The Rocourt Tephra (RT) is one of the widespread stratigraphic markers in western Germany, Belgium, and south Netherlands (Fig. 1). When the age of the eruption is known, either by dating the host sediments or the volcanic source itself, such tephra markers are precious guides for establishing reliable chronostratigraphic correlations from distinct sedimentary environments and for dating sedimentary or volcanic sequences. The RT is mainly used as an accurate chronostratigraphic marker in loess deposits, where its position inside the Humiferous Complex of Remicourt (Haesaerts et al., 1999; Haesaerts et al., 2016) allows to distinguish the MIS 5 interglacial and early glacial pedocomplex from Middle Pleistocene pedocomplexes (Meijs, 2002; Haesaerts et al., 2016; Pirson et al., 2018). It is also used for datingfluvial terraces (Juvigné et al., 2008), but also Middle Palaeolithic archaeological sites in several sedimentary environments including cave entrance sequences sometimes containing Neandertal remains (Pirson et al., 2006; Pirson and Di Modica, 2011; Juvigné et al., 2013; Di Modica et al., 2016).
Based on radiometric dating of their host sediments but also over- and underlying sediments, the age of the RT can be positioned between 61.5 and 90.3 ka (Pirson et al., 2006; Debenham, 2011; Pirson and Juvigné, 2011). Correlations of loess reference sequences and the INTIMATE event stratigraphy (high-resolution chronology for the Late Interglacial-Glacial cycles in the North Atlantic region refined by Rasmussen et al., 2014) allow the RT host sediment, the Humiferous Complex of Remicourt, to be estimated at around 80 − 78 ka (Juvigné et al., 2013; Haesaerts et al., 2016).
At this stage of the research, it becomes important to identify the volcano whose eruption is the source of the RT. On the one hand, this would reinforce or clarify the age of the fallout, because this volcano could be dated by various methods. On the other hand, it would be possible to determine the composition of the magma behind the RT's volcanic clasts, unfortunately altered, but also of the various minerals. All these data would give us more elements to better characterize the RT and facilitate its distinction from more or less contemporary tephras.
Geological setting and the Eifel sources
The Rocourt Tephra (RT) has been recorded in some thirty sites (Fig. 1). Most of the sites are emplaced in loess sections. The others are in sediments of alluvium terraces and in cave deposits (Pirson & Juvigné, 2011). The RT is a cryptotephra that is composed of explosively erupted ash-sized glass-shards and of crystals, preserved in sediments (including loess) or soils/paleosols but too widely scattered or too fine to be visible as a layer to the naked eye (Lowe, 2011). The RT is recognized by its concentration of volcanogenic minerals. Volcanic shards have been detected at three sites in loess sections, Rocourt, Kesselt and Remicourt (Fig. 1). They consist of blocky shards of hyaloclasts with a curviplanar surface and no vacuoles, which are typical of ash products from a major hydromagmatic eruptive event. Unfortunately, the volcanic glass is weathered. Alteration of the vitreous phase has led to the formation of illite, kaolinite and smectite (Pouclet et al., 2008; Juvigné et al. 2013). The average mineral composition of the volcanic clasts bearing sites (Rocourt, Kesselt and Remicourt) is: 70% ± 15 of clinopyroxene, 10% ± 5 of orthopyroxene, 17% ± 6 of amphibole, and 3% ± 1 of spinel. The appearance of these minerals is shown in Fig. 2. Clinopyroxenes are distributed in three groups: xenocrysts from peridotitic and pyroxenitic xenoliths, megacrysts from high pressure magmatic reservoirs, and phenocrysts from the magma chamber of the emitting volcano. These phenocrysts are characterized by the abundance of fassaitic diopside, a high-Ca pyroxene enriched in Ca-Tschermakite mole, which defined a high alkaline composition for the magma source (Pouclet et al., 2008).
The RT mineral and geochemical data suggest that the emitting volcano is in the West Eifel Volcanic Field (WEVF), where there are numerous large eruption centers produced by hydromagmatic explosive activities of alkaline magmas. Gullentops and Hocht (1998) attributed two occurrences of a tephra layer suspected to be the RT in the Lower Rhine Bay (Inden and Garzweiler), to the Dreiser Weiher, a monogenic crater of the WEVF (Fig. 1). However, Förster et al. (2020) placed the eruption of the Dreiser Weiher at 41 ka by ice-core tuning, too young to be the RT source. Instead, they correlated the RT with tephra layers reported in drill cores of lacustrine deposits in the WEVF, which they attributed to the Pulvermaar, a neighbouring monogenic crater (Fig. 1). They dated these tephra layers at ca. 75 ka, close to the bracketed age of the RT.
To test these assumptions, we sampled known tephra layers from the Dreiser Weiher and the Pulvermaar volcanoes. We analysed their mafic mineral composition and compared these with assemblages associated with the RT to determine whether either of these sources were potential source volcanoes.
Analytical procedures
Samples were taken from exposures in ancient quarries in the ramparts of the Dreiser Weiher and the Pulvermaar volcanic edifices. Dense minerals (δ g/cm3 > 2.8) were separated by boiling in HCl10%vol; sieving by 355/75 µm, and extracting the dense minerals with bromoform (δ = 2.8) in separating funnel by repeating agitation-decantation-harvest cycles, until no more harvest is obtained (generally 3 to 5 cycles). Aliquots were examined under the binocular magnifier, and smear slides were prepared in Canada balsam for identification with a petrological microscope. Examination of the material was focused on the volcanogenic mafic minerals (vmm). Feldspars have not been studied in the RT because it is not easy to distinguish juvenile volcanic feldspars from detrital ones. Consequently, feldspars are not considered in the present work.
Microprobe analyses were processed with a CAMECA SX Five electron probe microanalyser (EPMA) at Camparis, University Paris-Sorbonne, France, for the heavy minerals from the Dreiser Weiher and the Pulvermaar volcanoes. The reference materials were diopside for Si, Ca and Mg; Fe2O3 for Fe; MnTiO3 for Ti and Mn; Cr2O3 for Cr: albite for Na; and orthoclase for K and Al. The Ka X-ray was used for all the elements. The operating conditions included an accelerating voltage of 15 kV and beam current of 10 nA. Counting times were 20s for the peak and 10s for the background. Data corrections were made using the PAP method according to Pouchou and Pichoir (1991).