Neon isotopic composition of the mantle constrained by single vesicle analyses

© The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Earth and Planetary Science Letters 449 (2016): 145-154, doi:10.1016/j.epsl.2016.05.052.The origin of volatiles on Earth is still a matter of debate. Noble gases are an efficient geochemical tool to constrain Earth formation processes due to their inertness. Several studies have focused on the neon isotopic composition of the lower mantle because the 20Ne/22Ne ratio is thought to reflect that of Earth’s primordial components. Two models to explain the origin of light noble gases on Earth have been proposed: either solar wind implantation onto the Earth's solid precursors or dissolution into the mantle of a primordial atmosphere captured from solar nebula gas. In order to test these two models, we analyzed the noble gas compositions (helium, neon and argon) of two submarine oceanic island basalt glasses from Fernandina volcano (Galápagos archipelago), which have among the most primitive/unradiogenic terrestrial helium and neon isotopic compositions. Several sample pieces are studied both by step-crushing and by laser ablation analyses of single vesicles. Results of step-crushing are consistent with those of laser ablation analyses, but the latter results provide new insights into the origin of atmospheric contamination. The single-vesicle laser-ablation measurements overlap with the step crushing results, but have systematically higher 40Ar/36Ar, and 3He/36Ar, suggesting less atmospheric contamination using this method. The single vesicle data therefore suggest that atmospheric contamination is introduced by exposure to the modern atmosphere, after sample collection. 3He/4He values are about 23 times the atmospheric ratio (R/Ra) for the two Fernandina (Galápagos) samples, in agreement with previous studies. We obtain 20Ne/22Ne and 40Ar/36Ar isotopic ratios as high as 12.91 and 9400, respectively, for the mantle source of the Galápagos hotspot. The new data show that step-crushing and laser ablation analyses are complementary methods that should be used together to derive the noble gas ratios in uncontaminated samples. The results of neon compositions are consistent with previous hotspot studies and support the model of solar wind implantation associated with sputtering to explain helium and neon origins on Earth.M.D.K. acknowledges support from NSF OCE in collecting the samples and allowing his participation in this study (OCE-1259218 and OCE-1232985). M.M. acknowledges the financial support from the UnivEarthS Labex program of Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02).2018-06-0

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

Typical features of Parkinson disease and diagnostic challenges with microdeletion 22q11.2

Objective: To delineate the natural history, diagnosis, and treatment response of Parkinson disease (PD) in individuals with 22q11.2 deletion syndrome (22q11.2DS), and to determine if these patients differ from those with idiopathic PD. Methods: In this international observational study, we characterized the clinical and neuroimaging features of 45 individuals with 22q11.2DS and PD (mean follow-up 7.5 ± 4.1 years). Results: 22q11.2DS PD had a typical male excess (32 male, 71.1%), presentation and progression of hallmark motor symptoms, reduced striatal dopamine transporter binding with molecular imaging, and initial positive response to levodopa (93.3%). Mean age at motor symptom onset was relatively young (39.5 ± 8.5 years); 71.4% of cases had early-onset PD (<45 years). Despite having a similar age at onset, the diagnosis of PD was delayed in patients with a history of antipsychotic treatment compared with antipsychotic-naive patients (median 5 vs 1 year, p = 0.001). Preexisting psychotic disorders (24.5%) and mood or anxiety disorders (31.1%) were common, as were early dystonia (19.4%) and a history of seizures (33.3%). Conclusions: Major clinical characteristics and response to standard treatments appear comparable in 22q11.2DS-associated PD to those in idiopathic PD, although the average age at onset is earlier. Importantly, treatment of preexisting psychotic illness may delay diagnosis of PD in 22q11.DS patients. An index of suspicion and vigilance for complex comorbidity may assist in identifying patients to prioritize for genetic testing

Constraints on degassing processes at mid-oceanic ridges using volatile geochemistry and petrology

Afin de préciser l'origine des volatils terrestres et les flux actuels et passés vers l’atmosphère, de nombreuses études s’intéressent à la composition du manteau. Ce réservoir est échantillonné naturellement lors des éruptions volcaniques, mais une grande partie des gaz est alors émise dans l'atmosphère, de sorte que la composition des volatils du manteau reste peu contrainte. Nous cherchons à préciser les mécanismes de dégazage sous les dorsales océaniques afin de corriger ces fractionnements. L'analyse (He-Ne-Ar-CO2) de verres basaltiques issus de la dorsale des Galápagos, dans la zone d'influence du point chaud des Galápagos, montre que la composition en volatils des laves s'explique par distillation de Rayleigh d'une source unique. Cette source est distincte de celle du point chaud (isotopes du néon), impliquant un dégazage en profondeur du panache ou une hétérogénéité spatiale de sa composition.Plusieurs verres volcaniques issus de la dorsale Atlantique et Est Pacifique ont été imagés par micro-tomographie aux rayons X. L'étude met en évidence des mécanismes de nucléation et de croissance des bulles différents sous les deux dorsales. De la convection en périphérie de la chambre magmatique avant l’éruption a été mise en évidence par l’étude pétrologique des verres. Les vésicules imagées ont ensuite été ouvertes individuellement sous vide par ablation laser et analysées (CO2, 4He, isotopes de l'argon). La composition des bulles est hétérogène dans certains échantillons et compatible avec une distillation de Rayleigh. Les tendances de dégazage obtenues permettent d'obtenir localement la composition de la source mantellique, qui est hétérogène.L'étude permet également d'appréhender l'hétérogénéité des rapports 40Ar/36Ar dans les chambres magmatiques par la technique d’ablation laser qui diminue la contamination atmosphérique par rapport à la technique classique de broyageThe composition of mantle volatiles is related to the origin of Earth's volatiles and to the past and present volatile fluxes to the atmosphere. Although this reservoir is naturally sampled during volcanic eruptions, most of the volatiles are lost to the atmosphere during this event, thus the composition of mantle volatiles is still uncertain. We try here to precise the processes of degassing below mid-oceanic ridges to correct the lava compositions for degassing.The He-Ne-Ar-CO2 analyses of basaltic glasses sampled along the Galapagos Spreading Center, in the area of influence of the Galapagos hotspot, show that the volatile composition of lavas is fully explained by a Rayleigh distillation of a unique source distinct from the plume source. These results imply that the plume degasses at depth or is heterogeneous.Several volcanic glasses from Mid-Atlantic ridge and East Pacific Rise have been imaged by X-rays microtomography. The mechanisms of vesicle nucleation and growth appear to be different below the two ridges. A step of convection at the magma body margin has been evidenced by the petrologic study of the glasses. Imaged vesicles have been subsequently opened under vacuum by laser ablation and analysed (CO2, 4He, argon isotopes). We observe, depending on the samples, either a single composition for all bubbles, or variations in composition between bubbles consistent with a trend of equilibrium degassing in an open system. The trends of degassing allow extrapolating locally to the volatile composition of the mantle source, which is heterogeneous. We also studied the heterogeneity of 40Ar/36Ar ratios in magmatic chambers using the laser opening method, which lowers the contribution of atmospheric gases compared to the classical crushing metho

Contraintes sur les processus de dégazage des dorsales océaniques par la géochimie des volatils et la pétrologie des laves basaltiques

The composition of mantle volatiles is related to the origin of Earth's volatiles and to the past and present volatile fluxes to the atmosphere. Although this reservoir is naturally sampled during volcanic eruptions, most of the volatiles are lost to the atmosphere during this event, thus the composition of mantle volatiles is still uncertain. We try here to precise the processes of degassing below mid-oceanic ridges to correct the lava compositions for degassing.The He-Ne-Ar-CO2 analyses of basaltic glasses sampled along the Galapagos Spreading Center, in the area of influence of the Galapagos hotspot, show that the volatile composition of lavas is fully explained by a Rayleigh distillation of a unique source distinct from the plume source. These results imply that the plume degasses at depth or is heterogeneous.Several volcanic glasses from Mid-Atlantic ridge and East Pacific Rise have been imaged by X-rays microtomography. The mechanisms of vesicle nucleation and growth appear to be different below the two ridges. A step of convection at the magma body margin has been evidenced by the petrologic study of the glasses. Imaged vesicles have been subsequently opened under vacuum by laser ablation and analysed (CO2, 4He, argon isotopes). We observe, depending on the samples, either a single composition for all bubbles, or variations in composition between bubbles consistent with a trend of equilibrium degassing in an open system. The trends of degassing allow extrapolating locally to the volatile composition of the mantle source, which is heterogeneous. We also studied the heterogeneity of 40Ar/36Ar ratios in magmatic chambers using the laser opening method, which lowers the contribution of atmospheric gases compared to the classical crushing methodAfin de préciser l'origine des volatils terrestres et les flux actuels et passés vers l'atmosphère, de nombreuses études s'intéressent à la composition du manteau. Ce réservoir est échantillonné naturellement lors des éruptions volcaniques, mais une grande partie des gaz est alors émise dans l'atmosphère, de sorte que la composition des volatils du manteau reste peu contrainte. Nous cherchons à préciser les mécanismes de dégazage sous les dorsales océaniques afin de corriger ces fractionnements. L'analyse (He-Ne-Ar-CO2) de verres basaltiques issus de la dorsale des Galápagos, dans la zone d'influence du point chaud des Galápagos, montre que la composition en volatils des laves s'explique par distillation de Rayleigh d'une source unique. Cette source est distincte de celle du point chaud (isotopes du néon), impliquant un dégazage en profondeur du panache ou une hétérogénéité spatiale de sa composition.Plusieurs verres volcaniques issus de la dorsale Atlantique et Est Pacifique ont été imagés par micro-tomographie aux rayons X. L'étude met en évidence des mécanismes de nucléation et de croissance des bulles différents sous les deux dorsales. De la convection en périphérie de la chambre magmatique avant l'éruption a été mise en évidence par l'étude pétrologique des verres. Les vésicules imagées ont ensuite été ouvertes individuellement sous vide par ablation laser et analysées (CO2, 4He, isotopes de l'argon). La composition des bulles est hétérogène dans certains échantillons et compatible avec une distillation de Rayleigh. Les tendances de dégazage obtenues permettent d'obtenir localement la composition de la source mantellique, qui est hétérogène.L'étude permet également d'appréhender l'hétérogénéité des rapports 40Ar/36Ar dans les chambres magmatiques par la technique d'ablation laser qui diminue la contamination atmosphérique par rapport à la technique classique de broyag

Contraintes sur les processus de dégazage des dorsales océaniques par la géochimie des volatils et la pétrologie des laves basaltiques

Afin de préciser l'origine des volatils terrestres et les flux actuels et passés vers l atmosphère, de nombreuses études s intéressent à la composition du manteau. Ce réservoir est échantillonné naturellement lors des éruptions volcaniques, mais une grande partie des gaz est alors émise dans l'atmosphère, de sorte que la composition des volatils du manteau reste peu contrainte. Nous cherchons à préciser les mécanismes de dégazage sous les dorsales océaniques afin de corriger ces fractionnements. L'analyse (He-Ne-Ar-CO2) de verres basaltiques issus de la dorsale des Galápagos, dans la zone d'influence du point chaud des Galápagos, montre que la composition en volatils des laves s'explique par distillation de Rayleigh d'une source unique. Cette source est distincte de celle du point chaud (isotopes du néon), impliquant un dégazage en profondeur du panache ou une hétérogénéité spatiale de sa composition.Plusieurs verres volcaniques issus de la dorsale Atlantique et Est Pacifique ont été imagés par micro-tomographie aux rayons X. L'étude met en évidence des mécanismes de nucléation et de croissance des bulles différents sous les deux dorsales. De la convection en périphérie de la chambre magmatique avant l éruption a été mise en évidence par l étude pétrologique des verres. Les vésicules imagées ont ensuite été ouvertes individuellement sous vide par ablation laser et analysées (CO2, 4He, isotopes de l'argon). La composition des bulles est hétérogène dans certains échantillons et compatible avec une distillation de Rayleigh. Les tendances de dégazage obtenues permettent d'obtenir localement la composition de la source mantellique, qui est hétérogène.L'étude permet également d'appréhender l'hétérogénéité des rapports 40Ar/36Ar dans les chambres magmatiques par la technique d ablation laser qui diminue la contamination atmosphérique par rapport à la technique classique de broyageThe composition of mantle volatiles is related to the origin of Earth's volatiles and to the past and present volatile fluxes to the atmosphere. Although this reservoir is naturally sampled during volcanic eruptions, most of the volatiles are lost to the atmosphere during this event, thus the composition of mantle volatiles is still uncertain. We try here to precise the processes of degassing below mid-oceanic ridges to correct the lava compositions for degassing.The He-Ne-Ar-CO2 analyses of basaltic glasses sampled along the Galapagos Spreading Center, in the area of influence of the Galapagos hotspot, show that the volatile composition of lavas is fully explained by a Rayleigh distillation of a unique source distinct from the plume source. These results imply that the plume degasses at depth or is heterogeneous.Several volcanic glasses from Mid-Atlantic ridge and East Pacific Rise have been imaged by X-rays microtomography. The mechanisms of vesicle nucleation and growth appear to be different below the two ridges. A step of convection at the magma body margin has been evidenced by the petrologic study of the glasses. Imaged vesicles have been subsequently opened under vacuum by laser ablation and analysed (CO2, 4He, argon isotopes). We observe, depending on the samples, either a single composition for all bubbles, or variations in composition between bubbles consistent with a trend of equilibrium degassing in an open system. The trends of degassing allow extrapolating locally to the volatile composition of the mantle source, which is heterogeneous. We also studied the heterogeneity of 40Ar/36Ar ratios in magmatic chambers using the laser opening method, which lowers the contribution of atmospheric gases compared to the classical crushing methodNANCY-INPL-Bib. électronique (545479901) / SudocSudocFranceF

Timescales of convection in magma chambers below the Mid-Atlantic ridge from melt inclusions investigations

International audienceClosed hopper and complex swallowtail morphologies of olivine microcrysts have been described in the past in both mid-oceanic ridge basalts and subaerial tholeitic volcanoes and indicate fluctuations in magma undercooling. We describe similar morphologies in a Mid-Atlantic ridge pillow basalt (sample RD87DR10), and in addition we estimate the duration of temperature fluctuations required to produce these textures as follows: (1) Pairs of melt inclusions are arranged symmetrically around the centre of hopper crystals and each pair represents a heating-cooling cycle. Using the literature olivine growth rates relevant to the observed morphologies, and measuring the distance between two successive inclusions, we estimate the minimum time elapsed during one convection cycle. (2) The major element composition of melt inclusions (analysed by electron microprobe) was found to be in the range of the boundary layer measured in the glass surrounding the olivines, irrespective of their size. Several major elements demonstrate that this boundary layer results from rapid quenching on the seafloor, and not from crystal growth at depth, implying the inclusions had the same composition as the surrounding magma when they were sealed. Using diffusivity of slow diffusing elements such as Al2O3, we estimate the minimum time required for inclusion formation. These two independent approaches give concordant results: each cooling-heating cycle lasted between a few minutes and 1 h minimum. Thus, these crystals probably recorded thermal convection in small magmatic bodies (a dyke or shallow magma chamber) during the last hour or hours before eruption

An observed link between lithophile compositions and degassing of volatiles (He, Ar, CO2) in MORBs with implications for Re volatility and the mantle C/Nb ratio

International audienceThere are systematic variations between relative noble gas abundances and lithophile tracers such as 87Sr/86Sr, εNd and La/Sm in a suite of basaltic glasses from the South East Indian Ridge (SEIR). 4He/40Ar⁎ (where 40Ar⁎ is 40Ar corrected for atmospheric contamination) correlates positively with 87Sr/86Sr and La/Sm but anticorrelates with εNd. The large range in 4He/40Ar⁎ observed in the glasses is due to fractionation during magmatic degassing caused by the very different solubilities of He and Ar in silicate liquids, whereas 87Sr/86Sr, εNd, La/Sm, etc. are insensitive to magmatic processes but rather reflect mantle heterogeneity. Thus, there is a curious situation in this suite of basalts where tracers of mantle heterogeneity (87Sr/86Sr, εNd, La/Sm, etc.) correlate with a tracer of magmatic volatile processes (4He/40Ar⁎).Here, we propose that “enriched” mantle (with high La/Sm and 87Sr/86Sr, low εNd) also has a higher C concentration than “depleted” mantle. Magmas derived from enriched mantle will therefore have higher initial C concentrations, leading to a greater fraction of CO2 degassed and thus a higher 4He/40Ar⁎ ratio on eruption. Simple solubility-determined fractional degassing models show that the range in 4He/40Ar⁎ observed in SEIR basaltic glasses can be generated if the mantle C concentration varies by a factor of 2 over the length of the ridge, consistent with independent estimates of C concentration heterogeneity in the MORB mantle. The correlations between lithophile tracers and 4He/40Ar⁎ can be reproduced by mixing between a depleted endmember with 87Sr/86Sr = 0.70275, εNd = 8.2 and [C] = 12 ppm and an enriched endmember with 87Sr/86Sr = 0.70360, εNd = 5 and [C] = 24 ppm, followed by degassing.The proposed degassing model allows us to estimate the initial C concentration (i.e. prior to degassing) of each SEIR basalt (for which Sr or Nd isotopes are available); using independent Nb concentration data (Mahoney et al., 2002), we show that C/Nb ratios prior to degassing along the SEIR are relatively constant, probably with a C/Nb ratio of 200±100. However, although the constancy of C/Nb in these samples is a robust conclusion, the estimated C/Nb ratio itself is model dependent.We also use these data to evaluate volatility of Re during degassing of MORBs; Re is known to be moderately volatile during subaerial and shallow marine volcanism, although it is not known if this element is also volatile at conditions appropriate to MORB emplacement. Although there is a (poor) correlation between Re/Yb (Yb being a non-volatile element of similar apparent bulk compatibility to Re) and 4He/40Ar⁎ in these samples, it is more likely that this correlation results from Re/Yb variation in the mantle source and is not due to loss of Re during magmatic degassing

Mechanisms of magma degassing at mid-oceanic ridges and the local volatile composition (4He–40Ar⁎–CO2) of the mantle by laser ablation analysis of individual MORB vesicles

International audienceIndividual vesicles in <1 cm3 samples of MORB glasses commonly preserve significantly different volatile (He, Ar, CO2) compositions. Five separate MORB glass samples from different ridges were investigated, selectively opening targeted vesicles using a 193 nm excimer laser. One sample shows evidence of syn-emplacement selective helium loss from the vesicles in the glass rim close to the crystallised zone of the pillow lava, proving that He contents and low He/Ar ratios do not always reflect mantle and/or magmatic processes. However the composition of the different vesicles of three of these samples covers large ranges in He/Ar and Ar/CO2 with linear variations in plots of ln(He/Ar) vs. ln(Ar/CO2) which are consistent with a Rayleigh distillation at equilibrium and allow the relative abundances of the volatiles in the MORB mantle source to be estimated by correcting for degassing processes on a sample-by-sample basis. This technique presents a new tool for characterising and correcting for volatile fractionation processes that have modified the initial mantle source composition. The results confirm a heterogeneous CO2/3He ratio in the MORB source regions with the Azores mantle source enriched in C/3He compared to that of N-MORBs. The considerable heterogeneity in vesicle compositions within such a small volume (<1 cm3) requires injection of less degassed magma at shallow level before eruption. The maximum time interval between magma mixing and quenching on the seafloor is of the order of a few hours

Constraints on the noble gas composition of the deep mantle by bubble-by-bubble analysis of a volcanic glass sample from Iceland

International audienc