Experimental petrology constraints on the recycling of mafic cumulate: a focus on Cr-spinel from the Rum Eastern Layered Intrusion,Scotland

The ¹⁷⁶Lu–¹⁷⁶Hf and ¹⁴⁷Sm–¹⁴³Nd decay systems are routinely used to determine garnet (Grt)–whole-rock (WR) ages; however, the ¹⁷⁶Lu–¹⁷⁶Hf age of garnet is typically older than the ¹⁴⁷Sm–¹⁴³Nd age determined from the same aliquots. Here we present experimental data for Lu³⁺ and Hf⁴⁺ diffusion in garnet as functions of temperature, pressure and oxygen fugacity and show that the diffusivity of Hf⁴⁺ in almandine/spessartine garnet is significantly slower than that of Lu³⁺. The diffusive closure temperature (T _C) of Hf⁴⁺ is significantly higher than that of Nd³⁺, and although this property is partly responsible for the observed ¹⁷⁶Lu–¹⁷⁶Hf and ¹⁴⁷Sm–¹⁴³Nd Grt–WR age discrepancies, the difference between the T _C-s of Lu³⁺ and Hf⁴⁺ could lead to apparent Grt–WR ¹⁷⁶Lu–¹⁷⁶Hf ages that are skewed from the age of Hf⁴⁺ closure in garnet. In addition, the slow diffusivity of Hf⁴⁺ indicates that the bulk of metamorphic garnets retain a substantial fraction of prograde radiogenic ¹⁷⁶Hf throughout peak metamorphic conditions, a phenomenon that further complicates the interpretation of ¹⁷⁶Lu–¹⁷⁶Hf garnet ages and invalidates the use of analytical T _C expressions. We argue that the diffusion of trivalent rare earth elements in garnet becomes much faster when their concentration level falls below a few hundred ppm, as in the experiments of Tirone et al. (Geochim Cosmochim Acta 69: 2385–2398, 2005), and further argue that this low-concentration mechanism is appropriate for modeling the susceptibility of ¹⁴⁷Sm–¹⁴³Nd garnet ages to diffusive resetting.     

Unveiling the atmospheres of giant exoplanets with an EChO-class mission

More than a thousand exoplanets have been discovered over the last decade. Perhaps more excitingly, probing their atmospheres has become possible. With current data we have glimpsed the diversity of exoplanet atmospheres that will be revealed over the coming decade. However, numerous questions concerning their chemical composition, thermal structure, and atmospheric dynamics remain to be answered. More observations of higher quality are needed. In the next years, the selection of a space-based mission dedicated to the spectroscopic characterization of exoplanets would revolutionize our understanding of the physics of planetary atmospheres. Such a mission was proposed to the ESA cosmic vision program in 2014. Our paper is therefore based on the planned capabilities of the Exoplanet Characterization Observatory (EChO), but it should equally apply to any future mission with similar characteristics. With its large spectral coverage (0.4 ? 16 μm), high spectral resolution (λ/Δλ > 300 below 5 μm and λ/Δλ > 30 above 5 μm) and 1.5m mirror, a future mission such as EChO will provide spectrally resolved transit lightcurves, secondary eclipses lightcurves, and full phase curves of numerous exoplanets with an unprecedented signal-to-noise ratio. In this paper, we review some of today’s main scientific questions about gas giant exoplanets atmospheres, for which a future mission such as EChO will bring a decisive contribution.     

Modulation of soil moisture–precipitation interactions over France by large scale circulation

How soil moisture affects precipitation is an important question—with far reaching consequences, from weather prediction to centennial climate change—, albeit a poorly understood one. In this paper, an analysis of soil moisture–precipitation interactions over France based on observations is presented. A first objective of this paper is to investigate how large scale circulation modulates soil moisture–precipitation interactions, thanks to a weather regime approach. A second objective is to study the influence of soil moisture not only on precipitation but also on the difference between precipitation and evapotranspiration. Indeed, to have a total positive soil moisture–precipitation feedback, the potential decrease in precipitation associated with drier soils should be larger than the decrease in evapotranspiration that drier soils may also cause. A potential limited impact of soil moisture on precipitation is found for some weather regimes, but its sign depends on large scale circulation. Indeed, antecedent dry soil conditions tend to lead to smaller precipitation for the negative phase of the North Atlantic Oscillation (NAO) regime but to larger precipitation for the Atlantic Low regime. This differential response of precipitation to soil moisture anomalies depending on large scale circulation is traced back to different responses of atmospheric stability. For all circulation regimes, dry soils tend to increase the lifted condensation level, which is unfavorable to precipitation. But for the negative phase of the NAO, low soil moisture tends to lead to an increase of atmospheric stability while it tends to lead to a decrease of stability for Atlantic Low. Even if the impact of soil moisture anomalies varies depending on large scale circulation (it is larger for Atlantic low and the positive phase of the NAO), dry soils always lead to a decrease in evapotranspiration. As the absolute effect of antecedent soil moisture on evapotranspiration is always much larger than its effects on precipitation, for all circulation regimes dry soil anomalies subsequently lead to positive precipitation minus evapotranspiration anomalies i.e. the total soil moisture feedback is found to be negative. This negative feedback is stronger for the Atlantic Low and the positive phase of the NAO regimes.     

Evolution of the Parisian urban climate under a global changing climate

The evolution of the Parisian urban climate under a changing climate is analyzed from long-term offline numerical integrations including a specific urban parameterization. This system is forced by meteorological conditions based on present-climate reanalyses (1970–2007), and climate projections (2071–2099) provided by global climate model simulations following two emission scenarios (A1B and A2). This study aims at quantifying the impact of climate change on air temperature within the city and in the surroundings. A systematic increase of 2-meter air temperature is found. In average according to the two scenarios, it reaches +?2.0/2.4°C in winter and +?3.5/5.0°C in summer for the minimum and maximum daily temperatures, respectively. During summer, the warming trend is more pronounced in the surrounding countryside than in Paris and suburbs due to the soil dryness. As a result, a substantial decrease of the strong urban heat islands is noted at nighttime, and numerous events with negative urban heat islands appear at daytime. Finally, a 30% decrease of the heating degree days is quantified in winter between present and future climates. Inversely, the summertime cooling degree days significantly increase in future climate whereas they are negligible in present climate. However, in terms of accumulated degree days, the increase of the demand in cooling remains smaller than the decrease of the demand in heating.     

Impact of climate change on the hydrogeology of two basins in northern France

This study presents an analysis of climate-change impacts on the water resources of two basins located in northern France, by integrating four sources of uncertainty: climate modelling, hydrological modelling, downscaling methods, and emission scenarios. The analysis focused on the evolution of the water budget, the river discharges and piezometric heads. Seven hydrological models were used, from lumped rainfall-discharge to distributed hydrogeological models, and led to quite different estimates of the water-balance components. One of the hydrological models, CLSM, was found to be unable to simulate the increased water stress and was, thus, considered as an outlier even though it gave fair results for the present day compared to observations. Although there were large differences in the results between the models, there was a marked tendency towards a decrease of the water resource in the rivers and aquifers (on average in 2050 about ?14 % and ?2.5 m, respectively), associated with global warming and a reduction in annual precipitation (on average in 2050 +2.1 K and ?3 %, respectively). The uncertainty associated to climate models was shown to clearly dominate, while the three others were about the same order of magnitude and 3–4 times lower. In terms of impact, the results found in this work are rather different from those obtained in a previous study, even though two of the hydrological models and one of the climate models were used in both studies. This emphasizes the need for a survey of the climatic-change impact on the water resource.     

Magnetic properties of the LL5 ordinary chondrite Chelyabinsk (fall of February 15, 2013)

Here we characterize the magnetic properties of the Chelyabinsk chondrite (LL5, S4, W0) and constrain the composition, concentration, grain size distribution, and mineral fabric of the meteorite's magnetic mineral assemblage. Data were collected from 10 to 1073 K and include measurements of low‐field magnetic susceptibility (χ₀), the anisotropy of χ₀, hysteresis loops, first‐order reversal curves, Mössbauer spectroscopy, and X‐ray microtomography. The REM and REM′ paleointensity protocols suggest that the only magnetizations recorded by the chondrite are components of the Earth's magnetic field acquired during entry into our planet's atmosphere. The Chelyabinsk chondrite consists of light and dark lithologies. Fragments of the light lithology show logχ₀ = 4.57 ± 0.09 (s.d.) (n = 135), while the dark lithology shows 4.65 ± 0.09 (n = 39) (where χ₀ is in 10^?9 m³ kg^?1). Thus, Chelyabinsk is three times more magnetic than the average LL5 fall, but is similar to a subgroup of metal‐rich LL5 chondrites (Paragould, Aldsworth, Bawku, Richmond) and L/LL5 chondrites (Glanerbrug, Knyahinya). The meteorite's room‐temperature magnetization is dominated by multidomain FeNi alloys taenite and kamacite (no tetrataenite is present). However, below approximately 75 K remanence is dominated by chromite. The metal contents of the light and dark lithologies are 3.7 and 4.1 wt%, respectively, and are based on values of saturation magnetization.     

Density,porosity, mineralogy,and internal structure of cosmic dust and alteration of its properties during high‐velocity atmospheric entry

X‐ray microtomography (XMT), X‐ray diffraction (XRD), and magnetic hysteresis measurements were used to determine micrometeorite internal structure, mineralogy, crystallography, and physical properties at μm resolution. The study samples include unmelted, partially melted (scoriaceous), and completely melted (cosmic spherules) micrometeorites. This variety not only allows comparison of the mineralogy and porosity of these three micrometeorite types but also reveals changes in meteoroid properties during atmospheric entry at various velocities. At low entry velocities, meteoroids do not melt and their physical properties do not change. The porosity of unmelted micrometeorites varies considerably (0–12%) with one friable example having porosity around 50%. At higher velocities, the range of meteoroid porosity narrows, but average porosity increases (to 16–27%) due to volatile evaporation and partial melting (scoriaceous phase). Metal distribution seems to be mostly unaffected at this stage. At even higher entry velocities, complete melting follows the scoriaceous phase. Complete melting is accompanied by metal oxidation and redistribution, loss of porosity (1 ± 1%), and narrowing of the bulk (3.2 ± 0.5 g cm^?3) and grain (3.3 ± 0.5 g cm^?3) density range. Melted cosmic spherules with a barred olivine structure show an oriented crystallographic structure, whereas other subtypes do not.     

Comments on the low-wavenumber power of granulation brightness fluctuations

A comment is made on the Edmonds' study of the low-wavenumber power of solar granulation. It is concluded that its one-dimensional power spectrum is biased, and that the corresponding two-dimensional power spectrum shows negative parts.     

Metasomatism and melting history of a Variscan lithospheric mantle domain: evidence from the Puy Beaunit xenoliths (French Massif Central)

Mantle xenoliths from Puy Beaunit (French Massif Central) are compositionally varied, ranging from relatively fertile spinel lherzolites to refractory spinel dunites. Fertile peridotites have registered a modal (amphibole-bearing lherzolites) and cryptic metasomatic event that took place before the last Permian (257 Ma) melting episode. Depletion processes have been constrained by chemical modelling: the depletion is related to different degrees of partial melting, but two major melt extraction episodes are needed to explain the range of major element composition. The second event was responsible for the local large-scale dunitification of former residues. The first melting event (F25%) and metasomatic enrichment are attributed to an ancient fluid and/or liquid infiltration that could be related to a pre-Variscan regional subduction (located to the north of the Beaunit area). Texture acquisition and major deformation of the mantle xenoliths were sub-contemporaneous of the subduction and would result from lithospheric delamination. The second melting event (F17%) produced high-Mg basalts with calc-alkaline trace element signature that gave rise to the Permian underplating episode recognised in western Europe.