Climate changes caused by degassing of sediments during the emplacement of large igneous provinces: REPLY

International audienceReply to Grzegorz Racki for his comment on our manuscript and for his overall support for the hypothesis that the nature of the rocks in the substrate of large igneous province has an important influence on their environmental impact (Ganino and Arndt, 2009)

Textures in komatiites and variolitic basalts.

Komatiites and variolitic basalts are widespread in Archean volcanic sequences. Spinifex is a spectacular bladed olivine or pyroxene texture that characterizes komatiite, a rock almost exclusively restricted to the Archean; varioles are cm-scale leucocratic globular structures abundant in many Archean basalts. These striking textures provide valuable information about conditions during emplacement of the host magmas, particularly about how the magmas crystallized. Many komatiite flows have spinifex textures consisting of arrays of numerous subparallel olivine blades that extend 10's of cm to m's from the flow tops. The habit of the strongly anisotropic crystals is suggestive of fast cooling near the flow margin, yet the crystals form deep within the flows. The large temperature difference between solidus and liquidus of komatiites provides a partial explanation for their formation. In addition, the crystals are sharp-tipped and aligned so that their fastest growing faces were normal to the cooling contacts, suggesting that they grew in a strong chemical-potential gradient, in part created by the crystals themselves, as they modified the composition and temperature of the liquid from which they crystallized

Komatiite.

It is easy to explain roughly what a komatiite is but difficult to give a rigorous definition. The simple definition is that komatiite is an ultramafic volcanic rock (Arndt and Nisbet 1982). Alimit of 18% MgO separates komatiites from less magnesian volcanic rocks such as picrites,ankaramites or magnesian basalts. The term komatiitic basalt is applied to volcanic rocks containing less than 18% MgO that can be linked, using petrological, textural or geochemicalarguments, to komatiites

Mantle-derived magmas and magmatic Ni-Cu-(PGE) deposits

Magmatic Fe-Ni-Cu ± platinum-group element (PGE) sulfide deposits form when mantle-derived mafic and ultramafic magmas become saturated in sulfide and segregate immiscible sulfide liquid, commonly following interaction with crustal rocks. Although the metal contents of primary magmas influence ore compositions, they do not control ore genesis because the metals partition strongly into the sulfide liquid and because most magmas capable of segregating sulfide liquid contain sufficient abundances of ore metals. More important controls are the temperature, viscosity, volatile content, and mode of emplacement of the magma, which control the dynamics of magma emplacement and the degree of interaction with crust. By this measure, high-temperature, low-viscosity komatiites and tholeiitic picrites are most capable of forming Ni-Cu-(PGE) deposits, whereas lower-temperature, volatile-rich alkali picrites and basalts have less potential. In most deposits, ore formation is linked directly to incorporation of S-rich country rocks and only indirectly to contamination by granitic crust. However, the geochemical signature of contamination is easily recognized and is a useful exploration guide because it identifies magmas that had the capacity to incorporate crustal material. Several aspects of the ore-forming process remain poorly understood, including the control of mantle melting processes on the PGE contents of mafic-ultramafic magmas, the mechanisms by which sulfur is transferred from wall rocks to ores (bulk assimilation, incongruent melting, and/or devolatilization), the distances and processes by which dense sulfide melts are transported from where they form to where they become concentrated (as finely-dispersed droplets, as segregated layers, or by deformation-driven injection of massive sulfide accumulations), and the dynamic processes that increase the metal contents of the ores

Interaction of magma with sedimentary wall rock and magnetite ore genesis in the Panzhihua mafic intrusion, SW China

International audienceIn SW China, several large magmatic Fe-Ti-V oxide ore deposits are hosted by gabbroic intrusions associated with the Emeishan flood basalts. The Panzhihua gabbroic intrusion, a little deformed sill that contains a large titanomagnetite deposit at its base, concordantly intrudes late- Proterozoic dolostones. Mineralogical and chemical studies of the contact aureole in the footwall dolostones demonstrates that the metamorphism was largely isochemical, but for the release of large quantities of CO2 as the rocks were converted to marble and skarns during intrusion of the gabbroic magma. Petrological modelling of the crystallization of the intrusion, using H2O-poor Emeishan basalt as parent magma, shows that under normal conditions Fe-Ti-oxides crystallize at a late stage, after the crystallization of abundant olivine, clinopyroxene and plagioclase. In order for titanomagnetite to separate efficiently to form the ore deposit, this mineral must have crystallized earlier and close to the liquidus. We propose that CO2-rich fluids released during decarbonatization of sedimentary floor rocks passed up through the magma. Redox equilibria calculations show that when magma with the composition of Emeishan basalt is fluxed by a CO2-rich gas phase, its equilibrium oxygen fugacity (fO2) increases from FMQ to FMQ+1.5. From experimental constraints on magnetite saturation in basaltic magma under controlled fO2, such an oxidizing event would allow magnetite to crystallize near to the liquidus, leading to the formation of the deposit

ERA-MIN Research Agenda

European Research Area - Network on the Industrial Handling of Raw Materials for European Industriesroadmap of the "ERA-MIN" eranetNon-energy and non-agricultural raw materials underpin the global economy and our quality of life. They are vital for the EU's economy and for the development of environmentally friendly technologies essential to European industries. However, the EU is highly dependent on imports, and securing supplies has therefore become crucial. A sustainable supply of mineral products and metals for European industry requires a more efficient and rational consumption, enhanced substitution and improved recycling. Recycling from scrap to raw materials has been rapidly gaining in quantity and efficiency over the last years. However, continuous re-use cannot provide alone the necessary quantities of mineral raw materials, due to i) recycling losses, ii) the worldwide growing demand in raw materials, and iii) the need of "new" elements for the industry. To fully meet future needs, metals and mineral products from primary sources will still be needed in the future. Most of them will continue to be imported from sources outside Europe; but others can, and should, be produced domestically. Advanced research and innovation are required to improve the capacity of existing technologies to discover new deposits, to improve the efficiency of the entire geomaterials life cycle from mineral extraction to the use as secondary resource of products at the end of their industrial life, and to reduce the environmental footprint of raw materials extraction and use. Research and innovation must be made to acquire knowledge as well, and to improve our basic understanding of all engineering and natural processes involved in the raw materials life cycle, as well as the coupling of these processes. Finally, research has to go beyond the present-day economic and technological constraints, and it should be closely associated with training and education in order to maintain existing skills and to share the most recent developments with the industrial sector. A long-term vision of research is necessary in order to have the capacity of evaluating the environmental and societal impacts of present and developing industrial activities and to imagine tomorrow's breakthrough concepts and technologies that will create new industrial opportunities. These objectives require the input of contrasted scientific and technical skills and competences (earth science, material science and technology, chemistry, physics, engineer, biology, engineering, environmental science, economy, social and human sciences, etc). An important challenge is to gather all these domains of expertise towards the same objective. The ERA-MIN Research Agenda aims at listing the most important topics of research and innovation that will contribute to i) secure the sustainable supply and management of non-energy and non-agricultural raw materials, and ii) offer opportunities of investment and employment opportunities in the EU

Les cherts Archéens de la ceinture de roches vertes de Barberton (3.5-3.2Ga), Afrique du Sud. Processus de formation et utilisation comme proxys paleo-environnementaux

Les cherts archéens permettent de contraindre les environnements primitifs qui ont vu l apparition de la vie sur Terre. Ces roches siliceuses se forment selon trois processus : les C-cherts (cherts primaires) se forment par précipitation chimique de silice océanique sur le plancher, sous la forme d une boue siliceuse ou en tant que ciment dans les sédiments de surface; les F-cherts (cherts de fracture) précipitent dans les fractures de la crôute depuis les fluides circulant; les S-cherts (cherts secondaires) sont issus de la silicification de roches préexistantes lors de la percolation de fluides enrichis en silice. Ces processus sont largement acceptés mais des questions majeures subsistent : comment reconnaître ces différents types de chert ? Quelle est l origine de la silice et sous quelle forme a-t-elle précipité ? Quel signal chimique est porté par les cherts et comment s en servir pour les reconstructions paléo-environnementales ? Ces questions sont abordées à travers trois sites de la ceinture de roches vertes de Barberton, en Afrique du Sud. L approche adoptée combine l analyse des structures sédimentaires et de déformation, de la pétrologie et de la composition chimique et isotopique de ces unités. Dans ces sites, la formation des cherts est étroitement liée à l environnement de mise en place. La sédimentation clastique (turbidites) est à l origine des C-cherts de Komati River, déposés sous la forme d une boue siliceuse par adsorption de silice sur les particules argileuses en suspension. En absence de contribution continentale, les alternances de cherts noirs et blancs de Buck Reef sont interprétées comme issues de variations climatiques à l échelle saisonnières (chert noir), voire glaciaires/inter-glaciaires (chert blanc). Les cherts de fracture de Barite Valley sont liés à la précipitation de silice depuis une suspension colloïdale thixotrope remontant à travers la croûte. La composition chimique des cherts est contrôlée par leur environnement de mise en place, et représente un mélange entre une phase siliceuse et une phase contaminante, indépendamment des processus qui ont précipité la silice. Les cherts de Komati River et de Barite Valley sont enrichis en Al, K, Ti, HFSE et en REE, ce qui est attribué à la contamination de la matrice siliceuse par la présence de phyllosilicate. Une telle contribution clastique peut expliquer les larges gammes de 30Si dans les cherts de Komati River (-0.69 à +3.89 ), et la majorité des valeurs positives est probablement liée à la contribution de l eau de mer. Dans les dykes de Barite Valley, les 30Si très négatifs (-4.5 à +0.22 ) sont cohérents avec l origine hydrothermale basse température des fluides initiaux. A Buck Reef, l absence de contribution continentale s exprime dans les cherts blancs par une minéralogie exclusivement microquartzitique et par des concentrations extrêmement faibles en éléments traces (i.e. HFSE et REE<1ppm). 2% de carbonates et 3-4% de matériel continental (e.g. argiles) suffisent à masquer le signal siliceux dans ces cherts purs. Nous ne pouvons conclure sur la présence d un signal océanique dans ces cherts par manque de fiabilité des proxys océaniques modernes (appauvrissement en LREE, enrichissement en La et Y). Reconnus à la fois dans des quartz océaniques, hydrothermaux, magmatiques et pegmatitiques, ils ne permettent pas d identifier un signal d eau de mer dans les cherts archéens. Les 18O de ces cherts indiquent la présence de circulations fluides secondaires à moins de 100C, et leurs 30Si négatifs ou positifs (-2.23 et +1.13 en moyenne) montrent la contribution de fluides différents au moment de leur formation. Le couplage des observations pétrologiques et de terrain est la seule approche fiable pour reconnaître le mode de mise en place des cherts. Leur composition chimique dépend plus des conditions environnementales que des caractéristiques du fluide initial.Archean cherts potentially constrain the primitive environment in which life emerged and evolved. These siliceous rocks formed by three processes : C-cherts (primary cherts) formed by the chemical precipitation of oceanic silica, either as a siliceous ooze (or silica gel) on the seabed, or as cement within still soft sediments at the surface ; F-cherts (fracturefilling cherts) precipitated from circulating fluids in concordant or crosscutting veins in the shallow crust ; S-cherts (secondary cherts) are the result of the metasomatism (silicification) of preexisting rocks during the percolation of silica-rich fluids. These processes are generally accepted but major questions remain unsolved : how to recognize various chert types ? Where does the silica come from and how did it precipitate ? What chemical signal is hosted in cherts and how can it be used for paleo environmental reconstructions ? These questions are addressed here using three sites in the Barberton Greenstone Belt, South Africa, which contain a variety of cherts deposited in very different environments. The approach combines field description of sedimentary and deformation structures, the characterization of various chert petrologies, and the study of their chemical and isotopic composition. In these three sites, chert formation strongly depends on the environmental setting. Clastic sedimentation is directly linked to C-chert formation at Komati River, where the silica was deposited as a viscous, siliceous ooze by sorption process onto suspended clay particles. A continental contribution is absent at Buck Reef, and the black and white banded cherts (C-cherts) are interpreted to have formed by chemical precipitation of oceanic silica during seasonal (black chert) and maybe glacial/inter-glacial (white chert) climatic variations. The fracture-filling cherts from Barite Valley precipitated from a thixotropic colloidal suspension that migrated upward through the crust. The chemical compositions of cherts from these three sites are essentially controlled by the environment of deposition, and represent mixtures of a siliceous and contaminant phases, independent from the silica precipitation mode. Komati River C-cherts and Barite Valley F-cherts are both enriched in Al, K, Ti, HFSE and REE, which represents the contamination by phyllosilicates of the microquartzitic fabrics. Such a clastic contribution may account for the wide range of 30Si in Komati River cherts (-0.69 to +3.89 ) although the majority of positive values is attributed to seawater involvement. In the dykes, 30Si is strongly negative (-4.5 to +0.22 ) and is consistent with the low-temperature hydrothermal nature of these fluids. At Buck Reef, the lack of continental contribution is expressed in the white cherts, by a mineralogy exclusively composed of microquartz, and by extremely low trace element contents, i.e. HFSE and REE below 1ppm. We calculate that 2% of carbonates and 3-4% of clastic particles (i.e. clay, feldspar) would be enough to mask the silica composition in these high purity cherts. A marine signature was not recognized in their geochemistry because of the unreliability of commonly used modern proxys (i.e. LREE depletion, La and Y enrichment). These features were identified in oceanic, hydrothermal, magmatic and pegmatitic quartz and thus do not reliably identify an oceanic signal in Archean cherts. Because the 18O values in these white cherts indicates secondary fluid circulations at <100C, their negative or positive 30Si values (-2.23 and +1.13 in average) most probably represent different fluid contributions at the time they formed. The combination of field and petrological observations appears to be the most reliable approach to classify cherts and to deduce their origin, and we show here that their chemical composition depends more on the environmental conditions than on the primary fluid characteristics.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Gas emissions due to magma-sediment interactions during flood magmatism at the Siberian Traps: gas dispersion and environmental consequences.

International audienceWe estimate the fluxes of extremely reduced gas emissions produced during the emplacement of the Siberian Traps large igneous province, due to magma intrusion in the coaliferous sediments of the Tunguska Basin. Using the results of a companion paper (Iacono-Marziano et al. submitted to EPSL), and a recent work about low temperature interaction between magma and organic matter (Svensen et al., 2009), we calculate CO-CH4-dominated gas emission rates of 7×1015-2×1016 g/yr for a single magmatic/volcanic event. These fluxes are 7 to 20 times higher than those calculated for purely magmatic gas emissions, in the absence of interaction with organic matter-rich sediments. We investigate, by means of atmospheric modelling employing present geography of Siberia, the short and mid term dispersion of these gas emissions into the atmosphere. The lateral propagation of CO and CH4 leads to an important perturbation of the atmosphere chemistry, consisting in a strong reduction of the radical OH concentration. As a consequence, both CO and CH4 lifetimes in the lower atmosphere are enhanced by a factor of at least 3, at the continental scale, as a consequence of 30 days of magmatic activity. The short-term effect of the injection of carbon monoxide and methane into the atmosphere is therefore to increase the residence times of these two species and, in turn, their capacity of geographic expansion. The estimated CO and CH4 volume mixing ratios (i.e. the number of molecules of CO or CH4 per cm3, divided by the total number of molecules per cm3) in the low atmosphere are 2-5 ppmv at the continental scale and locally higher than 50 ppmv. The dimension of the area affected by these high volume mixing ratios decreases in the presence of a lava flow accompanying magma intrusion at depth. Complementary calculations for a 10-year duration of the magmatic activity suggest (i) an increase in the mean CH4 volume mixing ratio of the whole atmosphere up to values 3 to 15 times higher than the current one, and (ii) recovery times of 100 years to bring back the atmospheric volume mixing ratio of CH4 to the pre-magmatic value. Thermogenic methane emissions from the Siberian Traps has already been proposed to crucially contribute to end Permian-Early Triassic global warming and to the negative carbon isotopic shift observed globally in both marine and terrestrial sediments. Our results corroborate these hypotheses and suggest that concurrent high temperature CO emissions also played a key role by contributing to increase (i) the radiative forcing of methane and therefore in its global warming potential, and (ii) the input of isotopically light carbon into the atmosphere that generated the isotopic excursion. We also speculate a poisoning effect of high carbon monoxide concentrations on end-Permian fauna, at a local scale

Extremely reducing conditions reached during basaltic intrusion in organic matter-bearing sediments

International audienceRedox conditions in magma are widely interpreted as internally buffered and closely related to that of their mantle source regions. We use thermodynamic calculations to show that high-temperature interaction between magma and organic matter can lead to a dramatic reduction of the magma redox state, and significant departure from that of the original source. Field studies provide direct evidence of the process that we describe, with reported occurrences of graphite and native iron in igneous mafic rocks, implying very reducing conditions that are almost unknown in average terrestrial magmas. We calculate that the addition of 0.6 wt% organic matter (in the form of CH or CH2) to a standard basalt triggers graphite and native iron crystallisation at depths of few hundred meters. Interaction with organic matter also profoundly affects the abundance and the redox state of the gases in equilibrium with the magma, which are CO-dominated with H2 as the second most abundant species on a molar basis, H2O and CO2 being minor constituents. The assimilation of only 0.1 wt% organic matter by a basalt causes a decrease in its oxygen fugacity of 2-orders of magnitude. The assimilation of 0.6 wt% organic matter at depths < 500 m implies minimum CO content in the magma of 1 wt%, other gas components being less than 0.1 wt%. In the light of our calculations, we suggest that the production of native iron-bearing lava flows and associated intrusions was most likely accompanied by degassing of CO-rich gases, whose fluxes depended on the magma production rates

Instantaneous and ensemble average cavitation structures in Diesel micro-channel flow orifices

Cavitation developing upstream and inside the micro-channel orifices of transparent multi-hole fuel injector nozzles has been characterised using a high speed visualisation system. Images have been obtained with short exposure time and sufficiently high spatial and temporal resolution, freezing the formation of cavitation bubbles and their further development during every single injection cycle. Post processing of statistically large number of images collected from many successive injection events and numerous identical nozzles has allowed for the first time estimation of the ensemble average cavitation image and its standard deviation. The instantaneous images reveal the formation of a variety of complex and interacting two-phase flow regimes. Vapour bubbles have been found to exist inside the nozzle prior to start of injection. These bubbles originate from the previous injection cycle as they have not been evacuated from the nozzle and hence, they remain trapped, altering the flow of the subsequent injection event. During the opening and closing stages of the needle valve that controls the fuel flow through the nozzle, cavitation is found to form in the valve’s seat area. Subsequently, vortex or ‘string’ cavitation has been recorded to take place in a rather chaotic manner; its life time and most probable location of appearance have been estimated. The ensemble average images reveal the probability of cavitation appearance at a specific location within the nozzle and the micro-channel flow orifice. The standard deviation from the mean reveals locations with significant cycle-to-cycle variations of the flow. These are linked to significant deviations from the mean of the fuel spray dispersion angle forming downstream of the nozzle exit