Jean-Marie Dautria

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.

Contrasting lithospheric mantle domains beneath the Massif Central (France) revealed by geochemistry of peridotite xenoliths

We report major and trace element analyses for 82 coarse-grained peridotite xenoliths from 25 Cenozoic volcanic centres throughout the Massif Central (France). These data cover a region of about 150×150 km, allowing an investigation of large scale compositional variations in the subcontinental lithospheric mantle (SCLM). In agreement with textural variations, geochemical data define two contrasting lithospheric domains, situated north and south of latitude 45°30′. Peridotites of the northern domain show protogranular textures, characterised by clustered pyroxene–spinel distributions. They are rather refractory and depleted in MREE relative to HREE, but pervasively enriched in LREE and other highly incompatible elements. The samples show mantle-normalised patterns with negative anomalies of Nb, Ta, Zr and Hf, similar to enriched mantle xenoliths ascribed to carbonatitic metasomatism. In contrast, the peridotites of the southern domain are devoid of pyroxene–spinel clusters and are therefore referred to as coarse-granular. They are distinguished from the northern suite by more fertile compositions and relatively flat MREE–HREE patterns. In addition, only the harzburgites and a few lherzolites are enriched in LREE. Most southern domain lherzolites are depleted in these elements and the average composition of the southern suite is comparable to that of depleted MORB-source mantle (DMM). The main compositional differences between the two domains cannot be accounted for by a secular evolution of the Massif Central SCLM caused by Cenozoic plume upwelling. Instead, these differences record the existence of distinct lithospheric blocks assembled during the Variscan orogeny. To some degree, the northern and southern domains are reminiscent of cratonic and circumcratonic SCLM domains. Being relatively refractory and pervasively enriched in LREE, the northern domain displays similarities with cratonic SCLM. It is interpreted as a relatively ancient (pre-Variscan) lithospheric block involved in the Variscan belt. Conversely, the fertile composition and the DMM signature of the southern domain evoke more juvenile lithospheric mantle, possibly accreted or rejuvenated during the Variscan orogeny. Geophysical data indicate that asthenospheric upwelling beneath Massif Central is focused beneath the southern domain and follows a NW–SE trend, roughly parallel to Variscan structures in the crust. Though poorly constrained in direction, the limit between the two SCLM domains recognised in this study is consistent with this trend. This may suggest a link between the inherited architecture of the SCLM and channelling of asthenospheric upwelling. Secular variations in xenolith geochemistry, as well as correlations between trace element data and geophysical anomalies, suggest that the geochemical imprint of Cenozoic plume upwelling on SCLM xenoliths is limited to selective enrichments in U, Sr and Pb relative to Th and REE.

An experimental study of the brittle-ductile transition of basalt at oceanic crust pressure and temperature conditions

Received 21 September 2011; revised 10 January 2012; accepted 1 February 2012; published 23 March 2012. [1] The brittle to ductile transition (BDT) in rocks may strongly influence their transport properties (i.e., permeability, porosity topology…) and the maximum depth and temperature where hydrothermal fluids may circulate. To examine this transition in the context of Icelandic crust, we conducted deformation experiments on a glassy basalt (GB) and a glass-free basalt (GFB) under oceanic crust conditions. Mechanical and micro-structural observations at a constant strain rate of 10 � 5 s � 1 and at confining pressure of 100–300 MPa indicate that the rocks are brittle and dilatant up to 700–800 � C. At higher temperatures and effective pressures the deformation mode becomes macroscopically ductile, i.e., deformation is distributed throughout the sample and no localized shear rupture plane develops. The presence of glass is a key component reducing the sample strength and lowering the pressure of the BDT. In the brittle field, strength is consistent with a Mohr-Coulomb failure criterion with an internal coefficient of friction of 0.42 for both samples. In the ductile field, strength is strain rate- and temperature-dependent and both samples were characterized by the same stress exponent in the range 3 < n < 4.2 but by very different activation energy QGB =5 9� 15 KJ/mol and QGFB = 456 � 4 KJ/mol. Extrapolation of these results to the Iceland oceanic crust conditions predicts a BDT at � 100 � C for a glassy basalt, whereas the BDT might occur in non-glassy basalts at deeper conditions, i.e., temperatures higher than 550 � 100 � C, in agreement with the Icelandic

Thermodetrital and crystallodetrital magnetization in an Icelandic hyaloclastite

An Icelandic hyaloclastite, mostly composed of millimetric fragments of basaltic glass, that is fresh at the bottom of the unit but largely palagonitized in the upper part, has been studied by petrologic, mineralogical, and magnetic means, with the aim of determining the nature and characteristics of the natural remanent magnetization (NRM). The NRM was generally found to consist of two components: a thermodetrital remanent magnetization (thermo-DRM) and a crystallodetrital remanent magnetization (crystallo-DRM). Thermo DRM and crystallo-DRM are defined here as the remanences acquired as a result of the deposition of magnetic particles of detrital origin individually carrying either a TRM (thermoremanent magnetization) or a CRM (crystallization remanent magnetization), respectively. Regardless of the chemistry and size and of these particles, Thellier experiments carried out on samples carrying a thermo-DRM provide apparent paleointensities close to the expected geomagnetic paleointensity, which suggests that in the present case the fractional alignment of individual magnetic moments is similar for DRM and TRM. In the upper part of this outcrop, grain growth CRM was acquired by individual grains of magnetite which crystallized as a result of palagonitization of basaltic glass at low temperature (<100°C). No systematic difference could be observed between the directions of characteristic remanence in the layers richest in magmatic magnetic grains and the layer where secondary magnetite is the main remanence carrier. In both cases, the overall remanence exhibits a large inclination error (∼20°), and the samples have a marked anisotropy of magnetic susceptibility which is typical of sedimentary fabrics. Thus, secondary magnetite probably formed prior to the deposition of particles, and the bulk remanence in the palagonitized layers is a crystallo-DRM rather than a CRM. Throughout the entire stratigraphic thickness, Thellier paleointensity data are of good or excellent quality regardless of the nature of the primary remanence. In agreement with previous theoretical inferences and experimental results the layers carrying a crystallo-DRM provide a much lower (by a factor of 2) apparent paleointensity than the layers where the remanence is a thermo-DRM. This suggests that palagonitized basaltic glasses should not be used for paleointensity determinations. Our study shows that application of the Thellier thermal paleointensity method to sedimentary rocks can be a useful tool for distinguishing crystalline versus thermal blocking of the magnetic moments of the individual particles which, after deposition, carry a DRM. More generally, discrepancies between relative paleointensities obtained from sediments can be expected if rocks with different proportions of thermo-DRM, crystallo-DRM, or CRM are compared.

Rotation of the Semail ophiolite (Oman): Additional Paleomagnetic data from the volcanic sequence

Thirty-two flows (247 cores) were sampled in the V1 (Geotimes) and V2 (Lasail) volcanic units of the Semail ophiolite, Oman (Aswad, Fizh, Hilti, Sarami, Wuqbah, and Tayin massifs). Paleomagnetic analysis of the samples was complicated by a large overlap of the two components of magnetization carried by the rocks: a crystalline remanent magnetization (CRM) acquired in the present day field, probably during weathering, and an older CRM probably produced by oxidation of the original titanomagnetites during hydrothermal event(s). If the magnetization carried by the V1 samples was acquired during the hydrothermal event related to the emplacement of these lava, e.g., during and/or shortly after cooling, the tectonic unity of the northern domain has to be questioned and a differential rotation considered between the Aswad and Hilti-Sarami massifs but, by the time of emplacement of the V2 series, this northern area seems to behave as one large unit. As only one set of data is available for the southern Tayin-Sumail massif, it is premature but a possible relative rotation on the order of 90° can be suspected between the Hilti-Sarami and Tayin-Sumail massifs, rotation which would have occurred after emplacement of the V2 series.

Nouvelles données géochronologiques, géochimiques et isotopiques sur le volcanisme du Forez : relation avec l'évolution cénozoïque du manteau du Massif central

In the Forez district, two volcanic events are documented. The earlier one, of Paleocene age, predates the Oligocene rifting. The magmas (melilitite) are of carbonatitic affinity and they would correspond to small volume melts responsible for the chemical and mineralogical modifications previously identified in the lithospheric mantle of this region. The later lavas are Lower Miocene and of basanitic composition. Their isotopic signatures (Sr, Nd, Pb) suggest that they partly originated from partial melting of this modified lithosphere. The Paleocene magmatism would indicate the initiation of asthenospheric upwelling beneath the French Massif Central.

Rifting actif ou passif en Éthiopie ? éléments de réponse apportés par l'étude des xénolites péridotitiques de la région du lac Tana

The comparative study of peridotitic xenoliths from Oligocene and Quaternary volcanic formations of north-western Ethiopia shows that the textural, mineralogical, thermal and geochemical characteristics of the lithospheric mantle of this region were acquired in Oligocene times. These data suggest that the impinging of the Afar plume beneath the Ethiopian lithosphere occurred prior to the traps emplacement and the Plio-Quaternary rifting. This favours a model of active rifting for the Afar triple junction.

Le complexe annulaire d'âge Oligocène de l'Achkal (hoggar Central, Sud Algérie) : témoin de la transition au Cénozoïque entre les magmatismes tholéitique et alcalin. Évidences par les isotopes du Sr, Nd et Pb

The Achkal Oligocene ring complex cross-cuts the Upper Eocene tholeiitic traps located on the top of the Hoggar swell. The plutonic rocks range from tholeiitic gabbros to alkali essexites, monzonites and syenites, whereas the volcanites are restricted to late peralkaline rhyolites. The affinity change linked to the large isotopic heterogeneities (from EM1 to HIMU) suggests that the parental magmas are issued from two distinct mantle sources, first lithospheric then deeper. The Achkal has recorded the magmatic evolution of the Hoggar hot spot, between Eocene and Miocene.