Prof David Dobson
Along with colleagues at UCL and the Bayerisches Geoinstitut, I perform high-pressure experiments on deep Earth minerals and rocks. We have several high-pressure devices, including multi-anvil presses, Diamond cells and a modified Paris-Edinburgh cell for neutron diffraction.
|Internal view of a multi anvil press at UCL which can generate pressures to 25 GPa.||Diamond cell which can be used to 100 GPa or more, but has far smaller sample volumes.|
Earth’s Core Materials
core of the Earth is among the most inaccessible and least
understood regions of Earth. The solid inner core and liquid outer core consist
of iron plus several percent of a light element. Together with Drs Ian Wood and Lidunka Vocadlo, I have been investigating the iron-alloys which
are stable at high pressures. We have discovered a new high-pressure phase of FeSi
which is a prime core candidate material.
The outer core is responsible for the Earth’s magnetic field. Geodynamo models require a knowledge of the viscosity and diffusivity of the outer core liquid. We have an active programme to measure these properties at high pressure.
Stacked diffraction patterns from high-pressure CsCl-structured FeSi.
These patterns were collected from a sample in a Diamond-cell using synchrotron radiation at the European Synchrotron Research Facility.
Transport Properties of Mantle Materials
Subtle variations in the Earth’s magnetic field can be used to probe the electrical conductivity of the mantle. This, in turn, is a powerful tool for investigating variations in temperature and chemistry within the deep Earth. Prof John Brodholt and I been investigating the electrical conductivity of mantle minerals olivine, wadsleyite, magnesiowustite and perovskite. We have used these results to test whether the mantle convects in a layered or whole-mantle fashion.
The viscosity of the mantle is very poorly constrained. In the Laboratory for Mineral Ice and Rock Physics, we have a programme to develop new rheological testing apparatus for ultra-high-pressure experiments. These include the Gigapascal Deformation Cell and the modified Paris-Edinburgh-Belt cell. We will use Neutron diffraction to measure deviatoric stresses and strains in situ at the new ENGIN-X beamline at Rutherford Appleton Laboratory.
with Prof. Phil Meredith and Steve
Boon we are developing new techniques to investigate
the origins of deep earthquakes. Our recent work on serpentine dehydration was published
in Science and featured in the international press. See what the BBC has to say about our
research and Science.
True colour images of partially dehydrated serpentine recovered from 6 GPa (top) and 8 GPa (bottom).
The strikingly different colours are due to the different dehydration reactions at the two pressures:
Serpentine = olivine + enstatite + water (6 GPa) and:
serpentine = phase A + enatatite + water (8GPa).
Both samples showed seismic signals during dehydration, but the type of signal was significantly different.