Osmium isotope systematics of historical lavas from Piton de la Fournaise (Réunion Island, Indian Ocean)

Re–Os isotope and elemental data have been obtained for 20 historical picrites and basalts (1931–2006) from the Piton de la Fournaise volcano on Réunion Island and two old (>0.78?Ma) cumulates from a drill hole in the eastern part of the volcano. The ¹⁸⁷Os/¹⁸⁸Os ratios of the historical lava samples, selected to cover the MgO concentration and Pb isotopic ranges of Piton de la Fournaise lavas, range from 0.1311 to 0.1374. This result, together with previous results on 66-Ma-old lavas from the Deccan Traps (Allègre et al. in. Earth Planet Sci Lett, 170:197–204, 1999), supports the idea that the Os isotopic signature of the Réunion plume is relatively uniform and is at the less radiogenic end of the ocean island basalt spectrum. In detail, lavas erupted before 1992 seem to have higher ¹⁸⁷Os/¹⁸⁸Os than the lavas erupted after the 1992–1998 period of quiescence. Comparison of ¹⁸⁷Os/¹⁸⁸Os ratios with Pb, Sr and Nd isotopic data on the same set of samples shows no correlation between Os and Sr–Nd isotopes, whereas a broad positive relationship with Pb isotopes is observed, which is interpreted to reflect coupled fractionation of Re/Os and U–Th/Pb in the mantle due to the partitioning of Pb and Os into sulphides. Lavas inferred to be recording the Os isotopic signature of the Réunion plume source have higher ¹⁸⁷Os/¹⁸⁸Os ratios than the primitive mantle values. While this might be ascribed to melting of a lithologically heterogeneous source comprising recycled oceanic crust and/or continental sediment, the expected coupled Os–Sr–Nd–Pb isotopic variations are not observed. It is thus proposed that the mantle source for Piton de la Fournaise has inherently slightly radiogenic ¹⁸⁷Os/¹⁸⁸Os values that could reflect a mantle domain almost isolated from recycling processes.     

Effects of the October 25, 2010, Mentawai tsunami in La Réunion Island (France): observations and crisis management

On October 25, 2010, a Mw = 7.7 magnitude earthquake was recorded in the Kepulauan Mentawai archipelago, in Indonesia. Following the earthquake, the Pacific Tsunami Warning Center issued a tsunami bulletin to the countries of the Indian Ocean informing them that a local tsunami watch was in effect for Indonesia. La Réunion Island, a French territory located 5,000 km southwest of the seismic source, did not trigger its tsunami early warning protocol, but did suffer the impact of this tsunami 7h20 post-earthquake. This paper presents the post-tsunami survey conducted in La Réunion Island a few days after the event, providing run-up measurements that reach 1.72 m locally as well as information about the material losses and the crisis management. It provides a time sequence of the observations along the coast and the reactions of the population and authorities in dealing with this crisis. This paper also shows the local authorities’ lack of action in managing this event—thus increasing human and material vulnerabilities—the consequences for victims and the impact on the credibility of the local early warning system. These results provide geophysicists with information to better describe the source of this tsunami through far-field inversion and provide local decision makers with a comprehensive assessment of the weaknesses of the local early warning system and protocols.     

Evolution of melts of the Changbaishan Tianchi volcano (China–North Korea) as a model of ore–magmatic system formation: Data from melt inclusions studies

The composition and evolution of the melts of trachytes from the volcano were studied based on examining the inclusions of mineral-forming media by means of X-ray and ion microanalysis. A correlation was shown between the degree of enrichment of these melts in rare elements and the processes of magmatic differentiation. It was found that trachytes of the volcano were generated in highly differentiated alkaline melts enriched in Hf, Nb, Zr, Ta, U, Th, Rb, Y, and REEs under 1020–1060°C. The evolution of melts was determined by the processes of crystal fractionation. The main volatile components in the melts are water, fluorine, and chlorine with the concentrations of 0.1–0.5, 0.2–0.5, and 0.2–0.3 wt %, respectively. The melt crystallization was accompanied by degassing caused by the decrease in the outer pressure. The low concentrations of water and fluorine represent the melt composition by these components exclusively at the time of the melt movement towards the Earth’s surface just before the eruption.     

Geochemical characteristics of the high- and low-Ti basaltic rocks from the uplifted shoulder of the Ohře (Eger) Rift,Western Bohemia

High-Ti melanephelinite (3.8–5.9 wt% TiO₂), medium-Ti (phono)tephrite (2.7–3.1 wt% TiO₂), and low-Ti olivine melanephelinite/basanite (1.9–2.3 wt.% TiO₂) are subordinate rock types in the central European Cenozoic Volcanic Province. A contrasting melanephelinite to (phono)tephrite series occurs in the Lou?ná–Oberwiesenthal Volcanic Centre (37–28 Ma) and also as satellite volcanic bodies (26–12 Ma) together with olivine melanephelinite/basanite (17–20 Ma) on the southwestern periphery of the Kru?né hory mountains (Erzgebirge). The volcanic rocks intrude the Variscan basement of the uplifted shoulder of the Oh?e/Eger Rift in the Kru?né hory mountains of the Bohemian Massif. Low Mg# (44–59) and Cr, Ni contents and enrichment of LILE, Zr, Hf, Nb, Ta, U, Th and LREE in the high-Ti melanephelinites contrast with the composition of primitive low-Ti olivine melanephelinites/basanites displaying high Mg# (63–74) and high contents of compatible elements. The high-Ti melanephelinites reveal a wide range in initial ⁸⁷Sr/⁸⁶Sr of ca. 0.7034–0.7038 and ε_Nd of 2.4–4.9. The low-Ti melanephelinites show an overlapping range of initial ⁸⁷Sr/⁸⁶Sr of ca. 0.7035–0.7036 and ε_Nd of 4.3–5.5. The large variation in initial ⁸⁷Sr/⁸⁶Sr ratios at similar ε_Nd values in those rock types is interpreted as evidence for melting of metasomatized lithospheric mantle sources comprising K-bearing phases with radiogenic Sr. Modification of the olivine-free alkali basaltic magmas by differentiation or crustal contamination could give rise to the medium-Ti (phono) tephrites. The initial isotope ratios of all samples are consistent with HIMU-mantle sources and contributions from lithospheric mantle. The olivine-free melanephelinitic rocks often contain alkali pyroxenite–ijolite xenoliths with initial ⁸⁷Sr/⁸⁶Sr ratios of ca. 0.7036 and ε_Nd of 3.0. We interpret these xenoliths as samples of an intra-crustal alkali complex derived from similar mantle sources as those for the basaltic volcanic rocks.     

Caractérisation géostructurale des émergences et typologie des nappes d’altitude en milieu volcanique insulaire. (Ile de la Réunion)

Résumén

Sur l’Ile de la Réunion, l’alimentation en eau constitue un handicap majeur au développement agricole et urbain du versant sous le vent du massif volcanique du Piton des Neiges. En particulier dans les zones d’altitude (supérieure à 900m), les ressources en eau souterraine ne sont pas accessibles par forage. Sur les pentes externes du Piton des Neiges, quatre sites pilotes riches en émergences ont fait l’objet d’investigations hydrogéologiques dans le but d’évaluer la faisabilité de galeries de captage des eaux souterraines dans les hauts de l’île.

Le lever d’une carte géologique au 1/10 000 et la reconnaissance systématique du contexte géostructural des émergences ont permis : - de mettre en évidence la diversité des conditions hydrogéologiques du milieu volcanique,

- de montrer l’existence de sources profondément encaissées dans le massif, qui traduisent la présence de réserves aquifères significatives dont l’exploitation par galeries drainantes serait envisageable.

- de proposer une typologie des nappes d’altitude, appuyée et discutée en fonction des modèles hydrogéologiques existant sur les autres îles volcaniques.

Par ailleurs, l’intégration de ces résultats dans un modèle conceptuel décrivant les conditions de gisement des nappes d’altitude fournit l’architecture de base utilisable dans le cadre d’une simulation numérique de l’aquifère volcanique à l’échelle globale de l’île.     

Geochemical characteristics and age of metamorphic sole rocks within a Neotethyan ophiolitic mélange from Konya region (central southern Turkey)

Palaeo- and Neo-Tethyan-related magmatic and metamorphic units crop out in Konya region in the south central Anatolia. The Neotethyan assemblage is characterized by mélange and ophiolitic units of Late Cretaceous age. They tectonically overlie the Middle Triassic–Upper Cretaceous neritic to pelagic carbonates of the Tauride platform. The metamorphic sole rocks within the Konya mélange crop out as thin slices beneath the sheared serpentinites and harzburgites. The rock types in the metamorphic sole are amphibolite, epidote-amphibolite, garnet-amphibole schist, plagioclase-amphibole schist, plagioclase-epidote-amphibole schist and quartz-amphibole schist. The geochemistry of the metamorphic sole rocks suggests that they were derived from the alkaline (seamount) and tholeiitic (E-MORB, IAT and boninitic type) magmatic rocks from the upper part of the Neotethyan oceanic crust. Four samples from the amphibolitic rocks yielded ⁴⁰Ar/³⁹Ar isotopic ages, ranging from 87.04 ± .36 Ma to 84.66 ± .30 Ma. Comparison of geochemistry and geochronology for the amphibolitic rocks suggests that the alkaline amphibolite (seamount-type) cooled below 510 ± 25 °C at 87 Ma whereas the tholeiitic amphibolites at 85 Ma during intraoceanic thrusting/subduction. When all the evidence combined together, the intraoceanic subduction initiated in the vicinity of an off-axis plume or a plume-centered spreading ridge in the Inner Tauride Ocean at 87 Ma. During the later stage of the steady-state subduction, the E-MORB volcanics on the top of the down-going slab and the arc-type basalts (IAT/boninitic) detached from the leading edge of the overriding plate, entered the subduction zone after ~2 my and metamorphosed to amphibolite facies in the Inner Tauride Ocean. Duration of the intraoceanic detachment (~87 Ma) and ophiolite emplacement onto the Tauride-Anatolide Platform (Tav?anl? Zone), followed by subsequent HP/LT metamorphism (~82 Ma) spanned ~5 my in the western part of the Inner Tauride Ocean.     

La Formation glaciaire de la Mambéré (République Centrafricaine): Reconstitution paléogéographique et implications à l'échelle du Paléozoique africain

The Mambéré Formation constitutes a horizontal unit located in the western and southwestern part of the Central African Republic. Stratigraphical and sedimentological study provides strong argument to attribute these deposits a glacial origin. A palaeogeographical reconstruction has been outlined in order to precise the age of the formation. Two main categories of glacial deposits have been recognized:

glaciogenic deposits made of basal tills (with facetted pebbles) and flow tills (with flattened blunt pebbles);

reworked glacial deposits formed of sandstones and conglomeratic sandstones, in continuous beds, lenses or isolated blocks, together with siltstones and bedded sandstones.

The southerly provenance of the detrital material is demonstrated by quartz grain surface analysis and heavy-mineral study. This material results essentially from the dismantling of the Precambrian Schistoquartzitic Complex and secondarily from the Granitogneissic Complex. According to the palaeomagnetic polar paths and the migration of the glacial centers over the African continent during the Palaeozoic, the Mambéré Formation may be attributed a Lower Silurian age by reference to similar formations observed in Cameroon or a Lower Carboniferous age by comparison with the glacial formations reported from Niger and Egypt.     

Petrogenesis of layered pyroxenites from the Lherz,Freychinéde and Prades ultramafic bodies (Ariége,French Pyrénées)

The pyroxenite bands in spinel Iherzolites from three high-temperature peridotite massifs (Lherz, Freychinéde and Prades) in Ariège (French Pyrénées) have geochemical features including distributions of Mg, Ca, K, Ti, Zr and REE typical of cumulates derived by high P−T crystal segregation. However, these features of the whole-rocks are not reflected in the element distribution among the minerals which was partially modified during subsequent recrystallization. In addition, samples from the margins of the pyroxenite bands were affected by secondary processes such as metasomatism or subsolidus re-equilibration. Unlike pyroxenite xenoliths from basalts which are usually assumed to be derived from alkalic basaltic magmas, the pyroxenite layers are related to tholeiitic magmas similar to the parental liquids of the Triassic dolentes encountered throughout the Pyrénées. The data show that high P−T fractionation significantly affected the ascending tholeiitic magma and suggest that the continental tholeiites could have been derived from an upper mantle source with a chondrite shaped REE pattern.     

Identification and isotopic studies of early Cambrian magmatism (El Carancho Igneous Complex) at the boundary between Pampia terrane and the Río de la Plata craton,La Pampa province,Argentina

We have identified late Early Cambrian metaigneous rocks very poorly exposed at the Estancia El Carancho, in central La Pampa province, Argentina. They comprise calc–alkaline metadiorite and metagranite, and tholeiitic metapyroxenite and metagabbro. They are jointly referred to as the El Carancho Igneous Complex, and regarded to pertain to the Pampean magmatic arc and backarc, respectively. Titanite U–Pb SHRIMP dating of the metapyroxenite yielded 528 ± 5 Ma, and zircon U–Pb SHRIMP dating of the metadiorite yielded 520 ± 1.4 Ma. Hafnium isotope determinations on the dated zircons show ¹⁷⁶Hf/¹⁷⁷Hf ratios corresponding to positive εHf values from + 7.18 to + 9.37; Hf model ages of the Cambrian zircons yielded 884 Ma. It is interpreted that the metadiorites of the Complex crystallized from an Early Neoproterozoic (Tonian) juvenile source. We argue that the inferred occurrence of juvenile Tonian magmatic rocks in the (otherwise, mostly Paleo-Mesoproterozoic) substratum of the southern Pampia terrane could indicate a zone of thinned basement possibly associated with the early stage of Rodinia's breakup. In addition, the studied segment of the Pampean magmatic arc is contaminated by also juvenile, Late Mesoproterozoic crust, as evidenced by the presence of xenocrystic cores of 1140–1194 Ma – T_DM-Hf 1720 Ma and εHf values of + 3.24 to + 4.85 – in the Cambrian zircons, hence suggesting that the studied segment of the Pampean magmatic arc was intruded into juvenile Late Mesoproterozoic magmatic arc rocks. The El Carancho Igneous Complex would be located at the tectonic boundary between the Pampia terrane and the Río de la Plata craton. This boundary stands out in the aeromagnetic data as a change in the structural orientation about a roughly N-S line located approximately at 65° W and representing the suture zone between the Pampia terrane and the Río de la Plata craton. Our geotectonic model envisages westward dipping subduction of oceanic crust beneath the Pampia terrane; the El Carancho Igneous Complex would, therefore, have been originated on the Pampia side (upper plate) of the suture. Slivers of the arc- and backarc-type rocks would have been tectonically imbricated in the suture zone during the Pampean orogeny.     

Landuse and soil degradation in the southern Maya lowlands,from Pre-Classic to Post-Classic times: The case of La Joyanca (Petén,Guatemala)

This work focuses on the impact of Maya agriculture on soil degradation. In site and out site studies in the area of the city of La Joyanca (NW Petén) show that “Maya clays” do not constitute a homogeneous unit, but represent a complex sedimentary record. A high resolution analysis leads us to document changes in rates and practices evolving in time in relation with major socio-political and economic changes. It is possible to highlight extensive agricultural practices between Early Pre-classical to Late Pre-classical times. Intensification occurs in relation with reduction of the fallow duration during Pre-classic to Classic periods. The consequences of these changes on soil erosion are discussed. However, it does not seem that the agronomic potential of the soils was significantly degraded before the end of the Classic period.