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(2020) The Formation and Preservation of Chemical Heterogeneities in the Lower Mantle
Gülcher A, Yan J, Ballmer M & Tackley P
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02c: Room 1, View in program
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Anna, very interesting results. I have two questions: 1) are there any modern OIBs that may come from protected primitive "blobs" rather than the LLSVPs? How can you distinguish these 2 types of sources for modern OIBs using seismology or other geophysical means? 2) Where do recycled continental sediments etc. fit in the picture, are they part of the marble cake, and if so, do you think it will be possible to distinguish between mafic marble cake and felsic marble cake? (I actually I prefer the felsic flavor!)
Hi Kent Codie, thanks for your interest and good questions. I'll try to elaborate on them below: (1) This is definitely one of our key objectives for the near future. Ideally we would be able to directly image plume conduits and infer their sources by means of seismic tomography. Yet the mid-mantle is very ill-resolved with seismic tomographic methods and only significant seismic velocity anomalies may be evident (whereas our preliminary results indicate that these protected primitive blobs are rather undetectable in these seismic tomographic images). Another geophysical pathway is to use other tools of seismology, such as seismic vote maps (https://www.earth.ox.ac.uk/~smachine/cgi/index.php) to try to map out the “ambient” mantle (here: with low-amplitude seismic wavespeeds). This is work that is currently underway by colleagues of ours. We are presently also aiming to qualitatively assess the isotopic signatures this primordial material would have in comparison with ambient mantle or recycled material, mainly by discussing with and learning from geochemists and/or mineral physicists who work on magma ocean crystallisation/core-mantle interaction/isotope geochronology etc. Finally on a geodynamical note, plumes that entrain primordial material may either directly graze blobs, or just entrain previously eroded blob/primordial material near the CMB. The blobs get slowly eroded over time, and related small-scale heterogeneity might be entrained by plumes almost anywhere in the mantle (2) Felsic recycled heterogeneity is something we have not tried to assess (yet). On a technical note: we do not consider continental crust in our simplified numerical models. While our simulated marble cake is hence mafic (by model assumptions), we are open in terms of which materials might float through the mantle in this marble cake style. While dense, negatively buoyant ROC is expected to mainly reside in the lower mantle, recycling of continental material/sediments may more influence upper mantle chemistry and dynamics. Exactly how felsic heterogeneity would be distributed through the mantle, might be something to look into the future.
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