Basalte

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 minéralogique : plagioclases (50 %), de pyroxènes (25 à 40 %), d'olivine (10 à 25 %), et de 2 à 3 % de magnétite. Sur Terre, il a une origine volcanique et est un des constituants principaux de la croûte océanique. Sur la Lune, il constitue la surface des mers lunaires. Ce serait un constituant important des croûtes de Mars, Vénus et Mercure. Le mot basalte est emprunté au latin basaltes, terme dont on dit souvent qu'il est dérivé d'un terme éthiopien signifiant « roche noire ». Mais, d'une part ce terme n'évoque rien aux locuteurs amhariques, d'autre part une autre origine est citée, peut-être égyptienne. Le basalte est une roche mélanocrate à holomélanocrate (sombre à très sombre) à structure microlitique, qui est issue de la fusion partielle du manteau terrestre de composition péridotiti

A four-dimensional X-ray tomographic microscopy study of bubble growth in basaltic foam

Understanding the influence of bubble foams on magma permeability and strength is critical to investigations of volcanic eruption mechanisms. Increasing foam porosity decreases strength, enhancing the probability of an eruption. However, higher porosities lead to larger permeabilities, which can lessen the eruption hazard. Here we measure bubble size and wall thickness distributions, as well as connectivity, and calculate permeabilities and tensile strengths of basaltic foams imaged by synchrotron X-ray tomographic microscopy during bubble growth in hydrated basaltic melts. Rapid vesiculation produces porous foams whose fragmentation thresholds are only 5-6 MPa and whose permeabilities increase from approximately 1x10(-10) to 1x10(-9) m(2) between 10 and 14 s despite decreasing connectivity between bubbles. These results indicate that basaltic magmas are most susceptible to failure immediately upon vesiculation and at later times, perhaps only 10's of seconds later, permeability increases may lessen the hazard of explosive, basaltic, Plinian eruptions.

Nature Publishing Group2012

The Hadean upper mantle conundrum: Evidence for source depletion and enrichment from Sm-Nd, Re-Os, and Ph isotopic compositions in 3.71 Gy boninite-like metabasalts from the Isua Supracrustal Belt, Greenland

Reto Frei, Anders Meibom

Here we present Sm-Nd, Re-Os, and Pb isotopic data of carefully screened, least altered samples of boninite-like metabasalts from the Isua Supracrustal Belt (ISB, W Greenland)that characterize their mantle source at the time of their formation. The principal observations of this study are that by 3.7-3.8 Ga melt source regions existed in the upper mantle with complicated enrichment/depletion histories. Sm-Nd isotopic data define a correlation line with a slope corresponding to an age of 3.69+/-0.18 Gy and an initial epsilonNd value of +2.0+/-4.7. This Sm-Nd age is consistent with indirect (but more precise) U-Pb geochronological estimates for their formation between 3.69-3.71 Ga. Relying on the maximum formation age of 3.71 Gy defined by the external age constraints, we calculate an average epsilonNd [T = 3.71 Ga] value of +2.2+/-0.9 (n = 18, 1sigma) for these samples, which is indicative of a strongly depleted mantle source. This is consistent with the high Os concentrations, falling in the range between 1.9-3.4 ppb, which is similar to the estimated Os concentration for the primitive upper mantle. Re-Os isotopic data (excluding three outliers) yield an isochron defining an age of 3.76+/-0.09 Gy (with an initial gammaOs value of 3.9+/-1.2), within error consistent with the Sm-Nd age and the indirect U-Pb age estimates. An average initial gammaOs [T = 3.71 Ga] value of +4.4+/-1.2 (n = 8; 2sigma) is indicative of enrichment of their source region during, or prior to, its melting. Thus, this study provides the first observation of an early Archean upper mantle domain with a distinctly radiogenic Os isotopic signature. This requires a mixing component characterized by time-integrated suprachondritic Re/Os evolution and a Os concentration high enough to strongly affect the Os budget of the mantle source; modern sediments, recycled basaltic crust, or the outer core do not constitute suitable candidates. At this point, the nature of the mantle or crustal component responsible for the radiogenic Os isotopic signature is not known.

2004

Reto Frei, Anders Meibom

2004

A four-dimensional X-ray tomographic microscopy study of bubble growth in basaltic foam

Abrasivity of Icelandic basalts

The Hadean upper mantle conundrum: Evidence for source depletion and enrichment from Sm-Nd, Re-Os, and Ph isotopic compositions in 3.71 Gy boninite-like metabasalts from the Isua Supracrustal Belt, Greenland

A four-dimensional X-ray tomographic microscopy study of bubble growth in basaltic foam

Abrasivity of Icelandic basalts

The Hadean upper mantle conundrum: Evidence for source depletion and enrichment from Sm-Nd, Re-Os, and Ph isotopic compositions in 3.71 Gy boninite-like metabasalts from the Isua Supracrustal Belt, Greenland