Australia: The Land Where Time Began |
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Open-System Dynamics and Mixing in Magma
Mushes
Magma is present dominantly in a crystal-rich or mushy state that is
cooling slowly (1-3). Yet, when crystals (4) that are zoned complexly
are observed it is found that some had formed in 1-10 years (5-9), as
well as crystal fabrics that are time-transgressive (10) which implies
that magmas mix and transition rapidly from a crystal mush that is
locked to a mobile, eruptible fluid (5,6). In this study Bergantz et
al. resolved the
crystal-scale granular interactions by simulating the open-system
dynamics of a magma mush with a discrete-element numerical model. They
found that the existing magma responds as a viscoplastic material when
new magma is injected into a reservoir from below: around the edges of
the new injection fault-like surfaces form that act as a mixing bowl of
magma that can be unlocked and fluidised, which allows complex mixing.
Bergantz et al. identified 3
distinct dynamic regimes which depended on the rate of magma injection.
The intruded magma spreads by porous media flow through the crystal mush
if the injection rate of magma is slow. The entire mixing bowl becomes
fluidised at higher velocities. Crystals from the walls are entrained by
circulation within the mixing bowl which brings together crystals from
different parts of the reservoir that may have experienced different
physiochemical environments with the result that there is little melt
that is unmixed. Bergantz et al.
conclude that when both granular and fluid dynamics are considered
simultaneously observations of complex crystal fabrics and zoning that
have been observed in many magmatic systems can be explained.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |