Peridotite

This study presents bulk-rock major, trace, and platinum-group element data, as well as mineral chemistry for peridotites which form large enclaves (up to 500 × 1000 m) within Mesoarchaean orthogneisses of the Akia terrane in the Fiskefjord region, southern West Greenland. The largest peridotite body, known as Seqi, contains highly fosteritic olivine with a median Mg# of 92.6 and hosts extensive layers of chromitite, which can be traced for tens of metres with thicknesses of up to 30 cm. Thinner (<100 m thick), but extensive (up to 2000 m long) peridotite sheets are associated with coarse norite and orthopyroxenite with distinct cumulate textures in the Amikoq complex, located a few tens of kilometres south of Seqi. Intercalated amphibolites of tholeiitic basaltic composition show complementary geochemical evolution to the peridotites, consistent with igneous crystal fractionation trends. The U-shaped trace element patterns of the peridotites may either reflect the parental melt composition from which these olivine-rich rocks were derived, or alternatively this feature may be the result of melt-rock interaction. Overall, we interpret the Fiskefjord region peridotites to have formed as ultramafic cumulates derived from Archaean high-Mg, low Ca/Al magmas, although their geodynamic setting remains to be established.

Le massif péridotitique de cap Bougaroun se situe à une dizaine de kilomètres à l’ouest de Collo, et diffère du reste de la petite Kabylie par l’existence de larges affleurements des péridotites et serpentinites. Seules les variétés de serpentinites offrent une richesse géologique certaine pour les utiliser en tant que pierre décorative. Ces roches présentent tous les éléments qui leur donnent un aspect décoratif précieux ; variétés des couleurs, l'existence des veinules foncées et un lustre brillant après polissage ainsi que les réserves qui sont d’un volume de 107 m3.Notre étude cible, au fait une caractérisation des propriétés pétrographiques et minéralogiques de ces roches, aussi en mettant en évidence leur spécificité par une série d’essais physico-mécaniques pour confirmer que cette roche peut être utilisée comme une pierre décorative.

Major element and Re–Os isotope analysis of single sulfide inclusions in diamonds from the 240 Ma Jwaneng kimberlite has revealed the presence of at least two generations of eclogitic diamonds at this locality, one Proterozoic (ca. 1.5 Ga) and the other late Archean (ca. 2.9 Ga). The former generation is considered to be the same as that of eclogitic garnet

The Puy Beaunit maar presents a large variety of mantle xenoliths (spinel peridotites, pyroxenites and layered rocks). A detailed study of the textures and mineral equilibria shows the unusual character of this occurrence and the local complexity of the upper mantle beneath the French Massif Central. Ultramafic nodules have a metamorphic, magmatic or pyrometamorphic origin; they display different stages of

Petrographic and geochemical studies have been carried out on the peridotite xenoliths included in the pyroclastite deposits from the Lake Guinnadji and Ngao Djalsoka volcano close to the Dibi locality in the Adamawa plateau, central Cameroon. The peridotite xenoliths exhibit protogranular and porphyroclastic textures, and are composed of olivine, orthopyroxene and minor amount of clinopyroxene, amphibole (?) and spinel, typical of harzburgite type. ICP-MS and ICP-OES analyses of representative samples show that they are in the range of those of the primitive mantle origin (42.4-45.0 wt.% MgO, 65.7-79.0 wt.% normative olivine). Decreasing SiO 2 , TiO 2 , Al 2 O 3 and CaO contents vs increasing MgO trends evidence depletion from more fertile mantle by extraction of basaltic melts, leaving a refractory residue of harzburgite composition. High contents of REE and incompatible elements point out secondary enrichment processes which have affected the peridotite xenoliths. For this, metasomatism caused by silicate fluids is invoked. In that way, peridotites of Dibi area witness of the nature and the evolution of lithospheric mantle under the Adamawa plateau.Original Research Article

Melting processes beneath the Mid-Atlantic Ridge were studied in residual mantle peridotites sampled from a lithospheric section exposed near the Vema Fracture Zone at 11 N along the Mid-Atlantic Ridge. Fractional and dynamic melting models were tested based on clinopyroxene rare earth element and high field strength element data. Pure fractional melting (non-modal) cannot account for the observed trends, whereas dynamic melting with critical mass porosity <0Á01 fits better the measured values. Observed microtex-tures suggest weak refertilization with 0Á1–1% quasi-instantaneous or partially aggregated melts trapped during percolation. The composition of the melts is evaluated, together with their provenance, with respect to the garnet–spinel transition. Partial melts appear to be aggregated over short but variable intervals of the melting column. Deep melts (generated within the garnet stability field at the base of the melting column) escape detection, being separated from the residues by transport inside conduits or fractures. The temporal evolution of the melting process along the exposed section shows a steady increase of mantle temperature from 20 Ma to present.

The mantle structures observed by seismic tomography can only be linked with convection models by assuming some relationships between tempera- ture, density and velocity. These relationships are complex and non linear even if the whole mantle has a uniform composition. For example, the density variations are not only related to the depth dependent thermal expansivity and incompress- ibility, but also to the distribution of the mineralogical phases that are themselves evolving with temperature and pressure. In this paper, we present a stoichiometric iterative method to compute the equilibrium mineralogy of mantle assemblages by Gibbs energy minimization. The numerical code can handle arbitrary elemental composition in the system MgO, FeO, CaO, Al2O3 and SiO2 and reaches the thermodynamic equilibrium by choosing the abundances of 31 minerals belonging to 14 possible phases. The code can deal with complex chemical activities for min- erals belonging to solid state solutions. We illustr...

Abundances of U have been determined in minerals of the spinel peridotite xenoliths in
basaltic rocks from Hoggar (Southern Algeria) by the fission-track method. Olivine and spinel have the lowest U abundances (- 0.10-0.13 ppm) followed by orthopyroxene (- 0.22-0.24 ppm). The clinopyroxene has a large variation of U (0.23-0.74 ppm). The contents of U in amphibole are within the range of those of clinopyroxene (- 0.30-0.37 ppm). Although most of the U in the xenoliths is homogeneously distributed in the major rock-forming minerals, the whole-rock U abundances are at least twice as high as estimates of the upper mantle. The high contents of U even in rocks without amphibole is probably due to metasomatism by a fractionated fluid phase which took place before or during the recrystallization. The host basaltic rocks are not genetically related to the spinel peridotite.

A new approach is presented for modelling Cr-rich peridotite compositions and garnet–spinel compositions found in diamonds and xenoliths at conditions relevant to the deep continental Earth. Using recent experimental data, it is now possible to calculate phase relations and mineral compositions relevant to pressures, temperatures, and compositions of the deep lithospheric Earth using free energy minimization techniques. Here we present calculated phase relations in Cr-rich mantle compositions from pressures of 20–
60 kbar, and temperatures 800–1400 °C. The model is successful at modelling a wide range of natural mineral compositions which are found as xenoliths in diamond-bearing kimberlites from South Africa, and is illustrated using suites of Cr-rich xenoliths from near Kimberley, South Africa. The model can explain and quantify instances of garnet zonation in naturally occurring mantle rocks as a consequence of pressure– temperature re-equilibration; and shows that not all garnet zonations result from metasomatic processes. This sheds further light on peridotitic diamond inclusions and their abundances, and allows further quantitative constraints to be applied to diamond-indicator mineral chemistry.