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Reconciling Magma-Ocean Crystallization Models with the present-day Structure of the Earth's mantle | | Maxim D. Ballmer
; Diogo L. Lourenço
; Kei Hirose
; Razvan Caracas
; Ryuichi Nomura
; | | Date: |
3 Jan 2018 | | Abstract: | Terrestrial planets are thought to experience episode(s) of large-scale
melting early in their history. Fractionation during magma-ocean freezing leads
to unstable stratification within the related cumulate layers due to
progressive iron enrichment upward, but the effects of incremental cumulate
overturns during MO crystallization remain to be explored. Here, we use
geodynamic models with a moving-boundary approach to study convection and
mixing within the growing cumulate layer, and thereafter within the fully
crystallized mantle. For fractional crystallization, cumulates are efficiently
stirred due to subsequent incremental overturns, except for strongly
iron-enriched late-stage cumulates, which persist as a stably stratified layer
at the base of the mantle for billions of years. Less extreme crystallization
scenarios can lead to somewhat more subtle stratification. In any case, the
long-term preservation of at least a thin layer of extremely enriched cumulates
with Fe#>0.4, as predicted by all our models, is inconsistent with seismic
constraints. Based on scaling relationships, however, we infer that final-stage
Fe-rich magma-ocean cumulates originally formed near the surface should have
overturned as small diapirs, and hence undergone melting and reaction with the
host rock during sinking. The resulting moderately iron-enriched
metasomatized/hybrid rock assemblages should have accumulated at the base of
the mantle, potentially fed an intermittent basal magma ocean, and be preserved
through the present-day. Such moderately iron-enriched rock assemblages can
reconcile the physical properties of the large low shear-wave velocity
provinces in the presentday lower mantle. Thus, we reveal Hadean melting and
rock-reaction processes by integrating magmaocean crystallization models with
the seismic-tomography snapshot. | | Source: | arXiv, 1801.0991 | | Services: | Forum | Review | PDF | Favorites | |
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