Moyen J.-F.,Jean Monnet University |
Moyen J.-F.,CNRS Magmas and Volcanoes Laboratory |
Martin H.,CNRS Magmas and Volcanoes Laboratory
Lithos | Year: 2012
TTGs (tonalite-trondhjemite-granodiorite) are one of the archetypical lithologies of Archaean cratons. Since their original description in the 1970s, they have been the subject of many studies and discussions relating to Archaean geology. In this paper, we review the ideas, concepts and arguments brought forward in these 40. years, and try to address some open questions - both old and new. The late 1960s and the 1970s mark the appearance of "grey gneisses" (TTG) in the scientific literature. During this period, most work was focused on the identification and description of this suite, and the recognition that it is a typical Archaean lithology. TTGs were already recognised as generated by melting of mafic rocks. This was corroborated during the next decade, when detailed geochemical TTG studies allowed us to constrain their petrogenesis (melting of garnet-bearing metamafic rocks), and to conclude that they must have been generated by Archaean geodynamic processes distinct from their modern counterparts. However, the geodynamic debate raged for the following 30. years, as many distinct tectonic scenarios can be imagined, all resulting in the melting of mafic rocks in the garnet stability field. The 1990s were dominated by experimental petrology work. A wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to TTG liquids; whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate. From 1990s onwards, one of the key questions became the comparison with modern adakites. As originally defined these arc lavas are reasonably close equivalents to Archaean TTGs. Pending issues largely revolve around definitions, as the name TTG has now been applied to most Archaean plutonic rocks, whether sodic or potassic, irrespective of their HREE contents. This leads to a large range of petrogenetic and tectonic scenarios; a fair number of which may well have operated concurrently, but are applicable only to some of the rocks lumped together in the ever-broadening TTG "bin". © 2012 Elsevier B.V.
Bouvier A.,University of Western Ontario |
Boyet M.,CNRS Magmas and Volcanoes Laboratory
Nature | Year: 2016
The early evolution of planetesimals and planets can be constrained using variations in the abundance of neodymium-142 (142Nd), which arise from the initial distribution of 142Nd within the protoplanetary disk and the radioactive decay of the short-lived samarium-146 isotope (146Sm). The apparent offset in 142Nd abundance found previously between chondritic meteorites and Earth has been interpreted either as a possible consequence of nucleosynthetic variations within the protoplanetary disk or as a function of the differentiation of Earth very early in its history. Here we report high-precision Sm and Nd stable and radiogenic isotopic compositions of four calcium-aluminium-rich refractory inclusions (CAIs) from three CV-type carbonaceous chondrites, and of three whole-rock samples of unequilibrated enstatite chondrites. The CAIs, which are the first solids formed by condensation from the nebular gas, provide the best constraints for the isotopic evolution of the early Solar System. Using the mineral isochron method for individual CAIs, we find that CAIs without isotopic anomalies in Nd compared to the terrestrial composition share a 146Sm/144Sm-142Nd/144Nd isotopic evolution with Earth. The average 142Nd/144Nd composition for pristine enstatite chondrites that we calculate coincides with that of the accessible silicate layers of Earth. This relationship between CAIs, enstatite chondrites and Earth can only be a result of Earth having inherited the same initial abundance of 142Nd and chondritic proportions of Sm and Nd. Consequently, 142Nd isotopic heterogeneities found in other CAIs and among chondrite groups may arise from extrasolar grains that were present in the disk and incorporated in different proportions into these planetary objects. Our finding supports a chondritic Sm/Nd ratio for the bulk silicate Earth and, as a consequence, chondritic abundances for other refractory elements. It also removes the need for a hidden reservoir or for collisional erosion scenarios to explain the 142Nd/144Nd composition of Earth. © 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
Roche O.,CNRS Magmas and Volcanoes Laboratory
Bulletin of Volcanology | Year: 2012
The depositional processes and gas pore pressure in pyroclastic flows are investigated through scaled experiments on transient, initially fluidized granular flows. The flow structure consists of a sliding head whose basal velocity decreases backwards from the front velocity (U f) until onset of deposition occurs, which marks transition to the flow body where the basal deposit grows continuously. The flows propagate in a fluid-inertial regime despite formation of the deposit. Their head generates underpressure proportional to U f 2 whereas their body generates overpressure whose values suggest that pore pressure diffuses during emplacement. Complementary experiments on defluidizing static columns prove that the concept of pore pressure diffusion is relevant for gas-particle mixtures and allow characterization of the diffusion timescale (t d) as a function of the material properties. Initial material expansion increases the diffusion time compared with the nonexpanded state, suggesting that pore pressure is self-generated during compaction. Application to pyroclastic flows gives minimum diffusion timescales of seconds to tens of minutes, depending principally on the flow height and permeability. This study also helps to reconcile the concepts of en masse and progressive deposition of pyroclastic flow units or discrete pulses. Onset of deposition, whose causes deserve further investigation, is the most critical parameter for determining the structure of the deposits. Even if sedimentation is fundamentally continuous, it is proposed that late onset of deposition and rapid aggradation in relatively thin flows can generate deposits that are almost snapshots of the flow structure. In this context, deposition can be considered as occurring en masse, though not strictly instantaneously. © 2012 Springer-Verlag.
Ferot A.,CNRS Magmas and Volcanoes Laboratory |
Bolfan-Casanova N.,CNRS Magmas and Volcanoes Laboratory
Earth and Planetary Science Letters | Year: 2012
Experiments were performed under water-saturated conditions in the MFSH (MgO-FeO-SiO 2-H 2O) and MFASH (MgO-FeO-Al 2O 3-SiO 2-H 2O) systems at 2.5, 5, 7.5 and 9GPa, at temperatures from 1175 to 1400°C and H 2O initial abundance of 0.5-5wt%. One experiment was performed at 13.5GPa at a temperature of 1400°C in the MFSH system. Water contents were analyzed by Fourier transform infrared spectroscopy. Results show that Al contents in olivine and pyroxene in equilibrium with an aluminous phase decrease significantly with increasing pressure and decreasing temperature. The incorporation of Al enhances water incorporation in olivine and pyroxene, but only at pressures of 2.5 and 5GPa. At 7.5GPa (i.e. 225km depth) the pyroxene is monoclinic, indicating that in a hydrous mantle the orthoenstatite to clinoenstatite phase transition occurs at shallower depths than previously thought, which is more consistent with the Lehmann discontinuity than with the X discontinuity. The partitioning of water between pyroxene and olivine in the MFASH system decreases from a value of 2 at 2.5GPa (80km depth) to 0.9 at 9GPa (270km depth). At 13.5GPa and 1400°C, the water content of olivine is 1700±300ppmwt H 2O. The water partition coefficient between coexisting wadsleyite and olivine equals 4.7±0.7. We conclude that the water storage capacity of the upper mantle just above the 410km discontinuity is of 1500±300ppmwt H 2O. If we assume that the Low Velocity Layer observed near 350km is due to mantle melting, we can constrain the water content of the mantle at that depth to be ~850±150ppmwt H 2O. This new value is four times higher than previous estimates for the mantle source of Mid Oceanic Ridge Basalts.Finally, comparison of the depth ranges of the L and X seismic discontinuities and the water storage capacity of the upper mantle suggests that the L-discontinuity (180-240. km) is concomitant with a kink in the water storage due to the orthorhombic to monoclinic phase transition in enstatite, while the X-discontinuity (240-340. km) coincides with a kink in the water storage capacity due to dehydration of garnet. © 2012 Elsevier B.V.
Paquette J.-L.,CNRS Magmas and Volcanoes Laboratory |
Le Pennec J.-L.,CNRS Magmas and Volcanoes Laboratory
Geology | Year: 2012
Dating the oldest terrestrial rocks and minerals allows constraining the age and formation conditions of the ancient basement of our present-day continents. The oldest terranes, with ages ranging from 3.8 Ga to 4.0 Ga, have been identified in Canada, while Australian zircon crystals dated at 4.4 Ga represent the oldest dated minerals on Earth. The exceptional resistance of zircon grains to repeated geological cycles has allowed dating 3.3-3.9 Ga crystals in several Archean cratons worldwide. Identifying new, scarce but essential witnesses to this primitive Earth is still a challenging goal. Ignimbrites are silicic pyroclastic deposits produced by large-scale explosive eruptions. During magma residence and ascent, basement enclaves may be scavenged and mixed with the rhyolitic magma. Here we show that an ignimbrite eruption from Central Anatolia (Turkey) dated at 5.4 Ma has sampled Proterozoic and Archean zircons ranging from 2.3 Ga to 3.8 Ga in age. This is the first documented occurrence of Early Archean zircon crystals sampled by a recent volcanic event. These Central Anatolian zircon grains are among the oldest discovered on Earth and imply the presence at depth of remnants of an Early Archean basement that was hitherto unknown in Eurasia. © 2012 Geological Society of America.
Merle O.,CNRS Magmas and Volcanoes Laboratory
Tectonophysics | Year: 2011
A continental rift is conventionally described as a thinning process of the lithosphere ultimately leading to the rupture of the continent and the formation of a mid-oceanic ridge. Rifting is the initial and fundamental process by which the separation of two continents into two tectonic plates takes place. Previous classifications, particularly the one into "active" and "passive" rifting, are briefly presented, together with their limitations. The new classification presented here links continental rifts to the major plate tectonics structures which are at the origin of their formation. Thus, four types of rift can be defined: the subduction-related rift, the plume-related rift, the mountain-related rift and the transform-related rift. A number of examples representative of these four types of rift are then presented. This classification is shown to lie at the heart of our understanding of the major plate tectonic processes at work on Earth. © 2011 Elsevier B.V.
Kelfoun K.,CNRS Magmas and Volcanoes Laboratory
Journal of Geophysical Research: Solid Earth | Year: 2011
The rheology of volcanic rock avalanches and dense pyroclastic flows is complex, and it is difficult at present to constrain the physics of their processes. The problem lies in defining the most suitable parameters for simulating the behavior of these natural flows. Existing models are often based on the Coulomb rheology, sometimes with a velocity-dependent stress (e.g., Voellmy), but other laws have also been used. Here I explore the characteristics of flows, and their deposits, obtained on simplified topographies by varying source conditions and rheology. The Coulomb rheology, irrespective of whether there is a velocity-dependent stress, forms cone-shaped deposits that do not resemble those of natural long-runout events. A purely viscous or a purely turbulent flow can achieve realistic velocities and thicknesses but cannot form a deposit on slopes. The plastic rheology, with (e.g., Bingham) or without a velocity-dependent stress, is more suitable for the simulation of dense pyroclastic flows and long-runout volcanic avalanches. With this rheology, numerical flows form by pulses, which are often observed during natural flow emplacement. The flows exhibit realistic velocities and deposits of realistic thicknesses. The plastic rheology is also able to generate the frontal lobes and lateral levées which are commonly observed in the field. With the plastic rheology, levée formation occurs at the flow front due to a divergence of the driving stresses at the edges. Once formed, the levées then channel the remaining flow mass. The results should help future modelers of volcanic flows with their choice of which mechanical law corresponds best to the event they are studying. Copyright 2011 by the American Geophysical Union.
Druitt T.H.,CNRS Magmas and Volcanoes Laboratory
Bulletin of Volcanology | Year: 2014
The late-seventeenth century BC Minoan eruption of Santorini discharged 30-60 km3 of magma, and caldera collapse deepened and widened the existing 22 ka caldera. A study of juvenile, cognate, and accidental components in the eruption products provides new constraints on vent development during the five eruptive phases, and on the processes that initiated the eruption. The eruption began with subplinian (phase 0) and plinian (phase 1) phases from a vent on a NE-SW fault line that bisects the volcanic field. During phase 1, the magma fragmentation level dropped from the surface to the level of subvolcanic basement and magmatic intrusions. The fragmentation level shallowed again, and the vent migrated northwards (during phase 2) into the flooded 22 ka caldera. The eruption then became strongly phreatomagmatic and discharged low-temperature ignimbrite containing abundant fragments of post-22 ka, pre-Minoan intracaldera lavas (phase 3). Phase 4 discharged hot, fluidized pyroclastic flows from subaerial vents and constructed three main ignimbrite fans (northwestern, eastern, and southern) around the volcano. The first phase-4 flows were discharged from a vent, or vents, in the northern half of the volcanic field, and laid down lithic-block-rich ignimbrite and lag breccias across much of the NW fan. About a tenth of the lithic debris in these flows was subvolcanic basement. New subaerial vents then opened up, probably across much of the volcanic field, and finer-grained ignimbrite was discharged to form the E and S fans. If major caldera collapse took place during the eruption, it probably occurred during phase 4. Three juvenile components were discharged during the eruption-a volumetrically dominant rhyodacitic pumice and two andesitic components: microphenocryst-rich andesitic pumices and quenched andesitic enclaves. The microphenocryst-rich pumices form a textural, mineralogical, chemical, and thermal continuum with co-erupted hornblende diorite nodules, and together they are interpreted as the contents of a small, variably crystallized intrusion that was fragmented and discharged during the eruption, mostly during phases 0 and 1. The microphenocryst-rich pumices, hornblende diorite, andesitic enclaves, and fragments of pre-Minoan intracaldera andesitic lava together form a chemically distinct suite of Ba-rich, Zr-poor andesites that is unique in the products of Santorini since 530 ka. Once the Minoan magma reservoir was primed for eruption by recharge-generated pressurization, the rhyodacite moved upwards by exploiting the plane of weakness offered by the pre-existing andesite-diorite intrusion, dragging some of the crystal-rich contents of the intrusion with it. © 2014 Springer-Verlag Berlin Heidelberg.
Condamine P.,CNRS Magmas and Volcanoes Laboratory |
Medard E.,CNRS Magmas and Volcanoes Laboratory
Earth and Planetary Science Letters | Year: 2014
We have experimentally investigated the fluid-absent melting of a phlogopite peridotite at 1.0 GPa (1000-1300 °C) to understand the source of K2O- and SiO2-rich magmas that occur in continental, post-collisional and island arc settings. Using a new extraction technique specially developed for hydrous conditions combined with iterative sandwich experiments, we have determined the composition of low- to high-degree melts (Φ = 1.4 to 24.2 wt.%) of metasomatized lherzolite and harzburgite sources. Due to small amounts of adsorbed water in the starting material, amphibole crystallized at the lowest investigated temperatures. Amphibole breaks down at 1050-1075 °C, while phlogopite-breakdown occurs at 1150-1200 °C. This last temperature is higher than the previously determined in a mantle assemblage, due to the presence of stabilizing F and Ti. Phlogopite-lherzolite melts incongruently according to the continuous reaction: 0.49 phlogopite + 0.56 orthopyroxene + 0.47 clinopyroxene + 0.05 spinel = 0.58 olivine + 1.00 melt. In the phlogopite-harzburgite, the reaction is: 0.70 phlogopite + 1.24 orthopyroxene + 0.05 spinel = 0.99 olivine + 1.00 melt. The K2O content of water-undersaturated melts in equilibrium with residual phlogopite is buffered, depending on the source fertility: from ~3.9 wt.% in lherzolite to ~6.7 wt.% in harzburgite. Primary melts are silica-saturated and evolve from trachyte to basaltic andesite (63.5-52.1 wt.% SiO2) with increasing temperature. Calculations indicate that such silica-rich melts can readily be extracted from their mantle source, due to their low viscosity. Our results confirm that potassic, silica-rich magmas described worldwide in post-collisional settings are generated by melting of a metasomatized phlogopite-bearing mantle in the spinel stability field. © 2014 Elsevier B.V.
Ramsey M.S.,University of Pittsburgh |
Harris A.J.L.,CNRS Magmas and Volcanoes Laboratory
Journal of Volcanology and Geothermal Research | Year: 2013
Volcanological remote sensing spans numerous techniques, wavelength regions, data collection strategies, targets, and applications. Attempting to foresee and predict the growth vectors in this broad and rapidly developing field is therefore exceedingly difficult. However, we attempted to make such predictions at both the American Geophysical Union (AGU) meeting session entitled Volcanology 2010: How will the science and practice of volcanology change in the coming decade? held in December 2000 and the follow-up session 10. years later, Looking backward and forward: Volcanology in 2010 and 2020. In this summary paper, we assess how well we did with our predictions for specific facets of volcano remote sensing in 2000 the advances made over the most recent decade, and attempt a new look ahead to the next decade. In completing this review, we only consider the subset of the field focused on thermal infrared remote sensing of surface activity using ground-based and space-based technology and the subsequent research results. This review keeps to the original scope of both AGU presentations, and therefore does not address the entire field of volcanological remote sensing, which uses technologies in other wavelength regions (e.g., ultraviolet, radar, etc.) or the study of volcanic processes other than the those associated with surface (mostly effusive) activity. Therefore we do not consider remote sensing of ash/gas plumes, for example. In 2000, we had looked forward to a "golden age" in volcanological remote sensing, with a variety of new orbital missions both planned and recently launched. In addition, exciting field-based sensors such as hand-held thermal cameras were also becoming available and being quickly adopted by volcanologists for both monitoring and research applications. All of our predictions in 2000 came true, but at a pace far quicker than we predicted. Relative to the 2000-2010 timeframe, the coming decade will see far fewer new orbital instruments with direct applications to volcanology. However ground-based technologies and applications will continue to proliferate, and unforeseen technology promises many exciting possibilities that will advance volcano thermal monitoring and science far beyond what we can currently envision. © 2012 Elsevier B.V.