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| Titel |
The fluid dynamics of xenocryst formation in mafic enclaves |
| VerfasserIn |
Paul Jarvis, Jon Blundy, Katharine Cashman, Herbert Huppert, Heidy Mader |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250092906
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| Publikation (Nr.) |
 EGU/EGU2014-7271.pdf |
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| Zusammenfassung |
| Mafic enclaves produced by the mingling of felsic and mafic magmas commonly contain
xenocrysts; crystals akin to those in the felsic host. These crystals are interpreted as having
crossed the interface between the two magmas at some stage during the rock evolution. An
understanding of the physical conditions that allow this exchange would give insight into the
state of the system at the time of assimilation, providing information about the magmatic
history of the rock. Using both numerical models and analogue experiments, the low
Reynolds number gravitational settling of spheres on to fluid-fluid interfaces is studied as an
analogue to this problem.
Theoretical treatment suggests that whether or not a particle sinks or floats at an interface
depends on four dimensionless parameters; Bond number, the viscosity ratio, a modified
density ratio and the contact angle. Spheres are allowed to settle onto an interface for
different values of the dimensionless groups and the behavioural regime boundaries
are determined. Experimentally this consists of dropping spheres of varying radii
and density onto an interface between two density stratified fluids (silicon oil and
polyethylene glycol solution), both of which are lighter than the sphere. The spheres
are sputter coated in gold to ensure a constant surface interaction. The numerical
models are used to validate these results and apply them in geologic settings. Early
results suggest that the presence of even a small interfacial tension between the two
magmas is sufficient to inhibit the passage of crystals across interfaces in magmatic
systems.
An interesting feature of note in mafic enclaves is that the xenocrysts often occur in
clusters. This can be compared with observations from the analogue experiments where 6mm
nylon spheres were dropped onto the fluid interface. Although the spheres are light and small
enough to individually be supported by the interface, the successive addition of spheres leads
to the formation of an aggregate which, upon reaching a critical size or mass, sinks through
the interface. |
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