Guy Libourel

Homogeneous Distribution of 26Al in the Solar System from the Mg Isotopic Composition of Chondrules

Solar Chronometer The use of the short-lived radioactive isotope 26Al as a precise chronometer of early solar system processes relies on the assumption that it was uniformly distributed in the initial solar accretion disk. Villeneuve et al. (p. 985; see the Perspective by Davis) validate this assumption on the basis of high-precision isotopic analyses of primitive meteoritic materials. Furthermore, chondrules—constituents of the most common type of meteorites and among the first materials to have formed in the solar system—formed episodically over a period of more than one million years. High-precision isotopic analyses in chondrule minerals validate the use of 26Al as an early solar system chronometer. The timing of the formation of the first solids in the solar system remains poorly constrained. Micrometer-scale, high-precision magnesium (Mg) isotopic analyses demonstrate that Earth, refractory inclusions, and chondrules from primitive meteorites formed from a reservoir in which short-lived aluminum-26 (26Al) and Mg isotopes were homogeneously distributed at ±10%. This level of homogeneity validates the use of 26Al as a precise chronometer for early solar system events. High-precision chondrule 26Al isochrons show that several distinct chondrule melting events took place from ~1.2 million years (My) to ~4 My after the first solids condensed from the solar nebula, with peaks between ~1.5 and ~3 My, and that chondrule precursors formed as early as 0.87-0.16+0.19 My after.

Thermal fatigue as the origin of regolith on small asteroids

Space missions and thermal infrared observations have shown that small asteroids (kilometre-sized or smaller) are covered by a layer of centimetre-sized or smaller particles, which constitute the regolith. Regolith generation has traditionally been attributed to the fall back of impact ejecta and by the break-up of boulders by micrometeoroid impact. Laboratory experiments and impact models, however, show that crater ejecta velocities are typically greater than several tens of centimetres per second, which corresponds to the gravitational escape velocity of kilometre-sized asteroids. Therefore, impact debris cannot be the main source of regolith on small asteroids. Here we report that thermal fatigue, a mechanism of rock weathering and fragmentation with no subsequent ejection, is the dominant process governing regolith generation on small asteroids. We find that thermal fragmentation induced by the diurnal temperature variations breaks up rocks larger than a few centimetres more quickly than do micrometeoroid impacts. Because thermal fragmentation is independent of asteroid size, this process can also contribute to regolith production on larger asteroids. Production of fresh regolith originating in thermal fatigue fragmentation may be an important process for the rejuvenation of the surfaces of near-Earth asteroids, and may explain the observed lack of low-perihelion, carbonaceous, near-Earth asteroids.

The role of phosphorus in crystallisation processes of basalt: An experimental study

Abstract To evaluate the effects of phosphorus on differentiation of evolved basaltic magmas, a series of one-atmosphere experiments on a ferrobasaltic composition were carried out over a range of P 2 O 5 contents and at oxygen fugacities from 2 log 10 units below to 2 log 10 units above the fayalite-magnetite-quartz (FMQ) buffer curve. The experiments were performed isothermally, the investigated variables being the amount of added P 2 O 5 and the oxygen fugacity (f o 2 ). The results confirm the high solubility of phosphorus in basaltic magmas and show that, at fixed temperature, the progressive addition of phosphorus causes: 1. (1) the disappearance of olivine at reducing conditions, 2. (2) the disappearance of magnetite at oxidising conditions, 3. (3) the stabilisation of pigeonite throughout the studied range of f o 2 , 4. (4) an increase in the modal plagioclase/pyroxene ratio, and 5. (5) increased melt proportion and large changes in the composition of the coexisting melt, in particular the SiO 2 content. The destabilisation of magnetite with increasing P 2 O 5 content may be accounted for by the formation of Fe 3+ (PO 4 ) 3− complexes, rather than by a large change of the redox ratio of the melt; we suggest that the formation of Fe 3+ (PO 4 ) 3− complexes dominates that of P-O- M complexes (where M is a network-modifying cation). The effect of P on the modal proportions of plagioclase and pigeonite may help to explain the mineralogy of some anorthositic rocks and KREEP basalts, as well as the presence of pigeonite as an intercumulus phase in the Skaergaard intrusion (resulting from enrichment of the trapped liquid in phosphorus). These new results thus provide insights into the effects of phosphorus in lunar and terrestrial systems, as well as providing information regarding the structural role of phosphorus in silicate melts.

The extreme physical properties of the CoRoT-7b super-Earth

Photospheric stellar activity (i.e. dark spots or bright pl ages) might be an important source of noise and confusion in s tellar radialvelocity (RV) measurements. Radial-velocimetry planet se arch surveys as well as follow-up of photometric transit sur veys require a deeper understanding and characterization of the e ffects of stellar activities to di fferentiate them from planetary signals. We simulate dark spots on a rotating stellar photosphere. The variation s in the photometry, RV, and spectral line shapes are charact erized and analyzed according to the stellar inclination, the latitud e, and the number of spots. We show that the anti-correlation between RV and bisector span, known to be a signature of activity, requi s a good sampling to be resolved when there are several spot s on the photosphere. The Lomb-Scargle periodograms of the RV varia tions induced by activity present power at the rotational pe riod Prot of the star and its two first harmonics Prot/2 andProt/3. Three adjusted sinusoids fixed at the fundamental period a nd its two-first harmonics allow us to remove about 90% of the RV jitter amplit ude. We apply and validate our approach on four known active p lanethost stars: HD 189733, GJ 674, CoRoT-7, and ιHor. We succeed in fitting simultaneously activity and plane t ry signals on GJ674 and CoRoT-7. This simultaneous modeling of the activity and planetary parameters leads to slightly higher masses of CoR oT-7b and c of respectively, 5.7± 2.5 MEarth and 13.1± 4.1 MEarth. The larger uncertainties properly take into account the st ellar active jitter. We exclude short-period low-mass exoplanets around ιHor. For data with realistic time-sampling and white Gaussi an noise, we use simulations to show that our approach is e ffective in distinguishing reflex-motion due to a planetary co mpanion and stellar-activityinduced RV variations provided that 1) the planetary orbita l period is not close to that of the stellar rotation or one of i ts two first harmonics, 2) the semi-amplitude of the planet exceeds ∼30% of the semi-amplitude of the active signal, 3) the rotati nal period of the star is accurately known, and 4) the data cover more than o ne stellar rotational period.

Nitrogen solubility in basaltic melt. Part I. Effect of oxygen fugacity

Abstract The role of the oxygen fugacity on the incorporation of nitrogen in basaltic magmas has been investigated using one atmosphere high temperature equilibration of tholeiitic-like compositions under controlled nitrogen and oxygen partial pressures in the [C-N-O] system. Nitrogen was extracted with a CO2 laser under high vacuum and analyzed by static mass spectrometry. Over a redox range of 18 oxygen fugacity log units, this study shows that the incorporation of nitrogen in silicate melts follows two different behaviors. For log fO2 values between −0.7 and −10.7 (the latter corresponding to IW − 1.3), nitrogen dissolves as a N2 molecule into cavities of the silicate network (physical solubility). Nitrogen presents a constant solubility (Henry’s) coefficient of 2.21 ± 0.53 × 10−9 mol g−1 atm−1 at 1425°C, identical within uncertainties to the solubility of argon. Further decrease in the oxygen fugacity (log fO2 between −10.7 and −18 corresponding to the range from IW − 1.3 to IW − 8.3) results in a drastic increase of the solubility of nitrogen by up to 5 orders of magnitude as nitrogen becomes chemically bounded with atoms of the silicate melt network (chemical solubility). The present results strongly suggest that under reducing conditions nitrogen dissolves in silicate melts as N3− species rather than as CN− cyanide radicals. The nitrogen content of a tholeiitic magma equilibrated with N2 is computed from thermochemical processing of our data set as [N 2 ] (mol N 2 · g −1 ) = (2.21 ± 0.53) × 10 –9 · P N 2 +f O 2 –3/4 · (2.13 ± 0.11) × 10 –17 · P N 2 1/2 High nitrogen contents in primitive meteorites, especially in glass inclusions encapsulated in magnesian olivine of chondrites, are unlikely to result from nitrogen dissolution from the solar nebula gas, unless the pressure of the latter is underestimated by several orders of magnitude. Significant amounts of nitrogen, comparable to those estimated for the present-day mantle, could have been incorporated in the early Earth by dissolution in a magma ocean, under fO2 conditions relevant to those prevailing during metal segregation. The present results also imply that the N2/Ar ratio of tholeiitic basalts (e.g., MORBs) is not fractionated during magma generation and degassing, allowing to use argon as a geochemical proxy for nitrogen. It is probable that mantle nitrogen was degassed at rates and fluxes comparable to that of argon, as the oxygen fugacity of the mantle was unlikely to have been below IW from Archean to Present. Therefore, fractionation between nitrogen and argon in the Earth-atmosphere system is more probably the result of recycling rather than of mantle-derived magma degassing.

The effect of phosphorus on the iron redox ratio, viscosity, and density of an evolved ferro-basalt

Despite the abundant evidence for the enrichment of phosphorus during the petrogenesis of natural ferro-basalts, the effect of phosphorus on the physical properties of these melts is poorly understood. The effects of phosphorus on the viscosity, density and redox ratio of a ferro-basaltic melt have been determined experimentally. The viscosity measurements were obtained using the concentric cylinder method on a ferro-basaltic melt above its liquidus, at 1 atm, in equilibrium with air and with CO2. The density measurements were performed using the double Pt-bob Archimedean method at superliquidus conditions under 1 atm of air. The redox ratio was obtained by wet chemical analysis of samples collected during physical property measurements. Phosphorus pentoxide reduces ferric iron in ferro-basaltic melt. The reduction due to P2O5 is much larger than that for most other oxide components in basaltic melts. A coefficient for the reduction of ferric iron has been generated for inclusion in calculation schemes. The effect of P2O5 on the viscosity is shown to be complex. The initial reduction of ferric iron with the addition of P2O5 results in a relatively small change in viscosity, while further addition of P2O5 results in a strong increase. The addition of phosphorus to a ferro-basaltic melt also reduces the density. A partial molar volume of 64.5±0.7 cm3/mol for P2O5 in this melt has been obtained at 1300° C, yielding a volume of 12.9 cm3/mol per oxygen, consistent with a tetrahedral coordination for this high field strength cation. The effects of P2O5 on redox state, density and viscosity provide constraints on the structural role of phosphorus in these melts. The results suggest a complex interaction of phosphorus with the aluminosilicate network, and tetrahedral ferric iron. In light of the significant effects of phosphorus on the physical and chemical properties of ferro-basaltic liquids, and the extreme enrichments possible in these liquids in nature, the role of phosphorus in these melts should, in future, be considered more carefully.

Evolution of Oxygen Isotopic Composition in the Inner Solar Nebula

Changes in the chemical and isotopic composition of the solar nebula with time are reflected in the properties of different constituents that are preserved in chondritic meteorites. CR-group carbonaceous chondrites are among the most primitive of all chondrite types and must have preserved solar nebula records largely unchanged. We have analyzed the oxygen and magnesium isotopes in a range of the CR constituents of different formation temperatures and ages, including refractory inclusions and chondrules of various types. The results provide new constraints on the time variation of the oxygen isotopic composition of the inner (<5 AU) solar nebula—the region where refractory inclusions and chondrules most likely formed. A chronology based on the decay of short-lived 26 Al (t1=2 � 0:73 Myr) indicates that the inner solar nebula gas was 16 O-rich when refractory inclusions formed, but less than 0.8 Myr later, gas in the inner solar nebula became 16 O-poor, and this state persisted at least until CR chondrules formed � 1‐2 Myr later. We suggest that the inner solar nebula became 16 O-poor because meter-sized icy bodies, which were enriched in 17 Oa nd 18 O as a result of isotopic self-shielding during the ultraviolet photodissociation of CO in the protosolar molecular cloud or protoplanetary disk, agglomerated outside the snow line, drifted rapidly toward the Sun, and evaporated at the snow line. This led to significant enrichment in 16 O-depleted water,whichthenspreadthroughtheinnersolarsystem.Astronomicalstudiesofthespatialandtemporalvariations of water abundance in protoplanetary disks may clarify these processes. Subject headingg meteors, meteoroids — nuclear reactions, nucleosynthesis, abundances — planetary systems: formation — solar system: formation

Using stained glass windows to understand the durability of toxic waste matrices

Abstract Using stained glasses sampled from French and German cathedrals, and from different archaeological sites, this work presents an estimation of the effect of weathering conditions and glass composition on glass dissolution. Due to accurate dating, we also show that stained glass windows allow the determination of the average dissolution kinetics of many toxic elements contained in the glass (including transition and heavy metals, actinides, lanthanides). Therefore, a significant advantage of using stained glass windows, over other natural glasses, is that they permit the direct study of non-stoichiometric elemental dissolution rates, under natural conditions of alteration, averaged over more than a thousand years, and hence to constrain the long-term behaviour of vitrified waste matrices.

BORON PARTITIONING IN THE UPPER MANTLE : AN EXPERIMENTAL AND ION PROBE STUDY

Piston cylinder experiments were performed on two natural spinel lherzolites, from Lherz (French Pyrenees) and Kilbourne Hole (USA), in order to determine the distribution coefficients of boron between minerals and melt for upper mantle conditions. Additionally, a MORB glass and its phenocrysts were taken as a low pressure analogue. Boron concentration measurements were made by ion probe, the only technique suitable for in situ (∼ 10 μm) boron concentration measurements in the range 0.1–1 ppm. The two starting peridotites samples have low bulk boron concentrations of 0.96 and 0.81 ppm. Boron distribution coefficients were determined from (1) direct measurement of melt and crystals in contact, and (2) calculations from profiles of several tens of measurements made using 10μm diameter spots containing variable proportions of crystals and melt. Distribution coefficients between minerals and melt decrease in the following order: Dcpx-melt (0.117) >Dsp-melt (0.08) ⪢ Dol-melt (0.034) > = Dopx-melt (0.027). The limited ranges of composition, pressure, and temperature investigated mimic natural conditions for basalt genesis but sharply limit the present data for determining variations of partition coefficients with P, T, and X. However, boron behavior seems to follow roughly that of aluminum, since the Al-richest minerals are the ones with the highest boron distribution coefficients. The Dol-melt values increase with pressure, which is similar to what has been observed for the distribution coefficient of aluminum between olivine and melt. Boron is, therefore, strongly incompatible during partial melting in the mantle. Using these distribution coefficients and assuming that MORB have been produced by 5–20% partial melting, the depleted mantle source of MORB is estimated to contain ∼0.05–0.3 ppm B. These values correspond to the lower limit for the mean boron content of chondrites (0.55 ± 0.3 ppm) but are in accordance with a simple budget of boron between mantle, crust, and seawater. In fact, a simple mass balance calculation, where all the boron hosted in the crust and in seawater is presumed to have been extracted from an upper mantle of chondritic original boron content, yields a boron content for the depleted upper mantle of ∼0.3 ppm at present. Because in situ boron concentration measurements under the ppm level at the μm scale are now possible with the ion probe, and because of its strong partitioning in melts and possible partitioning in fluids, boron is a potentially very powerful geochemical tracer for the study of mantle processes such as basalt genesis or metasomatism.

Factors controlling compositions of cosmic spinels: application to atmospheric entry conditions of meteoritic materials

Abstract During their deceleration through the Earths atmosphere, meteoritic materials, i.e., interplanetary dust particles, micrometeorites and meteorites, experience thermal shocks which may alter their pristine mineralogy, texture or chemical characteristics. Among these changes, one of the most ubiquitous is the formation of spinels resulting from partial melting and subsequent crystallization of the meteoritic material. These “cosmic spinels” differ from terrestrial spinels by their high Ni and Fe3+ contents and show large variations in composition. In order to better understand the factors controlling their chemistry, pulse-heating experiments simulating atmospheric entry of extraterrestrial objects were carried out using Orgueil samples as proxies of meteoritic material. Covering a large range of experimental conditions (temperature 500°C We also show that, due to their fast crystallization kinetics, cosmic spinels can record through their composition, i.e., Al2O3 contents and FeO/Fe2O3 ratio, the diverse conditions of the atmosphere crossed by the extraterrestrial object during its fall towards the Earths surface. Chemistry of cosmic spinels is thus a powerful tool for constraining the entry conditions in the Earths atmosphere of any extraterrestrial object, including altitude of deceleration, entry angle and incident velocity. These in turn, may provide valuable information on the origin of the extraterrestrial material.