Thermal plumes in yield stress fluids : laboratory experiments and implications for diapirs and dike propagation in a soft crust or lithosphere
Abstract
Hot magma migrate through dikes in a solid/brittle crust or lithosphere, while hot instabilities rise as mushroom-shaped plumes in a viscous mantle. However, upwards migration of hot material in areas where the geological materials are "soft" (presenting solid as well as viscous properties) is still far from understood. These phenomena may play a crucial role in the dynamics of mid-ocean ridges, of volcanic systems or of the asthenosphere/lithosphere boundary. We present new laboratory experiments on the development of thermal plumes out of a localized source of heat in mixtures of water, glycerine and Carbopol. The resulting fluids are shear thinning and present a yield-stress, whereby flow occurs only if the local stress exceeds a critical value. Below this value, the fluid acts as an elastic solid. This is due to the existence of a continuous network of interacting micron-sized hydrogel particles. The fluid's rheological properties can be controlled by changing the proportion of Glycerol and water as well as the Carbopol concentration. We then systematically studied the influence of height and supplied heat. Depending on the Yield parameter Y0, which compares the thermally-induced stress to the yield stress, three different regimes are observed. For low Y0, no convection develops ; while for intermediate values, a small-scale convection cell appears and remains confined around the heater. For high Y0, thermal plumes develop. Their morphology differs from the mushroom-shape typically encountered in newtonian or in purely shear-thinning fluids. Combined temperature and velocity field measurements show that a plug flow develops within the plume thermal anomaly, therefore producing a rising finger- shape. Light scattering highlights the development of a damaged zone ahead of the diapir as it rises. 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. These are due to the interaction between the solid character of the gel matrix, the viscous character of the thermal plume and the diffusion of heat. The hot instability behavior seems therefore somewhere between dike and plume development. This type of hydrogels might therefore be good candidates to get some new insights into the complex rheological behavior of geological "soft" systems, especially into lithospheric and/or crustal plumbing.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T43C2334M
- Keywords:
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- 8159 TECTONOPHYSICS / Rheology: crust and lithosphere