Metasomatism and melting history of a Variscan lithospheric mantle domain: evidence from the Puy Beaunit xenoliths (French Massif Central)

Mantle xenoliths from Puy Beaunit (French Massif Central) are compositionally varied, ranging from relatively fertile spinel lherzolites to refractory spinel dunites. Fertile peridotites have registered a modal (amphibole-bearing lherzolites) and cryptic metasomatic event that took place before the last Permian (257 Ma) melting episode. Depletion processes have been constrained by chemical modelling: the depletion is related to different degrees of partial melting, but two major melt extraction episodes are needed to explain the range of major element composition. The second event was responsible for the local large-scale dunitification of former residues. The first melting event (F25%) and metasomatic enrichment are attributed to an ancient fluid and/or liquid infiltration that could be related to a pre-Variscan regional subduction (located to the north of the Beaunit area). Texture acquisition and major deformation of the mantle xenoliths were sub-contemporaneous of the subduction and would result from lithospheric delamination. The second melting event (F17%) produced high-Mg basalts with calc-alkaline trace element signature that gave rise to the Permian underplating episode recognised in western Europe.     

Formation of cratonic subcontinental lithospheric mantle and complementary komatiite from hybrid plume sources

Peridotitic sulphide inclusions in diamonds from the central Slave craton constrain the age and origin of their subcontinental lithospheric mantle (SCLM) sources. These sulphides align with either a ca. 3.5 Ga (shallow SCLM) or a ca. 3.3 Ga isochron (deep SCLM) on a Re–Os ischron diagram, with variably enriched initial ¹⁸⁷Os/¹⁸⁸Os. Since some Archaean to recent plume-derived melts carry a subducted crust (eclogite) signature and some cratonic SCLM may have been generated in plumes by extraction of komatiitic liquids, we explain these data by subduction of evolved lithospheric material (shallow SCLM) and melting in a hybrid mantle plume that contains domains of recycled eclogite (deep SCLM), respectively. In upwelling hybrid mantle, eclogite-derived melts react with olivine in surrounding peridotites to form aluminous orthopyroxene, convert peridotite to pyroxenite and confer their crustal isotope signatures. We suggest that it is subsequent to orthopyroxene enrichment of peridotite in an upwelling plume that partial melting of this Al- and Si- enriched source generated komatiites and complementary ultradepleted cratonic mantle residues. Although subduction is needed to explain some cratonic features, melting of a hybrid plume source satisfies several key observations: (1) suprachondritic initial ¹⁸⁷Os/¹⁸⁸Os in subsets of lithospheric mantle samples and in some coeval Archaean komatiites; (2) variable enrichment of cratonic mantle by high-temperature aluminous orthopyroxene; (3) high Mg# combined with high orthopyroxene content in cratonic mantle due to higher melt productivity of an Al- and Si-richer source; (4) variable orthopyroxene enrichment possibly linked to varying mantle potential temperatures (Tp), plume buoyancy and resultant eclogite load and/or variable availability of subducted material in the source; and (5) absence of younger analogues due to a secular decrease in Tp. Most importantly, this model also alleviates a mass balance problem, because it predicts a hybrid mantle source with variably higher SiO₂ and Al₂O₃ than primitive mantle, and, contrary to a primitive mantle source, is able to reconcile compositions of komatiites and complementary cratonic mantle residues.     

Thermal and redox state of the subcontinental lithospheric mantle of NE Spain from thermobarometric data on mantle xenoliths

Mantle xenoliths in within-plate Cenozoic alkaline mafic lavas from NE Spain are used to assess the local subcontinental lithospheric mantle geotherm and the influence of melting and metasomatism on its oxidation state. The xenoliths are mainly anhydrous spinel lherzolites and harzburgites and gradations between, with minor pyroxenites. Most types show protogranular textures, but transitional protogranular–porphyroclastic and equigranular lherzolites also exist. Different thermometers used in the estimates provide higher subsolidus equilibrium temperatures for harzburgites (1,062 ± 29°C) than for lherzolites (972 ± 89°C), although there is overlap; the lowest temperatures correspond to porphyroclastic lherzolites, whereas pyroxenites give the highest temperatures (up to 1,257°C). Maximum pressures for subsolidus equilibrium of peridotites are at 2.0–1.8 GPa. Later they followed adiabatic decompression and harzburgites registered lower pressures (1.02 ± 0.19 GPa) than lherzolites (1.41 ± 0.27 GPa). One pyroxenite gives values consistent with the spinel lherzolite field (1.08 GPa). The shallowest barometric data are in agreement with the highest local conductive geotherms, which implies that the lithosphere–asthenosphere boundary is at 70–60 km minimum depth. Higher equilibrium temperatures for the harzburgites could be explained by the existence of mafic magma bodies or dykes at the lower crust–mantle boundary. Paleo-fO₂ conditions during partial melting as inferred from the covariation between V and MgO concentrations are mainly between QFM−1 and QFM−2 in log units. However, most thermobarometric fO₂ estimates are between QFM−1 and QFM+1, suggesting oxidation caused by later metasomatism during uplift and cooling.     

Platinum-group element abundances in the upper mantle: new constraints from in situ and whole-rock analyses of Massif Central xenoliths (France)

Fourteen peridotite xenoliths collected in the Massif Central neogene volcanic province (France) have been analyzed for platinum-group elements (PGE), Au, Cu, S, and Se. Their total PGE contents range between 3 and 30 ppb and their PGE relative abundances from 0.01 to 0.001 × CI-chondrites, respectively. Positive correlations between total PGE contents and Se suggest that all of the PGE are hosted mainly in base metal sulfides (monosulfide solid solution [Mss], pentlandite, and Cu-rich sulfides [chalcopyrite/isocubanite]). Laser ablation microprobe-inductively coupled plasma mass spectrometry analyses support this conclusion while suggesting that, as observed in experiments on the Cu-Fe-Ni-S system, the Mss preferentially accommodate refractory PGEs (Os, Ir, Ru, and Rh) and Cu-rich sulfides concentrate Pd and Au. Poikiloblastic peridotites pervasively percolated by large silicate melt fractions at high temperature (1200°C) display the lowest Se (<2.3 ppb) and the lowest PGE contents (0.001 × CI-chondrites). In these rocks, the total PGE budget inherited from the primitive mantle was reduced by 80%, probably because intergranular sulfides were completely removed by the silicate melt. In contrast, protogranular peridotites metasomatized by small fractions of volatile-rich melts are enriched in Pt, Pd, and Au and display suprachondritic Pd/Ir ratios (1.9). The palladium-group PGE (PPGE) enrichment is consistent with precipitation of Cu-Ni-rich sulfides from the metasomatic melts. In spite of strong light rare earth element (LREE) enrichments (Ce/Yb_N < 10), the three harzburgites analyzed still display chondrite-normalized PGE patterns typical of partial melting residues, i.e., depleted in Pd and Pt relative to Ir and Ru. Likewise, coarse-granular lherzolites, a common rock type in Massif Central xenoliths, display Pd/Ir, Ru/Ir, Rh/Ir, and Pt/Ir within the 15% uncertainty range of chondritic meteorites. These rocks do not contradict the late-veneer hypothesis that ascribes the PGE budget of the Earth to a late-accreting chondritic component; however, speculations about this component from the Pd/Ir and Pt/Ir ratios of basalt-borne xenoliths may be premature.     

U-Th-Pb and Lu-Hf isotopic constraints on the evolution of sub-continental lithospheric mantle, French Massif Central

We have carried out a Pb double-spike and Lu-Hf isotope study of clinopyroxenes from spinel-facies mantle xenoliths entrained in Cenozoic intraplate continental volcanism of the French Massif Central (FMC). U-Th-Pb and Lu-Hf isotope systematics verify the existence of different lithospheric domains beneath the northern and southern FMC. Northern FMC clinopyroxenes have extreme Lu/Hf ratios and ultra-radiogenic Hf (ε_Hf = +39.6 to +2586) that reflect ∼15-25% partial melting in Variscan times (depleted mantle model ages ∼360 Ma). Zr, Hf and Th abundances in these clinopyroxenes are low and unaffected by hydrous/carbonatitic metasomatism that overprinted LILE and light REE abundances and caused decoupling of Lu/Hf-Sm/Nd ratios and Nd-Hf isotopes (ε_Nd = +2.1 to +91.2). Pb isotopes of northern FMC clinopyroxenes are radiogenic (²⁰⁶Pb/²⁰⁴Pb > 19), and typically more so than the host intraplate volcanic rocks. ²³⁸U/²⁰⁴Pb ratios range from 17 to 68, and most samples have distinctively low ²³²Th/²³⁸U (<1) and ²³²Th/²⁰⁴Pb (3-22). Clinopyroxenes from southern FMC lherzolites are generally marked by overall incompatible trace element enrichment including Zr, Hf and Th abundances, and have Pb isotopes that are similar to or less radiogenic than the host volcanic rocks. Hf isotope ratios are less radiogenic (ε_Hf = +5.4 to +41.5) than northern FMC mantle and have been overprinted by silicate-melt-dominated metasomatism that affected this part of FMC mantle. Major element and Lu concentrations of clinopyroxenes from southern FMC harzburgites are broadly similar to northern FMC clinopyroxenes and suggest they experienced similar degrees of melt extraction as northern FMC mantle. ²³⁸U/²⁰⁴Pb (53-111) and ²³²Th/²⁰⁴Pb ratios (157-355) of enriched clinopyroxenes from the southern FMC are extreme and significantly higher than the intraplate volcanic rocks. In summary, mantle peridotites from different parts of the FMC record depletion at ∼360 Ma during Variscan subduction, followed by differing styles of enrichment. Northern FMC mantle was overprinted by a fluid/carbonatitic metasomatic agent that carried elements like U, Pb, Sr and light REE. In contrast, much of the southern FMC mantle was metasomatised by a small-degree partial silicate melt resulting in enrichment of all incompatible trace elements. The extreme mantle ²³⁸U/²⁰⁴Pb (northern and southern FMC), ²³²Th/²³⁸U (northern FMC) and ²³²Th/²⁰⁴Pb ratios (southern FMC), coupled with unremarkable present-day Pb isotope ratios, constrain the timing of enrichment. Mantle metasomatism is a young feature related to melting of the upwelling mantle responsible for Cenozoic FMC volcanism, rather than subduction-related metasomatism intimately associated with mantle depletion during the Variscan orogeny. The varying metasomatic styles relate to pre-existing variations in the thickness of the continental lithospheric lid, which controlled the extent to which upwelling mantle could ascend and melt. In the northern FMC, a thicker and more refractory lithospheric lid (?80 km) only allowed incipient degrees of melting resulting in fluid/carbonatitic metasomatism of the overlying sub-continental lithospheric mantle. The thinner lithospheric lid of the southern FMC (?70 km) allowed larger degrees of melting and resulted in silicate-melt-dominated metasomatism, and also focused the location of the volcanic fields of the FMC above this region.     

Geochemical variation in peridotite xenoliths and their constituent clinopyroxenes from Ray Pic (French Massif Central): implications for the composition of the shallow lithospheric mantle

Anhydrous mantle peridotite xenoliths from a single volcanic vent in the French Massif Central are compositionally varied, ranging from relatively fertile lherzolites to refractory harzburgites. Fertile lherzolites closely resemble previous estimates of undepleted mantle compositions but the average of the Ray Pic xenoliths is much less enriched in LILE and LREE than McDonough's (1990) average mantle [McDonough, W.F., 1990. Constraints on the composition of the continental lithospheric mantle. Earth Planet. Sci. Lett., 101, 1–18]. The wide geochemical variation in the bulk rocks reflects significant heterogeneities that can be attributed to two major processes within the shallow lithospheric mantle. The first process is depletion, related to variable degrees of partial melting and melt extraction from an originally near-chondritic mantle. This process has largely controlled the major elements and much of the trace element variation between fertile lherzolites and refractory peridotites. LREE-depleted compositions are also produced by this process. During partial melting, HREE behaved coherently with the major oxides and the moderately incompatible trace elements (Y, V and Sc). A subsequent process of enrichment is indicated by high concentrations of incompatible trace elements in many of the xenoliths. Sr, Ba, K, Th, U, Nb and LREE abundance are independent of major oxide variations and reflect enrichment related to infiltration by alkaline silicate melts/fluids. Both fertile and refractory mantle were enriched but harzburgites were particularly affected. Modal metasomatism occurred only rarely and is indicated by Cr-diopside-rich veins and patches in a few samples. Their chemistry suggests that they were also formed by migration of similar magmas/fluids from the asthenospheric mantle, although the presence of wehrlitic patches may indicate interaction with carbonate melts. In both depleted and enriched xenoliths, trace element patterns for separated clinopyroxenes closely reflect those of the bulk rock, except for Rb, Ba and Nb, which are probably hosted by other phases.     

Geochemical characteristics of lava-field basalts from eastern Australia and inferred sources: Connections with the subcontinental lithospheric mantle?

 A large new database of major, trace elements and Sr-Nd isotopic ratios from 11 lava-field provinces in New South Wales and Queensland, eastern Australia allows detailed interpretation of the origin of these basaltic magmas. Isotopic signatures and trace element patterns identify an OIB-type (oceanic island basalt) source as a dominant component for most of these and some provinces appear to have additional significant components derived from the subcontinental lithospheric mantle (SCLM). The SCLM components have geochemical characteristics that overlap those observed in spinel lherzolite xenoliths (samples of shallow lithospheric mantle) from eastern Australia. These SCLM components show geochemical provinciality that indicates the occurrence of distinct geochemical lithospheric domains reflecting the timing and style of tectonic evolution of different regions. One component reflects modification by subduction-related processes during the late Paleozoic and Mesozoic, one records enrichment by fluids during old metasomatic events and another suggests a metasomatic event involving a distinctive amphibole and apatite-style enrichment. The composition and age distribution of volcanic lava-field provinces older than 10 Ma are consistent with a model involving a regional upwelling (elongated N–S along eastern Australia) of deep hot mantle related to marginal rifting and with OIB-type source geochemical characteristics. Thermal inhomogeneities within this plume swath resulted in small diapirs which may have undergone melt segregation at about 100 km and incorporated varying amounts of SCLM components there or from higher levels of the SCLM during ascent. Subsequent hot-spot generated central volcanoes overprinted this lava-field volcanism, tapped a similar OIB-type source component and truncated the thermal events. Accepted: 15 March 1995     

Rare earth elements,neodymium and strontium isotopic systematics in mineral waters: evidence from the Massif Central,France

Rare Earth Elements (REEs), and Sr and Nd isotope distributions, have been studied in mineralized waters from the Massif Central (France). The CO₂-rich springs are characterized by a neutral pH (6–7) associated with total dissolved solids (TDS) from 1 to 7 g l⁻¹. The waters result from the mixing of very mineralized water pools, thought to have equilibrated at a temperature of around 200°C with superficial waters. These two mineral water pools evidenced by Sr isotopes and dissolved REEs could reflect 2 different stages of water–rock interaction and an equilibrium with different mineral assemblages.The concentrations of individual dissolved REEs and total dissolved REEs (ΣREE), in the mineral waters examined, vary over several orders of magnitude but are not dependent on the main parameters of the waters (TDS, T°C, pH, Total Organic C). The dissolved REE concentrations presented as upper continental crust normalized patterns show HREE enrichment in most of the samples. The time evolution of REE patterns does not show significant fluctuations except in 1 borehole, located in the Limagne d’Allier area, which was sampled on 16 occasions over an 18 month period. Ten samples are HREE-enriched, whereas 6 samples show flat patterns.The aqueous speciation of REEs shows that CO²⁻₃ complexes dominate (>80%) over the free metal, F⁻, SO²⁻₄ and HCO⁻₃ complexes. The detailed speciation demonstrates that the fractionation of REEs (i.e. the HREE enrichment) in CO₂-rich and pH neutral fluids is due essentially to the predominance of the CO²⁻₃ complexes.The Sr isotopic composition of the mineral waters in the Massif Central shows different mixing processes; in the Cézallier area at least 3 end-member water types exist. The most dilute end-member is likely to originate as poorly mineralized waters with minimal groundwater circulation. Two other mineralized end-members are identified, although the link between the geographical location of spring outflow and the mixing proportion between the 2 end-members is not systematic. The range in ϵ_Nd(0) for mineralized waters in the Massif Central correlates well with that of the known parent rocks except for 4 springs. One way to explain the ϵ_Nd(0) in these instances is a contribution from drainage of volcanic rocks. The isotopic systematics help to constrain the hydrogeological models for this area.     

Mesoproterozoic differential metasomatism in subcontinental lithospheric mantle of Central Indian Tectonic zone: Evidence from major and trace element geochemistry of padhar mafic-ultramafic complex

Metasomatism above subduction zone is an important process capable of producing a heterogeneous mantle wedge with locally varying characters. This, in turn, leads to variation in character of the mafic and intermediate rocks produced through partial melting of lithospheric mantle. The Padhar mafic-ultramafic complex is situated in western part of the Betul belt, Central India, shows enrichment in Rb, Ba, Th and Pb and depletion in Nb, Hf and Zr. Major and trace element data along with REE, confirms the presence of a metasomatized mantle above a Mesoproterozoic subduction zone. Differential petrological evolution, corresponding to the different magma batches, as reflected in almost all of the binary element / oxide variation diagrams, testifies small scale metasomatic heterogeneity in the underlying suprasubduction zone mantle wedge.     

Lithospheric mantle beneath NE part of Bohemian Massif and its relation to overlying crust: new insights from Pilchowice xenolith suite,Sudetes, SW Poland

International Journal of Earth Sciences - The Oligocene/Miocene basanite from Pilchowice (Sudetes Mts., SW Poland) carries numerous small xenoliths of mantle peridotite, mostly harzburgite. The...     

Effects of melt percolation on highly siderophile elements and Os isotopes in subcontinental lithospheric mantle: A study of the upper mantle profile beneath Central Europe

The effects of melt percolation on highly siderophile element (HSE) concentrations and Re-Os isotopic systematics of subcontinental lithospheric mantle are examined for a suite of spinel peridotite xenoliths from the 4 Ma Kozákov volcano, Bohemian Massif, Czech Republic. The xenoliths have previously been estimated to originate from depths ranging from ∼32 to 70 km and represent a layered upper mantle profile. Prior petrographic and lithophile trace element data for the xenoliths indicate that they were variably modified via metasomatism resulting from the percolation of basaltic melt derived from the asthenosphere. Chemical and isotopic data suggest that lower sections of the upper mantle profile interacted with melt characterized by a primitive, S-undersaturated composition at high melt/rock ratios. The middle and upper layers of the profile were modified by more evolved melt at moderate to low melt/rock ratios. This profile permits an unusual opportunity to examine the effects of variable melt percolation on HSE abundances and Os isotopes.Most HSE concentrations in the studied rocks are significantly depleted compared to estimates for the primitive upper mantle. The depletions, which are most pronounced for Os, Ir and Ru in the lower sections of the mantle profile, are coupled with strong HSE fractionations (e.g., Os_N/Ir_N ratios ranging from 0.3 to 2.4). Platinum appears to have been removed from some rocks, and enriched in others. This enrichment is coupled with lithophile element evidence for the degree of percolating melt fractionation (i.e., Ce/Tb ratio).Osmium isotopic compositions vary considerably from subchondritic to approximately chondritic (γ_Os at 5 Ma from -6.9 to +2.1). The absence of correlations between ¹⁸⁷Os/¹⁸⁸Os and indicators of fertility, as is common in many lithospheric mantle suites, may suggest significant perturbation of the Os isotopic compositions of some of these rocks, but more likely reflect the normal range of isotopic compositions found in the modern convecting mantle. Osmium isotopic compositions correspondingly yield model Re-depletion (T_RD) ages that range from essentially modern to ∼1.3 Ga.Our data provide evidence for large-scale incompatible behavior of HSE during melt percolation as a result of sulfide dissolution, consistent with observations of prior studies. The degree of incompatibility evidently depended on melt/rock ratios and the degree of S-saturation of the percolating melt. The high Pt contents of some of these rocks suggest that the Pt present in this pervasively metasomatized mantle was controlled by a phase unique to the other HSE. Further, high Os concentrations in several samples suggest deposition of Os in a minority of the samples by melt percolation. In these rocks, the mobilized Os was characterized by similar to the ¹⁸⁷Os/¹⁸⁸Os ratios in the ambient rocks. There is no evidence for either the addition of Os with a strongly depleted isotopic composition, or Os with suprachondritic isotopic composition, as is commonly observed under such circumstances.     

Postdated melting of subcontinental lithospheric mantle by the Emeishan mantle plume: Evidence from the Anyi intrusion,Yunnan, SW China

The Anyi intrusion is located in the central zone of Emeishan large igneous province (ELIP), SW China. It outcrops in an area of about 0.65 km² and ~ 1 km thick and dips to the southwest. The Anyi intrusion consists of a lower clinopyroxenite zone, middle gabbro zone, and an upper monzonite–syenite zone. Up to 400 m thick stratiform disseminated Fe–Ti oxide layer with grades of 16–18 wt.% total Fe is hosted in the lower clinopyroxenite zone. Zircon SHRIMP U–Pb age (247 ± 3 Ma) indicates that the Anyi intrusion represents postdated mafic magmatism resulting from the ~ 260 Ma Emeishan mantle plume. Compared with the typical oxide-bearing intrusions (such as Panzhihua and Baima) formed at ~ 260 Ma in the ELIP, the Anyi intrusion is characterized by high alkaline contents and LREE/HREE ratios, extremely low εNd values (− 6.2 to − 7.6) and moderate high (⁸⁷Sr/⁸⁶Sr)_i values (0.7072 to 0.7086). These characteristics of the Anyi intrusion cannot be explained by fractional crystallization or crustal contamination, but may reflect a unique enriched continental lithospheric mantle source (a mantle source mixed between garnet pyroxenite and spinel peridotite). We propose that the postdated mafic magmatism associated with the formation of the Anyi intrusion and its Fe–Ti oxide ore may be the product of melting of a mantle source mixed between garnet pyroxenite and spinel peridotite in the shallow lithosphere caused by conductive heating combined with lithosphere thinning due to plume–lithosphere interaction.     

华北克拉通西北部显生宙岩石圈地幔属性与演化特征: 橄榄岩捕虏体证据

众所周知,华北克拉通东部岩石圈地幔的组成和性质在显生宙发生了显著变化,但由于西部出露含有捕虏体的火山岩较少,这在一定程度上限制了人们对该区岩石圈地幔属性与演化特征的认识。本文将华北克拉通西北部晚白垩世-新生代玄武岩中橄榄岩捕虏体的研究成果归纳总结,旨在进一步揭示该区岩石圈地幔的属性与演化特征。研究表明,华北克拉通西北部岩石圈地幔主要由低Mg^#的二辉橄榄岩和少量高Mg^#的方辉橄榄岩组成。高Mg^#橄榄岩代表该区受轻微再富集作用影响的古老岩石圈地幔残余,低Mg^#橄榄岩是软流圈来源熔体与高Mg^#橄榄岩反应的产物,代表地幔再富集作用对古老岩石圈地幔改造的结果。该区岩石圈地幔经历了多期地幔交代作用的改造,早期交代事件与古亚洲洋俯冲有关,近期与软流圈来源的玄武质熔体有关。这种广泛的地幔再富集作用对华北克拉通古老岩石圈地幔的转变做出了重要贡献。

     

Evidence for the multiple stage evolution of the subcontinental lithospheric mantle beneath the Eifel (Germany) from pyroxenite and composite pyroxenite/peridotite xenoliths

High temperature (1150–1250 °C), coarse-grained olivine-bearing clinopyroxenites occur in the ash-tuffs of the Dreiser Weiher maar-type volcano (West Eifel, Germany) as discrete xenoliths or as 1-5-cm-broad veins crosscutting anhydrous spinel peridotite host xenoliths. The clinopyroxenes (cpx) of these xenoliths have been analysed for trace element and Nd-Sr isotope compositions in order to document intra-suite variations and to constrain the processes involved in the formation of heterogeneities within a relatively well defined upper mantle section beneath the West Eifel. The patterns formed by cpx from the pyroxenites on multi-element diagrams are subparallel and convex-upward, showing troughs for high-field-strength elements (Nb, Zr, Hf, Ti) and Sr. Trace element modelling indicates that these pyroxenites represent high pressure precipitates of magmas that are more primitive or similar in compositions to the most undifferentiated Cenozoic alkali basaltic lavas from the West Eifel. The cpx cover the whole spectrum of Nd-Sr isotope compositions shown by the primitive lavas from the entire West Eifel volcanic field suggesting isotopic heterogeneity on the scale of an individual volcanic centre. Due to incomplete re-equilibration between the vein melts and the peridotitic wall rocks, cpx of the host peridotites of the composite xenoliths (that belong to the 1b-group of Stosch and Seck, 1980) have in some cases retained relics of a pre-vein host composition. The relic cpx range from LREE-depleted to LREE-enriched with isotope signatures indicating a time-integrated higher enrichment (lower ¹⁴³Nd/¹⁴⁴Nd and higher ⁸⁷Sr/⁸⁶Sr) than the cpx of the corresponding veins. The trace element and isotope compositions of the xenoliths support the perception that magmas generated from sub-lithospheric mantle sources beneath the West Eifel formed a system of narrow dike networks and differentiated during their ascent through the lithosphere (Duda and Schmincke 1985). The data provide evidence that: (1) melts parental to the Dreiser Weiher pyroxenites are genetically related to the young alkali basaltic volcanics; (2) these melts can be derived from distinct domains of the mantle beneath Dreiser Weiher ranging in Sr-Nd isotope signatures from HIMU-like to Bulk-Silicate-Earth values; (3) the enrichment process associated with the upwards migration of these magmas was spatially limited to a cm-scale in the case of the studied composite xenoliths; (4) parts of the Dreiser Weiher lithosphere have experienced an enrichment prior to the vein interaction by a metasomatic agent that is isotopically unrelated to the primitive West Eifel lavas. Received: 25 August 1997 / Accepted: 25 November 1997     

Petrological and geochemical relationships between pyroxene megacrysts and associated alkali-basalts from Massif Central (France)

Major, trace element, and REE analyses, as well as Sr isotopic ratios, have been obtained on twelve clinopyene megacrysts and phenocrysts and their alkali-basalt hosts from the French Massif Central. Equilibrium between crystals and host was examined based on petrographic and geochemical data.Two types of pyroxenes are recognized: the acmite-bearing clinopyroxenes, rich in incompatible elements and the salitic clinopyroxenes, poor in incompatible elements. ⁸⁷Sr/ ⁸⁶Sr isotopic data reveal no significant difference between clinopyroxenes and host lavas: they are in apparent isotopic equilibrium. The Sr isotopic ratios of the two types of pyroxenes are also quite similar. However pyroxene crystals from the first group are not in equilibrium with their host; they have crystallized at high-pressure from differentiated alkali-lavas and have been incorporated in a more primitive magma. Pyroxene crystals from the second group are in apparent equilibrium with their host lava; they have crystallized at various pressures. For the latter, distribution coefficients are proposed for compatible elements, trace elements and REE.     

Comparative magma/xenolith relationships in some volcanic and plutonic rocks from the French Massif Central

A comparative study of xenoliths from lavas and granitic rocks of central France indicates that:

1. The evolution of the xenoliths is essentially the same in intermediate to acid lavas as in granites. The major factor in this evolution is the assimilation by the host magmas of the quartzofeldspathic components of the xenoliths, which then become Al, Fe, Mg-rich restites.
2. Liquid immiscibility between melted xenoliths and host magmas can only be postulated for acid xenoliths in alkali basalts.
3. The basic microgranular xenoliths in some granites do not show important compositional gaps with their host rocks, though they are regularly more basic. They can be considered as early segregations (cumulates).

     

The late Cretaceous lithospheric mantle beneath the Central Andes: Evidence from phase equilibria and composition of mantle xenoliths

Temperature estimates and chemical composition of mantle xenoliths from the Cretaceous rift system of NW Argentina (26°S) constrain the rift evolution and chemical and physical properties of the lithospheric mantle at the eastern edge of the Cenozoic Andean plateau. The xenolith suite comprises mainly spinel lherzolite and subordinate pyroxenite and carbonatized lherzolite. The spinel lherzolite xenoliths equilibrated at high-T (most samples >1000 °C) and P below garnet-in. The Sm–Nd systematics of compositionally unzoned clino- and orthopyroxene indicate a Cretaceous minimum age for the high-T regime, i.e., the asthenosphere/lithosphere thermal boundary was at ca. 70 km depth in the Cretaceous rift. Major elements and Cr, Ni, Co and V contents of the xenoliths range between values of primitive and depleted mantle. Calculated densities based on the bulk composition of the xenoliths are <3280 kg/m³ for the estimated P–T conditions and indicate a buoyant, stable upper mantle lithosphere. The well-equilibrated metamorphic fabric and mineral paragenesis with the general lack of high-T hydrous phases did not preserve traces of metasomatism in the mantle xenoliths. Late Mesozoic metasomatism, however, is obvious in the gradual enrichment of Sr, U, Th and light to medium REE and changes in the radiogenic isotope composition of an originally depleted mantle. These changes are independent of the degree of depletion evidenced by major element composition. ¹⁴³Nd/¹⁴⁴Nd_i ratios of clinopyroxene from the main group of xenoliths decrease with increasing Nd content from >0.5130 (depleted samples) to ca. 0.5127 (enriched samples). ⁸⁷Sr/⁸⁶Sr_i ratios (0.7127–0.7131, depleted samples; 0.7130–0.7134, enriched samples) show no variation with variable Sr contents. Pb_i isotope ratios of the enriched samples are rather radiogenic (²⁰⁶Pb/²⁰⁴Pb_i 18.8–20.6, ²⁰⁷Pb/²⁰⁴Pb_i 15.6–15.7, ²⁰⁸Pb/²⁰⁴Pb_i 38.6–47) compared with the Pb isotope signature of the depleted samples. The large scatter and high values of ²⁰⁸Pb/²⁰⁴Pb_i ratios of many xenoliths indicates at least two Pb sources that are characterized by similar U/Pb but by different Th/Pb ratios. The dominant mantle type in the investigated system is depleted mantle according to its Sr and Nd isotopic composition with relatively radiogenic Pb isotope ratios. This mantle is different from the Pacific MORB source and old subcontinental mantle from the adjacent Brazilian Shield. Its composition probably reflects material influx into the mantle wedge during various episodes of subduction that commenced in early Paleozoic or even earlier. Old subcontinental mantle was already replaced in the Paleozoic, but some inheritance from old mantle lithosphere is represented by rare xenoliths with isotope signatures indicating a Proterozoic origin.     

Imaging global chemical and thermal heterogeneity in the subcontinental lithospheric mantle with garnets and xenoliths: Geophysical implications

Suites of mantle-derived xenoliths in volcanic rocks provide estimates of the geothermal gradient and composition of the subcontinental lithospheric mantle (SCLM) at the time of the volcanic eruption. The development of single-grain thermometry and barometry, applied to xenocryst minerals in volcanic rocks, has greatly expanded the number of localities for which such data can be obtained and made it feasible to map the geology of the SCLM on a broader scale, both vertically and laterally. From garnet xenocrysts, it is possible to derive profiles showing mean values of olivine composition, bulk-rock composition, density and seismic velocities, as well as geotherm parameters and constraints on the thickness of the SCLM. Geochemical profiles, coupled with Re–Os dating of peridotites and their enclosed sulfide minerals, show that Archean or Proterozoic SCLM is preserved at shallow levels beneath many areas of younger tectonothermal age; this implies rapid vertical variations in Vs and Vp with depth, which may affect seismic interpretations. Data from several hundred localities worldwide define a secular evolution in the composition of the SCLM, related to the tectonothermal age of the overlying crust. Archean SCLM is typically strongly depleted in basaltic components, highly magnesian and thick (160–250 km), and has low geotherms; Phanerozoic SCLM is typically fertile (rich in basaltic components), Fe-rich, thin (50–100 km) and has a range of high geotherms; Proterozoic SCLM (much of which may be reworked Archean mantle) tends to be intermediate in all respects. The correlated variations in SCLM fertility, lithospheric thickness and geotherm reinforce the effects of each on seismic velocity, and produce more rapid lateral variations in seismic response than would result from thermal effects alone. These correlations are the key to using seismic tomography images to map the lateral extent of different types of SCLM.