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The origin of maars at the type locality Eifel (Germany): H2O or CO2?

Mari Sumita1, Hans-Ulrich Schmincke1, Sumit Chakraborty2 and Thor H. Hansteen1

  • Affiliations: 1Dynamik des Ozeanbodens (FB4), GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel, Kiel, Germany; 2Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Bochum, Germany 

  • Presentation type: Poster

  • Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 208

  • Programme No: 3.2.31

  • Theme 3 > Session 2

Abstract

Maar deposits of the late glacial iconoclastic Pulvermaar (PM) at the type locality Eifel (Germany) are interpreted to have been initiated pyroclastically at midcrustal depth. Cognate mafic nodules (amphibole, phlogopite, clinopyroxene, apatite and minor olivine) up to 35 cm in diameter, and accompanying metamorphic nodules, are mantled by melilite nephelinite crusts. Nodules are probably derived from an almost fully crystallized intrusion, and gneiss carapace, respectively. Fluid inclusion barometry of the igneous nodules indicate depths of about 20 km, corresponding to the Conrad discontinuity. Replenishment of CO2-rich melilite nephelinite magma fragmented, abraded and rounded cognate and metamorphic rocks at depth. Rounded nodules were covered by compositionally identical lava rinds up to 3 cm thick consisting of minute sub-mm lava spheres interpreted to have formed by fluidized CO2-rich magma spray. Subrounded melilite-nephelinite lava pellets up to 5 cm in diameter, make up >50 vol.% in the bedded upper 8 m maar deposit. Tightly welded melilite nephelinite agglutinates inside the PM crater represent syn-eruptive lava fountains. CO2-jetting probably transported the nodule and pellet cargo at 7km/h to the surface, and was strongly enriched in Devonian sandstone and slate clasts during conduit formation. The nearby melilite nephelinite Daun maar cluster contain similar nodules and pellets. High CO2-concentration is indicated by the magma composition and by fragments of carbonatite consisting of alkali feldspar intergrown with carbonate in several maar cluster deposits. CO2 is regarded as the dominant factor inducing maar-explosions in these Eifel maars, as in silica-undersaturated, ultramafic and carbonatitic volcanic fields elsewhere.