Multiple stages of serpentinization in mantle derived peridotites of the South Armorican Variscan suture zone

<p>South Armorican mantle peridotites represent a great diversity of protoliths from supra-subduction zone to arc-fore arc ophiolites. In this study, we investigate the serpentinization of these protoliths. Numerous samples were collected in five different units, which represent ophiolitic dismembered pieces (Ty-Lan Peridotites (TLP) from the Audierne Complex, and Pont de Barel Peridotites (PBP), Folies Siffait Peridotites (FSP), l’Orgerais Peridotites (LOP) and Drain Peridotites (DP) from the Champtoceaux Complex). Field and microscopic observations together with Raman spectroscopy and electronic microprobe analysis (EMPA) allowed to identify several stages of serpentinization. All samples display a high rate of serpentinization, up to 80-90 %. Primary assemblage is represented by spinel (TLP, PBP, DP and LOP), olivine (TLP and FSP) and Ti-poor or Cr-rich pargasite (TLP and PBP). In all the samples, lizardite from olivine and bastites from pyroxene and amphibole characterize the first stage of serpentinization. It is associated with magnetite crystallization. No Al-rich lizardite meshe is identified by EMPA suggesting a low temperature (< 340°C) event. This serpentinization is followed by two generations of veins (V1 and V2). The V1 are Al-poor lizardite shear veins and crack-seal chrysotile veins characterize the V2. In PBP, microprobe mapping shows that V2 displays heterogeneous chemical chrysotile composition with significant variations of Al, Fe and Mg contents, suggesting metasomatism and/or variation of fluid composition during serpentinization. All these observations are closely similar to those of oceanic serpentinized peridotites. In the TLP, we identified a second stage of serpentinization characterized by antigorite after lizardite suggesting a high temperature event. In the OP, antigorite after lizardite was also identified. However, compared to the TLP ones, LOP antigorite is related to ductile (i.e., ultramylonite) deformations. This clearly indicates a high temperature stage of serpentinization (up to 500 °C). Furthermore, LOP ultramylonitized samples display one more chrysotile veins generation (V3) characterized by three distinct vein networks. The first one (V3a) is a crack-seal type vein network opened parallel to the main foliation. The second one (V3b) is perpendicular to the first one, whereas the third one (V3c) corresponds to tension gashes connected to C’ plans. This latter is perpendicular to V3a and V3b networks. The mylonitic foliation of LOP is similar to the surrounding micaschists schistosity, suggesting an orogenic high temperature stage of serpentinization. In the FSP, σ-type polycrystalline structures were identified. Lizardite meshes are progressively transposed and recrystallized into the foliation plan. This stage is associated with the crystallization of chlorite after tremolite, suggesting a retrograde stage of serpentinization during serpentinites exhumation. Finally, despite a great diversity of mantle-derived protoliths, our study shows that South-Armorican peridotites recorded a similar first low temperature oceanic stage of serpentinization. According to the Variscan history, it could have started during the Cambro-Ordovician for TLP, and during the Late Devonian for PBP, DP, LOP, FSP. Furthermore, some of these peridotites also recorded an orogenic serpentinization (LOP and FLP). Such observations provide new constraints that could be useful to a better understanding of the tectonometamorphic evolution of the South Armorican suture zones during the Variscan orogeny. </p>

Variation law of adsorption heat of methane and coal with inhomogeneous potential well

The non-uniform structures of coal deposits make potential well distribution on the coal surface inhomogeneous. Deep potential wells adsorb methane molecules more easily than shallow potential wells. The methane adsorption heat release differs according to the depths of the potential wells. During isobaric adsorption, the adsorption heat in the high-temperature stage is significantly higher than that in the low-temperature stage. A lower adsorption pressure results in greater adsorption heat variation during a temperature increase. During the isothermal adsorption process, the adsorption heat is higher in the low-pressure stage, with the preferential adsorption characteristics of deep potential wells.

Solid Phase Growth of Graphene on Silicon Carbide by Nickel Silicidation: Graphene Formation Mechanisms

This work presents experimental evidence of the formation mechanisms of few-layer graphene (FLG) films on SiC by nickel silicidation. FLG is formed by annealing of a 40 nm thick Ni layer on 6H-SiC at 1035ºC for 60 s, resulting in a Ni2Si layer which may be capped by any Ni that did not react during annealing. It has been proposed that FLG forms on top of the Ni during the high temperature stage. In contrast, during cooling, carbon atoms which were released during the silicidation reaction may diffuse back towards the Ni2Si/SiC interface to form a second FLG film. After annealing, layer-by-layer de-processing was carried out in order to unequivocally identify the FLG at each location using Atomic force microscopy (AFM) and Raman spectroscopy.

Effect of Co-Composting of Kitchen Waste and Agricultural Waste on H2S and NH3 Emission

NH3 and H2S emissions not only reduced the nutrient content of compost, and caused odor pollution during organic waste composting. In this study, composting kitchen waste, agricultural waste and co-composting kitchen waste and agricultural waste were conducted. The temperature, oxygen content and the typical odors emission were analyzed in all treatments. The results indicated that Co-composting kitchen waste and agricultural wastes significantly improve the pile temperature and shorten the mesophilic stage. Poor O2 transfer and pile temperature were the main reason for H2S production during waste composting. H2S release occurred mainly within 15 days in this study for all treatments. The highest concentration of H2S was observed in the treatment of T2. The productions of H2S were reduced by 24.8%, 42.3% and 55.1% for T1, T3 and T4 that compared to T2, respectively. The NH3 had a similar trend to that of H2S. About 66.4%-72.0% of the total NH3 released in the high temperature stage for all treatments. The highest concentration of NH3 was observed in the treatment of T4.The productions of NH3 were reduced by 36.5%, 19.8% and 21.3% for T1, T2 and T3 that compared to T4, respectively. In terms of H2S control, it is suitable to co-composting of kitchen waste and agricultural waste. In terms of NH3 control, it is better to compost kitchen waste and agricultural waste independently.