Christian Pin

Sequential separation of light rare-earth elements, thorium and uranium by miniaturized extraction chromatography: Application to isotopic analyses of silicate rocks

Abstract A new method for the concomitant separation of the light rare-earth elements (LREEs), thorium and uranium is described, and applied to the determination of 143 Nd 144 Nd ratios, and concentrations of Sm, Nd, Th and U in silicate rocks, using isotope dilution and thermal ionization mass spectrometry. The proposed scheme is based on two recently introduced extraction chromatographic materials, referred to as TRU.Spec (TRansUranic-element Specific) and Ln.Spec. For most geological samples, the TRU.Spec ‘resin’ affords a straightforward separation of LREE, U and Th with high yield, good purity and satisfactory blank levels, on a single small (0.25 ml) column. Only Fe3+ was found to have detrimental effects on the LREE yields. For iron-rich samples, this troublesome interference was overcome by pretreatment on a short cation exchange column. Besides, a tandem column arrangement with the Ln.Spec ‘resin’ enables the isolation of neodymium and samarium from bulk rock solutions be made in a single run. The validity of the method is illustrated by the measurement of Sm, Nd, Th and U concentrations along with 143 Nd 144 Nd ratios in 15 international standard reference materials.

Concomitant separation of strontium and samarium-neodymium for isotopic analysis in silicate samples, based on specific extraction chromatography

A separation scheme for strontium and light rare earth elements and its application to the isotopic analysis of strontium and neodymium in silicate rocks are presented. This method benefits from the selectivity and high capacity of two newly introduced extraction Chromatographic materials, referred to as Sr.Spec and TRU.Spec, respectively. These afford a straightforward separation of Sr and Sm + Nd with high yield, good purity and satisfactory blank levels, on very small (0.25 ml) columns using small volumes of solutions of a single mineral acid, HNO3. The validity of the method is illustrated by the measurement of 87Sr/ 86Sr and 143Nd/144Nd ratios in twelve international standard reference materials.

Granites, Granulites, and Crustal Differentiation

Partial melting and the ascent of granitoid magma are among the main processes leading to differentiation of the continental crust. Recent experimental studies and modelling indicate that fertile crustal rocks will typically produce 30 to 40 vol% melt at temperatures of 850°–900°C, even under fluid-absent conditions. Quartz-saturated pelitic rocks will yield 25 to 50 vol% melt at ~ 850°C, while metabasic and intermediate rocks will form 15 to 50 vol% melt at T ≤ 900°C. Production of a large quantity of melt in this temperature range will have the effect of buffering metamorphic temperature in the melting zone. The temperature will not exceed 850°–900°C until the partial fusion process is complete. Fusion of ~ 40% of the lower crust would consume a large amount of energy: the buffering capacity (?H) of a fluid-absent partial melting reaction (at 850°C) being about 50 cal per gram of melting rock. This means that metamorphic T will rarely exceed 850°–900°C during a first, major, thermal event. At this stage, crustal magma production will depend on the fertility of the source rock and the intensity of the thermal anomaly responsible for the metamorphism. With a fertile lower crust, such a thermal event will generate S- and I-type granitoid liquids and leave behind granulitic residues composed of Qtz + Kfs (or Pl) + Grt + Sil + Ru, Qtz + Pl + Opx + Grt, or Opx + Cpx + Pl (± Grt), depending on the composition of the protolith. During a second thermal event (affecting recycled, melt-depleted crust), high-temperature A-type magmas could be produced. During any subsequent thermal event (without introduction of aqueous fluids) there would be no buffering effects linked to either subsolidus or melting reactions. The buffer capacity of the crust would be exhausted, and temperatures of around 1000°C could readily be reached, given a sufficient heat supply. Depending on rock composition, unusual high-T assemblages such as spinel + quartz, sapphirine + quartz, orthopyroxene + sillimanite, and osumilite may develop.

Evaluation of a strontium-specific extraction chromatographic method for isotopic analysis in geological materials

Abstract The suitability of a recently published extraction chromatographic method to isolate from geological samples for isotopic analyses by thermal ionization mass spectrometry was evaluated. This method has several significant advantages over conventional ion-exchange techniques: high selectivity combined with high recoveries, overall simplicity and the use of very small reagent volumes of a single acid (HNO3). Satisfactory blank levels were obtained on new columns, but memory effects could not be totally eliminated from already used extraction chromatographic material. Determinations of 87Sr/86Sr ratios and Sr concentration by isotope dilution following the proposed method are reported for thirteen international rock standards.

Early Ordovician continental break-up in Variscan Europe: NdSr isotope and trace element evidence from bimodal igneous associations of the Southern Massif Central, France

Abstract The high-grade Marvejols Group and the low-grade Albigeois-Cevennes sedimentary sequence contain bimodal igneous rocks of Early Ordovician age which are representative of a widespread thermal event in the European Variscides. Comparison of their NdSr isotope and trace-element characteristics provides additional evidence for their origin in an ensialic extensional setting. As an alternative to the back-arc model proposed by previous authors, we propose that these associations record a continental break-up episode unrelated to contemporaneous subduction. In this model, the widespread Early Paleozoic bimodal magmatism marks the birth of a Mid-European oceanic arm. We propose that the break-up was controlled by both transtensional processes and mantle-plume activity. High grade associations, such as the Marvejols Group, and low-grade bimodal associations, such as the Albigeois-Cevennes, might be regarded as representing opposite rifted passive margins. The “northern”, Marvejols-type margin was involved in a Late Ordovician-Silurian subduction-like process, with HP-HT metamorphism. In contrast, the “southern” Albigeois/Cevennes margin remained largely unaffected, possibly as part of the overriding plate of the subduction zone. “Pseudo-calc-alkaline” signatures unrelated to subduction processes may occur in magmatic rocks associated with continental break-up episodes. In this case, negative Nb anomalies are produced by the addition of crustal components enriched in Th and LREE relative to Nb. This inferred alternative origin of Nb anomalies has important bearing on the paleogeodynamic settings based on geochemical data.

Variscan oceans : ages, origins and geodynamic implications inferred from geochemical and radiometric data

Abstract A review of the magmatic associations providing evidence for the existence of extensional regimes during the Palaeozoic evolution of Variscan Europe is presented. The available geochronological data allow recognition of two major rifting events, one in the Early Ordovician and the other in the Middle-Late Devonian. Scarce relicts of ophiolitic complexes indicate that both extensional episodes reached a true ocean spreading stage. It is suggested that the Variscan Belt resulted from a complex, two-stage collision process, through successive suturing (in the Silurian and the Early Carboniferous) of these oceanic domains. The superposition of two distinct opening-closure cycles involving geographically separate oceanic basins may partly account for the unusual width of the Variscan orogen.

U-Pb dating of an early paleozoic bimodal magmatism in the french Massif Central and of its further metamorphic evolution

In the Marvejols area (Southern french Massif Central), the gneissic Marvejols supergroup is overthrust on the metasedimentary “Série du Lot”, deposited in part prior to 540 Ma. The allochtonous terranes are characterized by the occurrence of a leptyno-amphibolitic group, a complex association of mafic and felsic rocks of igneous and sedimentary derivation. A 480±10 Ma age has been obtained by U-Pb dating of zircons, for the crystallization of both mafic and felsic meta-igneous rocks. These rocks were emplaced during an important extensional tectonics. Relics of eclogites, pyrigarnites, coronite gabbros and HP-trondhjemites are clear evidence for a further HP-HT event dated at 415±6 Ma on zircons from a HP trondhjemite. Subsequently, the Marvejols supergroup underwent an amphibolite facies metamorphism with incipient mobilization dated at 345±10 Ma.Rifting and thinning of the continental crust in Cambro-Ordovician times appears to be a major geodynamic feature which could account for the thermal events often referred to the “Caledonian” orogeny. The Silurian (415 Ma) age of the HP episode is clearly older than the main Variscan tectonometamorphic event; it is in agreement with Rb-Sr dating of the Moldanubian granulites and with some radiometric data from Southern Brittany (France). These results point to a compressive phase, probably in a subduction context, in view of the high pressures reached (15–20 Kb), after the Cambro-Ordovician distensive phase. The main final tectono-metamorphic paroxysm (blocking of subduction process and continental collison ?) is not prior to the end of Devonian (340–350 Ma) and is related to the Variscan orogeny s.s

Sources of Hercynian granitoids from the French Massif Central: Inferences from Nd isotopes and consequences for crustal evolution

The French Massif Central is characterized by a great abundance and variety of granitoids emplaced during Hercynian times (360-280 Ma). Distinct ϵNd(i) and (87Sr86Sr)i are not associated with particular petrographic types; however, lower ϵNd(i)-values (−8.5) occur among the younger leucogranites from the western part of the massif. Intermediate ϵNd(i) (−6) correlate with aluminous monzogranites and higher values (−6 to −3) with calc-alkaline granodiorites. In the eastern Massif Central ϵNd(i) are homogeneous at the regional scale. In the western Massif Central, the leucogranites and the monzogranite-granodiorites display different trends of ϵNd(i) with emplacement ages. From these results, extreme models involving a depleted mantle, or an old (Archean or Early Proterozoic) source may be ruled out. The trend of increasing ϵNd(i) with decreasing age observed in the monzogranite-granodiorite group suggests either an increasing contribution of a mantle component, or the progressive melting of more mafic, more refractory crustal sources, or both. The reverse trend displayed by the leucogranites might be explained in terms of the upward migration of partial melting across an isotopically zoned metasedimentary pile. The large amount of granites may be the result of favourable processes during the collision tectonics, combined with the unusual abundance of fertile materials, enriched in water and heat-producing elements, in the pre-Carboniferous crust.

Episodic and short-lived granitic pulses in a post-collisional setting: evidence from precise U–Pb zircon dating through a crustal cross-section in Corsica

Abstract In western Corsica, a large granitoid batholith was emplaced during Carboniferous and Permian times. These rocks display an evolution through time from high Mg–K calc-alkaline, to calc-alkaline, then alkaline affinities. Moreover, the magmatic processes can be observed at different crustal levels. New and precise Isotope Dilution and Thermo-Ionisation Mass Spectrometry (ID-TIMS) U–Pb dating suggests that granite magmatism in Corsica did not simply reflect the continuous thermal evolution of a thickened crust. A precise timing for the post-collisional magmatism is documented with four distinct short-lived episodes, respectively at 345, 338, 305 and 280 Ma, separated by periods of inactivity. These four magma pulses display variable Nd isotopic signatures related to mantle–crust interactions. Granite magma generation and emplacement is strongly connected to the thermal and tectonic activity of the lithosphere. An increasing contribution of a depleted mantle to the magmatic processes is documented for the last igneous event. This is interpreted to reflect the emplacement of depleted mantle materials in the lower crust, which enhanced heat flow and triggered partial melting of previously underplated mafic rocks. This increasing contribution of mantle materials marks the onset of a new geodynamic cycle characterised by an extensional tectonic regime, preluding to the opening of the Alpine orogenic tract.

Deciphering the petrogenesis of deeply buried granites: whole-rock geochemical constraints on the origin of largely undepleted felsic granulites from the Moldanubian Zone of the Bohemian Massif

The prominent felsic granulites in the southern part of the Bohemian Massif (Gfohl Unit, Moldanubian Zone), with the Variscan (∼340 Ma) high-pressure and high-temperature assemblage garnet+quartz+hypersolvus feldspar ± kyanite, correspond geochemically to slightly peraluminous, fractionated granitic rocks. Compared to the average upper crust and most granites, the U, Th and Cs concentrations are strongly depleted, probably because of the fluid and/or slight melt loss during the high-grade metamorphism (900–1050°C, 1·5–2·0 GPa). However, the rest of the trace-element contents and variation trends, such as decreasing Sr, Ba, Eu, LREE and Zr with increasing SiO 2 and Rb, can be explained by fractional crystallisation of a granitic magma. Low Zr and LREE contents yield ∼750°C zircon and monazite saturation temperatures and suggest relatively low-temperature crystallisation. The granulites contain radiogenic Sr ( 87 Sr/ 86 Sr 340 = 0·7106–0·7706) and unradiogenic Nd ( = − 4·2 to − 7·5), indicating derivation from an old crustal source. The whole-rock Rb–Sr isotopic system preserves the memory of an earlier, probably Ordovician, isotopic equilibrium. Contrary to previous studies, the bulk of felsic Moldanubian granulites do not appear to represent separated, syn-metamorphic Variscan HP–HT melts. Instead, they are interpreted as metamorphosed (partly anatectic) equivalents of older, probably high-level granites subducted to continental roots during the Variscan collision. Protolith formation may have occurred within an Early Palaeozoic rift setting, which is documented throughout the Variscan Zone in Europe.