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We provide evidence for a plume-like upwelling beneath the Eifel hotspot, Western Germany, by using teleseismic shear-wave splitting to resolve the anisotropy associated with upwelling flow that is spreading laterally into the asthenosphere. The variation in fast-polarization azimuth we find across the Eifel hotspot is explained by a model of slowly upwelling material that is horizontally being deflected or sheared in a parabolic asthenospheric flow (PAF) pattern toward west-southwest, a direction that correlates with Eurasian absolute plate motion. We suggest that the lack of an age progression for Eifel volcanism, which is expected for a fixed-upwelling model, is a result of (1) sporadic volcanism due to a low excess plume temperature and/or varying crustal stresses that periodically relax and facilitate eruption, and (2) complex upwelling flow pathways and/or Late Tertiary changes in the slow Eurasian plate motion. The success of the PAF model in fitting the data is remarkable given the small number of parameters (four) and the consistency with the plate motion direction determined from geology and/or geodesy. This suggests that a predictable mantle-anisotropy pattern may exist also for other hotspots driven by plume-like upwellings, and that splitting can be a useful diagnostic to differentiate between plume-like and alternative sources for mantle hotspots. [ABSTRACT FROM AUTHOR]

We study the gravity field of the western Rhenish Massif, Germany, to search for relationships to the mantle source of the recent Eifel volcanism. First we compile the available gravity data from the region and present them as Bouguer gravity map. This map does hardly show any anomaly univocally related to the Eifel plume, even after careful wavelength filtering the data. Forward modelling of the Bouguer gravity field with the seismological low-velocity anomaly in the upper mantle as input anomaly finds that the seismological model corresponds to an approximately -10 mGal anomaly (1 mGal is 10−5 m s−2). Such a tiny signal can be hardly recognized in the real data: Intrusions of magmatic high-density dykes and sills may even hide or reverse the expected negative mantle gravity signal which makes the situation even worse. As conclusion we find that a gravity signal of the Eifel plume cannot be seen in the available Bouguer data, but that this negative result is at least consistent with the seismological mantle models. Furthermore the forward modelling results suggest that small-scale upper mantle plumes may not be recognized as a Bouguer anomaly underneath a typical heterogeneous continental lithosphere. [ABSTRACT FROM AUTHOR]

We obtain Rayleigh wave dispersion curves and 1D models of the shear wave velocity vs underneath the Eifel region, Germany from teleseismic recordings. Using 31 broadband mobile stations and 10 permanent stations distributed across a nearly 400 km × 400 km wide area centred on the Eifel, an average regional model is obtained which shows a clear low vs anomaly (LVA) at 70 to 200 km depth with a maximum velocity reduction of 2.3 % at 135 km depth. To enhance the resolution below the volcanic Eifel region we calculated a local vs model based on a reduced data set with stations only from that area. This local model contains a more pronounced LVA starting at 45-50 km depth with a minimum vs at around 85 km depth (−2.5 % vs). The upper boundary of this local LVA is interpreted as the uppermost portion of the Eifel plume, and its location is in accordance with receiver function results (see Weber et al., this volume). [ABSTRACT FROM AUTHOR]

The average Moho depth in the Eifel is approximately 30 km, thinning to ca. 28 km under the Eifel volcanic fields. Receiver function (RF) images suggest the existence of a low velocity zone at about 60 to 90 km depth underneath the West Eifel. This observation is supported by P- and S-wave tomographic results and absorption (Ritter this volume). Indications for a zone of increased velocity near 200 km depth, again agree with S-wave and absorption tomographic results. This anomaly, surprisingly not visible in P-wave tomography, could be due to an area of S-wave anisotropy that compensates for elevated plume temperatures. All three RF anomalies — at the Moho, at 60 to 90 km and near 200 km depth — have a lateral extent of about 100 km. The aperture of the Eifel network limits the resolution of tomographic methods to the upper 400 km. The RF method does not suffer from this limitation and can resolve deeper structures. The 410 km discontinuity under the Eifel is depressed by 15 to 25 km. Lowering of the 410 km discontinuity could be explained by a maximum temperature increase of +200 to +300 °C. The second surprising feature in the 3-D RF image of the Eifel Plume is the occurrence of two additional, currently unexplained conversions between 410 and 550 km depth. They could represent remnants of previous subduction or anomalies due to delayed phase changes. The lateral extent of the two additional conversions and the depression of the 410 km discontinuity is about 200 km. The 660 km discontinuity, in contrast to the 410 km discontinuity, does not show any depth deviation from its expected value, a scenario also encountered in the western US. Based on these observations we present the following scenario for the Eifel plume. The Eifel plume is a plume with temperature excess relative to the surrounding mantle of about +200 to +300 °C. The plume is imaged in the upper mantle and might be fed by regions imaged as low velocity anomalies in the lower mantle under Central Europe. Seismological methods provide only a blurred present day snap-shot. Thus we can not exclude the possibility that ascent of plume material, possibly coming even from the lower mantle, is intermittent and we see only the present day effects and configuration of the plume. [ABSTRACT FROM AUTHOR]

In 1997/98 the seismological Eifel Plume experiment was conducted in Belgium, France, Germany and Luxembourg to record a comprehensive dataset for several analysis techniques. Here we summarise the main points about the field experiment, the 3D tomography studies using compressional (P) and shear (S) waves and the analysis of the P-coda for scattering characteristics. Our seismic models are compared with results from surface wave dispersion studies, teleseismic shear-wave splitting and receiver functions. The models contain as a common feature a low-velocity anomaly in the upper mantle. This anomaly starts at about 50-60 km depth and reaches down to at least 410 km depth. The velocity reduction reaches up to 2% for compressional waves and 5% for shear waves in its upper part (50-100 km depth). In the lower part of the upper mantle the reduction is about 1% for both wave types. The diameter of this anomaly is about 100-120 km. As cause for the velocity reduction we infer increased ambient temperatures of about 100-150 °C. At 50-100 km depth there may be also about 1% of partial melt. We interprete this seismic anomaly as small upper-mantle plume that buoyantly upwells from the transition zone and starts to melt just below the continental lithosphere and possibly also at greater depth. At the surface the plume activity is documented by the Quaternary Eifel volcanic fields. Solidified magma chambers and dykes in the lithosphere are inferred to cause strong scattering of seismic waves as discovered by coda analyses. [ABSTRACT FROM AUTHOR]

The Rhenish Massif was affected by uplift since the Early Tertiary. The uplift velocity increased abruptly during the Pleistocene, just after the paleomagnetic Brunhes/Matuyama boundary, i.e. about 0.8 Ma BP. Since that time the rivers were forced to cut deep valleys into the uprising crustal block. The height difference (δh) between an 0.8 Ma old valley floor, the Younger "Hauptterrasse" (Main Terrace), and the bottoms of the recent valleys allows us to quantify the amount of the young strong uplift. While in the central and eastern part of the Rhenish Massif the amount of uplift is between 50 and 100 m on average, the Eifel and the north-eastern part of the Hunsrück area show uplift of more than 200 m. This area of intense uplift contains a distinct anomaly with nearly 300 m uplift, situated just east of the Quaternary West Eifel volcanic field. The strong uplift in the Rhenish Massif is synchronous with the young volcanism of the Eifel region. [ABSTRACT FROM AUTHOR]

Neogene to Quaternary mafic alkaline volcanism in the Eifel is part of the Central European volcanic province. Timing and geographical distribution suggest a common, plate-scale origin, either by passive or active mantle upwelling. Seismic tomography suggests confined, narrow plume-structures in the upper mantle below the Eifel and the Massif Central, but a lower mantle connection is not approved. Geochemical characteristics (trace elements and Nd, Sr, Pb, He isotopes) of European Cenozoic volcanics indicate locally varying contributions with enriched mantle character and a common "European Asthenospheric reservoir" (EAR) signature, but a lower mantle component cannot be identified unambiguously. Combined high-precision helium, neon and argon isotope data of mantle xenoliths from mafic alkaline volcanic formations from the Eifel region and the Pannonian Basin show the presence of variably fractionated atmosphere- and mantle-derived components. The xenoliths have different generations of fluid inclusions hosting two distinct noble gas components: one component most probably originated from the local lithospheric mantle source with (21Ne/22Ne)mantle = 0.07 and 4He/3He ∼ 120,000 (∼6 RA), and dominates total helium and part of total neon. The second component is similar to hot spot-type noble gases, as observed in volcanic rocks from Hawaii or Réunion. It markedly influences neon isotopes and dominates the argon isotopic composition. Its isotopic structure is inferred to 40Ar/36Ar = 15,000 ± 1,200, (21Ne/22Ne)mantle = 0.042 ± 0.005 and a model-dependent 4He/3He ratio of ∼36,000 (∼20 RA). Elemental fractionation patterns of noble gases trapped in the xenolith inclusions indicate trapping of CO2-rich fluids during extended episodes of mantle metasomatism: early fluids have higher proportions of the local lithospheric component, while later fluids increasingly contain plume-derived volatiles. Noble gas isotope systematics indicate that a common deep mantle component is involved in European intra-plate volcanism, probably from a rising super-swell that feeds narrow upper mantle plumes beneath Europe. [ABSTRACT FROM AUTHOR]

Seismic tomography studies provide clear evidence for a small-scale, finger-like body of low-velocity material extending from about 70 to 400 km depth (Ritter et al. 2001, Keyser et al. 2002, Ritter this volume) or to 650 km (Montelli et al. 2004) beneath the Eifel. The causes of such strong "upper mantle plumes" and their significance with respect to intra-plate volcanism are still a matter of controversial debate. The instabilities might either represent active thermal uplift driven by base heating of the 670 km discontinuity or a response to external tectonic developments, i.e. lithospheric stress might induce regions of decompression melting of fertile fusible patches within a heterogeneous upper mantle (e.g. Courtillot et al. 2003, Wilson 2003, Wilson and Patterson 2001). Thus the fundamental issue is whether elevated temperatures, the occurrence of partial melts, variations in modal composition, mineral anisotropy, or distinct combinations of these effects contribute to the upper mantle seismic anomalies. [ABSTRACT FROM AUTHOR]

The two Quaternary volcanic fields in the Eifel region of Germany (West Eifel Volcanic Field - WEVF; East Eifel Volcanic Field - EEVF) resemble each other in their temporal, spatial, structural and compositional evolution but also differ significantly in several parameters. Most volcanoes in both fields erupted foiditic potassic (K2O/Na2O >1) lavas with phenocrystic phlogopite and microlitic leucite being mineralogically most diagnostic as are the corresponding major and trace element characteristics. Volcanoes are dominantly scoria cones, of which about half erupted lava flows, and maars, their formation being partly governed by magma-water interaction. Phreatomagmatic eruptive activity reflecting variable degrees of magma/water mixing occurred during the growth of many scoria cones especially during the initial growth stage. Volcanic activity in the WEVF started slowly less than 700 ka ago after the Rhenish shield had begun an accelerated phase of uplift with highly silica-undersaturated foiditic magmas near Ormont at the border with Belgium in the NW and peaked in the central part of the field between ca. 600 and 450 ka. Following a subsequent lull in activity, volcanism migrated to the SE, the frequency of volcano formation increasing during the past <100 ka, the youngest eruption having occurred at 11 ka. Most lavas are mafic with rare intermediate and local small highly evolved centers in the eastern central part of the field. Magma fractionation at high pressure, such as near the crust/mantle boundary, is reflected in common green-core clinopyroxene phenocrysts in many types of lavas - in both fields - and high temperature overprinting, partial melting and metasomatism of lower/middle crustal granulites. Very mafic and much less silica-undersaturated sodic olivine nephelinites and relatively LILE-poor sodic basanites with groundmass plagioclase, both being distinctly less isotopically enriched than the foidites, erupted in the southeastern WEVF during the past <50 ka side-by-side with foidites. Distinct suites of ultramafic xenoliths, each with many variants, are recognized: (1) depleted and enriched peridotites (lherzolites, dunites, harzburgites and wehrlites) comprising several groups (highly deformed porphyroclastic xenoliths in the periphery and high-temperature recrystallized anhydrous types and metasomatized types near the center) and (2) cumulate- textured hornblendites, glimmerites and pyroxenites. The fact that clastic maar deposits are especially rich in peridotite and other ultramafic xenoliths is explained by xenolith-rich mafic volatile-rich magmas rising from greater depth coupled with high expulsion speeds during phreatomagmatic explosions. The near-absence of peridotite xenoliths and the abundance of clinopyroxene-, phlogopite- and amphibole-rich ultramafic cumulates containing remnants of peridotites in maar deposits in the southeastern part of the WEVF is probably due to more efficient filtering and dissolution of mantle peridotite fragments in subcrustal to lower crustal magma reservoirs in which the cumulates formed. Volcanic activity in the EEVF started about 460 ka ago or slightly earlier. As in the WEVF, early activity in the western part of the EEVF is dominated by mafic foiditic lava compositions. A prominent phonolitic complex in its center (Rieden, ca. 430-360 ka) is represented by intrusions, domes, ignimbrites, and widespread fallout tephra. A younger eastern subfield beginning with the partly trachytic highly evolved phonolitic Wehr crater complex at about 215 ka ago was followed soon by the emplacement of potassic basanitic to tephritic scoria cones chiefly in the Neuwied tectonic basin. Volcanism extended east as far as Rhine River and south to close to the Moselle. Most EEVF basanite volcanoes formed ca. 215-190 ka ago.… [ABSTRACT FROM AUTHOR]

Major and trace element as well as isotopic compositions on 26 volcanic rocks from the Tertiary Hocheifel volcanic field and for comparison from Upper Rhine Graben occurrences (western Central European Volcanic Province) were measured in order to provide geochemical evidence for the geodynamic setting of the Tertiary Eifel volcanism. Except for a few differentiated lavas there are mainly basanitic compositions. These rocks were produced by low degree partial melting of a previously metasomatized garnet peridotite source at pressures and temperatures corresponding to depths of about 75 to 90 km. In contrast to the differentiated lavas, most of the basanites are not significantly affected by crustal contamination. In the Hocheifel, the crustally contaminated basanites and the differentiated lavas form an older age group (ca. 44-39 Ma). In contrast, the basanites not affected by crustal contamination belong to a younger age group (ca. 37-35 Ma) indicating a change in the petrogenetic style of evolution with time. Low-radiogenic Sr together with high-radiogenic Nd and Pb of the Hocheifel lavas show isotope characteristics similar to LVC (Low Velocity Composition) or FOZO (Focal Zone) mantle sources. Since FOZO is considered to be a common component in the entire mantle, a contribution from the deep mantle can be neither confirmed nor excluded. Plate reconstruction modelling indicates that the Hocheifel volcanic field at its time of activity ca. 40 m.y. ago was located ∼1000 km southwest relative to its recent position, and therefore a relation to a mantle-stationary Quaternary Eifel plume is not plausible. Integrating evidence from geochronology, geochemistry, isotopic compositions and plate reconstruction, we conclude that the Tertiary Hocheifel volcanism is not genetically related to a mantle plume but is caused by Middle to Upper Eocene pre-rift extensional decompression related to the Rhine Graben taphrogenetic evolution. [ABSTRACT FROM AUTHOR]

The Eifel volcanism is part of the Cenozoic Central European Volcanic Province and is located close to the Rhine Graben which has been formed by rifting and subsidence since the Eocene. Whereas the Quaternary volcanism of the Eifel appears to be genetically related to mantle plume activity, the cause of the Tertiary volcanism of the Hocheifel volcanic field is less clear. Here, we present geochronological evidence for the geotectonic setting of the Tertiary Eifel volcanism based on 40Ar/39Ar dating of 27 samples from 25 volcanic occurrences. Included are samples from the northern Upper Rhine Graben in order to evaluate a possible relationship between Hocheifel volcanism and Rhine Graben taphrogenesis. The geological relevance of the age data used for the geological discussion is indicated by plateau-type age spectra and tested by inverse isochron calculations. For the Hocheifel, two periods of activity at ca. 44 to 39 Ma and 37 to 35 Ma were inferred. Both age groups are represented by basanites as well as by more differentiated rocks and there is no clear relation between age and chemical composition. The time span dated for the northernmost Upper Rhine Graben volcanism is 59 to 47 Ma indicating foiditic to basanitic activity up to ca. 15 m.y. prior to the onset of rifting and subsidence. The Hocheifel volcanic activity is interpreted to represent the propagation of the pre-rift volcanism of the northern Upper Rhine Graben to the NW because the Hocheifel activity closely follows in time pre-rifting northern Upper Rhine Graben volcanism and the older period of Hocheifel activity shows propagation of younger volcanism to the north. In addition, the Hocheifel tectonic pattern derived from the time - space relation of volcanism corresponds to stress field conditions identical to those of the Upper Rhine Graben at the time of Hocheifel volcanic activity. Magma generation therefore appears to be related to decompression by extension during Middle to Upper Eocene. However, in contrast to the northern Upper Rhine Graben with graben formation subsequent to pre-rifting volcanism, no relevant rifting or subsidence occurred in the Hocheifel. The taphrogenetic evolution in between the Upper and Lower Rhine Graben regions was suspended in an early stage. [ABSTRACT FROM AUTHOR]

The Cenozoic volcanism in the French Massif Central region is fed by an upper mantle plume, which was revealed by teleseismic tomography about 10 years ago. This contribution reviews earlier studies and applies a new method to image the crust and upper mantle in the region. Since teleseismic tomography alone has only moderate ability to resolve crustal structures, we perform an integrated study by a joint teleseismic-gravimetric inversion to investigate the gross crustal imprints of the Massif Central. We use a 3-dimensional joint inversion code, which allows a variable model parameterisation, and 3D ray tracing to perform an iterative inversion. Travel time residuals are corrected for Moho topography and sedimentary influences to avoid mapping of known crustal structure into the mantle. Our study finds a prominent low-velocity structure in the upper mantle, which is interpreted as the thermal signature of the Massif Central plume. With a modelled diameter of about 100-120 km it reaches down to at least 330 km depth. The average determined seismic P-wave velocity contrast is −0.6% to −1.0% in the shallow asthenospheric mantle and deeper upper mantle. We found two low-velocity channels in the crustal layer beneath the Cantal/Monte Dore and south of the Devès volcanic fields. A zone of mainly high density and increased seismic velocity is determined in the crust south of the Limagne Graben between the two volcanic fields. Furthermore the Massif Central is characterised by increased seismic scattering in the lithosphere as found by studying the teleseismic P-wave coda. We interpret the detected high-velocity/high-density body and the lithospheric scatterers as cooled magmatic intrusions, produced during the Cenozoic volcanism. [ABSTRACT FROM AUTHOR]