J.-L. Bodinier

Evolution of LILE-enriched small melt fractions in the lithospheric mantle: a case study from the East African Rift

Abstract Spinel-peridotite xenoliths from Mega (East African Rift, Sidamo region, SE Ethiopia) show variable degrees of recrystallization coupled with trace-element variations. The less recrystallized samples (deformed xenoliths) consist of apatite-bearing porphyroclastic peridotites. They are strongly enriched in LILE (Ba, Th, U, Sr and LREE), with negative anomalies of the HFSE (Nb, Ta, Zr, Hf and Ti). The most recrystallized samples (granular xenoliths) consist of apatite-free peridotites with coarse-grained, granular textures. These samples are depleted or only slightly enriched in LILE and display no significant HFSE anomaly. We suggest that the inverse relationship between recrystallization and trace-element enrichment results from km-scale variation in volume and composition of melts pervasively infiltrated in the lithosphere. The deformed xenoliths record interaction with LILE-enriched small melt fractions, at low melt/rock ratio, while the granular xenoliths were extensively re-equilibrated with a higher fraction of basaltic melt, at higher melt/rock ratio. With a numerical simulation of reactive porous flow at the transition between adiabatic and conductive geotherms in the mantle, it is shown that these two processes were possibly coeval and associated with thermo-mechanical erosion of the lower lithosphere above a mantle plume.

Distribution of incompatible trace elements between the constituents of spinel peridotite xenoliths: ICP-MS data from the East African rift

To constrain the geochemical models of the lithospheric mantle, we have carried out a detailed study of the distribution of incompatible trace elements between the various constituents of spinel peridotites. Predominant and accessory minerals were separated in 12 mantle xenoliths from Mega (East African Rift, Sidamo region, SE Ethiopia). The samples range in composition from cpx-rich lherzolites to refractory harzburgites and are devoid of modal metasomatism, except for minor amount of apatite in some of them. Their trace element concentration encompasses almost the whole range reported in the literature for basalt-born xenoliths. Mineral separates (ol, opx, cpx, spinel and apatite) and their leachates were analyzed by ICP-MS, for rare earth elements (REE) and several incompatible trace elements (Rb, Sr, Ba, Zr, Hf, Nb, Ta, Th, U, and Ti). Spinel surfaces were investigated by SEM and EPMA to determine the composition of the attached micro-phases. n nMass-balance inversion shows that the trace element composition of whole rocks is controlled by five distinct components: n n1). the silicate minerals which account for the total HREE abundance, and 50–90% of LREE, Sr, and Zr-Hf, in the apatite-free peridotites; n n2). the mineral-hosted fluid inclusions which play a significant role for Rb (20–25%), and to a lesser degree for the other LILEs; n n3). a pervasive grain-boundary component selectively enriched in highly incompatible elements, which contributes 25–90% of the whole-rock budget for Ba, Th and U, and 10–50% for Nb and LREE, in the apatite-free samples; n n4). thin reaction layers (<10 μm thick) coating the surfaces of spinel grains and mainly composed of Ti-oxides and phlogopite. They are the predominant repository of Nb-Ta (45–60%) and Rb-Ba (30–80%) in all the studied xenoliths; n n5). apatite which largely predominates the budget of Th, U, Sr and LREE (25–75%) in the samples containing this mineral. n nCompared to the other peridotite constituents, fluid-derived inclusions in minerals provide minor contribution to the trace element budget of whole rocks. However, they strongly affect inter-mineral trace element partitioning. The latter strongly deviates from experimental distribution coefficients for the most incompatible elements and tends to one for elements such as Rb (±Ba, Th and U). This indicates that the different rock-forming minerals contain similar amounts of homogeneous inclusions. Fluid-derived inclusions may be responsible for the discrepancies that have been observed between inter-mineral trace-element partitioning derived from in situ and bulk mineral analyses.

Petrogenetic evolution of orogenic lherzolite massifs in the central and western Pyrenees

Abstract A petrological and geothermobarometric study of several tens of representative spinel peridotite and co-existing pyroxenite samples indicates that orogenic lherzolite massifs of the central-western Pyrenees (CWP) display significant differences from those of the eastern Pyrenees (EP). They are characterized by a predominance of clinopyroxene-rich spinel lherzolites, an abundance of coarse-granular textures, the absence of thick harzburgitic bands, lower proportions of pyroxenites, the absence of high-pressure amphibole-pyroxenite dykes, and a greater development of high-stress deformation textures which define 100m-scale mylonitic shear zones. The highly fertile compositions of the CWP peridotites can be accounted for by accretion of asthenospheric protoliths to the lithosphere via passive cooling or very limited degrees of adiabatic melting. This accretion event was followed by a thermal relaxation, resulting in the steady-state equilibrium stage (1050°C and 15–18 kbar) recorded by the garnet pyroxenites and granular lherzolites. Compared to the single decompression and cooling step identified in the EP lherzolite massifs, the CWP massifs record a two-step, non-adiabatic uplift. The earliest event is a nearly isothermal (1050-950°C) decompression from 60 km up to 25 km depth caused by a lithospheric thinning event, possibly related to the late Hercynian extension. The further uplift step from 25 km to 15 km depth was accompanied by cooling down to 600°C and high-stress mylonitic deformation, resulting in tectonic denudation and emplacement of the western Pyrenean massifs onto the floor of small Albo-Aptian pull-apart basins via large-scale shear zones. Geospeedometric considerations yield estimates around 5–10 Ma for the duration of cooling and mylonitic deformation, coeval with the mid-Cretaceous anticlockwise motion of the Iberian and European plates. So, the western Pyrenean peridotite massifs left the mantle between 109 and 117 Ma and were uplifted to shallower levels (15 km) than the EP massifs.

Geochemistry of metasomatism adjacent to amphibole-bearing veins in the Lherz peridotite massif

Abstract The Lherz peridotite massif, in the French Pyrenees, is intruded by a number of hornblendite and garnet-amphibole-pyroxenite (GAP) veins. New, high quality, elemental and isotopic data are presented for veins and their adjacent harzburgite wallrocks in order to evaluate the extent of reaction and the ability of fluids to permeate clinopyroxene-poor peridotites. Hornblendite and GAP veins have convex upward rare earth element (REE) profiles consistent with an origin as crystal segregates from alkali basalts. In all of the traverses Mg# increases away from the veins and MnO, TiO2, Zr, and the REE decrease away from the veins within a zone Ce Sm ratios and decrease in Ce contents away from the veins is consistent with equilibration of the calculated melt from the vein with the preexisting harzburgites. A region with high Ce Sm on the right of one vein may be the result of chromatographic fractionation of melt during percolation from the amphibole-bearing veins. However, this is not observed on the opposite side of the vein and ratios are variable within the zone, so an asymmetrical and irregular chromatographic front would be required. The high Ce Sm ratios, therefore, most likely reflect pre-vein REE heterogeneity in the harzburgites. Data for the Lherz massif suggest that dramatic variations in incompatible element concentrations can develop metasomatically in the continental lithospheric mantle over a relatively short length scale.

Growth of the European lithospheric mantle-dependence of upper-mantle peridotite facies and chemical heterogeneity on tectonics and age

Abstract In Europe, during the Phanerozoic, collision of microplates caused the juxtaposition of disparate lithospheres of variable age and provenance. The complex prehistory of these plates, together with the present-day tectonic regime, generated considerable topography at the lithosphere-asthenosphere boundary. From north to south across Europe there exists a considerable variation in lithosphere thickness, seismic velocity and heat flow, with concomitant changes in the mantle helium flux, the extent, type and source of Cenozoic volcanism, and the age and origin of the lithospheric mantle protolith. Consideration of Moho depth and lithosphere thickness reveals that the lithospheric mantle should be dominated by garnet-diamond facies mantle beneath stable shield areas (e.g. the Baltic Shield) and young mountain belts (e.g. the Alps and Betics), spinel-garnet facies mantle beneath Variscan Europe and spinel-plagioclase facies mantle in the western Mediterranean, Pannonian Basin and Rhinegraben. However, consideration of mantle xenolith data reveals that garnet peridotites are rare beneath Variscan Europe and that plagioclase peridotites are unreported from the Pannonian Basin and the Rhinegraben. A tectonic dimension to lithosphere thickness, as well as a function that relates to the initial stabilisation age, is illustrated by the presence of thick lithosphere (diamond facies) beneath old tectonically stable areas such as the Baltic Shield, and young tectonically active regions such as the Alps. Thermo-tectonic processes have also produced lithosphere under the Archaean of NW Scotland that is as thin as that under parts of the Alpine Orogen (e.g. in western Mediterranean and the Pannonian Basin). Extreme chemical heterogeneity in the lithospheric mantle can result from both time-integrated effects over several billion years (e.g. in the North Atlantic craton) and mixing over several tens of millions of years along the tectonically active southern margin of Europe (e.g. in the Betics). Volcanic rocks provide a valuable probe of the lithospheric mantle and the asthenosphere, and it is apparent that throughout the Phanerozoic the lithospheric mantle evolved in response to repeated cycles of collision-subduction and intraplate extension. Volcanic rocks erupted during the Caledonian, Variscan and Alpine orogenies were derived from shallow lithospheric reservoirs containing a sedimentary component. In contrast, during syn- or post-orogenic extension volcanic rocks were derived from sub-lithospheric sources or by passive reactivation of young lithospheric mantle.

Petrology and geochemistry of a cumulate xenolith suite from Bute: evidence for late Palaeozoic crustal underplating beneath SW Scotland

Xenoliths from Hawks Nib (Bute, SW Scotland), entrained in alkali basalt of late Carboniferous–early Permian age, generally possess unrecrystallized igneous cumulate textures. Most are inferred to derive from lower crustal (20–30 km) depths, with a small minority from the underlying restitic mantle. Compositions range from ultramafic cumulates (dunite, wehrlite, websterite, clinopyroxenite) to gabbros and anorthosites. The suite is unique in being dominated by poikilitic wehrlites and olivine clinopyroxenites. Major element variation in the cumulates is controlled by modal mineral variations. The ultramafic cumulates all have high REE contents and light REE (LREE)-enriched patterns, with websterites being the least enriched. Gabbroic xenoliths have uniform patterns in which both LREE and heavy REE are depleted relative to the middle REE, with small positive Eu anomalies. Present-day Sr–Nd isotopic data for the suite are the most depleted among Scottish xenoliths. The xenoliths are inferred to be products of young replenishments of the lower crust by basaltic magmas during continental underplating. Orthopyroxene-bearing lithologies crystallized from tholeiitic magmas whereas alkali basalts formed the clinopyroxene-rich samples; both parental magma-types were derived from similar mantle sources.

Origin and significance of poikilitic and mosaic peridotite xenoliths in the western Pannonian Basin: geochemical and petrological evidences

Peridotite xenoliths erupted by alkali basaltic volcanoes in the western Pannonian Basin can be divided into two fundamentally contrasting groups. Geochemical characteristics of the abundant protogranular, porphyroclastic and equigranular nodules suggest that these samples originate from an old consolidated and moderately depleted lithospheric mantle domain. In contrast, the geochemical features of the worldwide rare, but in the Pannonian Basin relatively abundant, poikilitic xenoliths attest to a more complex evolution. It has been argued that the origin of the peculiar texture and chemistry may be intimately linked to melt/rock reactions at successively decreasing liquid volumes in a porous melt flow system. The most likely site where such reactions can take place is the asthenosphere–lithosphere boundary. In this context, poikilitic xenoliths may provide petrological and geochemical evidence for reactions between magmatic liquids issued from the uprising asthenosphere and the solid mantle rocks of the lithosphere. These reactions are important agents of the thermal erosion of the lithosphere; thus, they could have considerably contributed to the thinning of the lithosphere in the Pannonian region. We suggest that in the Pannonian Basin, there could be a strong relation between the unusual abundance of poikilitic mantle xenoliths and the strongly eroded lithosphere.