| Abstract | dc.description.abstract | This contribution presents a numerical model of time-dependent
physical processes and differentiation in a cooling magma chamber
to obtain the magma and liquid composition and model the distribution
of minerals and H2O gas bubbles at any moment and place.
The numerical experiments were mainly carried out simulating 10
kyr of differentiation of a 50 km3 cylindrical stock-like reservoir as
one of the end-members of a wide spectrum of magma body geometries.
A detailed space^time distribution of compositional varieties
and exsolved phases in a chamber of initially superheated basaltic
composition (corresponding to a lava of the HudsonVolcano, Chile)
was obtained through numerical simulation using the finite-element
method.The roof of the chamber is located at 2 km depth in a crust
with a geothermal gradient of 308C/km. The model considers two
spatial directions (z and r) and assumes the magma to be an incompressible
non-Newtonian liquid, following the Navier^Stokes formulation,
where the density and viscosity depend on the temperature
and exsolved solid and volatile phase content, but viscosity also
depends on the shear rate. The temperature transfer equation used
includes conduction, convection and latent heat. The setting of the
model considers that: (1) an extension of 1km into the country rocks
around the chamber imposed on the heat flow across the margins is
appropriate; (2) the reaction rates for phase (solid and gas) exsolution
correspond to a Gaussian probability function; (3) the involved
phases, calibrated from MELTS, are olivine, clinopyroxene, magnetite,
plagioclase, orthopyroxene and H2O gas; (4) the velocity of the
exsolved phase movements with respect to the hosting liquid is given
by the Stokes’ velocity; (5) the sizes of crystals and bubbles in the
melt vary in space and time up to a maximum given by the modal
crystal size observed in the Hudson Volcano. Our results indicate
that the convection dynamics of the reservoir are characterized by
three distinct flows of decreasing velocity with time: convective cells,
plumes and layer flows along the walls. The along-wall magma
flow, which persists during most of the 10 kyr of magma cooling, contributes
to the thermal insulation of the chamber interior, giving
rise to a nearly permanent coexistence of liquids of contrasting composition.
A strong compositional stratification is generated in the
upper half of the chamber, with a downward-increasing thickness of
layers. Such stratification is mainly the result of continuous upward
flow and storage on top of the sidewall residual melts. The lower
half of the chamber exhibits an independent convective pattern dominated
by ascending plumes. A less pronounced stratification is generated
here as a consequence of mixing between the residual liquid in
the ascending plumes and the surrounding melt. Crystal accumulations
of olivine, clinopyroxene and magnetite at the bottom generate
a significant volume of ultrabasic magma at the end of 10 kyr.
A plagioclase-rich solidification front along the walls is formed
during the last stages of crystallization, when the along-wall
magma flow diminishes in velocity. An application of the numerical
modeling to a cylindrical sill-like chamber is presented to emphasize
the role of the aspect ratio in the magma fluid dynamics, compositional
gradients and exsolved phase distribution. The stock-like
chamber favors the formation of steep compositional gradients and
H2O gas concentration at the roof. Concentration of the exsolved
phases along the margins is favored in a sill-like chamber. From the
simulation results, it is possible to infer that stock-like chambers
would be more eruptible and would exhibit a wider compositional
spectrum of eruptive materials than sill-like chambers. Because
solid and liquid dispersion follow different patterns in a convective
chamber, the record of crystal^liquid equilibrium would be an
exception. | en_US |
