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Journal of Petrology | Volume 44 | Number 6 | Pages 1077-1095 | 2003
© Oxford University Press 2003
Lithospheric Mantle Evolution beneath the Eifel (Germany): Constraints from Sr–Nd–Pb Isotopes and Trace Element Abundances in Spinel Peridotite and Pyroxenite Xenoliths
1 INSTITUT FÜR MINERALOGIE UND GEOCHEMIE DER UNIVERSITÄT KÖLN, ZÜLPICHER STR. 49B, D-50674 COLOGNE, GERMANY
2 MINERALOGISCHES INSTITUT DER UNIVERSITÄT HEIDELBERG, IM NEUENHEIMER FELD 236, D-69120 HEIDELBERG, GERMANY
3 INSTITUT FÜR MINERALOGIE DER UNIVERSITÄT MÜNSTER, CORRENSSTR. 24, D-48149 MÜNSTER, GERMANY
4 RESEARCH SCHOOL OF EARTH SCIENCES, THE AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, A.C.T. 0200, AUSTRALIA
Present address: Institut für Mineralogie und Geochemie der Universität Köln, Zülpicher Str. 49B, D-50674 Cologne, Germany. Telephone: ++49 221 470 3170. Fax: ++49 221 470 5199. E-mail: Gudrun.Witt-Eickschen{at}uni-koeln.de
RECEIVED MARCH 22, 2002; ACCEPTED DECEMBER 13, 2002
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONANALYTICAL PROCEDURESPETROGRAPHY OF THE XENOLITHSTRACE ELEMENT CHARACTERISTICS...DISCUSSIONREFERENCES |
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The Pb isotope compositions of amphiboles and clinopyroxenes in spinel peridotite and pyroxenite mantle xenoliths from the intra-plate Quaternary volcanic fields of the Eifel province (Germany) are strongly correlated with their Sr–Nd isotope and trace element compositions. High-temperature anhydrous xenoliths from a depth of around 60 km have trace element and Sr–Nd–Pb isotope compositions similar to the depleted source of mid-ocean ridge basalts (Depleted MORB Mantle, DMM). Amphibole-bearing xenoliths from shallower depths (<45 km) provide evidence for three temporally distinct episodes of mantle metasomatism in the subcontinental lithosphere: (1) aqueous fluids from an isotopically enriched (EM-like) mantle reservoir caused amphibole formation during deformation in the shallow continental lithospheric mantle and may be subduction related, probably associated with the last major tectonic event that influenced the area (Hercynian orogeny). (2) During a second phase of mantle metasomatism the EM-like lithospheric mantle was affected by melts from an ancient, HIMU-like (high time-integrated µ = 238U/204Pb) mantle source. The HIMU-like component introduced by these fluids had a much more radiogenic Pb isotope composition than the asthenospheric source of the widespread Cenozoic magmatism in Europe and may be linked to reactivation of ancient subducted crustal domains during the Hercynian orogeny or to early Cretaceous deep-sourced mantle plumes. (3) During a brief final stage the heterogeneously enriched EM–HIMU subcontinental lithosphere was locally modified by basaltic melts migrating along fractures and veins through the upper mantle as a consequence of the Cenozoic Eifel volcanism. Although a DMM component is completely lacking in the metasomatic fluids of the metasomatic episodes 1 and 2, the vein melts of episode 3 and the Cenozoic Eifel lavas require mantle sources containing three end-member components (DMM–HIMU–EM). Thus, mobilization of the more depleted mantle material occurred at the earliest in the Tertiary, contemporaneously with the development of the extensive rift system and main melt generation in Europe. Alternatively, the variety of Sr–Nd–Pb isotope signatures of the metasomatic agents may have been produced by melting of isotopically distinct mantle domains in a heterogeneous uprising mantle plume.
KEY WORDS: Eifel; Europe; mantle xenoliths; metasomatism; Pb isotopes
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONANALYTICAL PROCEDURESPETROGRAPHY OF THE XENOLITHSTRACE ELEMENT CHARACTERISTICS...DISCUSSIONREFERENCES |
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The Cenozoic western and central European volcanic provinces are spatially and temporally linked to the development of an extensive intra-continental rift system and domal uplift of Hercynian basements in connection with the collision of the African and Eurasian plates (Wilson & Downes, 1991; Wilson & Patterson, 2001). The most primitive mafic rocks within these volcanic fields share many of the Sr–Nd–Pb isotope and geochemical characteristics of plume-related ocean island basalts (OIB) (e.g. Wedepohl & Baumann, 1999). Thus, the chemical composition of the lavas appears to be consistent with an origin from a mantle plume, but seismic or geological evidence for partial melting in a large ascending mantle plume is lacking. Instead, seismic tomographic studies indicate the presence of small, finger-like bodies of low-velocity material extending from
70 km to at least 400 km in the upper mantle beneath these areas (Granet et al., 1995; Ritter et al., 2001; Keyser et al., 2002). Whereas previous studies emphasized the difficulty in reconciling partial melting in such small mantle diapirs with plume dynamics (Wilson & Downes, 1991; Wedepohl et al., 1994; Hegner et al., 1995), more recent models recognize this as a distinct type of mantle convection (e.g. Wilson & Patterson, 2001). Cenozoic magma generation is associated with these small-scale zones of diapiric upwelling and thus the source of the OIB-like component is inferred to be located at the base of the upper mantle or even in the lower mantle (Wedepohl & Baumann, 1999; Wilson & Patterson, 2001). Although the abundant spinel peridotite mantle xenoliths exhumed from the subcontinental lithosphere beneath Europe by the Cenozoic magmas do not necessarily provide information about the source region of the most primitive basalts, they might provide a component within the lithospheric mantle that contributed to the isotope and geochemical characteristics of asthenosphere-derived magmas. These mantle xenoliths show a much wider range in Nd–Sr isotope compositions than their Cenozoic host basalts and provide important constraints on the nature and geochemical evolution of the lithospheric upper mantle beneath Europe (e.g. Downes, 2001). This raises the following questions: (1) Is the variable metasomatic enrichment of the subcontinental lithosphere related to the Cenozoic intra-plate magmatism of Europe or is it the fingerprint of much earlier metasomatic events? (2) Are the metasomatic agents plume or asthenosphere derived, or released from recycled ancient mafic subducted crust? (3) Did a lithospheric mantle, similar to that represented by the xenoliths, play a role in the petrogenesis of the Cenozoic lavas?
To address these questions we present a study of Pb isotope compositions in conjunction with new trace element data for different types of lithospheric mantle xenoliths (Table 1) from the Quaternary volcanic fields of the Eifel (Germany), for which, with the exception of some samples, Nd–Sr isotopic data have already been published (Stosch et al., 1980; Stosch & Lugmair, 1986; Witt-Eickschen & Kramm, 1998b; Witt-Eickschen et al., 1998). Supplementing these data with Pb isotopes has the distinct advantage that the mantle components are much better constrained than by Nd–Sr isotope correlations alone. Based on the extreme Sr–Nd–Pb isotope compositions of young oceanic basalts, Zindler & Hart (1986) distinguished four end-member components in the Earth's mantle: DMM (depleted upper mantle representing the source of mid-ocean ridge basalts), EM1 and EM2 (enriched mantle), and HIMU (high time-integrated µ = 238U/204Pb; widely considered to reflect ancient recycled oceanic crust). These reservoirs and isotope signatures are thought to be produced by the formation and recycling of oceanic crust and lithosphere, plus small amounts of recycled continental crust. Wörner et al. (1986) proposed an additional end-member composition for continental basalts (PREMA = prevalent mantle), which is isotopically enriched relative to DMM and interpreted as the reactivated ‘fossilized’ heads of mantle plumes that were unable to penetrate the continental lithosphere (Stein & Hofmann, 1992). Depleted DMM-like components present in plumes seem to differ from the present-day DMM source by having a higher Hf isotope ratio (e.g. Iceland lavas, Kempton et al., 2000) and are probably generated by ancient melting events. Moreover, most hotspots have distinct isotopic ‘flavours’ (Hofmann, 1997) that are not necessarily related to mixing of melts or material from the isotopically distinct end-member components but may be due to melting of mantle domains with specific isotopic and geochemical characteristics.
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The Sr–Nd–Pb isotope compositions of the mantle xenoliths are used to constrain the geochemical characteristics and source of the metasomatic agents that enriched the shallow subcontinental lithospheric mantle beneath the Eifel. In addition, we have studied pyroxenite and hornblendite veins occurring in composite mantle xenoliths that crystallized from parental melts generated from sub-lithospheric mantle sources. As the vein melts are clearly unmodified by crustal effects and genetically related to the Cenozoic alkali basalts from the Eifel (Witt-Eickschen & Kramm, 1998b; Witt-Eickschen et al., 1998), they retain the original isotopic characteristics of the mantle source of the intra-plate lavas.
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ANALYTICAL PROCEDURES |
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TOPABSTRACTINTRODUCTIONANALYTICAL PROCEDURESPETROGRAPHY OF THE XENOLITHSTRACE ELEMENT CHARACTERISTICS...DISCUSSIONREFERENCES |
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Incompatible trace element abundances in amphiboles and clinopyroxenes were measured in situ by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the Research School of Earth Sciences (ANU) in Canberra. Ablation was performed in a He atmosphere using an ArF EXIMER laser (193 nm) at 100 mJ/pulse and a 5 Hz pulse repetition rate using an ablation time of 60 s. The ablated material was flushed in a continuous argon flow into the torch of an Agilent 7500 Series ICP-MS system. The silicate glass reference material NIST 612 was analysed and background count rates were measured before and after 10 unknowns for calibration purposes and instrumental drift corrections. To correct for differences in the ablation yield between standard and samples, 43Ca was used as an internal standard, based on electron microprobe measurements of CaO in the minerals. Replicate routine analyses (n = 307) of trace element abundances in basalt glass standard USGS BCR-2G are compared with the data of Norman et al. (1998) in Table 2. These analyses yielded the following standard deviations (2
) of the average concentrations: rare earth elements (REE) (La to Yb) 4–6%, Lu 8%; large ion lithophile elements (LILE) 3–6%; Y 4%, high field strength elements (HFSE) 3–6%; Pb, U and Th 6%; W 16%; Mo 4%. The trace element concentrations reported in Tables 2 and 3 were collected from the grain cores with an 84 µm laser spot size and are averages of 3–9 analyses performed for each mineral. Multiple analyses of grain cores and rims with a 29 µm spot demonstrated that the trace element abundances are homogeneous within individual amphibole and clinopyroxene grains. Further information about analytical details including correction procedures, limits of detection, and instrumental errors has been given by Eggins et al. (1998).
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Sr–Nd–Pb isotopic analyses of inclusion-free amphiboles, clinopyroxenes, and phlogopites separated by handpicking under a binocular microscope (50–200 mg) were carried out at the Zentrallabor für Geochronologie der Universität Münster. The Sr–Nd isotope data were obtained using the same etching, separation, and analytical methods, including measurement conditions and correction procedures for mass fractionation, given in detail by Witt-Eickschen & Kramm (1998a). Repeat analyses yielded an 87Sr/86Sr ratio of 0·71024 ± 0·00002 (n = 55) for the NBS-987 standard and a 143Nd/144Nd ratio of 0·51184 ± 0·00002 (n = 24) for the La Jolla Standard. For the Pb isotope analysis amphibole and clinopyroxene separates were leached for 2 h in hot (80°C) 2N HCl and washed in ultra-pure water, whereas phlogopite was decomposed without pretreatment. A 205Pb spike was added before dissolution of the minerals in HF–HNO3 (5:1). The Pb fraction of the samples was separated by a wash and elution procedure on AG1-X8 anion exchange Teflon columns using 1N HBr and 6N HCl. The sample solution was evaporated down for loading after addition of a drop of 0·25N HNO3. Pb was measured on a single Re filament using the silica gel–H3PO4 emitter technique. The isotope ratios were measured in static multi-collection mode on a VG Sector 54 mass spectrometer at temperatures ranging from 1350 to 1450°C. Where concentrations of Pb were very low (<8 ng Pb) mass 204 was measured with a Daly detector, performing three Daly–Faraday gain calibrations before and after each sample run. All Pb isotope ratios are corrected for a 0·10% fractionation per a.m.u. based on repeat analyses of NBS 982. Total procedural blanks for Pb did not exceed 30 pg Pb during the period of the analytical work and were allowed for in the correction of the data (blank composition: 206Pb/204Pb = 17·72, 207Pb/204Pb = 15·52, 208Pb/204Pb = 37·70). The data determined exclusively by the Faraday cups have a total error of ±0·1%; those determined by the use of the electron multiplier have a total error of ±0·2%. Repeat analyses (n = 15) of the NBS 982 standard yielded 206Pb/204Pb = 36·738 ± 0·032, 207Pb/204Pb = 17·153 ± 0·020, and 208Pb/204Pb = 36·745 ± 0·054.
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PETROGRAPHY OF THE XENOLITHS |
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TOPABSTRACTINTRODUCTIONANALYTICAL PROCEDURESPETROGRAPHY OF THE XENOLITHSTRACE ELEMENT CHARACTERISTICS...DISCUSSIONREFERENCES |
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The samples analysed in this study include spinel peridotite and pyroxenite xenoliths from basaltic tephra layers of the Quaternary volcanic fields of the West and East Eifel [Fig. 1; West Eifel: Dreiser Weiher (DW), Meerfelder Maar (MM); East Eifel: Olbrück (EE)] that have previously been characterized petrologically and geochemically and for the most part for their Sr–Nd-isotopic composition (Stosch et al., 1980; Stosch & Lugmair, 1986; Stosch, 1987; Witt & Seck, 1989; Witt-Eickschen et al., 1993, 1998; Witt-Eickschen & Harte, 1994; Witt-Eickschen & Kramm, 1998b). The modal mineralogy of the samples is listed in Table 1. LA-ICP-MS incompatible trace element abundances and Sr–Nd–Pb isotope data for amphiboles and clinopyroxenes are presented in Tables 2–4. On the basis of their thermal and geochemical characteristics three groups of mantle xenoliths can be identified (Stosch & Seck, 1980; Witt-Eickschen et al., 1993): (1) anhydrous, high-temperature, light REE (LREE)-depleted peridotites; (2) two types of modally metasomatized xenoliths with texturally equilibrated Ti-poor amphiboles; (3) magmatic clinopyroxenites and hornblendites from vein systems within the lithospheric mantle of the West Eifel.
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Anhydrous, high-temperature, LREE-depleted xenoliths
High-temperature (1150–1250°C) coarse-grained (Fig. 2a) to recrystallized spinel peridotites from the West Eifel are anhydrous and were derived from a depth of around 60–65 km (Köhler & Brey, 1990). The lherzolites (Table 1) selected for this investigation lack textural evidence for the existence of a precursor protolith that has equilibrated at even greater depths (e.g. spinel–pyroxene clusters as an indicator of the former presence of garnet).
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On the basis of the depletion of LREE in the clinopyroxenes [(La/Yb)n = 0·4–0·6, where subscript n means normalized to the primitive mantle (PM) values of Hofmann (1988)] and their Sr–Nd isotope compositions (Table 4), Stosch et al. (1980) suggested that these xenoliths represent fragments of the subcontinental mantle lithosphere with geochemical and isotopic affinities of the mid-ocean ridge basalt source mantle (DMM; Depleted MORB Mantle). Their Sr and Nd model ages were attributed to multi-stage melting episodes in the sub-Eifel mantle with a main low-degree partial melting event at
2 Ga as a result of the formation of continental crust (Stosch, 1987).
Modally metasomatized xenoliths with texturally equilibrated Ti-poor amphiboles
In contrast to the LREE-depleted high-temperature peridotites, the majority of the mantle xenoliths of the West and East Eifel provide strong evidence for metasomatic overprint, which produced compositional and isotopic heterogeneities in the mantle part of the lithosphere. Two texturally distinct types of amphibole-bearing peridotites can be recognized: equigranular recrystallized and porphyroclastic.
Equigranular recrystallized amphibole-bearing xenoliths
Depleted lherzolites to harzburgites (Table 1) with tabular (Fig. 2b) or mosaic equigranular recrystallized textures, which occur in the West Eifel volcanic rocks, are considered to be the products of extensive shearing and recrystallization (e.g. Mercier & Nicolas, 1975). Temperatures of most samples (900–970°C) selected for this study plot within the low-temperature range typical for this xenolith suite (Sachtleben & Seck, 1981). Some xenoliths with chemically zoned orthopyroxene (MM262, MM766, DW194) yield higher temperatures (1020–1030°C) and belong to the group of reheated hydrous peridotites (Witt-Eickschen et al., 1993). CO2-rich fluid inclusions in the pyroxenes covering a variety of densities and homogenization temperatures suggest an equilibration of the peridotites immediately below the Moho (Witt-Eickschen et al., 2003), which lies at a depth of 28 km below the Eifel (Mechie et al., 1983; Raikes & Bonjer, 1983). The xenoliths contain 1–5 wt % tabular pargasite that had attained textural and major and trace element chemical equilibrium with all of the other mineral phases since its formation (Witt & Seck, 1989; Witt-Eickschen & Harte, 1994).
Porphyroclastic amphibole-bearing xenoliths
Deformation during cooling has resulted in the development of porphyroclastic textures in xenoliths from the East Eifel (Witt & Seck, 1989). The xenoliths are characterized by large deformed orthopyroxene porphyroclasts surrounded by a matrix of fine-grained, polygonal-shaped olivine, pyroxene and amphibole neoblasts (Fig. 2c). The equilibration temperatures of an early, high-temperature stage (>1050°C) have been reconstructed from the bulk composition of the now-exsolved pyroxene porphyroclasts by reintegrating the composition of the host pyroxene with that of the exsolution lamellae (Witt & Seck, 1989; Witt-Eickschen & Harte, 1994). The now-exsolved orthopyroxene porphyroclasts and the clinopyroxene lamellae, as well as the orthopyroxene and clinopyroxene neoblasts, record distinctly lower temperatures (800°C) that indicate cooling during recrystallization. The presence of 2–5 wt % pargasite to edenite amphiboles (Table 1) as polygonal neoblasts in the recrystallized matrix and in broken kink bands within the pyroxene porphyroclasts indicates a temporal association between deformation and metasomatic enrichment processes (Witt & Seck, 1989).
Clinopyroxenite and hornblendite magmatic veins and adjacent peridotitic wall rocks
Clinopyroxenite and mica-bearing hornblendite veins of presumed magmatic origin present in composite mantle xenoliths from the West Eifel provide evidence for high-pressure infiltration of silicate melts into the lithospheric mantle (Witt-Eickschen et al., 1993). The effect of these infiltrating melts on the mineralogy, major and trace element and Sr–Nd isotope composition of their peridotitic wall rocks is in most cases limited to distances of several centimetres from the veins (Witt-Eickschen et al., 1993, 1998; Witt-Eickschen & Kramm, 1998b). Both clinopyroxenite and hornblendite veins and their peridotitic wall rocks were selected for Pb isotope analysis.
Clinopyroxenite veins
The olivine-bearing clinopyroxenites occur as coarse-grained veins of 1–5 cm width (Fig. 2d) crosscutting anhydrous, high-temperature, peridotite host xenoliths and as discrete xenoliths (Fig. 2e) up to 25 cm in diameter. The vein clinopyroxenes have major element compositions similar to the clinopyroxenes from the non-composite high-temperature lherzolites from the West Eifel, but their Ti contents are distinctly higher (Witt-Eickschen et al., 1993). Witt-Eickschen & Kramm (1998b) estimated a depth interval of 50–70 km for the pyroxenite precipitation by estimating temperatures and pressures for the host peridotite using the two-pyroxene geothermometer of Brey & Köhler (1990) in combination with the Ca-olivine–clinopyroxene geothermobarometer of Köhler & Brey (1990). The relatively high pressures obtained (1·7–2·2 GPa) are still 0·1–0·5 GPa lower than the corresponding maximum pressures of the stability of the Cr-bearing spinel in these peridotites (Witt-Eickschen & Kramm, 1998b).
Micaceous hornblendite veins
Small (<1·5 cm) veins consisting of Ti-rich pargasite (up to 3·8 wt % TiO2) and phlogopite occur exclusively in contact with the equigranular recrystallized peridotites from shallow mantle depths (Fig. 2f). As a result of the interaction with the vein melts the geochemical and isotopic characteristics of the peridotitic wall rocks were altered in a transition area of a very limited distance of 1 cm from the vein contact. Within this area LREE, Sr and Nb were ‘leached’ from, whereas Ti, Zr and Hf were added into the wall rock, and the 87Sr/86Sr ratio of the host peridotite changed towards the values of the hornblendite vein (Witt-Eickschen et al., 1998).
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TRACE ELEMENT CHARACTERISTICS AND Sr–Nd–Pb ISOTOPE SIGNATURES OF THE XENOLITHS |
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TOPABSTRACTINTRODUCTIONANALYTICAL PROCEDURESPETROGRAPHY OF THE XENOLITHSTRACE ELEMENT CHARACTERISTICS...DISCUSSIONREFERENCES |
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Modally metasomatized xenoliths with texturally equilibrated Ti-poor amphiboles
Amphibole and clinopyroxene from the equigranular recrystallized xenoliths are strongly enriched in the LREE and middle REE (MREE) and relatively depleted in the HFSE Ti, Zr, and Hf (Figs 3b and 4b). In contrast, the concentrations of the highly incompatible HFSE Nb and Ta are extremely high in the amphibole, resulting in (Nb/La)n and (Ta/La)n ratios of 2–6 and 2–3, respectively. With the exception of one sample (MM278), amphibole and clinopyroxene reveal a trough for Pb relative to REE.
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The REE patterns of amphibole (Fig. 3a) and clinopyroxene (Fig. 4a) from the porphyroclastic xenoliths display a continuous decrease from Lu to Dy and range from V-shaped with minima at Sm, Eu or Gd to moderately enriched in both LREE and MREE. Compared with the pargasites from the equigranular recrystallized xenoliths, the amphiboles have lower K contents (Table 2), a strong depletion of Nb and Ta relative to REE [(Nb/La)n < 1], higher concentrations of Ba, Sr, Pb and U relative to REE (Fig. 3a), and distinctly higher U/Th ratios.
The Sr–Nd isotope compositions of coexisting clinopyroxene and Ti-poor amphibole are indistinguishable (MM766 in Table 4) and plot in an Sr–Nd isotope diagram significantly below the field for the primitive Cenozoic Eifel volcanic rocks (Fig. 5). The positively correlated 208Pb/204Pb and 206Pb/204Pb ratios define a linear array indicating that the time-integrated Th/U ratio in the source of the metasomatic agents was higher than that for ocean island basalts (OIB) from the northern hemisphere (Fig. 6a). A less coherent but stronger trend away from the Northern Hemisphere Reference Line (NHRL) of Hart (1984) is seen in the 207Pb/204Pb vs 206Pb/204Pb diagram (Fig. 6a).
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Broad positive and negative correlations, respectively, exist between 206Pb/204Pb and 143Nd/144Nd and 87Sr/86Sr (Fig. 7). The data plot between the two end-member mantle components EM1 and HIMU that define the LoNd (‘low Nd’) array of Hart et al. (1986). The strongest isotopic influence of a HIMU-like component is manifested in the equigranular recrystallized xenoliths from the Dreiser Weiher (samples DW194, DW211). Clinopyroxenes from these samples have 206Pb/204Pb ratios of >20 that fall on or close to the NHRL in the Pb–Pb isotope covariation diagrams (Fig. 6). Rosenbaum & Wilson (1996) reported similar high 206Pb/204Pb ratios for xenoliths from another West Eifel locality. The amphiboles from the porphyroclastic xenolith from the East Eifel plot closer towards the isotopic end-member EM1 along the EM1–HIMU trend (Fig. 7).
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Clinopyroxenite and hornblendite magmatic veins
Discrete, 10–16 cm diameter clinopyroxenite xenoliths (DW906, DW918) and 2–5 cm wide clinopyroxenite veins of composite xenoliths (DW327, DW328) from depths of
50–70 km were selected for the Pb isotope study. Their clinopyroxenes display convex-upward REE patterns and troughs for HFSE and Sr (Witt-Eickschen & Kramm, 1998b). The Pb, Th and U concentrations are significantly lower (Fig. 4c) compared with the clinopyroxenes from the porphyroclastic and equigranular recrystallized peridotites (Fig. 4a and b).
The Ti-rich amphiboles from relatively wide hornblendite veinlets (1 cm) occurring in contact with the hydrous equigranular recrystallized host peridotites from shallower mantle depths display convex-upward REE patterns combined with high HFSE concentrations and significant troughs for Th, U and Pb relative to REE (Fig. 3c). Ti-rich amphibole from a very thin hornblendite veinlet (<2 mm in size) has an REE pattern with a steep negative slope from LREE to heavy REE (HREE) combined with very high Nb, Zr and Hf concentrations (sample MM251 in Fig. 3c; not analysed for Sr–Nd–Pb isotopes because of the small vein volume). This veinlet has been interpreted to represent virtually complete crystallization of small volumes of melt that formed the amphiboles present in the other veins (Witt-Eickschen et al., 1998).
Despite the significant mineralogical and geochemical differences between the clinopyroxenite and hornblendite veins, the two types of magmatic veins are isotopically similar, suggesting a genetic link between their parental melts (Table 4). The Sr–Nd isotopic characteristics of the hornblendite veins are identical and lie within the range obtained for the clinopyroxenite veins (Fig. 5). The 206Pb/204Pb (18·95–19·43), 207Pb/204Pb (15·53–15·67) and 208Pb/204Pb ratios (38·85–39·39) of vein clinopyroxene, amphibole and phlogopite cover, within the limits of error, the entire range observed for the Cenozoic lavas from the Eifel volcanic field (Fig. 6). The vein and wall-rock minerals plot off the HIMU–EM1 trend shown by the hydrous peridotite xenoliths within the DMM–HIMU–EM1 space in the Sr–Nd–Pb diagrams (Fig. 7).
High-temperature, LREE-depleted xenoliths
Clinopyroxene from the most fertile LREE-depleted lherzolite with respect to mineralogy (DWK1: 10 wt % cpx) has the lowest Pb concentration (20 ppb) and the most unradiogenic Pb isotope composition (206Pb/204Pb 17·3) of all investigated clinopyroxene samples. It represents depleted mantle material with Sr–Nd–Pb isotope signatures similar to DMM that defines the source region of mid-ocean ridge basalts from the North Atlantic. The Pb isotope composition of clinopyroxene from the other high-temperature xenolith (DW58) is more radiogenic and plots in the 208Pb/204Pb vs 206Pb/204Pb covariation diagram on the NHRL (Fig. 6a). This supports the conclusion of Stosch & Lugmair (1986) that this peridotite may have experienced old (>1·1 Ga) multi-stage enrichment and depletion processes.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONANALYTICAL PROCEDURESPETROGRAPHY OF THE XENOLITHSTRACE ELEMENT CHARACTERISTICS...DISCUSSIONREFERENCES |
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Nature of the metasomatic agents: evidence from trace element compositions
Porphyroclastic xenoliths
The strong depletion from HREE to MREE in the clinopyroxenes and amphiboles (Figs 3a and 4a) of the porphyroclastic xenoliths suggests that these peridotites were initially depleted in their incompatible trace element abundances. During the enrichment process elements fairly soluble in aqueous fluids such as the LILE (Ba, Sr) and Pb have been added to the clinopyroxenes and amphiboles as manifested by their high Pb/Ce and Sr/Sm ratios, whereas Ta and Nb concentrations are depleted relative to U, REE and LILE (Figs 3a and 4a). The solubility of Th in aqueous fluids is lower than that of U at high oxygen fugacities (fO2 > FMQ – 2, where FMQ is fayalite–magnetite–quartz) and the resulting excess of U in the fluids can be further enhanced during the transit through the upper mantle (e.g. Brenan et al., 1995). Thus, the elevated primitive mantle normalized U/Th ratios in the amphiboles (3·1–5·9) and clinopyroxenes (2·7–5·5) from the porphyroclastic xenoliths compared with those from the equigranular recrystallized peridotites and magmatic veins (U/Th 0·4–1·3) are also compatible with a hydrous metasomatic agent. The remarkable amounts of W and Mo (0·1–0·9 ppm W; 0·5–1·2 ppm Mo) found in the amphiboles and clinopyroxenes (Tables 2 and 3) provide additional evidence for the involvement of aqueous fluids. These highly incompatible siderophile elements are commonly mobilized by aqueous fluids, as can be inferred from the composition of black smokers and hydrothermal ore-deposits and from melt–aqueous fluid partition coefficients (Keppler & Wyllie, 1991).
Equigranular recrystallized xenoliths
The element patterns and Sr–Nd–Pb isotope compositions of clinopyroxene and amphibole in the equigranular recrystallized xenoliths from the West Eifel are distinctly different from those of the minerals from the magmatic vein systems and at variance with an origin by spatially limited wall-rock reaction postulated by other workers (e.g. Nielson & Wilshire, 1993). Hence this type of mantle metasomatism may be related to pervasive melt flow through a porous medium rather than to wall-rock reactions.
Rare extrusive Quaternary carbonatitic and calcite-bearing rocks from the West and East Eifel (Liebsch et al., 1996; Riley et al., 1999) point to the existence of carbonatitic melts in the underlying lithosphere of the Eifel as their Sr–Nd isotope systematics are consistent with a mantle origin (Fig. 5). Thus we evaluated whether melts parental to carbonatites might be responsible for the enrichment in the peridotites by calculating the trace element concentrations of the hypothetical metasomatic agents in equilibrium with the equigranular recrystallized mantle xenoliths. Representative trace element patterns of the hypothetical melts, shown in Fig. 8, were calculated from the trace element compositions of amphibole (Fig. 3b) and clinopyroxene (Fig. 4b) from sample WE211, which has a highly radiogenic Pb isotope signature. For the calculation different sets of mineral–silicate melt and mineral–carbonatitic melt partition coefficients from the literature were used (Table 5). The metasomatic agents in equilibrium with clinopyroxene and amphibole are enriched in LREE and Sr and reveal marked negative anomalies of Ti, Zr and Hf relative to REE (Fig. 8b and c). Their trace element compositions are very similar to those of oceanic carbonatites (Fig. 8a) attributed to an asthenospheric mantle origin (Hoernle et al., 2002). In contrast, the Eifel carbonatites, which are petrogenetically linked to the Quaternary silicate Eifel lavas (e.g. Laacher See sövite shown in Fig. 8a; Liebsch, 1996), lack the relative Sr enrichment. Other marked features of the calculated melts are the Nb/Ta (on average 37) and Zr/Hf (on average 269) ratios distinctly higher and Ti/Eu (on average 130) ratios lower than the primitive mantle values (17·6, 34·2 and 7452, respectively), which are also consistent with the trace element characteristics of carbonatites (e.g. Woolley & Kempe, 1989). Because of their low viscosity and low dihedral wetting angles, carbonatitic melts can escape their source regions at melt fractions as low as 0·1% and percolate upwards through the peridotite matrix as a result of formation of interconnected grain-edge networks (e.g. Minarik, 1998). Thus such melts are considered as very effective and mobile metasomatic agents in the lithospheric mantle (e.g. Yaxley et al., 1998). The petrography and major element chemistry of the equigranular recrystallized peridotites, however, provide no further support for a carbonatite-induced metasomatism. This makes the interpretation considering interaction of the peridotites with carbonatitic melts difficult, but does not preclude this possibility.
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Clinopyroxenite and hornblendite veins
The clinopyroxene from the clinopyroxenite and the amphibole from the hornblendite veins differ significantly from those in the metasomatized peridotite host rocks by their higher Ti concentrations and convex-upward REE patterns (Figs 3 and 4) that are typical for amphibole and clinopyroxene megacrysts precipitated from basic melts (Irving & Frey, 1984). Thus the new trace element data obtained by LA-ICP-MS for selected veins support our previous conclusions (Witt-Eickschen & Kramm, 1998b; Witt-Eickschen et al., 1998) that the melts from which the clinopyroxenite and hornblendite veinlets crystallized clearly have a silicate melt origin. Using experimental mineral–silicate melt partition coefficients (Table 5) yields trace element compositions for a hypothetical melt in equilibrium with vein clinopyroxene DW906 (Table 3) perfectly overlapping in its shape with those of the Eifel silicate volcanic rocks (Fig. 9). The melt pattern calculated from the composition of vein amphibole MM333 as well as the pattern of vein amphibole MM251, representing complete crystallization of a small volume of melt (Witt-Eickschen et al., 1998), display a significant enrichment of Zr and Hf over Sm not recognized in the Eifel lavas.
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Metasomatic events in the lithospheric mantle beneath the Eifel: constraints from the Sr–Nd–Pb isotope signatures
Formation of a heterogeneous EM1–HIMU lithosphere
The combined Sr–Nd–Pb isotopic signatures of the mantle xenoliths display a large range of variations indicating isotopic heterogeneity in the lithospheric mantle underneath the Eifel (Table 4; Fig. 7). A DMM component occurs as an old protolith in the LREE-depleted, high-temperature lherzolites from the deeper regions of the Eifel lithosphere. These xenoliths may represent former asthenospheric material although there is no textural evidence for this conclusion. In contrast, the Sr–Nd–Pb isotope and trace element compositions of the hydrous tabular recrystallized and porphyroclastic peridotites provide evidence that highly enriched components infiltrated the initially depleted shallow continental mantle lithosphere, which might be related to the Cenozoic volcanism. According to current hypotheses (Wilson & Downes, 1991; Cebriá & Wilson, 1995; Hoernle et al., 1995) the Cenozoic primitive magmas of the continental volcanism in western and central Europe are thought to represent two distinct mantle sources. The most primitive sodium-rich mafic lavas from the Cenozoic European volcanic fields, as well as from the contemporaneous magmatism in the western Mediterranean and eastern Atlantic, have a common mantle source variously referred to as the EAR (European Asthenospheric Reservoir) or LVC (Low Velocity Composition) located deep in the asthenospheric mantle with isotope characteristics plotting between DMM and HIMU. Potassium-rich basalts have been attributed to EM1-like sources reflecting small-degree melts derived from phlogopite- (± amphibole)-rich parts of the lithospheric mantle whose variable isotope composition corresponds to the local tectono-magmatic history (e.g. Wilson & Downes, 1991). In the Quaternary Eifel volcanic fields both types of lavas occur: an Na-rich (K2O/Na2O < 0·5) olivine nephelinite and basanite suite recording Sr–Nd–Pb similar to, but less extreme than the EAR source and a K-rich (K2O/Na2O > 0·5) leucitite–melilite–nephelinite suite with Sr–Nd isotopic characteristics close to Bulk Earth (Kramers et al., 1981; Wörner et al., 1986).
However, on the basis of our data there is no indication of the involvement of the EAR component during the HIMU–EM1-like enrichment of the shallow subcontinental lithosphere manifested in the hydrous mantle xenoliths (Fig. 7). Thus the metasomatic agents responsible for this enrichment event do not have their isotopic equivalent in the young alkaline magmatism of Europe. Rosenbaum & Wilson (1996) proposed a two-stage enrichment model for the Eifel mantle: (1) subduction modification of a mantle protolith; (2) a later enrichment by asthenosphere-derived melts. Mantle xenoliths with an EM1-like isotopic signature occur in both the East Eifel (this study) and West Eifel (Rosenbaum & Wilson, 1996) volcanic fields, implying a relatively large-scale metasomatic event. The mobilization of the volatile-rich EM1-like fluids is temporally linked with a deformation process in the shallow lithospheric mantle (Witt & Seck, 1989). This suggests that fluids carrying an isotope component from a crustal source might be related to a major tectonic episode in this area, i.e. the Hercynian orogeny (Rosenbaum & Wilson, 1996). Evidence for an episode of subduction beneath the Eifel area in the Hercynian orogeny is provided by the Sr–Nd–Pb isotope signatures of amphibole-bearing granulites sampled from Quaternary tephra deposits of the East Eifel volcanic field. These granulites are considered to occupy the base of the East Eifel crust down to a Moho depth of 29–34 km (Okrusch et al., 1979) and exhibit EM1-like present-day Nd–Pb isotope signatures (Loock et al., 1990; Rudnick & Goldstein, 1990). Rudnick & Goldstein (1990) attributed their formation to underplating of mantle-derived basaltic magmas that mixed with material in the pre-existing Precambrian lower crust 450 Myr ago. A protolith age of 400 Ma is recorded by amphibolite-facies metagranodiorites from 10–20 km under the Eifel that formed by partial melting of a lower-crustal source (Stosch et al., 1992). Rudnick & Goldstein (1990) explained the discrepancy between the Proterozoic Nd model ages of the granulites (1·5 Ga; Stosch et al., 1986) and their much younger age of 450 Ma inferred from the Pb isotope compositions by a tectonic transport of upper-crustal lithologies into the lower crust during the Variscan orogeny. The Sr–Nd–Pb isotope signature of some granulite xenoliths was not reset by infiltration of melts or fluids associated with the Cenozoic magmatic event (Stosch et al., 1992). Thus the isotopic EM1-like signature seems to be a primary feature of the Hercynian lower crust and possibly also of the lithospheric mantle into which mafic mantle-derived lavas intruded later.
If EM1-like subduction-related fluids affected the initially LREE-depleted lithospheric mantle, this requires a second stage of enrichment by melts with HIMU-like isotope signatures that reacted during their ascent with the EM1-like subcontinental mantle lithosphere. This process resulted in small-scale isotopic heterogeneities within the metasomatized upper mantle characterized by varying EM1–HIMU-like isotope signatures. The most extreme (i.e. radiogenic) 206Pb/204Pb and 208Pb/204Pb isotope ratios in the West Eifel mantle xenoliths are unique for the European lithospheric mantle (Downes, 2001). The HIMU signature is even stronger than that of the uniform EAR or LVC end-member composition postulated for the Cenozoic European and eastern Atlantic volcanic provinces (Fig. 6). Xenoliths with the most radiogenic Pb isotope compositions converge in the 207Pb–206Pb covariation diagram on the NHRL (Fig. 6b), which represents a 1·77 Ga secondary isochron for oceanic basalts. However, the provenance of the HIMU-like metasomatic agent is unconstrained: it may be either plume derived or an unrelated ancient metasomatic component incorporated into ageing continental lithosphere. In addition, the timing of the melt infiltration remains uncertain and several geodynamic scenarios seem to be possible: (1) the input of the HIMU-like component into the mantle lithosphere is relatively old and associated with the Hercynian orogeny. (2) The metasomatic agents record the very low-degree partial melts initiated by an early Cretaceous deep-sourced mantle plume (Wilson, 1996) with a HIMU-like signature such as the ocean island basalts of St. Helena. (3) The fluid mobilization is young. It began shortly before the main volcanism in the Cenozoic and may be due to the upwelling of a small-scale mantle ‘hot-blob’ from a layer deeper than 400 km (Ritter et al., 2001). Alternatively, the fluid generation may have started during early stages of rifting in connection with the collision of the African and European plates. This requires a reactivation of subducted old crustal domains that may have been deposited at the base of the ‘post-Hercynian’ lithosphere.
On the other hand, the well-developed HIMU–EM1 trend may be the result of source heterogeneity, rather than mixing of metasomatic agents from isotopically distinct mantle reservoirs. Bell & Tilton (2001) considered that both HIMU and EM1 sources are stored within the deep mantle and attributed the formation of an EM1–HIMU-like mantle lithosphere beneath East Africa (Cohen et al., 1984) to the addition of material from a heterogeneous mantle plume covering the complete isotope spectrum between HIMU and EM1 signatures. Thus melting of isotopically distinct mantle domains in a heterogeneous uprising mantle plume could have produced all the metasomatic agents responsible for the HIMU–EM1 enrichment in the deformed mantle lithosphere beneath the Eifel.
Vein metasomatism by young EM1–DMM–HIMU basaltic melts
Witt-Eickschen et al. (1998) inferred from textural, geochemical and Sr–Nd-isotopic evidence that the highly LREE-enriched amphibole grains of tabular habit present in the equigranular recrystallized peridotite wall rocks adjacent to the hornblendite veins formed during an episode of modal metasomatism pre-dating the vein melt injection. As a consequence of the subsequent vein metasomatic event new amphiboles precipitated in some host peridotites. These are, in contrast to the pre-existing amphiboles, not in textural equilibrium with the constituent phases and have chemical compositions similar to the Ti-rich vein amphiboles (Witt-Eickschen et al., 1993). In addition, small-scale compositional gradients within the pre-existing individual Ti-poor amphibole grains developed (Witt-Eickschen et al., 1993) that were used to estimate time constraints for the vein formation (Witt-Eickschen et al., 1998) by applying the diffusion-controlled chromatographic fractionation model of Bodinier et al. (1990). The trace element modelling of these zoning profiles in the tabular amphiboles implies an extremely brief event of melt–wall-rock interaction, which occurred within a time interval of only 10–1000 years before the rapid transport of the xenoliths to the surface (Witt-Eickschen et al., 1998). Thus the vein metasomatism established in the composite xenoliths represents the brief final stage of the multiple enrichment processes beneath the Eifel, obviously as a consequence of the Cenozoic volcanism.
The present isotope study supports this view of a young vein metasomatic event, as the Sr–Nd–Pb data of the minerals from the magmatic clinopyroxenite and hornblendite veins and from their peridotitic wall rocks and the Quaternary Eifel lavas share nearly the same diversity of Sr–Nd–Pb isotopic signatures (Fig. 7). This indicates common sources for the young Eifel volcanism and for the melts parental to the veins that migrated upwards through a system of dyke networks during their ascent through the mantle lithosphere. Because a DMM-like or PREMA-like component is completely lacking in the EM1–HIMU-like metasomatic agents that have pre-enriched the shallower subcontinental Eifel mantle, the mobilization of the DMM–HIMU–EM1 material recognized in the young lavas occurred at the earliest in the Tertiary, contemporaneously with the development of the extensive rift system in Europe and the main melt generation. The diversity of the isotope signatures recorded by the Eifel lavas and the vein melts genetically related to the young Eifel volcanism can have been pro- uced by the interaction of the primary asthenospheric PREMA- or EAR-like melts with components derived from the highly heterogeneous EM1–HIMU-like lithospheric mantle (Fig. 7).
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ACKNOWLEDGEMENTS |
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G.W.-E. thanks Hugh O'Neill and David H. Green for access to the LA-ICP-MS system in Canberra, and Mike Shelley and Charlotte Allen for their kind help with the analyses. Many thanks go also to Heidi Baier for her help with the mass spectrometer in Münster, and to Heinz-Günter Stosch and Wim Bausen for providing unpublished data for Eifel basalts. We thank Else-Ragnhild Neumann, Heinz-Günter Stosch, Ernst Hegner and Marjorie Wilson for their constructive comments on the manuscript, helpful suggestions and improving the English. Financial support for G.W.-E. from the ‘Deutsche Forschungsgesellschaft’ is gratefully acknowledged.
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