HR: 16:30h
AN: V44B-03    [Abstracts]
TI: Successful and Failing plumes in a Heterogeneous Mantle: the Icelandic Case
AU: Kumagai, I
EM: kumagai@ipgp.jussieu.fr
AF: Institut de Physique du Globe, 4 Place Jussieu, PARIS cedex 05, 75 252, France
AU: Kumagai, I
EM: kumagai@ipgp.jussieu.fr
AF: Earthquake Research Institute, The University of Tokyo, 1-1-1, Yayoi, Bunkyo, Tokyo, 113- 0032, Japan
AU: * Davaille, A
EM: davaille@ipgp.jussieu.fr
AF: Institut de Physique du Globe, 4 Place Jussieu, PARIS cedex 05, 75 252, France
AU: Kurita, K
EM: kurikuri@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, The University of Tokyo, 1-1-1, Yayoi, Bunkyo, Tokyo, 113- 0032, Japan
AU: Stutzmann, E
EM: stutz@ipgp.jussieu.fr
AF: Institut de Physique du Globe, 4 Place Jussieu, PARIS cedex 05, 75 252, France
AB: Although Iceland is always cited as an exemple of hot spot volcanism produced by a deep mantle plume, an increasing number of observations cannot be explained by the classical plume model of a mushroom-shaped plume out of a sustained localized heat source. Volcanic episodes with moderate temperature predate the major episode of mafic magma emplacement (~60Ma) containing hot picrite magma with strong rare gas anomalies. Present-day Iceland shows moderate temperatures, a strong rare gas anomaly, and an apparent disconnection between slow seismic anomalies in the upper and lower mantle. Noteworthy, the same mixture of geochemical ingredients are found in Icelandic lavas during its 80 Myr of activity. We present a new experimental study of the more realistic case of thermochemical convective instabilities developping out of a heterogeneous bottom hot thermal boundary layer. Depending on the buoyancy ratio B, two end-member regimes are observed. For large B, a thermal plume develops above the denser layer and only a small amount of denser fluid is entrained in the plume. For small B, the dense layer can be sufficently heated to become buoyant and rise: the thermo-chemical plume is therefore mainly constituted of material from the chemically denser layer. The fate of the heterogeneous material in the plume then depends on time since the instability cools as it ascends. As a result, the core of the plume head, which consists of initially hotter but chemically heavier material, can cool enough to become denser than the ambient fluid before reaching the surface of the tank: the heterogeneous material then sinks back and a new thermal plume with a lower temperature anomaly is generated from the top edge of the heavier collapsing blob. In this "failing-plume" mode, the thermo-chemical plume fails to deliver most of the chemical heterogeneity to the surface. Hence, the thermal and compositional structure of a thermo-chemical plume changes with time and is quite irregular. In particular, it is not because a region is hot that it is buoyant and rising. The interplay between thermal convection and at least 3 layers (or "reservoirs") of different densities could therefore well explain the time- dependence and morphology of the Icelandic melting anomaly. And in this framework, geochemical data can be explained if each reservoir is a mixture of the same mantle components, albeit in different proportions.
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting