Garnet and clinopyroxene megacrysts from the Cretaceous (70 Ma) Mbuji-Mayi kimberlites and one garnet megacryst from the lower Oligocene (32 Ma) Kundelungu kimberlites in Democratic Republic of Congo have been investigated for combined Nd and Hf isotope compositions. These megacrysts are thought to result from the metasomatic (re)crystallization of lithospheric mantle peridotites during the infiltration of a proto-kimberlitic melt/fluid. In addition, zircon and baddeleyite megacrysts from the Mbuji-Mayi kimberlites have been investigated for Hf isotope composition. Although baddeleyites are uncommon in kimberlite megacryst suites, their origin is most probably related to that of zircon megacrysts from Mbuji-Mayi. Mbuji-Mayi garnet megacrysts display ranges of εNd(t) from −0.6 to +6.1 and εHf(t) from +6.6 to +12.1; the Kundelungu garnet megacryst has overlapping isotopic compositions (+0.8 and +6.0, respectively). By contrast, Mbuji-Mayi clinopyroxene megacrysts display a more restricted range of εNd(t) values (+2.7 to +4.6) and extend toward lower εHf(t) compositions (+3.0 to +9.1). Mbuji-Mayi zircon and baddeleyite megacrysts have similar εHf(t) values (+6.5 to +7.1 and +6.0 to +8.4, respectively). Differences in initial isotopic composition between garnets on the one hand and clinopyroxenes, zircons, and baddeleyites on the other have been confirmed by various Hf and Nd model ages calculations. Clinopyroxene, zircon, and baddeleyite megacrysts plot close to the worldwide kimberlite field in a combined εHf(t)-εNd(t) plot, which favors recent (i.e. at or shortly before the time of kimberlite eruption) crystallization through interaction between the infiltrating proto-kimberlite melt/fluid and peridotite wall rocks. On the other hand, the wide range of εNd(t) values and the higher εHf(t) values for garnet megacrysts suggest that they have been formed through recrystallization of old garnet-bearing peridotitic protoliths in the subcontinental lithospheric mantle. Calculated rare earth element patterns of liquids in equilibrium with clinopyroxene megacrysts confirm the direct relationship to Group I kimberlites, while those in equilibrium with garnet megacrysts show more variability, which could also reflect that their formation results from more complex processes.

Mbuji-Mayi (east Kasai province) and Kundelungu (Shaba province) are the two kimberlite fields known for a long time in the Democratic Republic of Congo (DRC). Mbuji-Mayi intrudes the Archean basement (Congo-Kasai Craton) and is diamond-rich, whereas Kundelungu cuts across Paleoproterozoic basement (Bangweulu block) and is diamond-poor. The megacryst suites (or discrete nodule suites) of both fields include garnet and clinopyroxene megacrysts. The pyrope-rich megacrysts can be subdivided in three groups on the basis of their Cr contents: low-Cr (0.00-1.79 wt.%Cr(2)O(3): Mg(#): 72.8-84.0); medium-Cr (1.93-5.16 wt.%Cr(2)O(3): Mg#: 76.2-86.3) and high-Cr (5.42-7.10 wt.%Cr(2)O(3): Mg(#): 79.2-84.6). There are no significant geochemical differences between the garnets from Mbuji-Mayi and from Kundelungu. Polymineral inclusions composed of K-rich hydrated phases (phlogopite and amphibole), fresh glass and Cr-spinels are identified in garnets from all three groups, in both localities, which suggest a common origin. Two groups of diopside megacrysts from Mbuji-Mayi are distinguished on the basis of their Ca content: low-Ca (Ca(#): 39.5-42.1; 0.61-0.92 wt.% Cr(2)O(3)) and medium-Ca (Ca(#): 44.1-48.5; 0.41-1.09 wt.%Cr(2)O(3)); they differ from a third group of high-Cr diopsides (Ca(#): 47.1-49.4; 1.31-2.77 wt.%Cr(2)O(3)). The major element compositions of DRC megacrysts are distinct from those of many other megacryst suites worldwide: the clinopyroxenes are lower in Fe and Ti and higher in Mg and the garnets contain more Cr and significantly less Ti, Fe and Al. These DRC megacryst compositions are intermediate between those of peridotite minerals and those of kimberlite megacrysts from other localities. Most garnets have "normal" REE profiles ((La/Yb)(N) = 0.003-0.027), whereas clinopyroxenes display relative LREE enrichment ((La/Yb)(N) = 5.1-43.2). The REE patterns of garnet and clinopyroxene megacrysts are similar to those from metasomatized South African mantle lherzolites. The differences in composition between DRC megacrysts and those from other kimberlites might reflect different modes of formation. Some megacryst suites are related by fractional crystallization processes, the DRC garnet and clinopyroxene megacrysts display geochemical similarities with peridotites and may originate by metasomatic transformation and recrystallization of mantle peridotites. (C) 2009 Elsevier B.V. All rights reserved.

The Saghro Cenozoic lavas form a bimodal suite of nephelinites (with carbonatite xenoliths) and phonolites emplaced in the Anti-Atlas belt of Morocco. Despite the paucity of samples with intermediate composition between the two main types of lava (only one phonotephrite flow is reported in this area), whole-rock major element modelling shows that the two main lithologies can be linked by fractional crystallization. The most primitive modelled cumulates are calcite-bearing olivine clinopyroxenites, whereas the final stages of differentiation are characterized by the formation of nepheline-syenite cumulates. This evolution trend is classically observed in plutonic alkaline massifs associated with carbonatites. Late-stage evolution is responsible for the crystallization of hainite- and delhayelite-bearing microdomains, for the transformation of aegirine-augite into aegirine (or augite into aegirine-augite), and for the crystallization of lorenzenite and a eudialyte-group mineral as replacement products of titanite. These phases were probably formed, either by crystallization from late residual peralkaline melts, or by reaction of pre-existing minerals with such melt, or hydrothermal peralkaline fluid.

Summary The natural Sr isotope composition of acid leached soils developed on loess, under beech forest, in central Belgium was used as a tracer of soil forming processes, in conjunction with physico‐chemical and quantitative mineralogical investigations. Attention was focused on weathering and exchange processes, with special emphasis on the origin of the current soil exchangeable fraction and the influence of the atmospheric deposition and biological cycling on the calcium exchangeable pool (Sr acts as a proxy for Ca). The determination of 87 Sr/ 86 Sr ratios was made on the bulk soil, on the clay‐ and silt‐size soil separates, on 0.1 m HCl extracts, on the labile pool, on the soil solution and on the bulk precipitation. The acid leached soil profiles are characterized by a sequence of weathering processes that is highlighted by both mineralogical and isotopic changes. From the calcareous unweathered loess (pH 7.5) to the uppermost soil horizons (pH < 4.0) the evolution of the 87 Sr/ 86 Sr isotope ratio clearly reflects: (i) the selective weathering of Ca‐plagioclase (small 87 Sr/ 86 Sr ratio) and the increasing proportion of resistant K‐ and Rb‐rich minerals (large 87 Sr/ 86 Sr ratio) in the uppermost soil horizons; and (ii) a downward translocation of clay minerals with a large isotopic ratio, a physical breakdown of muscovite and a non‐congruent chemical weathering of K‐feldspar. The influence of organic restitutions or atmospheric deposition is not significant. The comparison between the Sr isotopic signature of the soil solution, and the exchangeable and HCl‐extractible soil fractions provides information about cation exchange efficiency, soil–water interaction and the origin of the exchangeable pool.

The Limousin ophiolite (French Massif Central) occurs as elongate bodies forming a (nearly) continuous suture zone between two major lithotectonic units of the French Variscan belt. The mantle section of the ophiolite is made of diopside-bearing harzburgite, harzburgite and dunite characteristic of a lherzolite-harzburgite ophiolite type (LHOT). The plutonic section is essentially composed of troctolites, wehrlites and gabbros locally intruded by ilmenite-rich mafic dykes. All the rocks were strongly affected by an ocean-floor hydrothermal metamorphism. The composition and evolution of primary magmatic phases (olivine, clinopyroxene, plagioclase and spinel) throughout the lowermost magmatic sequence correspond to those described in oceanic cumulates (ODP data). The Limousin ophiolite is thus of MOR type instead of SSZ type. The whole lithological section, the mineral chemistry, the extensive hydrothermal oceanic alteration and the relatively thin crustal section are typical of a slow-spreading ridge ocean (i.e. Mid-Atlantic ridge). Comparison of the Limousin ophiolite with other ophiolites from European Variscides suggests that the oceanic domain was actively spreading during the Late Palaeozoic and extended from the Armorican massif to the Polish Sudetes.

Abstract A Late Pan-African calc-alkaline dike swarm (basalt-andesite-dacite-rhyolite) has been investigated in a region of over 2000 km2 in the Alpine Danubian window, South Carpathians (Romania). Amphibole phenocrysts and microphenocrysts have been investigated by wavelength-dispersive microprobe analysis and BSE imaging. The Ca-amphibole population, represented in all the lithologies, displays a large compositional range, interpreted as the result of two processes: (1) magmatic evolution (kaersutite → Ti-pargasite → pargasite → Ti-magnesiohastingsite → magnesiohastingsite → edenite → tschermakite → magnesiohornblende) linked to magmatic differentiation from andesitic basalt to rhyolite; and (2) deuteritic alteration of the primary amphibole related to late-emplacement hydrothermal activity (yielding numerous varieties comprising those cited above). In all rock types, amphibole phenocrysts equilibrated at a nearly constant pressure of about 0.6 ± 0.1 GPa, but their temperatures of crystallization ranged from 1000.900 °C for basaltic andesites to 700.600 °C for dacites. In rhyolites, edenite to magnesiohornblende crystals reflect a continuous range of P-T conditions from 700 °C/0.6 GPa to 600 °C/0.1 GPa, in agreement with their change of habit from euhedral to subhedral. Complex zonations in pargasite-magnesiohastingsite (including resorption) are interpreted in term of self-organization of oscillatory zoning without significant heating and/or magma mixing.

The origin of magmatic layering is still hotly debated. To try to shed some light on this problem, two ultramafic–mafic layered xenoliths from Puy Beaunit (French Massif Central) were investigated in detail. The nodules belong to a stratiform intrusion emplaced in the deep crust during the Permian (257±6 Ma; Féménias, O., Coussaert, N., Bingen, B., Whitehouse, M., Mercier, J.-C., Demaiffe, D., 2003. A Permian underplating event in late- to post-orogenic tectonic setting. Evidence from the mafic–ultramafic layered xenoliths from Beaunit (French Massif Central). Chem. Geol. 199 293–315.). The 3 to 5 cm thick nodules have, in common, a central orthopyroxenite layer; the succession of layers is, respectively, norite–orthopyroxenite–norite (PBN 00-01) and norite–orthopyroxenite–harzburgite (PBN 00-03). The variations of both major (by electron microprobe) and trace, essentially the RE, elements (by LA-ICP-MS) were measured in major mineral phases (orthopyroxene, clinopyroxene, plagioclase, spinel) along cross-section perpendicular to the layering. Strong grain size, chemical and textural variations occur along these sections: they can be continuous or discontinuous, symmetrical or asymmetrical. Such complex variations cannot be solely related to a single magmatic history (fractional crystallisation, mineral sorting). Other processes such as element enrichment by residual liquid channelling along layer boundaries and/or sub-solidus recrystallisation and element redistribution must be invoked. It appears, in particular, that element distribution in the central orthopyroxenite layer could result from the injection of micro-sills of orthopyroxene-rich liquid between previously consolidated layers.

The Limousin ophiolite is located at the suture zone between two major thrust sheets in the western French Massif Central. This ophiolitic section comprises mantle-harzburgite, mantle-dunite, wehrlites, troctolites and layered gabbros. It has recorded a static metamorphic event transforming the gabbros into undeformed amphibolites and the magmatic ultramafites into serpentinites and/or pargasite-bearing chloritites. With various thermobarometric methods, it is possible to show that the different varieties of amphibole have registered low-P (c. 0.2 GPa) conditions with temperature ranging from high-T, late-magmatic conditions to greenschist-zeolite metamorphic facies. The abundance of undeformed metamorphic rocks (which is typical of the lower oceanic crust), the occurrence of Ca-Al (-Mg) metasomatism illustrated by the growth of Ca-Al silicates in veins or replacing the primary magmatic minerals, the P-T conditions of the metamorphism and the numerous similarities with oceanic crustal rocks from Ocean Drilling Program and worldwide ophiolites are the main arguments for an ocean-floor hydrothermal metamorphism in the vicinity of a palaeo-ridge. Among the West-European Variscan ophiolites, the Limousin ophiolites constitute an extremely rare occurrence that has not been involved in any HP (subduction-related) or MP (orogenic) metamorphism as observed in other ophiolite occurrences (i.e. France, Spain and Germany).

Dykes of Cenozoic age (37.5 ± 2.3 Ma) crop out in the Tchircotché area (Garoua rift, north Cameroon). They consist of a lamprophyric (monchiquite) series with diopside, subsilicic kaersutite and apatite phenocrysts, Ba–Ti-rich biotite microphenocrysts and Cr-diopside xenocrysts scattered in a matrix of analcitic composition containing oligoclase, albite and sanidine microlites and carbonate ocelli.

The nature of the petrogenetic links between carbonatites and associated silicate rocks is still under discussion (i.e., [Gittins J., Harmer R.E., 2003. Myth and reality of the carbonatite–silicate rock “association”. Period di Mineral. 72, 19–26.]). In the Paleozoic Kola alkaline province (NW Russia), the carbonatites are spatially and temporally associated to ultramafic cumulates (clinopyroxenite, wehrlite and dunite) and alkaline silicate rocks of the ijolite–melteigite series [Kogarko, 1987, Kogarko et al., 1995, Verhulst et al., 2000, Dunworth and Bell, 2001, Woolley, 2003]. In the small (≈20 km2) Vuoriyarvi massif, apatite is typically a liquidus phase during the magmatic evolution and so it can be used to test genetic relationships. Trace elements contents have been obtained for both whole rocks and apatite (by LA-ICP-MS). The apatites define a single continuous chemical evolution marked by an increase in REE and Na (belovite-type of substitution, i.e., 2Ca2+=Na++REE3+). This evolution possibly reflects a fractional crystallisation process of a single batch of isotopically homogeneous, mantle-derived magma.The distribution of REE between apatite and their host carbonatite have been estimated from the apatite composition of a carbonatite vein, belonging to the Neskevara conical-ring-like vein system. This carbonatite vein is tentatively interpreted as a melt. So, the calculated distribution coefficients are close to partition coefficients. Rare earth elements are compatible in apatite (D>1) with a higher compatibility for the middle REE (DSm:6.1) than for the light (DLa:4.1) and the heavy (DYb:1) REE.

The Lovozero alkaline massif (Kola Peninsula, Russia) is composed of three major units. The central unit (80% of the volume) comprises numerous well developed layers composed, from bottom to roof, of an urtite–juvite–foyaite–lujavrite continuous lithological sequence (ijolite–foid-bearing alkali feldspar syenite in IUGS nomenclature). The mode of emplacement of the massif and the mechanism of formation of the layering are still under debate. Petrological, mineralogical (two stages of crystallisation) and structural evidence from the detailed analysis of one of these layers (unit II-7) is interpreted in terms of both mechanical (magmatic to sub-solidus, non-coaxial deformation) and thermal differentiation operating on a crystal-laden (alkali feldspar, high T nepheline, aegirine-augite) material of foyaitic composition. Textural and mineralogical data suggest that a sheet of foiditic magma intruded into solidified earlier units of the Lovozero layered sequence and acquired a sill-like structure on cooling.

Many studies made in Europe and North America have shown an increasing depletion of exchangeable base cations that may cause tree nutritional deficiencies in sensitive soils. We use radial variation of strontium isotope in tree-rings (Sr-87/Sr-86 ratio) to monitor possible changes in Ca sources for tree nutrition (Sr is used as an analog to Ca). The two main sources of Ca in forest stands are mineral weathering release and atmospheric inputs. Measurements in several forest stands in temperate regions show a steep decrease from pith to outer wood of the Sr isotope ratio from similar to 1870 to similar to 1920 except for stands developed on soils with a higher Ca status. This suggests a decrease of the weathering contribution (high Sr-87/Sr-86 ratio) when cations are displaced from the soil exchange complex by acid deposition at a rate faster than the replenishment of the cation pool by mineral weathering. This displacement enhances the atmospheric contribution, which is characterized by a low Sr-87/Sr-86 ratio. Tree-ring chronologies are an exceptional historic-timing record of chemical changes in the soil environment induced by atmospheric pollution. The reliability of the tree-ring recorder has been verified with a well-controlled nutritional perturbation in the context of a limed forest stand (with a known liming Sr isotopic signature). Our data suggest that forest ecosystems were affected by atmospheric inputs of strong acids earlier than previously thought.

The two main sources of Ca in forest ecosystems are mineral weathering release and atmospheric inputs. We use the Sr-87/Sr-86 isotopic ratio (Sr is used as a proxy for Ca) to determine the Ca contribution from atmospheric deposition input in two forest ecosystems (beech stands) growing on soils formed from parent materials with contrasting total Ca contents and isotopic ratios: Pleistocene loess in Central Belgium (acid leached soil) and Lower Devonian shales in the Ardenne Massif of High Belgium (ochreous brown earth). The Sr-97/Sr-86 ratio and the Ca and Sr contents were measured in bulk precipitation, in vegetation (beechwood growth rings and leaves) and in main soil horizons (total, acid-extractable and labile pools). The relative contributions of atmospheric input and soil mineral weathering to vegetation were calculated using mixing equations.Calculations based on Sr isotope ratios of bulk precipitation (end-member 1; Sr-87/Sr-86 close to ocean water: 0.7090), 0.1 M HCl-extractable soil fraction (end-member 2) and beech wood (mixing compartment) indicate that about 43% (Central Belgium) and 39% (Ardennes) of Sr uptake originates from atmospheric inputs. For Ca, the contributions of atmospheric input are 75% and 78%, respectively. These estimations are, however, very sensitive to the choice of the appropriate Sr-87/Sr-86 and Sr/Ca ratios for the weathering end-member. The isotopic composition of the mineral source is estimated from the soil mineralogical composition and the relative weatherability of the Sr-bearing minerals. Due to soil processes (mineral weathering, element recycling) and geomorphological events (addition of allochtonous minerals to the autochtonous pool inherited from the local bedrock), the distribution of minerals in both soil profiles is heterogeneous and varies from horizon to horizon. Which horizons are relevant and which soil pool (total soil, acid-extractable fraction or labile pool) should be selected for isotopic measurement of the weathering end-member, is therefore open to uncertainty. The choice of the appropriate Sr-87/Sr-86 and Sr/Ca ratios for the weathering end-member is discussed.Our results emphasize the importance of the atmospheric contribution of Ca for tree mineral nutrition in the studied forest stands, suggesting that these ecosystems on acid soils are sensitive to chemical changes in the atmospheric environment, e.g. acid depositions associated with decreasing input of atmospheric cations. Our results support therefore the conclusion of Hedin et al. [Hedin, L.O., Granat, L., Likens, G.E., Bulshand, T.A., Galloway, J.N., Butler, T., Rodhe, H., 1994. Steep declines in atmospheric base cations in regions of Europe and North America. Nature, 367: 351-354], suggesting that the atmospheric base cation trends in Europe and North America are ecologically relevant on the scale of decades for poorly buffered ecosystems. This is a serious possibility of depletion of the soil available cation pool of the High Belgium beech forest, which grows on a bedrock with extremely low levels of total calcium. &COPY; 2004 Elsevier B.V. All rights reserved.

The fabric in a dike is representative of the magmatic flow, considered as Newtonian. The anisotropy of magnetic susceptibility of the rocks gives a good representation of the shape-preferred orientation which, in turn, is a marker of the magmatic flow. Generally, a symmetrical pattern of the fabric across the dike is in agreement with a flow of magma within a channel: the flow direction is then reliable with this imbrication. An asymmetrical fabric is dependent on the flow and displacement of the wall. We present the case of both symmetrical and asymmetrical dike fabrics recording different emplacements. From a Pan-African calc–alkaline dike swarm (of basaltic–andesitic–dacitic–rhyolitic composition) of the Alpine Danubian window from South Carpathians (Romania), two populations of dikes have been described: thick (1–30 m) N–S-trending dikes and thin (<1 m) E–W dikes. The first display asymmetrical fabric and record the regional sinistral movement of the walls. In contrast, the thin dikes are symmetrical and frequently display an arteritic morphology that limits the dike length, with no cartographic extension. We propose to relate the two types of dikes to the same regional stress field in a continuum of emplacement during a regional brittle event.

Thirty-one plugs of alkaline volcanic rocks of Cenozoic age (37 Ma in mean) occur in the Upper Benue valley, northern Cameroon (Central Africa). The complete alkaline series (alkaline basalts, hawaiites, mugearites, phonolites, trachytes and rhyolites) is represented. Basalts contain phenocrysts of olivine, Al-Ti-rich diopside, and Ti-magnetite, and hawaiites-abundant microphenocrysts of plagioclase. Mugearites have a trachytic texture and contain xenocrysts of K-feldspar, apatite, quartz and unstable biotite. Phonolites are peralkaline. Trachytes (peralkaline and non-peralkaline) and rhyolites are characterised by their sodic mineralogy with aegirine-augite, richterite, and arfvedsonite phenocrysts. There is a large compositional gap between basaltic and felsic lavas, except the mugearites. Despite this gap, major- and trace-element distributions are in favour of a co-magmatic origin for the basaltic and felsic lavas. The Upper Benue valley basalts are similar in their chemical and isotopic features to other basalts from both the continental and oceanic sectors of the Cameroon Line. The Upper Benue valley basaltic magmas (87Sr/86Sr≈0.7035; ε Nd=+3.9) originate from an infra-lithospheric reservoir. The Sr–Nd isotopic composition and high Sr contents of the mugearites suggest that they are related to mantle-derived magmas and that they result from the mixing, at shallow crustal levels, of a large fraction of trachytic magma with a minor amount of basaltic magma. Major-element modelling of the basalt–trachyte evolution (through hawaiite and mugearite compositions) does not support an evolution through fractional crystallization alone. The fluids have played a significant role in the felsic lavas genesis, as attested by the occurrence of F-rich minerals, calcite and analcite. An origin of the Upper Benue valley rhyolitic magmas by fractional crystallization of mantle-derived primitive magmas of basaltic composition, promoted or accompanied by volatile, halogen-rich fluid phases, may be the best hypothesis for the genesis of these lavas. These fluids also interact with the continental crust, resulting in the high Sr-isotope initial ratios (0.710) in the rhyolites, whereas the Nd isotopic composition has been less affected (ε Nd=+0.4).