Global-scale modelling of melting and isotopic evolution of Earth's mantle: melting modules for TERRA
Many outstanding problems in solid-Earth science relate to the geodynamical explanation of geochemical observations. Currently, extensive geochemical databases of surface observations exist, but satisfying explanations of underlying mantle processes are lacking. One way to address these problems is...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-04-01
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| Series: | Geoscientific Model Development |
| Online Access: | http://www.geosci-model-dev.net/9/1399/2016/gmd-9-1399-2016.pdf |
| Summary: | Many outstanding problems in solid-Earth science relate to the geodynamical
explanation of geochemical observations. Currently, extensive geochemical
databases of surface observations exist, but satisfying explanations of
underlying mantle processes are lacking. One way to address these problems is
through numerical modelling of mantle convection while tracking chemical
information throughout the convective mantle.<br><br>
We have implemented a new way to track both bulk compositions and
concentrations of trace elements in a finite-element mantle convection code.
Our approach is to track bulk compositions and trace element abundances via
particles. One value on each particle represents bulk composition and can be
interpreted as the basalt component. In our model, chemical fractionation of
bulk composition and trace elements happens at self-consistent, evolving
melting zones. Melting is defined via a composition-dependent solidus, such
that the amount of melt generated depends on pressure, temperature and bulk
composition of each particle. A novel aspect is that we do not move particles
that undergo melting; instead we transfer the chemical information carried by
the particle to other particles. Molten material is instantaneously
transported to the surface layer, thereby increasing the basalt component
carried by the particles close to the surface and decreasing the basalt
component in the residue.<br><br>
The model is set to explore a number of radiogenic isotopic systems, but as an
example here the trace elements we choose to follow are the Pb isotopes and
their radioactive parents. For these calculations we will show (1) the
evolution of the distribution of bulk compositions over time, showing the
buildup of oceanic crust (via melting-induced chemical separation in bulk
composition), i.e. a basalt-rich layer at the surface, and the transportation
of these chemical heterogeneities through the deep mantle; (2) the amount of
melt generated over time; (3) the evolution of the concentrations and
abundances of different isotopes of the trace elements (U, Th, K and Pb)
throughout the mantle; and (4) a comparison to a semi-analytical theory relating
observed arrays of correlated Pb isotope compositions to melting age
distributions (Rudge, 2006). |
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| ISSN: | 1991-959X 1991-9603 |