Supervisors: John Rudge (Earth Sciences) and Sanne Cottaar (Earth Sciences)
Importance of the area of research:
At subduction zones, cold slabs sink back into the mantle, and the descent of these slabs through the upper mantle has been well-imaged by seismic tomography. Slabs appear to differ in their behaviour as they reach the base of the upper mantle: some slabs seem to stagnate or pond at 660 km depth, while others seem to continue to descend into the lower mantle. We still do not fully understand the dynamical reasons why some slabs pond and others do not. Knowing the ultimate fate of subducting slabs is a key part of understanding global geochemical cycles, as sinking slabs control the exchange of mass between the upper and lower mantle.
This project has three main goals: 1, To use seismology to characterise the progress of sinking slabs through the mantle, and in particular to place constraints on present-day slab flux into the lower mantle. 2, To combine the present-day slab flux information with models of slab thermal evolution and mineral physics to understand the dynamical controls on slab ponding. 3, To look at the implications of mass transfer between the upper and lower mantle on geochemical cycles throughout Earth's history, particularly in terms of transport of volatiles and noble gases.
What the student will do:
The student will be involved in the analysis and interpretation of existing data (both seismological and geochemical), and the development of new theory. The student will develop numerical models of the thermal structures of slabs that will be combined with mineral physics models of phase transitions in order to examine the buoyancy forces acting on subducting slabs, and in turn their dynamics as they pass through the transition zone. The consequences of slab fate for the global thermal and chemical evolution of Earth will be examined using simple box models that the student will develop.
Fukao, Y. & Obayashi M. 2013. Subducted slabs stagnant above, penetrating through, and trapped below the 660 km discontinuity, J. Geophys. Res. Solid Earth, 118, pp 5920–5938. doi:10.1002/2013JB010466
Hofmann, A.W. 1997. Mantle geochemistry: the message from oceanic volcanism. Nature, vol. 385, pp 219-229. doi:10.1038/385219a0
King S.D., Frost D.J. & Rubie D.C. 2015. Why cold slabs stagnate in the transition zone. Geology 43, pp 231-234. doi:10.1130/G36320.1
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