten basalt up to 60 GPa by means of
in situ
X-ray diffraction is described, with the coordination of silicon increasing from four under ambient conditions to six at 35 GPa, and subsequent reduced melt compressibility, which seems to affect siderophile-element partitioning.
Changes in molten basalt at depth
Chrystèle Sanloup and co-authors report on the structure of molten basalt at deep mantle conditions determined using
in situ
X-ray diffraction experiments in laser-heated diamond anvil cells. They find that silicon coordination increases from 4 at ambient conditions to 6 at a pressure of 35 GPa, with the compressibility of the melt becoming lower above this transition. Given that this pressure coincides with a marked change in the pressure evolution of nickel partitioning between molten iron and molten silicates, the authors conclude that melt compressibility may control siderophile-element partitioning. These findings provide data that can be incorporated into quantitative models of the behaviour of silicate liquids deep in the Earth's mantle.
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 glass
1
,
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.