Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
This study presents microstructural evidence for deformation-controlled melt organization and for changes in olivine deformation associated with the presence of melt in an 80 m vertical section of the crust-mantle transition zone in the Oman ophiolite. This zone represents an 'end member' case for analyzing feedbacks between deformation and melt distribution in the upper mantle, since it experienced strong shear strains in presence of large melt fractions. It is characterized by a subhorizontal compositional layering at the mm to meter scale, from weakly impregnated dunites to olivine-rich gabbros, which parallels a pervasive foliation containing a strong stretching lineation. The parallelism between the compositional layering and the foliation, the diffuse limits of the layers, the alignment of elongated plagioclase-rich aggregates devoid of internal deformation structures with the elongation of olivine crystals in the dunitic layers, and the sharp compositional changes across some, but not all layer limits suggest deformation plays an essential role on the development of the layering. The variation on a mm-scale of the olivine crystal preferred orientation (CPO) symmetry as a function of the modal content: from axial-[100] symmetry in layers with <70% modal olivine to axial-[010] in more gabbroic levels (<40% olivine), which is repeated over the entire section, implies deformation in presence of variable melt fractions. Axial-[100] olivine CPO in olivine-rich layers is consistent with deformation by dislocation creep under high temperature, low pressure, dry conditions. Axial-[010] olivine CPO patterns imply additional sliding along preferentially wetted (010) grain boundaries, increase in the activity of [001] glide, or transpression localized in the melt-rich layers. Since the change in CPO symmetry is not accompanied by dispersion, instantaneous melt fractions must have remained <30-40% in all layers. The continuous variation in olivine CPO symmetry with decreasing olivine content implies therefore that the former depends on the cumulated strain in presence of melt rather than on the instantaneous melt fraction.
ISSN: 0012-821X

hal-00795508
https://hal.archives-ouvertes.fr/hal-00795508
DOI : 10.1016/j.epsl.2012.10.003

Éditeur(s) : Elsevier
Résumé : As top predators of pelagic food webs, large fish naturally bioaccumulate mercury (Hg). Determining Hg burdens in commercialized fish is essential considering the concern about effects of contaminants on human health and the legal thresholds that are therefore set for local consumption and/or exportation. Total Hg levels were measured in the muscular tissue of 183 fish of five commercially important species from the tropical zone of the Western Indian Ocean. All individuals were measured and sexed in order to study the impregnation of Hg with size and sex within each species. Values of Hg found in this part of the Indian Ocean were comparable to Hg in muscular tissue of the same species studied in other areas. The highest Hg levels were noted in Swordfish (Xiphias gladius) caught in waters surrounding Reunion Island (3.97 +/- 2.67 mu g g(-1) dry weight). Following the Swordfish, in decreasing order of Hg content, were the Yellowfin Tuna (Thunnus albacares) and the Skipjack (Katsuwonus pelamis), then the Common Dolphinfish (Coryphaena hippurus) and the Wahoo (Acanthocybium solandri). In the North of the Mozambique Channel, Swordfish had higher Hg levels than Yellowfin Tunas, and Dolphinfish exhibited intermediate Hg levels. The size of a fish was a determining factor of its Hg burden, as was the species. Differences in size-normalized Hg levels were observed between the two study zones for Swordfish and Common Dolphinfish. Sex, in contrast, did not influence Hg levels suggesting that females and males have similar feeding habits. The muscular Hg levels presented here suggest that consumers of fish originating from the Western Indian Ocean should limit themselves to one Swordfish based meal per week, or one fish meal a day if they choose to eat tuna or Common Dolphinfish. (c) 2006 Elsevier B.V. All rights reserved.
The Science of The Total Environment (0048-9697) (Elsevier), 2006-08 , Vol. 366 , N. 2-3 , P. 688-700

Droits : 2006 Elsevier
http://archimer.ifremer.fr/doc/2006/publication-1862.pdf
DOI:10.1016/j.scitotenv.2006.02.006
http://archimer.ifremer.fr/doc/00000/1862/

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
Analysis of the microstructures in the km-scale mantle shear zone that separates the northern and the central parts of the Lanzo peridotite massif provides evidence of an evolution in time and space of deformation processes accommodating shearing in the shallow mantle within an extensional setting. This shear zone displays an asymmetric distribution of deformation facies. From south to north, gradual reorientation of the foliation of coarse porphyroclastic plagioclase-bearing peridotites is followed by development of protomylonites, mylonites, and mm-scale ultramylonite bands. A sharp grain size gradient marks the northern boundary. Early deformation under near-solidus conditions in the south is recorded by preservation of weakly deformed interstitial plagioclase and almost random clinopyroxene and plagioclase crystal orientations. Feedback between deformation and melt transport probably led to melt focusing and strain weakening in the shear zone. Overprint of melt-rock reaction microstructures by solid-state deformation and decrease in recrystallized grain size in the protomylonites and mylonites indicate continued deformation under decreasing temperature. Less enriched peridotite compositions and absence of ultramafic dykes or widespread melt-impregnation microstructures north of the shear zone and clinopyroxene and amphibole enrichment in the mylonites and ultramylonites suggest that the shear zone acted as both a thermal barrier and a high-permeability channel for late crystallizing fluids. These observations, together with chemical data indicating faster cooling of central Lanzo relative to the northern body, corroborate that this shear zone is a mantle detachment fault. All deformation facies have crystal preferred orientations consistent with deformation by dislocation creep with dominant activation of the (010)[100] and (100)[001] systems in olivine and orthopyroxene, respectively. Dynamic recrystallization produces dispersion of olivine CPO but not a change of dominant deformation mechanism. Evidence for activation of grain boundary sliding is limited to mm-scale ultramylonite bands, where solid-state reactions produced very fine grained polymineralic aggregates. Except for these latest stages of deformation, strain localization does not result from the microstructural evolution; the grain size decrease is a consequence of the need to deform a rock volume whose strength continuously increases because of decreasing temperature conditions. Strain localization in the intermediate levels thus essentially results from the more localizing behavior of both the deep, partially molten, and shallow parts of this extensional shear zone distribution.
ISSN: 1525-2027

hal-00644844
https://hal.archives-ouvertes.fr/hal-00644844
DOI : 10.1029/2011GC003627