The origin of the glass-coated crustal xenoliths Rockeskyllerkopf volcanic complex, West Eifel, Germany

Abstract

The Rockeskyllerkopfvolcano in the Quaternary West Eifel Volcanic field is composed of maar, spatter and scoria deposits, and lava flows that erupted at various stages throughout the volcano's history. The xenoliths that occur in the silicaundersaturated nephelinitic and tephritic lavas are clinopyroxenite + peridotite, as well as Devonian sedimentary and lower crustal rocks. The quartz-rich Devonian sedimentary xenoliths are divided into two categories. The first are unaltered xenoliths that occur in maar and poorly-welded scoria deposits. The second are glass-coated xenoliths; these are found in welded, bomb-rich deposits related to a feeder dyke that, once it broke through to the surface, formed a fire fountain type eruption. The glass-coated xenoliths range from 1 cm up to 10cm in diameter. The majority are rounded with some having folded on themselves. The colour of the glass coating varies from green to brown, it is on average less than a millimeter thick, except where the xenoliths are folded, or have drips at their base. In the latter case the glass coating can be up to 3 millimeters thick. Back-scattered electron images show that the xenoliths can be divided into three zones. The first is the glass-coating that contains partly dissolved, rounded quartz crystals and quench crystals of alkali feldspar and a few vesicles. The second zone contains more rounded quartz grains and vesicles with less glass. The third zone is the inner most zone; it is made up of angular to subrounded quartz grains and broken down biotite grains that formed glass+ oxides+/- alkali feldspar+/- cordierite. This zone contains the most vesicles of the three and the least glass. Veins are present in several sample; these run through zones two and three, and appear to be the source of the glass in the outer most zone. The glass coating is interpreted to have formed by the breakdown of biotite and reaction of the melt with the quartz grains in the xenolith. The heat source that led to melting and breakdown of the biotite is the dyke and spatter deposits. Through capillary action the melt concentrated on the edges of the xenoliths. This was tested by experiments on similar Devonian sedimentary rock. The rock was heated in a furnace to temperatures between 1001°C and 1276°C for up to 180 minutes. The textures and mineralogy are similar to those observed in the samples from the volcano, this supports the hypothesis for the petrogenesis of these samples.