Panache (géologie)

Résumé

vignette|L'archipel d'Hawaï montre ses volcans alimentés par le point chaud, manifestation en surface de l'activité d'un panache. Un panache est, en géologie, une remontée de roches anormalement chaudes, provenant du manteau terrestre. Comme la partie haute des panaches peut fondre partiellement en atteignant des profondeurs faibles, ils sont souvent à l'origine de centres magmatiques tels que les points chauds, et peuvent être à l'origine de grandes coulées basaltiques appelées trapps. C'est un phénomène permettant à la Terre d'évacuer sa chaleur interne, mais qui reste d'une importance secondaire par rapport à la chaleur perdue en limite de plaques lithosphériques. . En 2016, une modélisation thermomécanique suggère que les panaches sont à l'origine des premières plaques tectoniques en fracturant la croûte terrestre il y a 4 milliards d’années. Le modèle de panache–point chaud de Wilson–Morgan Le modèle de panache–point chaud de Wilson–Morgan (Wilson, 1963 ; Morgan, 19

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https://fr.wikipedia.org/wiki/Panache_(g%C3%A9ologie)

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Concepts associés (28)

vignette|Carte des principales plaques tectoniques terrestres. La tectonique des plaques (du latin tardif tectonicus, dérivé du grec , « de construction ») est un modèle scientifique expliquant la dy

vignette|Les cratères de la caldeira Tengger et au loin le Semeru à Java, en Indonésie. vignette|Le Pico do Fogo au Cap-Vert. vignette|Volcans sur Oahu, à Hawaï. vignette|Le Vésuve en Italie. vignette

thumb|Lave en coussins de basalte vu au fond de l'océan, au niveau de Hawaï. Le basalte est une roche magmatique volcanique issue d'un magma refroidi rapidement et caractérisée par sa composition min

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Composition and Pressure Effects on Partitioning of Ferrous Iron in Iron-Rich Lower Mantle Heterogeneities

James Badro, Charles-Edouard Boukaré, Marco Cantoni, Susannah McGregor Dorfman, Philippe Gillet, Farhang Nabiei

Both seismic observations of dense low shear velocity regions and models of magma ocean crystallization and mantle dynamics support enrichment of iron in Earth’s lowermost mantle. Physical properties of iron-rich lower mantle heterogeneities in the modern Earth depend on distribution of iron between coexisting lower mantle phases (Mg,Fe)O magnesiowüstite, (Mg,Fe)SiO3 bridgmanite, and (Mg,Fe)SiO3 post-perovskite. The partitioning of iron between these phases was investigated in synthetic ferrous-iron-rich olivine compositions (Mg0.55Fe0.45)2SiO4 and (Mg0.28Fe0.72)2SiO4 at lower mantle conditions ranging from 33–128 GPa and 1900–3000 K in the laser-heated diamond anvil cell. The resulting phase assemblages were characterized by a combination of in situ X-ray diffraction and ex situ transmission electron microscopy. The exchange coefficient between bridgmanite and magnesiowüstite decreases with pressure and bulk Fe# and increases with temperature. Thermodynamic modeling determines that incorporation and partitioning of iron in bridgmanite are explained well by excess volume associated with Mg-Fe exchange. Partitioning results are used to model compositions and densities of mantle phase assemblages as a function of pressure, FeO-content and SiO2-content. Unlike average mantle compositions, iron-rich compositions in the mantle exhibit negative dependence of density on SiO2-content at all mantle depths, an important finding for interpretation of deep lower mantle structures.

2021

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Terrestrial Accretion Under Oxidizing Conditions

James Badro

The abundance of siderophile elements in the mantle preserves the signature of core formation. On the basis of partitioning experiments at high pressure (35 to 74 gigapascals) and high temperature (3100 to 4400 kelvin), we demonstrate that depletions of slightly siderophile elements (vanadium and chromium), as well as moderately siderophile elements (nickel and cobalt), can be produced by core formation under more oxidizing conditions than previously proposed. Enhanced solubility of oxygen in the metal perturbs the metal-silicate partitioning of vanadium and chromium, precluding extrapolation of previous results. We propose that Earth accreted from materials as oxidized as ordinary or carbonaceous chondrites. Transfer of oxygen from the mantle to the core provides a mechanism to reduce the initial magma ocean redox state to that of the present-day mantle, reconciling the observed mantle vanadium and chromium concentrations with geophysical constraints on light elements in the core.

2013

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An early geodynamo driven by exsolution of mantle components from Earth's core

James Badro

Recent palaeomagnetic observations(1) report the existence of a magnetic field on Earth that is at least 3.45 billion years old. Compositional buoyancy caused by inner-core growth(2) is the primary driver of Earth's present-day geodynamo(3-5), but the inner core is too young(6) to explain the existence of a magnetic field before about one billion years ago. Theoretical models(7) propose that the exsolution of magnesium oxide-the major constituent of Earth's mantle-from the core provided a major source of the energy required to drive an early dynamo, but experimental evidence for the incorporation of mantle components into the core has been lacking. Indeed, terrestrial core formation occurred in the early molten Earth by gravitational segregation of immiscible metal and silicate melts, transporting iron-loving (siderophile) elements from the silicate mantle to the metallic core(8-10) and leaving rock-loving (lithophile) mantle components behind. Here we present experiments showing that magnesium oxide dissolves in core-forming iron melt at very high temperatures. Using core-formation models(11), we show that extreme events during Earth's accretion (such as the Moon-forming giant impact(12)) could have contributed large amounts of magnesium to the early core. As the core subsequently cooled, exsolution(7) of buoyant magnesium oxide would have taken place at the core-mantle boundary, generating a substantial amount of gravitational energy as a result of compositional buoyancy. This amount of energy is comparable to, if not more than, that produced by inner-core growth, resolving the conundrum posed by the existence of an ancient magnetic field prior to the formation of the inner core.

Nature Publishing Group2016

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