A comparative experimental kinetic study of spontaneous and plasma-assisted cool flames in a rapid compression machine

International audiencePlasma-assisted cool flames of n- heptane were generated in the combustion chamber of a rapid compression machine coupled with a nanosecond dielectric barrier discharge, at a pressure of 1.5 bar and temperature T = 650 K. Increasing of the voltage pulse amplitude at the electrode resulted in a transition from no reactivity to induced cool flame and then to fast ignition. Sampling of the reacting mixture was performed at selected times during the experiments to draw mole fraction profiles of the fuel and major low temperature stable intermediates, showing a gradual increase in the mole fraction of these species after the discharge. Comparison with a spontaneous cool flame case at a slightly higher pressure shows that no new species are formed in the plasma-assisted case, and that after the initiation of reactivity by the discharge at the nanosecond timescale, the distribution and relative importance of the main reaction pathways is conserved at the millisecond timescale. Differences in the shape of the mole fraction and light emission profiles however suggest that the plasma-assisted cool flame is propagative

Low intra-crystalline closure temperatures of Cr-bearing spinels from the mantle xenoliths of the Middle Atlas Neogene-Quaternary Volcanic Field (Morocco): A mineralogical evidence of a cooler mantle beneath the West African Craton

The crystal chemistry of nine Cr-spinels from lherzolite and harzburgite xenoliths from the Middle Atlas Neogene-Quaternary Volcanic Field of Morocco have been studied by means of X-ray single crystal diffraction and electron microprobe analyses. Cell edges are usually within the range 8.13-8.14 \uc5, but there are three samples with longer a value, so that the whole analyzed series is within the range 8.1334 (4) \u2013 8.2021 (2) \uc5, while the oxygen positional parameter values are very similar ranging between 0.2626 (1) and 0.2629 (2) for all of them. The cation distribution shows that the crystal structure is ordered with almost all divalent cations in the tetrahedral site and trivalent cations in the octahedral site. The determined intracrystalline temperatures are in the range 550-750\ub0C, that are the lowest values ever found for Cr-spinels from mantle xenoliths as these are usually higher than 730\ub0C. If we consider the behavior of some geotherms from literature, the determined temperatures are confined in a depth range of about 20-40 Km. Lithospheric models for the studied area indicate that the lower crust reaches its deepest value in a range between 30 and 40 Km. Consequently, we can assume that the studied xenoliths were emplaced at a \u201cshallow\u201d depth of about 20-30 Km, just beneath the lower crust, where they were disrupted and brought to the surface from the ascending alkaline lavas. This assumption is consistent with the concomitant presence of some crustal xenolith

Physical volcanology and emplacement mechanism of the Central Atlantic Magmatic Province (CAMP) lava flows from the Central High Atlas, Morocco

The best preserved and most complete lava flow sequences of the Central Atlantic Magmatic Province (CAMP) in Morocco are exposed in the Central High Atlas and can reach up to 300 m in thickness. Four distinct formations, emplaced in subaerial environments, are classically recognized: the Lower, Intermediate, Upper and Recurrent formations. These formations are separated by paleosoils and sedimentary sequences (mudstones, siltstones, sandstones, limestones), that are in general less than two meter-thick and may exceptionally reach a thickness of 80 m, representing minor periods of volcanic quiescence. CAMP lava flows of the Central High Atlas can be grouped into two main categories: subaerial compound pahoehoe flows and simple flows. The former type is exclusively confined to the Lower and Intermediate Formations, while simple flows occur in the Upper and Recurrent Formations. The dominance of compound flows in the two lowermost units of the CAMP suggests a slow emplacement during successive sustained eruptive episodes. Instead the thick single flows characterizing the Upper and Recurrent units indicate higher effusive rates. Basaltic pillow lavas (always of short lateral extent: 10 to 100 m), showing radial jointing and vitreous rinds, identical to those found in the Western Meseta, are occasionally associated with hyaloclastites in the base of the Intermediate Formation, immediately above clastic sediments, or in the Upper Formation. The occurrence of pillow lavas does not imply a generalized subaqueous environment at the time of the lava emission. Instead, they represent subaerial flows that entered small lakes occupying depressions on the volcanic topography of the Lower and Intermediate Formations. The short lateral extent of the pillow lavas and their constant stratigraphic position, the existence of lava flows with unequivocal subaerial characteristics associated to sediments containing fossilized wood, clearly indicate onshore emplacement

U\u2013pb zircon geochronological and petrologic constraints on the post-collisional variscan volcanism of the tiddas-souk es-sebt des a\ueft ikko basin (Western meseta, morocco)

The NE\u2013SW trending Tiddas Souk Es-Sebt des Ait Ikko (TSESDAI) basin, located at 110 km southeast of Rabat, in the region of Khmesset between the village of Tiddas Souk Es-Sebt des Ait Ikko, is the third largest late Palaeozoic continental trough in the northern Central Moroccan Meseta. It is a ~20 km long and ~2\u20133 km wide basin, comprising mainly mixed volcano-sedimentary reddish-purple continental Permian rocks laying with an angular unconformity on Visean deep marine siliciclastic sediments and unconformably overlain by the Triassic and Cenozoic formations. In this study we aim to better determine the age of Permian volcanics and their chemical and mineralogical characteristics, as well as assess the provenance of inherited zircons, thus contributing to the understanding of the late stages of the Variscan orogeny in Morocco. The standard volcanic succession includes the following terms: (i) andesites, lapilli tuffs and andesitic ash deposits; (ii) accumulations of rhyolitic lavas; (iii) lapilli tuffs and rhyolitic ash (formation F1); (iv) flows and breccias of dacites; (v) andesite flows; and (vi) basaltic flows. The various volcanic and subvolcanic studied rocks display calc-alkaline-series characteristics with high contents of SiO2, Al2O3, CaO, MgO, and relatively abundant alkalis, and low contents of MnO. In the classification diagram, the studied facies occupy the fields of andesites, trachy-basalts, dacites, trachydacites, and rhyolites and display a sub-alkaline behavior. These lavas would be derived from a parental mafic magma (basalts) produced by partial fusion of the upper mantle. Specific chemical analyses that were carried out on the mineralogical phases (biotite and pyroxene) revealed that the examined biotites can be classified as magnesian and share similarities with the calc-alkaline association-field, while the clinopyroxenes are mainly augites and plot on the calc-alkaline orogenic basalt field. Andesites and dacites of TSESDAI show similarities with the rocks of the calc-alkaline series not linked to active subduction and which involve a continental crust in their genesis. The existence of enclaves in the lavas of the TSESDAI massif; the abnormally high contents of Rb, Ba, Th, and La; and the systematic anomalies in TiO2 and P2O5 indicate also a crustal contamination mechanism. Three magmatic episodes are distinguished with two episodes that correspond to an eruptive cycle of calc-alkaline andesites and rhyolites followed by a basaltic episode. The SHRIMP U\u2013Pb geochronologic data of zircons recovered from the rhyolite dome of Ari El Mahsar in TSESDAI basin show a Concordia age of 286.4 \ub1 4.7 Ma interpreted to date the magmatic crystallization of this dome. Thus, the rhyolite likely belongs to the third magmatic episodes of TSESDAI

Geochemistry and geochronology of the neoproterozoic Backarc basin Khzama ophiolite (Anti-atlas mountains, morocco): Tectonomagmatic implications

The Khzama ophiolite is a highly dismembered complex located in the Siroua inlier of the Moroccan Anti-Atlas Belt. It consists of ultramafic rocks, cumulate gabbros, sheeted dikes, pillow lavas, and an overlying volcano-sedimentary sequence. Three main tectonic slices of sheeted dike complexes are studied in detail along three rivers, exposing well preserved outcrops where individual dikes are clearly distinguishable from the intruded host rock (Assif n\u2019Tinzla, Assif n\u2019Tasriwine, and Assif n\u2019Iriri). Sheeted dikes of the Khzama ophiolitic complex are basaltic to andesitic in composition, displaying a clear sub-alkaline nature. We identify two sets of dikes that originate from lower High-Ti series (HTS) lavas and overlying upper Low-Ti series (LTS) lava. The immobile trace-element signatures of these rocks point to a genesis on a backarc environment with magmas sourced in a supra-subduction zone (SSZ) at the spinel peridotite zone. The obtained SHRIMP U-Pb data of the gabbro represent the first radiometric age of zircon extracted from the mafic rocks that were intruded by the sheeted dike complex of the Khzama ophiolite. These grains yield a concordia age of 763 \ub1 5 Ma, which is consistent with the 761.1 + 1.9/ 121.6 and 762 + 1/ 122 Ma U-Pb zircon ages of plagiogranites of Siroua. Based on their mineralogy, modal proportions, and major element chemistry, the felsic dikes are classified as high silica\u2013low alumina trondhjemites or plagiogranites. These plagiogranites were likely formed by the partial melting of mafic rocks rather than by extreme fractional crystallization. A plagiogranite dated at 777 \ub1 4.7 Ma (U-Pb on zircon) is significantly older than the ca. 762 Ma plagiogranites previously recorded for the Khzama locality, suggesting a long-lived supra-subduction zone (SSZ) with conditions for the hydrous melting of mafic rocks

U-Pb Zircon geochronological and petrologic constraints on the post-collisional Variscan volcanism of the Tiddas-Souk Es-Sebt des Aït Ikko (Tsesdai) basin (Western Meseta, Morocco)

Trabajo presentado en el 2nd International Congress on Permian and Triassic, celebrado en Casablanca (Marruecos), del 24 al 27 de abril de 2018The NE-SW Tsesdai basin (110 km to the SE of Rabat) is the third largest Late Palaeozoic continental trough in the northern Central Moroccan Meseta. It is <20 km long and ~2-3 km wide, comprising mixed volcano-sedimentary reddish-purple Permian rocks that rest on Visean deep marine siliciclastic sediments, overlain by the Triassic and Cenozoic formations. The lower and upper boundaries correspond to angular unconformities. The Permian age of these formations was obtained with the help of plant remains and vertebrate ichnofossils (e.g., Voigt et al., 2011, etc.). The occurrence of volcanics in this basin was first reported by Termier (1936) who considered them (as well as those in Khenifra) as forming the “Central Moroccan volcanic axis” stretching from Khenifra to Tiddas and displaying felsic and mafic volcanics emplaced in the Neogene/Quaternary. This “axial-model” has been accepted by several authors (e.g. Michard, 1976) until the 1980s. Later contributions largely improved the geological knowledge of this basin (e.g., Gonord et al.,1980; Cailleux et al., 1983; Zouine, 1986; etc.). The conclusions of these authors are:(i) the volcanic complex is Permian s.l. (post-Carboniferous and pre-Triassic, presumably Stephanian/Permian) a percept based on (a) the intrusion of rhyolites into the Visean deep marine siliciclastics of the basement (shark-fin rhyolitic dome of Ari el Mahsar extrusive and its satellite dykes); (b) the angular unconformity between the red detritic sediments and the Visean siliciclastics folded during the late Westphalian A; (c) the presence of reworked rhyolitic/andesitic pebbles in the red sediments of the Tsesdai basin and their absence in the detrital sediments of the Westphalian C/ D preserved nearby in the Sidi Kacem graben; (d) the angular unconformity between the Triassic and the red detritals; and (d) strike-slip fault that postdates the basin sedimentary/volcanic formations and predates the Triassic/Cenozoic. (ii) the distinction of 3 major volcanic pulses: the first one (mainly andesitic) predates the Permian series; witnesses of this pulse can still be observed in Tiddas, but most material is reworked in the conglomerates; the second pulse is synsedimentary, displays different petrographic characteristics, and it is remarkable for its andesitic lava flows interbedded with the detritals. The third pulse of intrusive rhyolitic dykes precedes the Ari el Mahsar rhyolitic dome emplacement and the dacitic flows. (iii) identification of three intra-permian tectonic events. The close relationship between volcanism and fracturation is obvious: dykes and effusive centers are located in relation with N40-N70 faults, mostly strike-slip. We present new insights on the petrology and geochronology of the Tsesdai Permian volcanism affecting the northwestern Gondwanaland margin. The studied volcanic has typical orogenic calc-alkaline signatures. For the last volcanic pulse we determined U-Pb ages of zircon by Sensitive High-Resolution Ion Microprobe (SHRIMP). The shark fin rhyolitic extrusive dome of Ari el Mahsar (sample ARM15/GPS: 33°39'51.48"N, 6° 4'2.06"W) yielded two predominant ages (603 ± 25 Ma and 286.4 ± 4.7 Ma): the young age is more concordant, being interpreted as the crystallization age of rhyolites, whilePeer reviewe