Eifel Volcanic Field Research Articles

Seismological monitoring of magmatic and tectonic earthquakes in the East Eifel Volcanic Field, Germany

Seismological monitoring of magmatic and tectonic earthquakes in the East Eifel Volcanic Field, Germany

An adaptive 6-dimensional floating-search multi-station seismic-event detector (A6-DFMSD) and its application to low-frequency earthquakes in the East Eifel Volcanic Field, Germany

We introduce a seismic event detector that applies signal analysis in the time and frequency domains. Signals are searched for with matching coincidences at neighbouring recording stations in space and time. No a priori waveform information is needed for the Adaptive 6-Dimensional Floating-search Multi-station Seismic-event Detector (A6-DFMSD). It combines a short / long time average algorithm (STA/LTA), frequency range selection, energy envelope matching, and backprojection techniques to find a robust detection model. As a challenging test example, the new detector is tuned and applied to a dataset with five months of microearthquake (ML < 2) recordings in the East Eifel Volcanic Field (EEVF), Germany. There, both magmatic and tectonic earthquakes occur in a depth range between 3 km and 43 km. A6-DFMSD detected 4.3 times as many events as were already known and it discovered a previously unknown event cluster. After manual localization and classification of the events, we show that A6-DFMSD finds events of different origins: tectonic, magmatic, atmospheric, and anthropogenic. In particular, low-frequency (LF) earthquakes of magmatic origin with a complicated waveform coda are very well identified. We suggest that seismological networks monitoring local seismicity in similar target zones would benefit from the use of A6-DFMSD to allow the detection of a wide range of different seismic signals.

Rotherkopfite, KNa2(Fe2+2.5Ti4+1.5)Fe2+(Si4O12)2, a new neptunite-group mineral without essential lithium, from Rother Kopf, Eifel volcanic fields, Germany

Abstract. Rotherkopfite, KNa2(Fe2.52+Ti1.54+)Fe2+(Si4O12)2, is a new member of the neptunite group, from Rother Kopf, Roth, near Gerolstein, Eifel volcanic fields, Rhineland-Palatinate, Germany. It is found in cavities in a quartz–sanidine xenolith embedded in a vesicular alkaline basalt and is associated with fluorophlogopite and an amphibole supergroup mineral that is zoned from potassic-magnesio-fluoro-arfvedsonite on the exterior to potassic-fluoro-richterite in the core. It is presumed to have formed as the result of contact metasomatism of the xenolith by the alkaline basalt melt. Rotherkopfite occurs as brownish-red equant or tabular crystals, up to about 0.2 mm in maximum dimension. The mineral has a light-orange streak, a vitreous lustre, a Mohs hardness of ∼4.5, a brittle tenacity, a curved fracture and a density of 3.20(2) g cm−3. Optically, rotherkopfite crystals are biaxial (+), with α=1.668(5), β=1.678(5), γ=1.720(5) (white light) and 2V(meas) = 53.2(6)°. The empirical formula from electron microprobe analyses, secondary ion mass spectrometry and structure refinement is (K0.87Na0.20)Σ1.07(Na1.99Ca0.01)Σ2.00M1+M2(Fe1.662+Ti1.48Mg0.79Mn0.02)Σ3.95M3(Fe0.642+Li0.16Ti0.15Al0.01)Σ0.96(Si8.00O24). Rotherkopfite is monoclinic with space group C2/c and unit-cell parameters a=16.4599(17), b=12.5457(6), c=10.0487(7) Å, β=115.669(7)°, V=1870.3(3) Å3 and Z=4. The crystal structure (R1=0.0268 for 1324 reflections with I&gt;2σI) is based on two interwoven three-dimensional frameworks: (1) a silicate framework made up of pyroxene-like chains of corner-sharing SiO4 tetrahedra and (2) an octahedral framework made up of chains of edge-sharing metal–oxygen octahedra. The two interwoven frameworks are bound to one another by corner sharing. K and Na are hosted in channels in the combined framework.

Petrogenesis of volcanic rocks from the Quaternary Eifel volcanic fields, Germany: detailed insights from combined trace-element and Sr–Nd–Hf–Pb–Os isotope data

Quaternary rocks from the East and West Eifel volcanic fields in western Germany are a key suite of intraplate volcanic rocks that can provide insights into volcanism of the Central European Volcanic Province (CEVP) and into continental intraplate volcanism in general. We present a comprehensive dataset for Eifel lavas including isotope as well as major and trace element data for 59 samples covering representative compositions of the different volcanic fields. In line with previous studies, the lavas are all SiO2-undersaturated, alkaline-rich and mainly comprise primitive basanites, melilitites, and nephelinites (Mg# ≥ 57). Geochemical compositions of samples from both volcanic subfields display distinct differences in their trace-element as well as radiogenic isotope compositions, largely confirming previous subdivisions. Coupled trace-element and radiogenic Sr–Nd–Hf–Pb–Os isotope compositions can now provide firm evidence for spatially heterogeneous mantle sources and compositionally distinct magmatic pulses. Within the West Eifel Field, Sr–Nd–Pb isotope compositions of the younger (≤80 ka), ONB-suite (olivine-nephelinite-basanite) are similar to FOZO (FOcal ZOne) or the EAR (European Asthenospheric Reservoir) and resemble compositions that have been previously reported from plume-sourced ocean island basalts (OIB). In marked difference, older (700 Ma to 80 ka) volcanic rocks from the F-suite (Foidite) in the West Eifel field and from the entire east Eifel Field tap a more enriched mantle component, as illustrated by more radiogenic Sr isotope (86Sr/87Sr up to 0.705408) and variable Pb isotope compositions (206Pb/204Pb = 18.61–19.70, 207Pb/204Pb = 15.62–15.67 and 208Pb/204Pb = 38.89–39.76). Combined trace-element compositions of ONB-suite samples are in good agreement with results from batch melting models suggesting a hybrid composition of Eifel magmas formed through mixing 10% of a FOZO-like melt with 90% of a DMM-like melt, similar to melts from the Tertiary HEVF. However, radiogenic Sr–Nd–Pb isotope compositions of F-suite and EEVF and some ONB lavas require the admixture of melts from lithospheric mantle sources. Elevated Nb/Ta and Lu/Hf ratios in combination with variable 187Os/188Os ratios can now demonstrate the presence of residual carbonated eclogite components, either in the lithosphere or in the asthenospheric mantle. Finally, by combining geochemical and temporal constraints of Tertiary and Quaternary volcanism it becomes evident that CEVP volcanism in central and western Germany has resulted from compositionally distinct magmatic pulses that tap separate mantle sources. Although the presence of a mantle plume can neither be fully confirmed nor excluded, plume-like melt pulses which partially tap carbonated eclogite domains that interact to variable extents with the lithosphere provide a viable explanation for the temporal and compositional cyclicity of CEVP volcanism.

Reconstruction of warm-season temperatures in central Europe during the past 60 000 years from lacustrine branched glycerol dialkyl glycerol tetraethers (brGDGTs)

Abstract. Millennial-scale climate variations during the last glacial period, such as Dansgaard–Oeschger (DO) cycles and Heinrich events, have been extensively studied using ice core and marine proxy records. However, there is a limited understanding of the magnitude of these temperature fluctuations in continental regions, and questions remain about the seasonal signal of these climate events. This study presents a 60 000-year-long temperature reconstruction based on branched glycerol dialkyl glycerol tetraethers (brGDGTs) extracted from lake sediments from the Eifel Volcanic Field, Germany. brGDGTs are bacterial membrane-spanning lipids that are known to have a strong relationship with temperature, making them suitable for temperature reconstructions. We test several temperature calibration models on modern samples taken from soils and multiple maar lakes. We find a negative bias in brGDGT-based temperature estimates associated with water depth and anoxic conditions that can be corrected for by accounting for a brGDGT isomer that is only produced in anoxic conditions. The corrected temperature reconstruction correlates with proxy and climate model estimates of temperature spanning the same time period, validating the calibration approach we selected. However, millennial-scale variability is significantly dampened in the brGDGT record, and in contrast to other Northern Hemisphere climate records, during several Heinrich stadials, temperatures actually increase. We demonstrate that these apparent discrepancies can be explained by the unique seasonal response of the brGDGT paleothermometer to temperatures of months above freezing (TMAF). Our data support the view that warm-season temperatures in Europe varied minimally during the last glacial period and that abrupt millennial-scale events were defined by colder, longer winters. Our continuous high-resolution temperature reconstruction provides important information about the magnitude of seasonal climate variability during the last glacial period that can be used to test climate models and inform studies of paleoecological change.

Clinopyroxenite Xenoliths Record Magma Transport and Crystallization in the Middle and Upper Crust: A Case Study from the Rockeskyllerkopf Volcanic Complex, West Eifel, Germany

Abstract Clinopyroxenite xenoliths comprising cumulus clinopyroxene ± amphibole together with intercumulus phlogopite ± olivine ± apatite ± titanite form a large part of the xenolith load in the oldest deposits of the Rockeskyllerkopf Volcanic Complex (RVC) in the West Eifel volcanic field. The xenoliths also contain xenocrysts of olivine and clinopyroxene derived from mantle peridotite and clinopyroxene from lower crustal granulite. The clinopyroxenite xenoliths are divided into five groups on the basis of their modal mineralogy and mineral compositions. Groups 1 to 4 define a continuous compositional trend indicative of fractionation of a mafic alkaline magma. Group 5 xenoliths are compositionally distinct and have been tentatively linked to high pressure crystallization of phonolitic magma within the RVC system. Thermobarometry of the group 1 to 4 xenoliths indicates that they crystallized between 1 and 4 kilobars, equivalent to a depth of 4 to 14 km. Group 1 to 3 xenoliths all crystallized at between 1050°C and 1150°C, whereas the amphibole-rich group 4 xenoliths give temperature estimates of ~900°C. The clinopyroxenites share a common parent magma with clinopyroxene–phlogopite veins found in subcontinental lithospheric mantle xenoliths. However, the vein forming mama was richer in incompatible elements, in particular Zr and Hf and is interpreted to be an early formed batch of magma with the clinopyroxenites crystallizing from magma derived from the same mantle, which had been depleted by the earlier phase of melting. Intrusion of magma began around 155 ky prior to the eruption of the RVC. Fe–Mg interdiffusion profiles in zoned clinopyroxene show that the magma that formed the xenoliths was present in the crust for up to 28 ky prior to the eruption. However, most samples give interdiffusion times between 1.5 and 9.9 ky. Based on xenocryst residence times and the calculated P–T conditions for clinopyroxene, there were at least seven separate batches of magma emplaced below Rockeskyllerkopf, probably as sills.

Identifying Neotectonic Motions in Germany Using Discontinuity-corrected GNSS Data

The crustal motions throughout Germany have not yet been fully understood because the research scope of previous studies often focuses only on some active grabens. Thus, we investigate it in detail to identify the neotectonic motion characteristics and specific deformation-ongoing regions. High accuracy for monitoring and data analyses is required because the expected crustal deformation in Germany is small. For this reason, we use high-precision GNSS time series processing techniques and interdisciplinary data to reflect actual motions and determine the causes of deformation. Also, an advanced technique of discontinuity correction is introduced to unify the fragments of the GNSS coordinate time series for better velocity field reliability. Our findings show that the crustal motions in Germany tend to increase at a maximum speed of +1.0 mm/year. Meanwhile, local subsidence of around -\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-$$\\end{document}0.8 mm/year is concentrated in the river basins (e.g., the Rhine, Ems, Elbe, Northern Oder, and Danube) and extensive mining regions. The Earth’s crust here also behaves with noticeable compressions. The intra-plate motion in Germany is ∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim $$\\end{document}0.8 mm/year. A special region with an extension rate of +4.3 nstrain/year is observed along the North–South trending Regensburg-Leipzig-Rostock shear zone. Machine Learning clusters the 3D plate velocity field in Germany into three distinct regions with increasing speeds: Northwest, East, and Southwest. Significant surface deformations are detected mainly in the Upper Rhine graben, Eifel volcanic field, and Thuringian-Vogtland slate mountains. The harmonic motions of the Earth’s crust in Germany have an amplitude of ∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim $$\\end{document}4.7 mm, in which the surface loads contribute half to this type of motion. The findings will contribute to the overall picture of neotectonics here.

Lithospheric Sill Intrusions and Present‐Day Ground Deformation at Rhenish Massif, Central Europe

AbstractThe Rhenish Massif in Central Europe, which includes the Eifel Volcanic Fields, has shown ongoing ground deformation and signs of possible unrest. A buoyant plume exerting uplift forces at the bottom of the lithosphere was proposed to explain such deformation; the hypothesis of (possibly concurrent) melt accumulation in the crust/lithospheric mantle has not been explored yet. Here, we test deformation models in an elastic half‐space considering sources of varying aspect ratio, size and depth. We explore the effects of data coverage, noise and uncertainty on the inferred source parameters. We find that the observed deformation would require melt accumulation in sub‐horizontal sill‐like structures expanding at the rate of up to ∼0.045 km3/yr. We discuss feasibility, limitations and possible interpretations of our resulting models and elaborate on further observations which may help constrain the structure of the Rhenish Massif magmatic system.

Depth and timing of magma evolution underneath the Emmelberg scoria cone in the West Eifel Volcanic Field

Depth and timing of magma evolution underneath the Emmelberg scoria cone in the West Eifel Volcanic Field

Bayesian age–depth modelling applied to varve and radiometric dating to optimize the transfer of an existing high-resolution chronology to a new composite sediment profile from Holzmaar (West Eifel Volcanic Field, Germany)

Abstract. This study gives an overview of different methods to integrate information from a varve chronology and radiometric measurements in the Bayesian tool Bacon. These techniques will become important for the future as technologies evolve with more sites being revisited for the application of new and high-resolution scanning methods. Thus, the transfer of existing chronologies will become necessary because the recounting of varves will be too time consuming and expensive to be funded. We introduce new sediment cores from Holzmaar (West Eifel Volcanic Field, Germany), a volcanic maar lake with a well-studied varve record. Four different age–depth models have been calculated for the new composite sediment profile (HZM19) using Bayesian modelling with Bacon. All models incorporate new Pb-210 and Cs-137 dates for the top of the record, the latest calibration curve (IntCal20) for radiocarbon ages as well as the new age estimation for the Laacher See Tephra. Model A is based on previously published radiocarbon measurements only, while Models B–D integrate the previously published varve chronology (VT-99) with different approaches. Model B rests upon radiocarbon data, while parameter settings are obtained from sedimentation rates derived from VT-99. Model C is based on radiocarbon dates and on VT-99 as several normal distributed tie points, while Model D is segmented into four sections: sections 1 and 3 are based on VT-99 only, whereas sections 2 and 4 rely on Bacon age–depth models including additional information from VT-99. In terms of accuracy, the parameter-based integration Model B shows little improvement over the non-integrated approach, whereas the tie-point-based integration Model C reflects the complex accumulation history of Holzmaar much better. Only the segmented and parameter-based age integration approach of Model D adapts and improves VT-99 by replacing sections of higher counting errors with Bayesian modelling of radiocarbon ages and thus efficiently makes available the best possible and most precise age–depth model for HZM19. This approach will value all ongoing high-resolution investigations for a better understanding of decadal-scale Holocene environmental and climatic variations.

Magma accumulation underneath Laacher See volcano from detrital zircon in modern streams

The detrital zircon grains from two catchments in the East Eifel Volcanic Field consist of crystals sourced from proximal deposits of Quaternary trachytic–phonolitic volcanic centres, Hocheifel Paleogene trachytic rocks and the Paleozoic basement. The outcrop patterns indicate the derivation of a significant detrital zircon population from deposits of the c. 13 ka Lower and Upper Laacher See Tephra. Intercalated Middle Laacher See Tephra, consisting of valley-filling ignimbrites, by contrast, is a minor zircon source despite thick ignimbrite deposits in one of the catchments. This reflects zircon undersaturation in the hotter and less evolved phonolite tapped from deeper parts of the magma reservoir as the eruption unfolded, as well as fewer antecrysts recycled from the plutonic carapace. A previously unrecognized zircon age population of c. 63 ka, mostly derived from Lower Laacher See Tephra, marks the onset of the presence of evolved magma at the top of the reservoir. Correlative Hf and O isotopic analysis reveals significant crustal interactions during phonolite differentiation in a shallow reservoir, with the assimilation of cogenetic, but hydrothermally modified, plutonic margins. Raman spectra indicate that magmatically heated crustal zircon xenocrysts are absent. Detrital zircon can thus provide more integrated insights into the evolution of Quaternary magma systems than the punctuated sampling of volcanic or cogenetic plutonic rocks. Supplementary material: Data tables presenting U–Th, U–Pb, O–Hf and Raman geochronology results are available at https://doi.org/10.6084/m9.figshare.c.6123823

Fingerprinting secondary mantle plumes

Many vertical seismic velocity anomalies observed below different parts of the Eurasian plate are rooted in the transition zone between the upper and lower mantle (410–660 km), forming so-called secondary plumes. These anomalies are interpreted as the result of thermal effects of large-scale thermal upwelling (primary plume) in the lower mantle or deep dehydration of fluid-rich subducting oceanic plates. We present the results of thermo-mechanical numerical modelling to investigate the dynamics of such small-scale thermal and chemical (hydrous) anomalies rising from the lower part of the Earth's upper mantle. Our objective is to determine the conditions that allow thermo-chemical secondary plumes of moderate size (initial radius of 50 km) to penetrate the continental lithosphere, as often detected in seismo-tomographic studies. To this end, we examine the effect of the following parameters: (1) the compositional deficit of the plume density due to the presence of water and hydrous silicate melts, (2) the width of the weak zone in the overlying lithosphere formed because of plume-induced magmatic weakening and/or previous tectonic events, and (3) a tectonic regime varied from neutral to extensional. In our models, secondary plumes of purely thermal origin do not penetrate the overlying plate, but flatten at its base, forming “mushroom”-shaped structures at the level of the lithosphere-asthenosphere boundary. On the contrary, plumes with enhanced density contrast due to a chemical (hydrous) component are shown to be able to pass upwards through the lithospheric mantle to shallow depths near the Moho when (1) the compositional density contrast is ≥ 100 kg m−3 and (2) the width of the lithospheric weakness zone above the plume is ≥ 100 km. An extensional tectonic regime facilitates plume penetration into the lithosphere but is not mandatory. Our findings can explain observations that have long remained enigmatic, such as the “arrow”-shaped zone of low seismic velocities below the Tengchong volcano in south-western China and the columnar (“finger”-shaped) anomaly within the lithospheric mantle discovered more than two decades ago beneath the Eifel volcanic fields in north-western Germany. It appears that a chemical component is a characteristic feature not only of conventional hydrous plumes located over presently downgoing oceanic slabs, but also of upper mantle plumes in other tectonic settings.

Vein and Wehrlite Formation in the Lithospheric Mantle Below the West Eifel Volcanic Field: Modelling the Effects of Pressure, Temperature and Peridotite/Melt Ratio on Magma-Peridotite Interaction Using pMELTS and a Melt Infiltration Experiment

Abstract The mantle peridotite xenoliths of the West Eifel Volcanic Field in Germany preserve evidence of multiple phases of metasomatism. The most recent metasomatic episode produced a variety of veins in the peridotite: high temperature (&amp;gt;1150°C), phlogopite–clinopyroxenite intermediate temperature (~1150°C), olivine clinopyroxenite and relatively low temperature (&amp;lt;1100°C) hornblendite together with marginal reaction zones of wehrlite and dunite. The veins and associated reaction zones have been interpreted as products of crystallization of magmas similar to those that transported the xenoliths to surface. We describe a high temperature melt infiltration experiment and thermodynamic (pMELTS) models that examine the origin of high temperature phlogopite–clinopyroxenite veins and the evolution of the wall rock adjacent to the veins and compare the result to the major and trace element signatures of the minerals in the veins. The infiltration experiment replicates the common reaction textures such as partially dissolved orthopyroxene and sieved secondary clinopyroxene that are found associated with veins. In the thermodynamic model, we calculated the equilibrium assemblages and mineral compositions for peridotite–melt mixtures ranging from peridotite only to melt only over the range of 1150 to 1350°C and 1 GPa to 2 GPa. The models reproduce the composition of vein minerals at a small peridotite/melt ratio, whereas at larger peridotite/melt ratios they produce wehrlite/dunite assemblages that are similar both in modal mineralogy and composition to the natural samples. The models show that olivine clinopyroxenite veins may have been produced at a higher pressure than the phlogopite–clinopyroxenite veins. Our models show that interaction of magma with a trace element signature indicative of a garnet-bearing source with spinel facies mantle will result in a dilution of the trace element garnet signature with little to no variation in the major oxide composition.

&amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C, CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; ∕ &amp;lt;sup&amp;gt;3&amp;lt;/sup&amp;gt;He and &amp;lt;sup&amp;gt;3&amp;lt;/sup&amp;gt;He ∕ &amp;lt;sup&amp;gt;4&amp;lt;/sup&amp;gt;He ratios reveal the presence of mantle gas in the CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;-rich groundwaters of the Ardennes massif (Spa, Belgium)

Abstract. Although natural CO2-rich groundwaters of eastern Belgium have been known for centuries, the exact origin of their gas is still unclear. This paper presents the results of a sampling campaign in Belgium (Spa, Stoumont, Malmedy): 30 samples of both carbogaseous and non-carbogaseous groundwaters were analyzed for major elements, CO2 content and carbon isotopic composition. Among them, 13 samples were also analyzed for 3He/4He and 4He/20Ne ratios. The combination of δ13C (between ca. −9 ‰ VPDB1 and −2 ‰ VPDB), CO2/3He ratio (between 1.9×108 and 2.9×109) and 3He/4He (between 0.92 and 2.70 Ra) shows with a high level of confidence that the CO2 in the carbogaseous groundwater of Spa and Bru has a mantle origin. It can likely be attributed to the degassing of mantle from the neighboring Eifel volcanic fields, located at a distance of 100 km eastwards. The identity and nature of the deep-rooted fractures that act as CO2 transport pathways to the surface are still to be clarified, but several major thrust faults exist in the Rhenish Massif and could connect the Eifel volcanic fields with the studied area.

Dissolution - reprecipitation reactions as a mechanism for magma contamination: An example from interaction of partially melted sanidine megacrysts and clinopyroxene phenocrysts in nephelinite from Graulei, West Eifel Volcanic Field, Germany

Sanidine – diopside – aegirine-augite enclaves in a nephelinite from the West Eifel Volcanic Field in Germany provide new insights into the process of xenolith assimilation. The enclaves originated as sanidine megacrysts which were entrained by the rising nephelinite magma and partly melted at ~1190 °C and 0.5 GPa. The partly melted sanidine captured clinopyroxene phenocrysts (Type 1 cpx) from the nephelinite. These captured phenocrysts underwent dissolution and reprecipitation to form sieve-textured to euhedral near end member diopside crystals (Type 3 cpx) that grew epitaxially on the original phenocryst. The components of the dissolved phenocryst that were not taken up by the diopside were enriched in the potassic melt that formed the enclave, and during quenching formed aegirine-augite crystals (Type 4 cpx) that locally overgrow the Type 3 diopside. Only enclaves > ~ 5 mm preserve the diopside – aegirine-augite – sanidine assemblage. Small Type 2 clinopyroxene-and leucite-rich clusters in the nephelinite are the remnants of assimilation of mm-sized sanidine crystals. The mechanism of phenocryst capture by molten enclaves shows that the assimilation process is more complex than simple dispersal of melt and crystals from disaggregated xenoliths and that crystal transfer from the magma to partly molten xenoliths may play a significant role in the process.

New evidence for upper Permian crustal growth below Eifel, Germany, from mafic granulite xenoliths

Abstract. Granulite xenoliths from the Quaternary West Eifel Volcanic Field in Germany record evidence of magmatism in the lower crust at the end of the Permian. The xenoliths sampled two distinct bodies: an older intrusion (ca. 264 Myr old) that contains clinopyroxene with flat, chondrite-normalised rare earth element (REE) profiles and a younger (ca. 253 Myr old) intrusion that crystallised middle-REE-rich clinopyroxene. The younger body is also distinguished based on the negative Sr, Zr and Ti anomalies in primitive mantle-normalised multi-element plots. REE-in-plagioclase–clinopyroxene thermometry records the magmatic temperature of the xenoliths (1100–1300 ∘C), whereas Mg-in-plagioclase and Zr-in-titanite thermometry preserve an equilibration temperature of ca. 800 ∘C. These temperatures, together with a model of the mineral assemblages predicted from the composition of one of the xenoliths, define the pressure of crystallisation as ∼1 GPa. The xenoliths also preserve a long history of reheating events whose age ranges from 220 to 6 Myr. The last of these events presumably led to breakdown of garnet; formation of symplectites of orthopyroxene, plagioclase and hercynite; and redistribution of heavy rare earth elements into clinopyroxene. The data from the West Eifel granulite xenoliths, when combined with the existing data from granulites sampled in the East Eifel, indicate that the lower crust has a long a complex history stretching from at least 1.6 Ga with intrusive events at ca. 410 and 260 Ma and reheating from the Triassic to late Miocene.

Quantification of excess 231Pa in late Quaternary igneous baddeleyite

AbstractInitial excess protactinium (231Pa) is a frequently suspected source of discordance in baddeleyite (ZrO2) geochronology, which limits accurate U/Pb dating, but such excesses have never been directly demonstrated. In this study, Pa incorporation in late Holocene baddeleyite from Somma-Vesuvius (Campanian Volcanic Province, central Italy) and Laacher See (East Eifel Volcanic Field, western Germany) was quantified by U-Th-Pa measurements using a large-geometry ion microprobe. Baddeleyite crystals isolated from subvolcanic syenites have average U concentrations of ~200 ppm and are largely stoichiometric with minor abundances of Nb, Hf, Ti, and Fe up to a few weight percent. Measured (231Pa)/(235U) activity ratios are significantly above the secular equilibrium value of unity and range from 3.4(8) to 14.9(2.6) in Vesuvius baddeleyite and from 3.6(9) to 8.9(1.4) in Laacher See baddeleyite (values within parentheses represent uncertainties in the last significant figures reported as 1σ throughout the text). Crystallization ages of 5.12(56) ka (Vesuvius; MSWD = 0.96, n = 12) and 15.6(2.0) ka (Laacher See; MSWD = 0.91, n = 10) were obtained from (230Th)/(238U) disequilibria for the same crystals, which are close to the respective eruption ages. Applying a corresponding age correction indicates average initial (231Pa)/(235U)0 of 8.8(1.0) (Vesuvius) and 7.9(5) (Laacher See). For reasonable melt activities, model baddeleyite-melt distribution coefficients of DPa/DU = 5.8(2) and 4.1(2) are obtained for Vesuvius and Laacher See, respectively. Speciation-dependent (Pa4+ vs. Pa5+) partitioning coefficients (D values) from crystal lattice strain models for tetra- and pentavalent proxy ions significantly exceed DPa/DU inferred from direct analysis of 231Pa for Pa5+. This is consistent with predominantly reduced Pa4+ in the melt, for which D values similar to U4+ are expected. Contrary to common assumptions, baddeleyite-crystallizing melts from Vesuvius and Laacher See appear to be dominated by Pa4+ rather than Pa5+. An initial disequilibrium correction for baddeleyite geochronology using DPa/DU = 5 ± 1 is recommended for oxidized phonolitic melt compositions.

Timescales from magma mixing to eruption in alkaline volcanism in the Eifel volcanic fields, western Germany

Diffusion profiles in olivine crystals from the final mafic eruption products of the compositionally zoned Laacher See tephra deposit were measured to identify recharge and eruption-triggering events prior to the eruption of the Laacher See volcano (12.9 kyr). These products represent the hybrids of mixing between phonolite and intruding basanite at the bottom of the reservoir, which is likely related to the eruption-triggering event. Additionally, olivine crystals from ten basanitic scoria cones and maar deposits (East Eifel) and two nephelinites (West Eifel) were analyzed to constrain histories of olivine in Quaternary basanite magmas. Olivine crystals from the Laacher See hybrids vary in core composition (Fo83–89) and show reversely zoned mantles with high Fo87.8–89 compared to olivine in East Eifel basanites erupted in nearby, older scoria cones. Towards the crystal margin, olivine in the hybrids develop a normally zoned overgrowth (Fo86.5–87.5). Olivine from East Eifel basanites show similar zonation and core compositions (Fo80–88) but have less forsteritic mantles (Fo83–88) indicating that these basanites are less primitive than those recharging the Laacher See reservoir (> Fo89). Olivine in the West Eifel nephelinites show mantles similar to those from Laacher See (Fo87.5–90), but have normal zoning and high-Fo cores (Fo88–92). This indicates that olivine in the Laacher See hybrids were entrained by a near-primary basanite from older cumulates just before hybridization of the basanite with the phonolite. Diffusion modeling indicates maximum timescales between entrainment and eruption of Laacher See of 30–400 days that are comparable to those calculated for olivine from basanitic scoria cones (10–400 days).

Geodetic evidence for a buoyant mantle plume beneath the Eifel volcanic area, NW Europe

SUMMARY The volcanism of the Eifel volcanic field (EVF), in west-central Germany, is often considered an example of hotspot volcanism given its geochemical signature and the putative mantle plume imaged underneath. EVF's setting in a stable continental area provides a rare natural laboratory to image surface deformation and test the hypothesis of there being a thermally buoyant plume. Here we use Global Positioning System (GPS) data to robustly image vertical land motion (VLM) and horizontal strain rates over most of intraplate Europe. We find a spatially coherent positive VLM anomaly over an area much larger than the EVF and with a maximum uplift of ∼1 mm yr−1 at the EVF (when corrected for glacial isostatic adjustment). This rate is considerably higher than averaged over the Late-Quaternary. Over the same area that uplifts, we find significant horizontal extension surrounded by a radial pattern of shortening, a superposition that strongly suggests a common dynamic cause. Besides the Eifel, no other area in NW Europe shows significant positive VLM coupled with extensional strain rates, except for the much broader region of glacial isostatic adjustment. We refer to this 3-D deformation anomaly as the Eifel Anomaly. We also find an extensional strain rate anomaly near the Massif Central volcanic field surrounded by radial shortening, but we do not detect a significant positive VLM signal there. The fact that the Eifel Anomaly is located above the Eifel plume suggests that the plume causes the anomaly. Indeed, we show that buoyancy forces induced by the plume at the bottom of the lithosphere can explain this remarkable surface deformation. Plume-induced deformation can also explain the relatively high rate of regional seismicity, particularly along the Lower Rhine Embayment.

Mullite-2&amp;lt;i&amp;gt;c&amp;lt;/i&amp;gt; &amp;amp;#8211; a natural polytype of mullite

Abstract. A single crystal (∼ 20 µm × 20 µm × 330 µm) of mullite-2c, a natural polytype of mullite, was separated from a radially grown cluster of acicular crystals from Ettringer Bellerberg (Quarternary Eifel volcanic fields, Germany). The chemical composition determined from electron microprobe analysis (EMPA) is Na0.01Mg0.05Al8.52Fe0.293+Si3.13Ti0.02O19.55, corresponding to x=0.22(8) in the generalised mineral formula My+Mgz2+M8+4x+y-2z3+M4-4x-y+z4+O20-2x. Only Fe3+ as foreign cation was considered in the refined structure model, partially replacing Al3+ in the octahedral chains. A crystal of a similar type, though exhibiting a significantly different composition with x=0.02, was first described in 2015, tentatively named “sillimullite” by Fischer et al. (2015). This crystal and our new sample have similar structural properties, now classified as a polytype of mullite, designated mullite-2c. Single-crystal X-ray diffraction showed that the mullite-2c crystal investigated here exhibits partial Si ∕ Al ordering in the double chains of (Si,Al)O4 tetrahedra in contrast to the sample described in 2015 as being completely ordered. The ordering in mullite-2c results in a doubled c lattice parameter with respect to mullite. It crystallises in space group Pnam, with cell parameters for the new sample of a=7.5432(5) Å, b=7.7048(5) Å, c=5.7965(3) Å, V=336.89(6) Å3 and Z=1. X-ray powder diffraction data are presented with a detailed discussion of the differences between the diffraction patterns of sillimanite, mullite and mullite-2c. Crystals of mullite-2c are translucent to lightly violet, they possess a vitreous lustre and the calculated density is 3.199 g cm−3. The optical character is biaxial (+), with refractive indices determined by spindle-stage microscopy of nx=1.6673, ny=1.6687 and nz=1.680(4) (adjusted to conform to 2VZ=39(4)∘). Applying the Gladstone–Dale approach, the compatibility index is 0.007, representing superior compatibility. In terms of chemical composition and structural features mullite-2c is an outstanding example of mullite-type compounds falling into the postulated miscibility gap between sillimanite and mullite. Its crystal structure combines characteristics from both mullite (oxygen vacancies, triclusters of tetrahedral building units) and sillimanite (high degree of Si ∕ Al ordering in the tetrahedral building units, causing the doubled c parameter). The lattice parameters (normalised to 1c) of the new sample lie between those of sillimanite and 3 / 2 mullite; the chemical composition is close to 3 / 2 mullite and thus differs significantly from the silica-rich composition of the species previously determined by Fischer et al. (2015), indicating a relatively large compositional variation.