The explosive origin of Pulvermaar in the West Eifel Volcanic Field (WEVF): H2O or CO2?

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Maars represent striking and touristically attractive (e.g. Eifel volcanic field) tephra-rimmed volcanic craters, tephra fragments being dominated by near-surface rock fragments. Originally thought to represent magmatic gas (CO2) explosion craters, maars have been generally interpreted since the early 1970ies as having resulted from physical interaction between rising magma and (ground-) water. Closer inspection of the rim deposits of the prominent Pulvermaar in the western Eifel shows that the tephra exposed at the rim consist to ca 50 % of round lava-coated fragments of a slowly cooled igneous rock originally emplaced at ca. 20km beneath the surface as deduced from fluid inclusions in clinopyroxene. Following fragmentation of the plutonic rocks and surrounding metamorphic host rocks at depth possibly by injected high-p CO2, the fragments were rounded, coated by new magma and likely transported by high velocity turbulent CO2-jets to the uppermost crust. Here they contributed to fragmentation of upper crustal Devonian sandstones/slates, fragmentation possibly enhanced by heated groundwater. A 10 m-wide outcrop of agglutinates at the southern inner crater wall shows that fluid melilite-nephelinite magma participated in the emplacement process. This hypothesis suggests a fundamental participation of magmatic CO2 in the fragmentation, upward transport and eruption of largely solid igneous pellets. This model, if substantiated by the proposed research, would represent a basic change in paradigm with respect to maar genesis. It would also imply that the origin of at least some maar structures in the Eifel resembles that of prevailing models of multistage emplacement processes of kimberlite systems rather than of phreatomagmatic maar formation, the main source of energy and site of fragmentation being magmatic (CO2) sourced deep in the crust/mantle. High-resolution field and lab analyses will be carried out to test the proposed model.