Slab-derived metasomatism in the Carpathian-Pannonian mantle revealed by investigations of mantle xenoliths from the Bakony-Balaton Highland Volcanic Field

International audienceA suite of fifteen peridotite xenoliths from the Bakony-Balaton Highland Volcanic Field (BBHVF, Pannonian Basin, Central Europe) that show abundant petrographic evidence of fluid and melt percolation were studied in order to decipher the formation of their melt pockets and veins. The suite mainly consists of “fertile” lherzolites (5.8–19.9 vol.% clinopyroxene) and a few harzburgites (1.9–5.4 vol.% clinopyroxene) from well-known localities (Szentbékkálla, Szigliget) and two previously unreported localities (Füzes-tó and Mindszentkálla). Major and trace element data indicate that most of the peridotites record variable degrees of partial melt extraction, up to > 15% for the harzburgites. Subsequently, the xenoliths experienced at least two stages of metasomatic modification. The first stage was associated with percolation of a volatile-bearing silicate melt and resulted in crystallization of amphibole, enrichment in the most incompatible trace elements (Ba, Th, U, Sr), and development of negative Nb-Ta anomalies in clinopyroxene. The second and last metasomatic event, widespread beneath the BBHVF, is associated with the formation of silicate melt pockets, physically connected to a network of melt veins, with large and abundant CO2 vesicles. The glass in these veins has sub-alkaline trachy-andesitic composition and displays an OIB-like trace element signature. Its composition attests to the migration through a supra-subduction zone mantle wedge of silicic melt highly enriched in volatiles (CO2, H2O, Cl, F), LILE, REE and HFSE and consistent with compositions of natural and experimental examples of slab melting-derived magma. In the present case, however, melt was likely derived from melting of oceanic crust and carbonated sediments under conditions where Nb-rich mineral phases were not stable in the residue. A likely scenario for the origin such melts involves melting after subduction ceased as the slab thermally equilibrated with the asthenosphere. Melt-rock reactions due to ascent of hot, CO2-rich, siliceous melt to near-Moho depths triggered destabilization of amphibole and primary clinopyroxene, spinel, and possibly olivine. The resulting andesitic glass in melt pockets evolved to more mafic compositions due to mantle mineral assimilation but has heterogeneous trace element signatures mostly inherited from preexisting amphibole. The present example of melt-rock reactions between highly volatile-enriched siliceous slab-derived melt and peridotite from the upper part of the lithospheric mantle ultimately produced derivative melt with major element composition akin to calc-alkaline basaltic andesite, with generally low trace elements concentrations but selective pronounced enrichments in LILE's such as Ba, Sr, Pb

Evidence of a shallow magma reservoir below Los Humeros volcanic complex: Insights from the geochemistry of silicate melt inclusions

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New method for initial composition determination of crystallized silicate melt inclusions

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Three-dimensional distribution of glass and vesicles in metasomatized xenoliths : A micro-CT case study from Nógrád–Gömör Volcanic Field (Northern Pannonian Basin)

International audienceIn this study, three clinopyroxene-enriched upper mantle xenoliths, petrographically classified as wehrlite,were investigated from the Nógrád–Gömör Volcanic Field with the use of X-ray microtomography. Our main goal was toquantify the volume of the glass phase and the vesicles to reveal their three-dimensional distribution. Among the studiedwehrlite xenoliths, one is weakly and two are strongly metasomatized. The two latter wehrlite xenoliths are characterizedby higher modal amount of glass and vesicles, which suggests a genetic connection between glass and concomitantvesicles, and the metasomatic agent. The glass, which was a melt at mantle conditions, forms an interconnected network.This may explain the presence of the electromagnetic anomaly with high electrical conductivity beneath the study area.Our study contributes to the better understanding of melt migration and its metasomatic effect in the lithospheric mantlebeneath monogenetic volcanic field

Is the "Azores Hotspot" a Wetspot ? Insights from the geochemistry of fluid and melt inclusions in olivine of Pico basalts

International audienceThe concept of an 'Azores mantle plume' has been widely debated, and the existence of an Azores hotspot questioned. In an effort to shed new light on this controversy, we present He isotope and major, trace and volatile element compositions for basaltic scoriae from five monogenetic cones emplaced along the fissure zone of Pico Island, the youngest island of the Azores archipelago. The bulk scoriae and lavas are moderately alkaline basalts, and their He isotope ratios, determined on olivine crystals, vary between 10*2 and 11*1 ± 0*1 Ra. In contrast, melt inclusions hosted in olivine (Fo76-83*5) span a large range of compositions (K2O = 0*7-1*7 wt %; Ce = 32-65 ppm; Nb = 21-94 ppm), which extends the compositional field of lavas erupted along the Pico fissure zone. This chemical evolution is predominantly controlled by polybaric fractional crystallization. Most melt inclusions share similar enrichments in large ion lithophile and light rare earth elements, and trace element ratios (La/Sm, La/Yb, Sr/Nd, Ta/Th, Zr/Y) with their bulk-rocks. Only a few of them differ in their lower contents of incompatible elements and La/Sm, Li/Ta and Na/K ratios, a feature that is ascribed to distinct conditions of melting. As a whole, the melt inclusions preserve high and variable volatile contents, and contain up to 1*8-2*0 wt % of H2O and 0*4 wt % of CO2. The total fluid pressures, retrieved from the dissolved CO2 and H2O concentrations, and the PCO2 from fluid inclusions, indicate magma ponding and crystallization at the crust-mantle boundary (ca. 18 km deep). The H2O/Cl and H2O/Ce ratios in the inferred parental undegassed basalts of the Pico fissure zone average 0*036 ± 0*006 and 259 ± 21, respectively. The latter value is significantly higher than that reported for typical mid-ocean ridge basalts from the southern Mid-Atlantic Ridge, but is similar to published ratios for submarine undegassed basalts from the Azores platform. Combining the calculated compositions of Pico primary magmas formed by low degrees of melting with recent geophysical data for the Azores, we propose a model for Azores magma generation involving the decompression melting of a water-enriched mantle domain (H2O = 680-570 ppm) with an estimated temperature excess of ≤120°C with respect to the Mid-Atlantic Ridge

First-principles modeling of chlorine isotope fractionation between chloride-bearing molecules and minerals

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High resolution X–ray computed tomography and scanning electron microscopy studies of multiphase solid inclusions in Oman podiform chromitite: implications for post–entrapment modification

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