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| Titel |
Combined Laboratory and Numerical Study on Thermal Plumes in Yield-Stress Fluids : Implications for Midocean-Ridge Systems |
| VerfasserIn |
A. Massmeyer, A. Davaille, T. Rolf, P. Tackley, E. Di Giuseppe |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250062996
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| Zusammenfassung |
| Diapirs form by hot magma migrating through the lithosphere. While hot instabilities in a
Newtonian mantle are known to rise as a mushroom-shaped plume, the upwards
migration of hot material in “soft“ geological materials (presenting solid as well
as viscous properties) is still far from understood. We are therefore studying this
problem experimentally and numerically, to shed some light into this problem. The
results give an idea about which basic properties are needed in mid-ocean ridge
systems.
The Laboratory setup consists of a localized heat-source placed in a plexiglas-tank
containing a mixture of water, glycerine and Carbopol. Due to the existence of a continuous
network of interacting micron-sized hydrogel particles, the solution presents a yield stress
and is shear-thinning. The rheology is described by a Herschel-Bulkley model. The fluid’s
rheological properties can be controlled by changing either the proportion of Glycerol and
water or the Carbopol concentration.
In our experiments we systematically studied the influence of rheological properties and
supplied heat. Depending on the Yield parameter Y 0, which compares the thermally-induced
stress to the yield stress, three different regimes are observed. For low Y 0, no convection
develops. For intermediate values, a small-scale convection cell appears and remains confined
around the heater. Only for high Y 0, a thermal plume develops. The morphology differs from
the mushroom-shape typically encountered in Newtonian or in purely shear-thinning
fluids.
The experiments show that the onset time is delayed compared to a Newtonian case and
depends on Y 0. Combined temperature and velocity field measurements show that a plug
flow develops within the plume thermal anomaly, therefore producing a rising finger-shape.
Moreover, although the heat supply is continuous through time, the uplift of such
plumes is not: the thermal anomaly marks some ”pauses” as it rises through the gel
column.
In addition we implemented a regularized Herschel-Bulkley model into a viscoplastic
numerical code. Despite some differences in the spatio-temporal evolution, the results equally
show a long onset-time and a morphology comparable to the one observed in the
laboratory.
Our experiments are able to explain several features obeserved for an off-axis
diapir in Oman, especially the strong shear localization along the diapir edges. It
further suggests that this diapir was emplaced in a partially molten lithospheric
matrix. Therefore this type of hydrogels might be a good candidate to get some new
insights into the complex rheological behavior of such geological ”soft” systems. |
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