Origin of maar clusters at the type locality Eifel (Germany): H2O or CO2?

Abstract The closely spaced late glacial Daun and Gillenfeld maar clusters at the maar type locality in western Eifel (Germany) are commonly interpreted to be of phreatomagmatic origin powered by thermohydraulic explosions occurring initially at a depth of 200–300 m below the surface. Our reconnaissance work focusing on the iconoclastic water-filled Pulvermaar (PM) deposits (a funnel-shaped crater 74 m deep and 700 m in diameter surrounded by a tephra ring) and other nearby maars has provided intriguing evidence that CO 2 -dominated pyroclastic processes at a depth of several kilometers may have been a fundamental factor in generating the volumetrically abundant volcanic pellets ( aka subspherical lava lapilli) and the rounded, lava-coated fragments of plutonic and metamorphic rocks (named nodules here) in these deposits. Phreatomagmatic explosions probably contributed to near-surface country rock fragmentation and crater foundering. Supporting lines of evidence for the role of CO 2 include the following: (1) the likely high CO 2 -concentration of the melilite-nephelinite magma; (2) fragments of carbonatite (alkali feldspar intergrown with carbonate) in several maar deposits including the nearby ca. 11 000-year-old Ulmen maar deposits, the youngest volcano in Germany; (3) strongly rounded plutonic and gneiss nodules up to ca. 35 cm in diameter, both interpreted to be of mid-crustal derivation at ca. 20 km depth. The nodules record a complex multiphase dynamic history at depth that commences with an earlier phase of intruded melilite nephelinite into a metamorphic gneissic carapace that was largely crystallized at the time of the eruption. Subsequently, these plutonic as well as the metamorphic rocks were fragmented, followed by thorough milling and rounding of the fragments and eventual lava spray-coating of both types of nodules by a later phase of intruded gas-rich nephelinite magma that was compositionally identical to the older resident intrusion. The prominent role of these processes is reflected by the abundance of volcanic pellets in the lower exposed PM tephra ring deposits and by their dominance in the upper finer-grained well-bedded maar deposits. The pellets are composed of agglutinated smaller nephelinite lapilli and crystal fragments that were possibly derived from collided and fragmented plutonic nodules. The collection of nodules and pellets was most likely transported upward, largely by CO 2 -flushing and by continuous milling in the transport system, and joined by Devonian rock fragments closer to the surface. In the final stages of ascent, this was probably accompanied by near-surface phreatomagmatic processes including crater-formation.