Abstract
Abstract. 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. 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. 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).
Highlights
A big question in solid-Earth sciences is, what are the interior dynamics of the mantle? A related question that might help to find answers is, what processes are responsible for the geochemical heterogeneity observed in magmatic outputs
Some aspects of the geochemical observations are constraints on mantle dynamics, because the dynamics are partly responsible for the heterogeneity in geochemical observations
Segregated material has reached the core–mantle boundary (CMB) within 500 million years (seen in time series of bulk compositional field and values of the domain root mean square velocity which is above 1 cm year−1; not shown)
Summary
A big question in solid-Earth sciences is, what are the interior dynamics of the mantle? A related question that might help to find answers is, what processes are responsible for the geochemical heterogeneity observed in magmatic outputs (recorded in databases, e.g. Lehnert et al, 2000). J. van Heck et al.: Global-scale modelling of melting and isotopic evolution of Earth’s mantle crust is produced by partial melting at oceanic spreading centres where most mantle melting occurs, and where most chemical heterogeneity is generated This heterogeneous material is brought into the deeper mantle via subduction of oceanic lithosphere. To a lesser extent, melting happens on continents and beneath oceanic lithosphere to create ocean island basalts (OIBs), providing a second mechanism for creating heterogeneity In addition to this continuous generation of heterogeneities, chemically distinct material might have survived for billions of years, originating much earlier in Earth history, e.g. linked to core formation processes, mantle magma oceans, or asteroid bombardment. This makes melting a first-order feature to be implemented in thermo-chemical convection codes
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