Petrological geodynamics of mantle melting III. AlphaMELTS þ multiphase
flow: The effect of water
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Abstract
The influence of water is evaluated in this last contribution of a series aiming to study the petrological and dynamic
evolution of mantle melting. Water is considered to be either a chemical component in the melt or solid
assemblage but it can also be present as a pure water phase in a oversaturated environment. A three-phase-flow
model was developed for this purpose.
Only a limited set of conditions has been applied to the 1-D upwelling mantle column. A range of fixed temperatures
(1150–1450 C) and water contents in the solid mantle (0, 0.02 wt.%, 0.2 wt.%) have been imposed at
the entry point (120 km deep) for the two melting models introduced in the previous installments, dynamic
equilibrium melting (DEM) and dynamic fractional melting (DFM) model.
As expected, for a given temperature at the base of the mantle column, the depth of the first melt formation
increases with higher water content in the mantle. After the first melt is created, very negligible amount of melt is
formed over a certain depth interval which approximately ends at the depth where the first melting of the dry
mantle would take place. However melt is present as a dynamic phase thorough the entire region regardless
whether the DEM or DFM model has been applied.
Under a quasi-steady state regime, the melt and residual mantle compositions vary significantly over depth,
depending on the conditions imposed to the model (DEM, DFM, bottom temperature and water content). Several
distinctions can be made at the extraction point (top of the mantle column ¼ 15 km deep). For DEM and DFM
models at this lowest depth, the most influential factor affecting the melt composition after the quasi-steady state
condition has been reached is the temperature at the base of the column. In general, for a high temperature model,
the input water in the mantle does not seem to play a significant role on the bulk composition of the melt (except
for the water content in melt). But at low temperature water does have some noticeable influence on the variation
of some chemical components in melt (SiO2, Fe2O3, CaO, Na2O at T ¼ 1250 C or lower). A similar conclusion can
be made also for the residual mantle composition. The presence of a dynamic free water phase is detected only in
absence of melt or in coexistence with a melt phase when the mantle is relatively cold (bottom temperature
1250 C) and the input water content at the base of the model is relatively high (0.2 wt.%).
Complete output data for several numerical simulations and six animations illustrating various melting models
are available following the instructions in the supplementary material.
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