Evidence for Slab-derived Silicate Melt in the Sub-Arc Mantle

The sources of arc-related lavas are commonly modified by metasomatic fluids and/or melts believed to be extracted from parts of the subducting slab (e.g., the basaltic crust and/or overlying sediments). Evidence from phase equilibria, trace-element geochemistry, and volatile contents arc-related lavas and melt inclusions strongly suggest that aqueous fluid derived from subducted basalt is one of these phases. In contrast, the properties and even presence of slab-derived melts in the sub-arc mantle are relatively uncertain. Commonly, a role for slab-derived melt is implicated where arc lavas are enriched in sediment-born and fluid-insoluble trace elements such as Th (e.g., Elliott et al., 1997). However, this evidence is contradicted by or fails to address other constraints; for instance: Solvi between water-rich silicate melts and solute-rich aqueous fluids 'close' at T-P conditions within the range of those in the slab and mantle wedge, suggesting that the dichotomy between 'slab fluid' and 'slab melt' may be poorly posed (e.g., Bureau and Keppler, 1999); furthermore, thermal models of subduction zones suggest that no section of the slab should melt except under special circumstances or through generally unexpected physical effects (Peacock, 1991); finally, experimental and empirical constraints on the expected geochemical properties of slab fluids and melts are inconsistent with one another in several respects, adding ambiguity to trace-element arguments for or against the role of slab melt (e.g., Keppler, 1996; Johnson and Plank, 1999)

Melting of Amphibole-bearing Wehrlites: an Experimental Study on the Origin of Ultra-calcic Nepheline-normative Melts

Olivine + clinopyroxene ± amphibole cumulates have been widely documented in island arc settings and may constitute a significant portion of the lowermost arc crust. Because of the low melting temperature of amphibole (∼1100°C), such cumulates could melt during intrusion of primary mantle magmas. We have experimentally (piston-cylinder, 0·5-1·0 GPa, 1200-1350°C, Pt-graphite capsules) investigated the melting behaviour of a model amphibole-olivine-clinopyroxene rock, to assess the possible role of such cumulates in island arc magma genesis. Initial melts are controlled by pargasitic amphibole breakdown, are strongly nepheline-normative and are Al2O3-rich. With increasing melt fraction (T > 1190°C at 1·0 GPa), the melts become ultra-calcic while remaining strongly nepheline-normative, and are saturated with olivine and clinopyroxene. The experimental melts have strong compositional similarities to natural nepheline-normative ultra-calcic melt inclusions and lavas exclusively found in arc settings. The experimentally derived phase relations show that such natural melt compositions originate by melting according to the reaction amphibole + clinopyroxene = melt + olivine in the arc crust. Pargasitic amphibole is the key phase in this process, as it lowers melting temperatures and imposes the nepheline-normative signature. Ultra-calcic nepheline-normative melt inclusions are tracers of magma-rock interaction (assimilative recycling) in the arc crus

Zonation of H_(2)O and F Concentrations around Melt Inclusions in Olivines

Studies of both naturally quenched and experimentally reheated melt inclusions have established that they can lose or gain H_(2)O after entrapment in their host mineral, before or during eruption. Here we report nanoSIMS analyses of H2O, Cl and F in olivine around melt inclusions from two natural basaltic samples: one from the Sommata cinder cone on Vulcano Island in the Aeolian arc and the other from the Jorullo cinder cone in the Trans-Mexican Volcanic Belt. Our results constrain olivine/basaltic melt partition coefficients and allow assessment of mechanisms of volatile loss from melt inclusions in natural samples. Cl contents in olivine from both samples are mostly below detection limits (≤0·03 ± 0·01 ppm), with no detectable variation close to the melt inclusions. Assuming a maximum Cl content of 0·03 ppm for all olivines, maximum estimates for Cl partition coefficients between olivine and glass are 0·00002 ± 0·00002. Olivines from the two localities display contrasting H_(2)O and F compositions: Sommata olivines contain 27 ± 11 ppm H_(2)O and 0·28 ± 0·07 ppm F, whereas Jorullo olivines have lower and proportionately more variable H_(2)O and F (11 ± 12 ppm and 0·12 ± 0·09 ppm, respectively; uncertainties are two standard deviations for the entire population). The variations of H_(2)O and F contents in the olivines exhibit clear zonation patterns, increasing with proximity to melt inclusions. This pattern was most probably generated during transfer of volatiles out of the inclusions through the host olivine. H_(2)O concentration gradients surrounding melt inclusions are roughly concentric, but significantly elongated parallel to the crystallographic a-axis of olivine. Because of this preferential crystallographic orientation, this pattern is consistent with H_(2)O loss that is rate-limited by the ‘proton–polaron’ mechanism of H diffusion in olivine. Partition coefficients based on olivine compositions immediately adjacent to melt inclusions are 0·0007 ± 0·0003 for H_(2)O and 0·0005 ± 0·0003 for F. The H_(2)O and F diffusion profiles most probably formed in response to a decrease in the respective fugacities in the external melt, owing to either degassing or mixing with volatile-poor melt. Volatile transport out of inclusions might also have been driven in part by increases in the fugacity within the inclusion owing to post-entrapment crystallization. In the case of F, because of the lack of data on F diffusion in olivine, any interpretation of the measured F gradients is speculative. In the case of H_(2)O, we model the concentration gradients using a numerical model of three-dimensional anisotropic diffusion of H, where initial conditions include both H2O decrease in the external melt and post-entrapment enrichment of H_(2)O in the inclusions. The model confirms that external degassing is the dominant driving force, showing that the orientation of the anisotropy in H diffusion is consistent with the proton–polaron diffusion mechanism in olivine. The model also yields an estimate of the initial H_(2)O content of the Sommata melt inclusions before diffusive loss of 6 wt % H_(2)O. The findings provide new insights on rapid H_(2)O loss during magma ascent and improve our ability to assess the fidelity of the H_(2)O record from melt inclusions

The petrology of a hazardous volcano: Calbuco (Central Southern VolcanicZone, Chile)

peer reviewedThe recurrent explosive eruptions of Calbuco (Andean Southern Volcanic Zone (SVZ)) threat a rapidly expanding tour- istic and economic region of Chile. Providing tighter constraints on its magmatic system is therefore important for better monitoring its activity. Calbuco is also distinguished by hornblende-bearing assemblages that contrast with the anhydrous parageneses of most Central SVZ volcanoes. Here we build on previous work to propose a detailed petrological model of the magmatic system beneath Calbuco. Geochemical data acquired on a hundred samples collected in the four units of the volcano show no secular compositional change indicating a steady magmatic system since ~ 300 ka. A tholeiitic Al2O3-rich (20 wt. %) basalt (Mg# = 0.59) is the parent magma of a differentiation trend straddling the tholeiitic/calc-alkaline fields and displaying a narrow compositional Daly gap. Amphibole crystallization was enabled by the higher H2O content of the basalt (3–3.5 wt. % H2O at 50 wt. % SiO2) compared to neighboring volcanoes. This characteristic is inherited from the primary mantle melt and possibly results from a lower degree of partial melting induced by the mantle wedge thermal structure. Although macrocrysts are not all in chemical equilibrium with their host rocks and were thus presumably unlocked from the zoned crystal mush and transported in the carrier melt, the bulk-rock trend follows both experimental liquid lines of descent and the chemical trend of calculated melts in equilibrium with amphibole (AEMs). These contradictory observations can be reconciled if minerals are transported in near cotectic proportions. The AEMs overlap the Daly gap revealing that the missing liquid compositions were present in the storage region. Geothermobarometers all indicate that the chemical diversity from basalt to dacite was acquired at a shallow depth (210–460 MPa). We suggest that differentiation from the primary magma to the parental basalt took place either in the same storage region or at the MOHO.CDR J.00066.14, PDR T.0079.18, Odysseus grant (ON

Grain scale processes recorded by oxygen isotopes in olivine-hosted melt inclusions from two MORB samples

International audienceAlthough olivine-hosted melt inclusions from mid ocean ridge basalts (MORB) are commonly used as a proxy formantle composition, these melt inclusions generally show larger elemental and isotopic compositional variationthan their host lavas and the origin of these heterogeneities remains disputed. Here we present oxygen isotopedata from melt inclusions hosted in olivine from two samples from the Mid-Atlantic ridge. Melt inclusions fromdifferent crystals within the same sample show>2.5‰ δ18O variation within each sample, which is nearly eighttimes the analytical error of 0.3‰ (2 standard deviations) and five times the δ18O range in unaltered MORB.Measured δ18O in melt inclusions do not correlate with common magmatic tracers, and δ18O measured in thehost olivines suggest a maximum of 1‰ δ18O source heterogeneity. Less than half of the melt inclusions fromeach sample are in equilibrium with their host crystals; the remaining melt inclusions have either lower or higherolivine-melt oxygen isotope partition coefficients compared to the theoretical equilibrium values. Here wediscuss several potential processes that could contribute to these observations, but none satisfactorily explain theolivine-melt inclusion oxygen disequilibrium that we observe in these samples. Nevertheless, it seems clear thatthe variability of δ18O in melt inclusion from two MORB samples do not record only common magmatic process(es), but rather a localized grain scale process. Any δ18O variation in melt inclusions should thus be interpretedwith caution

Calbuco (Central Southern Volcanic Zone, Chile): a hazardous volcano

CDR J.00066.14, PDR T.0079.18, Odysseus grant (ON

Deep-Learning Assessed Muscular Hypodensity Independently Predicts Mortality in DLBCL Patients Younger Than 60 Years.

[en] BACKGROUND: Muscle depletion (MD) assessed by computed tomography (CT) has been shown to be a predictive marker in solid tumors, but has not been assessed in non-Hodgkin's lymphomas. Despite software improvements, MD measurement remains highly time-consuming and cannot be used in clinical practice. METHODS: This study reports the development of a Deep-Learning automatic segmentation algorithm (DLASA) to measure MD, and investigate its predictive value in a cohort of 656 diffuse large B cell lymphoma (DLBCL) patients included in the GAINED phase III prospective trial (NCT01659099). RESULTS: After training on a series of 190 patients, the DLASA achieved a Dice coefficient of 0.97 ± 0.03. In the cohort, the median skeletal muscle index was 50.2 cm2/m2 and median muscle attenuation (MA) was 36.1 Hounsfield units (HU). No impact of sarcopenia was found on either progression free survival (PFS) or overall survival (OS). Muscular hypodensity, defined as MA below the tenth percentile according to sex, was associated with a lower OS and PFS, respectively (HR = 2.80 (95% CI 1.58-4.95), p < 0.001, and HR = 2.22 (95% CI 1.43-3.45), p < 0.001). Muscular hypodensity appears to be an independent risk factor for mortality in DLBCL and because of DLASA can be estimated in routine practice

Origine des basaltes des archipels de la Societe et des Tuamotu: approche geochimique et petrologique

SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : T 79901 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Genèse et évolution des magmas primitifs de la chaîne des Puys (Massif Central) (une approche par l'étude des inclusions magmatiques)

Les basaltes émis dans la Chaîne des Puys, dernière manifestation volcanique du Massif Central, ne correspondent pas à des magmas primaires. Ces derniers ne pouvant pas être identifiés à partir des laves, une approche plus directe a été utilisée, à savoir l'étude des inclusions magmatiques piégées dans les olivines magnésiennes. Les compositions en éléments en traces de ces liquides primitifs inclus, de type basanite-basalte alcalin, seraient le résultat de différents degrés de fusion d'une source magmatique à amphibole résiduelle, c'est-à-dire au niveau du manteau lithosphérique métasomatisé. D'autre part, les liquides piégés dans les olivines d'enclaves mantelliques retrouvées dans un des volcans étudiés sont riches en silice et en alcalins. Ils représenteraient les premiers degrés de fusion d'une péridotite à 1 GPa ; sans relation pétrogénique directe avec les liquides piégés dans les olivines magmatiques et les laves émises en surfaceCLERMONT FD-BCIU Sci.et Tech. (630142101) / SudocRENNES-Géosciences (352382209) / SudocSudocFranceF

Melt inclusions in scoria and associated mantle xenoliths of Puy Beaunit volcano, Chaîne des Puys, Massif Central, France

International audienc