The Crust–Mantle Transition of the Khantaishir Arc Ophiolite (Western Mongolia)

The crust–mantle transition of the Khantaishir ophiolite in western Mongolia is well exposed. The mantle section shows an up to 4 km thick refractory harzburgitic mantle with local dunite channels and lenses. Towards its top, the mantle is increasingly replaced by discrete zones of pyroxenite, which form a kilometre-wide and hundreds of metres-thick horizon at the contact with the overlying crustal section. The plutonic crustal section is composed of gabbros, gabbronorites, tonalites and minor plagiogranites. The lower part of the crustal section is intercalated with pyroxenite lenses, forming a layered sequence, whereas the upper part is cut by volcanic dykes associated with the overlying basalt–andesitic volcanic section. Most of the ultramafic rocks and gabbronorites show a depletion in high field strength elements and positive anomalies for Sr and Pb, whereas gabbros, tonalites and plagiogranites are enriched in large ion lithophile elements and have slightly enriched rare earth element patterns. Non-modal fractional melting models indicate that the most depleted harzburgites of the ophiolite originated after 20–25% of melt extraction from the mantle. Leached minerals and whole-rocks from the crust–mantle transition of the Khantaishir ophiolite define a Sm–Nd isochron at 540 ± 12 Ma, which is interpreted as the formation age of the crust–mantle transition. Additionally, minerals and whole-rocks display a restricted εNd(t=540 Ma) composition (+3·5 to +7·0) and a large scatter in εSr(t=540 Ma) (–19·8 to +14·2). Clinopyroxenes in the crust–mantle transition rocks indicate that they were in equilibrium with a boninitic-like melt, consistent with the lavas observed in the volcanic section of the ophiolite. It is therefore inferred that the Khantaishir ophiolite represents a slice of an incipient oceanic island-arc formed in a suprasubduction environment.

Electron Microprobe and Raman Spectroscopy Investigation of an Oxygen-Bearing Pt–Fe–Pd–Ni–Cu Compound from Nurali Chromitite (Southern Urals, Russia)

One grain, about 100×80μm in size, occurring in chromitite associated with the layered sequence of the Nurali mafic-ultramafic complex (Southern Urals, Russia) was investigated by electron-microprobe analyses and Raman spectroscopy. The grain is characterized by a spotty, rugged appearance and chemical zoning from which two compositions were calculated: (Pt0.35Pd0.26Fe0.22Cu0.01Ni0.05)0.98O1.02and (Fe0.90Pt0.58Ni0.28Pd0.13Cu0.08Rh0.01)1.98O1.02. In the lack of X-ray data, Raman spectroscopy established the presence of a diffuse 500–700 band and a sharp peak at 657 cm−1of relative wavenumber that strongly resemble the Raman spectra of synthetic PtO and PdO (palladinite). It is concluded that the Nurali grain probably represents a platinum group element (PGE) oxide, and does not consist of a mixture of PGE alloys with Fe oxide or hydroxide as reported for other natural PGE-O compounds.

Kûngnât, revisited. A review of five decades of research into an alkaline complex in South Greenland, with new trace-element and Nd isotopic data

The Kûngnât Complex (1275±1.8 Ma) in the Gardar Alkaline Province, South Greenland, cuts Archaean gneisses and comprises two intersecting syenitic stocks and a gabbroic ring-dyke. The magmas, with increasingly more primitive compositions, were emplaced successively by ring-faulting and roof stoping. The syenites are orthocumulates (cumulus alkali feldspar, olivine, pyroxene, titanomagnetite and apatite; intercumulus phases include alkali amphibole, biotite, quartz and calcite). In the well dissected earlier stock, a 2.2 km-thick layered sequence displays graded modal layering, feldspar lamination and cryptic layering. Modal layering in both stocks is directed mainly inwards at 35° – 50°. Heterogeneous nucleation of the cumulus assemblage, close to steep thermal boundary layers, is inferred. The modal layering is ascribed primarily to gravitational sorting aided by the large density differential between a) feldspar and b) Fe-rich silicates and oxides. Episodic collapse of cumulus + melt slurries contributed to inward-dipping crystal pediments on the chamber floors. The Ring-Dyke (up to 100 m wide) is nearly continuous through 360°. Kûngnât exhibits a compositional nearcontinuum from olivine gabbro through syenite intermediaries to alkali granite, ascribed to protracted assimilation/fractional crystallization processes. The most radiogenic Nd isotope data from Kûngnât (εNdi values between –3.3 and –1.0) point to a lithospheric mantle source, whereas the most unradiogenic values imply enrichment in LREE by crustal contamination of the magmas.

Prolonged magma emplacement as a mechanism for the origin of the marginal reversal of the Fongen–Hyllingen layered intrusion, Norway

The ~100 m thick marginal zone of the Fongen–Hyllingen Intrusion (FHI) consists of non-layered, highly iron-enriched ferrodiorites that are overlain by a ~ 6 km thick layered sequence of gabbroic to dioritic rocks of the Layered Series. From the base upwards the marginal zone becomes more primitive as exemplified by a significant increase in whole-rock MgO, Mg-number and normative An. The reverse trends are also evident from an upward increase in An-content of plagioclase (from ~ 30 to ~ 43 at.%) and Mg-number of amphibole (from ~ 9 to ~ 23 at.%) and clinopyroxene (from ~ 23 to ~ 33 at.%). The marginal zone is abruptly terminated at the contact with the overlying Layered Series as is evident from a step-like increase in Mg-number of mafic minerals and An-content of plagioclase, as well as a sharp increase in whole-rock MgO and Mg-number in overlying olivine gabbronorites of the Layered Series. Based on these features the marginal zone of the FHI can be interpreted as an aborted marginal reversal. Reverse trends in whole-rock and mineral compositions, as well as a sharp break in these parameters are indicative of its formation in an open system with the involvement of the prolonged emplacement of magma that became increasingly more primitive. Such development of the marginal reversal was interrupted by the emplacement of a major influx of more primitive magma that produced the Layered Series. The open system evolution of a basaltic magma chamber may represent a general mechanism for the origin of marginal reversals in mafic sills and layered intrusions.

Iron-rich ultramafic pegmatite replacement bodies within the Upper Critical Zone, Rustenburg Layered Suite, Northam Platinum Mine, South Africa

Discordant veins, pipes and occasionally subconcordant sheets of iron-rich ultramafic pegmatite disrupt the layered cumulate sequence of the Upper Critical Zone, Rustenburg Layered Suite, Bushveld Complex. These pegmatite bodies have been studied where they replace the Merensky Reef footwall at Northam Platinum Mine, situated in the Swartklip Facies of the western lobe of the Rustenburg Layered Suite. Composed chiefly of ferroaugite and fayalitic olivine, the pegmatites appear to be formed by the preferential replacement of plagioclase-rich cumulates within the layered sequence. Fe-Ti oxides, sulphide (pyrrhotite and chalcopyrite) and plagioclase also occur in variable quantities. Differentiation within the pegmatite is observed where it has spread laterally beneath the impervious Merensky chromitite layer, with the development of subparallel cm-scale layers of massive magnetitite, massive sulphide and sulphide pegmatite. While some Fe-rich mobile phase must have been responsible for the pegmatites, it is concluded that the pegmatite bulk composition does not represent the original liquid. Furthermore the mode of occurrence precludes the injection of a crystal mush. Rather it is argued, mainlyon geochemical and isotopic grounds, that Fe-rich residual melts derived from the Upper Zone in the downward crosscutting gap areas migrated laterallyand upwards into the adjacent Upper Critical Zone. Variable reaction with the layered cumulates produced the anastomosing pegmatite bodies.

Anisotropic parameters of layered media in terms of composite elastic properties

In elastic earth models, at a wide variety of scales, the subsurface is a layered sequence of different constituent media. It is therefore important to understand the elastic properties of such a sequence, in particular to determine the response of the layering to an investigating seismic wave. It can be shown that if the individual layer thicknesses are much less than the wavelength of a seismic wave passing through the stack, the wave will propagate as though it were traversing a homogenous, anisotropic medium (Postma, 1955). This property has been subjected to rigorous testing both experimentally (Melia and Carlson, 1984) and numerically (Carcione et al., 1991). The elastic properties of this “equivalent medium” can be derived algebraically from the elastic properties of the materials that compose the layers (Backus, 1962). The homogenous equivalent medium will be transversely isotropic (hereafter referred to as TI), the axis of symmetry lying perpendicular to the layering.