Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Quartz is a common crustal mineral that deforms plastically in a wide range of temperatures and pressures, leading to the development of different types of crystallographic preferred orientation (CPO) patterns. In this contribution we present the results of an extensive modeling of quartz fabric transitions via a viscoplastic self-consistent (VPSC) approach. For that, we have performed systematic simulations using different sets of relative critical resolved shear stress of the main quartz slip systems. We have performed these simulations in axial compression and simple shear regimes under constant Von Mises equivalent strain of 100% (γ = 1.73), assuming that the aggregates deformed exclusively by dislocation glide. Some of the predicted CPOs patterns are similar to those observed in naturally and experimentally deformed quartz. Nevertheless, some classical CPO patterns usually interpreted as result from dislocation glide (e.g. Y-maxima due to prism < a > slip) are clearly not developed in the simulated conditions. In addition we reported new potential preferred orientation patterns that might happen in high temperature conditions, both in axial compression and simple shear. We have demonstrated that CPOs generated under axial compression are usually stronger that those predicted under simple shear, due to the continuous rotation observed in the later simulations. The fabric strength depends essentially on the dominant active slip system, and normally the stronger CPOs result from dominant basal slip in < a >, followed by rhomb < a > and prism [c] slip, whereas prism < a > slip does not produce strong fabrics. The opening angle of quartz [0001] fabric used as a proxy of temperature seems to be reliable for deformation temperatures of ~ 400 °C, when the main slip systems have similar behaviors.
ISSN: 0040-1951

hal-01115674
https://hal.archives-ouvertes.fr/hal-01115674
DOI : 10.1016/j.tecto.2014.08.011

Éditeur(s) : Elsevier
Résumé : The combined use of Lu-Hf and Sm-Nd isotope systems potentially offers a unique perspective for investigating continental erosion, but little is known about whether, and to what extent, the Hf-Nd isotope composition of sediments is related to silicate weathering intensity. In this study, Hf and Nd elemental and isotope data are reported for marine muds, leached Fe-oxide fractions and zircon-rich turbidite sands collected off the Congo River mouth, and from other parts of the SE Atlantic Ocean. All studied samples from the Congo fan (muds, Fe-hydroxides, sands) exhibit indistinguishable Nd isotopic composition (epsilon(Nd)similar to - 16), indicating that Fe-hydroxides leached from these sediments correspond to continental oxides precipitated within the Congo basin. In marked contrast, Hf isotope compositions for the same samples exhibit significant variations. Leached Fe-hydroxide fractions are characterized by epsilon(Hf) values (from -1.1 to +13) far more radiogenic than associated sediments (from -7.1 to -12.0) and turbidite sands (from -27.2 to -31.6). epsilon(Hf) values for Congo fan sediments correlate very well with Al/K (i.e. a well-known index for the intensity of chemical weathering in Central Africa). Taken together, these results indicate that (1) silicate weathering on continents leads to erosion products having very distinctive Hf isotope signatures, and (2) a direct relationship exists between epsilon(Hf) of secondary clay minerals and chemical weathering intensity. These results combined with data from the literature have global implications for understanding the Hf-Nd isotope variability in marine precipitates and sediments. Leached Fe-hydroxides from Congo fan sediments plot remarkably well on an extension of the 'seawater array' (i.e. the correlation defined by deep-sea Fe-Mn precipitates), providing additional support to the suggestion that the ocean Hf budget is dominated by continental inputs. Fine-grained sediments define a diffuse trend, between that for igneous rocks and the,seawater array', which we refer to as the 'zircon-free sediment array' (epsilon(Hf)=0-91 epsilon(Nd)+3.10). Finally, we show that the Hf-Nd arrays for seawater, unweathered igneous rocks, zircon-free and zircon-bearing sediments (epsilon(Hf)= 1.80 epsilon(Nd)+2.35) can all be reconciled, using Monte Carlo simulations, with a simple weathering model of the continental crust. (C) 2008 Elsevier B.V. All rights reserved.
Earth and Planetary Science Letters (0012-821X) (Elsevier), 2009 , Vol. 277 , N. 3-4 , P. 318-326

Droits : 2009 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/2009/publication-6451.pdf
DOI:10.1016/j.epsl.2008.10.028
http://archimer.ifremer.fr/doc/00000/6451/