Pervasive melt percolation reactions in ultra-depleted refractory harzburgites at the Mid-Atlantic Ridge, 15° 20′N : ODP Hole 1274A

Author Posting. © The Authors, 2006. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Contributions to Mineralogy and Petrology 153 (2007): 303-319, doi:10.1007/s00410-006-0148-6.ODP Leg 209 Site 1274 mantle peridotites are highly refractory in terms of lack of residual clinopyroxene, olivine Mg# (up to 0.92) and spinel Cr# (~0.5), suggesting high degree of partial melting (>20%). Detailed studies of their microstructures show that they have extensively reacted with a pervading intergranular melt prior to cooling in the lithosphere, leading to crystallization of olivine, clinopyroxene and spinel at the expense of orthopyroxene. The least reacted harzburgites are too rich in orthopyroxene to be simple residues of low-pressure (spinel field) partial melting. Cu-rich sulfides that precipitated with the clinopyroxenes indicate that the intergranular melt was generated by no more than 12% melting of a MORB mantle or by more extensive melting of a clinopyroxene-rich lithology. Rare olivine-rich lherzolitic domains, characterized by relics of coarse clinopyroxenes intergrown with magmatic sulfides, support the second interpretation. Further, coarse and intergranular clinopyroxenes are highly depleted in REE, Zr and Ti. A two-stage partial melting/melt-rock reaction history is proposed, in which initial mantle underwent depletion and refertilization after an earlier high pressure (garnet field) melting event before upwelling and remelting beneath the present-day ridge. The ultra-depleted compositions were acquired through melt re-equilibration with residual harzburgites.Funding for this research was provided by Centre National de la Recherche Scientifique-Institut National des Sciences de l’Univers (Programme Dynamique et Evolution de la Terre Interne)

Formation and deformation of hyperextended rift systems: Insights from rift domain mapping in the Bay of Biscay-Pyrenees

International audienceThe Bay of Biscay and the Pyrenees correspond to a Lower Cretaceous rift system including both oceanic and hyperextended rift domains. The transition from preserved oceanic and rift domains in the West to their complete inversion in the East enables us to study the progressive reactivation of a hyperextended rift system. We use seismic interpretation, gravity inversion, and field mapping to identify and map former rift domains and their subsequent reactivation. We propose a new map and sections across the system illustrating the progressive integration of the rift domains into the orogen. This study aims to provide insights on the formation of hyperextended rift systems and discuss their role during reactivation. Two spatially and temporally distinct rift systems can be distinguished: the Bay of Biscay-Parentis and the Pyrenean-Basque-Cantabrian rifts. While the offshore Bay of Biscay represent a former mature oceanic domain, the fossil remnants of hyperextended domains preserved onshore in the Pyrenean-Cantabrian orogen record distributed extensional deformation partitioned between strongly segmented rift basins. Reactivation initiated in the exhumed mantle domain before it affected the hyperthinned domain. Both domains accommodated most of the shortening. The final architecture of the orogen is acquired once the conjugate necking domains became involved in collisional processes. The complex 3-D architecture of the initial rift system may partly explain the heterogeneous reactivation of the overall system. These results have important implications for the formation and reactivation of hyperextended rift systems and for the restoration of the Bay of Biscay and Pyrenean domain

Les types paragénétiques des amphiboles sodiques dans les roches magmatiques

In sodic amphiboles of igneous origin, Fe3+ ions are more abundant than Al3+ in the occupancy of the octahedral sites, even in varieties showing a great substitution of Al for Si in tetrahedral sites. On the other hand the Mg/Fe2+ ratio ranges are much more extended. The magnesium-rich alkali amphiboles are essentially richteritic : (1) potassic richterites in some ultramafic xenoliths of kimberlites and in highly potassic volcanic rocks (orenditic lamproites) ; (2) richterite-magnesioarfvedsonite solid solutions in carbonatites and related rocks. The paragenetic types of iron-rich or intermediate alkali amphiboles follow three main fractionation trends. In the series that follow a miaskitic evolution, the amphibole changes hastingsite to taramite, where NaFe3+ mainly substitutes for CaFe2+. The agpaitic alkaline series show the sequence kataphorite arfvedsonite → riebeckite-arfvedsonite, corresponding to NaSi → CaAl and F- OH-substitutions ; the Ca/Al ratio hardly deviates from one in all the amphiboles of the sequence. The amphiboles from supersatured alkaline series follow a fractionation trend richterite-katophorite → arfvedsonite → riebeckite-arfvedsonite, governed by the two main substitutions NaR3+ → CaR2+ and NaSi → CaAl. The Ca/Al ratio in this case is around two. Therefore SiO2 activity, as well as temperature, total pressure and oxygen fugacity, plays a major role in controlling the solubility of aluminium in sodic amphiboles, as well as in hornblendes.Les amphiboles sodiques dans les roches ignées sont caractérisées par une large prédominance de Fe3+ sur Al3+ dans les sites octaédriques de la structure, même dans les variétés où AlIV remplace fortement Si dans les sites tétraédriques. La gamme de variation des proportions Mg/Fe2+ est au contraire beaucoup plus étendue. Les amphiboles alcalines et magnésiennes sont essentiellement richtéritiques : (1) richtérites potassiques dans certains nodules ultramafiques des kimberlites et dans les roches volcaniques fortement potassiques (lamproïtes orenditiques) ; (2) solutions solides richtérite-magnésioarfvedsonite dans les carbonatites et roches associées. Les types paragénétiques des amphiboles ferrifères ou intermédiaires se groupent suivant trois suites de fractionnement. Dans les séries alcalines évoluant vers des termes sous-saturés, à chimisme miaskitique, la suite est hastingsite taramite ; les séries sous-saturées à chimisme agpaïtique présentent la séquence kataphorite → arfvedsonite → riebeckite-arfvedsonite, le rapport Ca/Al des amphiboles restant toujours voisin de 1. Les séries alcalines à évolution sursaturée sont caractérisées par la suite richtérite-kataphorite → arfvedsonite → riebeckite-arfvedsonite, le rapport Ca/Al des amphiboles restant voisin de 2. La solubilité de l'aluminium dans les amphiboles sodiques, comme dans les hornblendes, semble donc contrôlée par l'activité de SiO2 dans le magma, à côté d'autres facteurs tels que température, pression totale et fugacité d'oxygène.Fabriès Jacques. Les types paragénétiques des amphiboles sodiques dans les roches magmatiques. In: Bulletin de Minéralogie, volume 101, 2, 1978. 1878-1978, centenaire de la Société Française de Minéralogie et Cristallographie (SFMC) pp. 155-165

Fernand Conquéré (1936-1983)

Fabriès Jacques. Fernand Conquéré (1936-1983). In: Bulletin de Minéralogie, volume 106, 6, 1983. Eclogites et ultrabasites à grenat - First International Eclogite Conference

Nouvelles observations sur les amphiboles orthorhombiques

A re-examination of the mineralogical characters has been enterprised on samples of orthorhombic amphiboles coming from the Collection of Mineralogy of the National Museum of Natural History in Paris. The results we obtained with chemical analysises, X Rays and by the infra-red spectrometry have been confronted with the optical properties, in order to lay down diagnostic criterias for the definition of types inside this family.Une révision des caractères minéralogiques a été entreprise sur des échantillons d'amphiboles orthorhombiques provenant de la Collection de Minéralogie du Muséum national d'histoire naturelle de Paris. Les résultats obtenus à l'aide des analyses chimiques, des rayons X et par la spectrométrie infrarouge ont été confrontés avec les propriétés optiques, afin d'établir des critères quant à la définition de types à l'intérieur de cette famille.Fabriès Jacques, Perseil Eléna Adriana. Nouvelles observations sur les amphiboles orthorhombiques. In: Bulletin de la Société française de Minéralogie et de Cristallographie, volume 94, 4, 1971. pp. 385-395