Biogenic Iron Preserves Structures during Fossilization: A Hypothesis

International audienc

In situ Viscometry of Primitive Lunar Magmas at High Pressure and High Temperature

Understanding the dynamics of the magmatic evolution of the interior of the Moon requires accurate knowledge of the viscosity (η) of lunar magmas at high pressure (P) and high temperature (T) conditions. Although the viscosities of terrestrial magmas are relatively well-documented, and their relation to magma composition well-studied, the viscosities of lunar titano-silicate melts are not well-known. Here, we present an experimentally measured viscosity dataset for three end member compositions, characterized by a wide range of titanium contents, at lunar-relevant pressure-temperature range of ∼1.1–2.4 GPa and 1830–2090 K. In situ viscometry using the falling sphere technique shows that the viscosity of lunar melts varies between ∼0.13 and 0.87 Pa-s depending on temperature, pressure and composition. Viscosity decreases with increasing temperature with activation energies for viscous flow of Ea = 201 kJ/mol and Ea = 106 kJ/mol for low-titanium (Ti) and high-Ti melts, respectively. Pressure is found to mildly increase the viscosity of these intermediate polymerized melts by a factor of ∼1.5 between 1.1 and 2.4 GPa. Viscosities of low-Ti and high-Ti magmas at their respective melting temperatures are very close. However at identical P-T conditions (∼1.3 GPa, ∼1840 K) low-Ti magmas are about a factor of three more viscous than high-Ti magmas, reflecting structural effects of Si and Ti on melt viscosity. Measured viscosities differ significantly from empirical models based on measurements of the viscosity of terrestrial basalts, with largest deviations observed for the most Ti-rich and Si-poor composition. Viscosity coefficients for these primitive lunar melts are found to be lower than those of common terrestrial basalts, giving them a high mobility throughout the lunar mantle and onto the surface of the Moon despite their Fe and Ti-rich compositions

Experimental investigation of the stability of Fe-rich carbonates in the lower mantle

International audienceThe fate of carbonates in the Earth's mantle plays a key role in the geodynamical carbon cycle. Although iron is a major component of the Earth's lower mantle, the stability of Fe-bearing carbonates has rarely been studied. Here we present experimental results on the stability of Fe-rich carbonates at pressures ranging from 40 to 105 GPa and temperatures of 1450-3600 K, corresponding to depths within the Earth's lower mantle of about 1000-2400 km. Samples of iron oxides and iron-magnesium oxides were loaded into CO2 gas and laser heated in a diamond-anvil cell. The nature of crystalline run products was determined in situ by X-ray diffraction, and the recovered samples were studied by analytical transmission electron microscopy and scanning transmission X-ray microscopy. We show that Fe-(II) is systematically involved in redox reactions with CO2 yielding to Fe-(III)-bearing phases and diamonds. We also report a new Fe-(III)-bearing high-pressure phase resulting from the transformation of FeCO3 at pressures exceeding 40 GPa. The presence of both diamonds and an oxidized C-bearing phase suggests that oxidized and reduced forms of carbon might coexist in the deep mantle. Finally, the observed reactions potentially provide a new mechanism for diamond formation at great depth

The Weaklaw Vent, SE Scotland:Metasomatism of eruptive products by carbo-hydro-fluids of probable mantle origin

This is the author accepted manuscript. The final version is available from CUP via the DOI in this record The Weaklaw vent in SE Scotland (East Lothian coast), inferred to be Namurian, produced lava spatter and volcanic bombs. The latter commonly contained ultramafic xenoliths. All were metasomatised by carbonic fluids rich in incompatible elements. The lavas and xenoliths are inferred to have been basanites and lherzolites prior to metasomatism. The abundance and size of (carbonated) peridotite xenoliths at Weaklaw denotes unusual rapidity of magma ascent and high-energy eruption making Weaklaw exceptional in the British Isles. The lavas and xenoliths were altered subsequently by low-temperature (<200°C) carbo-hydrous fluids to carbonate, clay and quartz assemblages. A small irregular tuffisite 'dyke' that transects the ejecta is also composed dominantly of carbonates and clays. The peridotitic xenoliths are typically foliated, interpreted as originating as pre-entrainment mantle shear-planes. Analyses of the relic spinels shows them to be compositionally similar to spinels in local unaltered lherzolites from near-by basanitic occurrences. Chromium showed neither significant loss nor gain but was concentrated in a di-octahedral smectite allied to volkonskoite. It is in the complex association of smectite with other clays, chlorite and possibly fuchsite that the diverse incompatible elements are concentrated. We conclude that late Palaeozoic trans-tensional fault movement caused mantle shearing. Rapid ascent of basanite magma entrained large quantities of sheared lithospheric mantle. This was followed by ascent of an aggressive carbonate-/ hydroxyl-rich fluid causing pervasive metasomatism. The vent is unique in several ways: in its remarkable clay mineralogy and in displaying such high Cr-clays in a continental intra-plate setting; in being more productive in terms of its 'cargo' of peridotite xenoliths; in presenting an essentially un-eroded sequence of Namurian extrusives; and, not least, for giving evidence for post-eruptive, surface release of small-melt, deep-source fluids

Kinetics of high-pressure mineral phase transformations using in situ time-resolved X-ray diffraction in the Paris-Edinburgh cell: a practical guide for data acquisition and treatment

Abstract Synchrotron X-ray diffraction (XRD) is a powerful technique to study in situ and in real-time the structural and kinetic processes of pressure-induced phase transformations. This paper presents the experimental set-up developed at beamline ID27 of the ESRF to perform time-resolved angle dispersive XRD in the Paris-Edinburgh cell. It provides a practical guide for the acquisition of isobaric-isothermal kinetic data and the construction of transformation-time plots. The interpretation of experimental data in terms of reaction mechanisms and transformation rates is supported by an overview of the kinetic theory of solid-solid transformations, with each step of data processing illustrated by experimental results of relevance to the geosciences. Reaction kinetics may be affected by several factors such as the sample microstructure, impurities or differential stress. Further high-pressure kinetic studies should investigate the influence of such processes, in order to acquire kinetic information more akin to natural or technological processes

Evolution petrologique des lithospheres en subduction: approche experimentale in situ des transformations mineralogiques et de leurs cinetiques

The evolution of a subduction zone is intimately linked with the petrological reactions in the subducting plate and their kinetics. Several examples showing this relation have been experimentally investigated using HP-HT devices (large volume press, diamond anvil cell) and synchrotron in situ X-ray diffraction. (i) The transformation rates of coesite into its low-pressure polymorph, quartz, have been determined. These kinetic data are used to discuss the preservation of coesite during its ascent to the Earth's surface, and the tectonic processes related to the exhumation of ultra-high pressure rocks. The use of the percentage of retrogression of natural coesite inclusions to constrain P-T-t paths is examined. (ii) The breakdown of antigorite under low H2O activity conditions results in a fluid discharge rate of the order of 3.10-6 to 3.10-8 m3fluid.m-3rock.s-1. This is faster than the viscous relaxation of serpentinites, and could lead to brittle failure or weakening of pre-existing faults. The dehydration of antigorite could thus provide an explanation for the seismicity in the lower plane of double seismic zones. (iii) The petrological assemblage of a mid ocean ridge basalt (MORB) under lower mantle conditions consists mostly of Mg-rich perovskite, Ca-rich perovskite and stishovite. Between 800 and 1150 km, two Al-rich phases occur, the Ca-ferrite type and the new aluminum-rich phase (NAL), and represent 20 wt% of the assemblage. At 1200 km approximately, the NAL phase disappears whereas all other phases are stable up to 1400 km depth. From 800 to 1400 km, the density of a thermally equilibrated oceanic crust is higher than that of the surrounding mantle. Moreover, the NAL disappearance triggers a 1% density rise, increasing the dragging effect of the oceanic crust in the uppermost lower mantle. This petrological change might be related to the seismic heterogeneities detected at ~1200 km depth beneath Pacific subduction zones. (iv) Finally, investigations on the chemical analysis of experimental samples, at a sub-micron scale, using a nano-SIMS ion probe are presented.L'évolution d'une zone de subduction est reliée aux transformations pétrologiques de la plaque plongeante et à leurs cinétiques. Plusieurs exemples illustrant cette relation ont été étudiés expérimentalement, à l'aide des techniques de HP-HT (presse large volume, cellule à enclumes de diamant chauffage laser) et de la diffraction de rayons X in situ source synchrotron. (i) La vitesse de transformation de la coésite vers son polymorphe de basse pression, le quartz, a été déterminée. Cette cinétique de rétromorphose permet de discuter les modalités de préservation de la coésite lors de son retour vers la surface, et par là les processus tectoniques à l'origine de l'exhumation des roches de ultra-haute pression. L'utilisation du taux de rétromorphose de coésites naturelles pour la modélisation des chemins P-T-t d'exhumation est discutée. (ii) La déstabilisation de l'antigorite (serpentine), dans des conditions de faible activité d'H2O, libère des fluides à une vitesse de 10-6 à 10-8 m3fluide.m-3roche.s-1. Ces taux de production de fluides seraient susceptibles d'occasionner une augmentation de la pression de fluides et une hydrofracture de la matrice rocheuse. La déshydratation de l'antigorite pourrait ainsi expliquer la séismicité du plan inférieur des zones à doubles plans de Bénioff. (iii) L'assemblage minéralogique d'un basalte de ride médio-océanique (MORB) dans le manteau inférieur est constitué majoritairement de Mg-pérovskite, Ca-pérovskite et stishovite. De 800 à 1150 km de profondeur, deux phases alumineuses sont présentes, l'une de structure calcium ferrite et l'autre nommée "new aluminum phase" (NAL), et représentent 20% pds de l'assemblage. A ~1200 km, la phase NAL disparaît alors que toutes les autres phases restent stables jusqu'à 1400 km au moins. De 800 à 1400 km, la densité de la croûte océanique à l'équilibre thermique est plus élevée que celle du manteau environnant. En outre, la disparition de la NAL conduit à un saut de densité de +1% qui pourrait être responsable de réflecteurs sismiques profonds observés dans les zones de subduction péri-Pacifique. (iv) Enfin, des investigations sur l'analyse chimique à l'échelle sub-micronique d'échantillons de pétrologie expérimentale par sonde ionique nanoSIMS sont présentées

Évolution pétrologique des lithosphères en subduction (approche expérimentale in situ des transformations minéralogiques et de leurs cinétiques)

L évolution d une zone de subduction est reliée aux transformations pétrologiques de la plaque plongeante et à leurs cinétiques. Plusieurs exemples illustrant cette relation ont été étudiés expérimentalement, à l aide des techniques de haute-pression haute température et de la diffraction X source synchrotron. (i) La vitesse de transformation de la coésite en quartz a été déterminée. Cette cinétique de rétromorphose permet de discuter l exhumation des roches de ultra-haute pression. (ii) La déstabilisation de l antigorite (serpentine), dans des conditions de faible activité d H2O, libère des fluides à une vitesse de 3.10-6 à 3.10-8 m3fluide.m3roche.s-1. Ces taux de production de fluides pourraient occasionner une hydrofracture de la matrice rocheuse, expliquant la séismicité du plan inférieur des zones à doubles plans de Bénioff. (iii) L assemblage minéralogique d un basalte de ride médio-océanique (MORB) dans le manteau inférieur est constitué majoritairement de Mg-pérovskite, Ca-pérovskite et stishovite. De 800 à 1150 km de profondeur, deux phases alumineuses sont présentes, la calcium ferrite et la new aluminum phase (NAL), et représentent 20% pds de l assemblage. A 1150 km, la phase NAL disparaît alors que les autres phases restent stables au moins jusqu à 1400 km. De 800 à 1400 km, la densité de la croûte océanique à l équilibre thermique est plus élevée que celle du manteau environnant. La disparition de la NAL conduit à un saut de densité de +1% qui pourrait expliquer les réflecteurs profonds observés dans les subductions péri-Pacifiques. (iv) Enfin, des investigations sur l analyse chimique d échantillons de pétrologie expérimentale par sonde ionique nano-Sims sont présentéesLYON1-BU.Sciences (692662101) / SudocSudocFranceF

Kinetics of the coesite-quartz transition: Application to the exhumation of ultrahigh-pressure rocks

International audienceThe kinetics of the quartz-coesite phase transition has been studied in situ by X-ray diffraction in the 2.1-3.2 GPa, 5001010 C pressure-temperature range. Analysis of the data within Cahn's model of nucleation and growth at grain boundaries reveals that the prograde and retrograde reactions have different kinetics. The quartz --> coesite transformation is one order of magnitude faster than coesite --> quartz. Both reactions are characterized by high nucleation rates, so that the overall reaction kinetics is controlled by crystal growth processes. For the coesite --> quartz transformation, growth rates are extrapolated using Turnbull's equation with an activation energy for the transition of 163 +/- 23 kF/mol. This kinetic law is combined with an 'inclusion in a host' elastic model to study the contribution of kinetics in coesite preservation. This numerical modelling shows that above 400degreesC retrograde transformation of coesite to quartz is mainly controlled by the 'pressure vessel' effect of the host phase, whereas reaction kinetics is the controlling factor at lower temperatures. The influence of the shape of the P-T path and the exhumation rate upon the retrogression of coesite to quartz are investigated to use the percentage of unretrogressed coesite inclusions to constrain P-T-t paths

Kinetics of the coesite-quartz transition: Application to the exhumation of ultrahigh-pressure rocks

International audienceThe kinetics of the quartz-coesite phase transition has been studied in situ by X-ray diffraction in the 2.1-3.2 GPa, 5001010 C pressure-temperature range. Analysis of the data within Cahn's model of nucleation and growth at grain boundaries reveals that the prograde and retrograde reactions have different kinetics. The quartz --> coesite transformation is one order of magnitude faster than coesite --> quartz. Both reactions are characterized by high nucleation rates, so that the overall reaction kinetics is controlled by crystal growth processes. For the coesite --> quartz transformation, growth rates are extrapolated using Turnbull's equation with an activation energy for the transition of 163 +/- 23 kF/mol. This kinetic law is combined with an 'inclusion in a host' elastic model to study the contribution of kinetics in coesite preservation. This numerical modelling shows that above 400degreesC retrograde transformation of coesite to quartz is mainly controlled by the 'pressure vessel' effect of the host phase, whereas reaction kinetics is the controlling factor at lower temperatures. The influence of the shape of the P-T path and the exhumation rate upon the retrogression of coesite to quartz are investigated to use the percentage of unretrogressed coesite inclusions to constrain P-T-t paths

Orbital control on exceptional fossil preservation

International audienceExceptional fossil preservation is defined by the preservation of soft to lightly sclerotized organic tissues. The two most abundant types of soft-tissue preservation are carbonaceous compressions and replicates in authigenic minerals. In the geological record, exceptionally preserved soft fossils are rare and generally limited to only a few stratigraphic intervals. In the Fezouata Shale (Lower Ordovician, southern Morocco), we found that deposits yielding pyritized soft tissues contain iron-rich silicate minerals. These minerals played a crucial role in inhibiting the decay of dead individuals and are comparable to those found in formations yielding carbonaceous soft parts around the world. Furthermore, we found that iron-rich minerals show a cyclic pattern of occurrence (of ~100 k.y. periodicity) implicating a short-period eccentricity control on iron availability through the general oceanic and atmospheric circulations. Our results identify, for the first time, an external climate forcing on exceptional preservation and show that orbital forcing may be a level-selective parameter responsible for the discontinuous occurrence of horizons preserving soft parts around the world