Earth Sciences School Seminar - Dr Sanne Cottaar - University of Cambridge - The mantle transition beneath Europe; from slab to plume and beyond
Dr Sanne Cottaar - University of Cambridge
G25, Reynolds Lecture Theatre, School of Earth Sciences, Wills Memorial Building
We are pleased to welcome Dr Sanne Cottaar from the University of Cambridge who will be delivering the School Seminar:-Title: The mantle transition beneath Europe; from slab to plume and beyondAbstract
The mantle transition zone discontinuities at 410 and 660 km are generally related to mineral phase transitions, and thus their depths at which they occur are indicators of temperature, composition and water content. Here we map the topography of the '410' and '660' beneath Europe, which tectonically is a natural laboratory to look at the effect of the slabs in Southern Europe and the effect of a plume beneath Iceland.
Seismic studies of the conversions of pressure to shear waves (Pds phases) are an important tool to observe lateral variations in these discontinuities. Here we collect a Pds data set across all European seismic stations since 2000 that are available through ORFEUS or IRIS; resulting in >500,000 event-station pairs. We construct receiver functions through iterative deconvolution and after quality control keep ~40,000 high quality receiver functions. We combine all receiver functions in common conversion point and slowness stacks at different frequencies to map discontinuities down to 1400 km. We correct for velocity structures using recent tomographic models. There are three main conclusions from this:
- In the topography of the discontinuity around 660 km, we find broadscale depressions of 30 km beneath central Europe and around the Mediterranean. These depressions do not correlate with any topography on the discontinuity around 410 km. Temperature or the pressure of water cannot solely explain the strong depressions. Our preferred hypothesis is the dissociation of ringwoodite into akimotoite and periclase in cold downwelling slabs at the bottom of the transition zone. The strongly negative Clapeyron slope predicted for the subsequent transition of akimotoite to bridgmanite explains the depression with a temperature reduction of 200–300 K and provides a mechanism to pond slabs in the first place.
- Beneath the Icelandic plume, we see depressions in the 660 which is opposite from what is expected for an excess temperature. No realistic plume structure (increasing the corrections for mantle velocity) can change the sign in the topography. Our hypothesis is that this is a signature of a majorite dominated mantle transition zone at high temperatures. Both cases show a sign of complexity when interpreting the '660’.
- When looking at deeper structures, we see local conversions from 950-1050 km. Frequency dependence analysis of these observations suggest they come from a sharp discontinuity.
Lunch will be provided in the Earth Sciences Common Room after this seminar.
All staff and students welcome.
For further information please contact Dr. Robert Myhill.