Alkaline hybrid mafic magmas of the Yampa area,NW Colorado,and their relationship to the Yellowstone mantle plume and lithospheric mantle domains

The Yampa volcanic field (late Miocene) consists of about 70 outcrops of monogenetic cinder cones, lavas, dykes, volcanic necks and hydrovolcanic pyroclastic deposits and is situated in the most northerly part of the Rio Grande rift. Contemporaneous extension in this part of the rift was small, but there is geological and geophysical evidence that, by the late Miocene, the area was underlain by hot asthenosphere convected by the Yellowstone mantle plume. The Yampa rocks are mafic and chemically diverse, including basanites, alkali basalts, potassic trachybasalts, hawaiites and shoshonites. About half the rocks bear the xenocryst suite feldspar, pyroxene, Fe–Ti oxide, amphibole, biotite. There is a tendency for xenocryst-free rocks to be the most mafic, interpreted to indicate that the xenocrysts are cognate, and represent cumulate material from fractional crystallization of the magmas in deep crustal magma chambers. The elemental and isotopic (Nd and Sr) variations can be modelled by mixing variable proportions of partial melts of local lithospheric mantle with an OIB end-member formed by partial melting of asthenosphere. The OIB end-member appears to have the elemental and isotopic composition of typical Northern Hemisphere OIB, in particular the plume-derived basanites of Loihi seamount, Hawaii. The OIB end-member at Yampa is interpreted to have been derived from mantle convected in the Yellowstone mantle plume.     

Heavy metal abundances in the Kandy lake—An environmental case study from Sri Lanka

The Kandy lake, situated in the heart of Sri Lanka's second largest city with a population of nearly 120,000, has been monitored to probe the extent of heavy metal pollution. Although the lake is a source of drinking water to the city, a large number of effluent canals drain into the lake carrying a continuous flow of industrial and domestic waste matter. A total of 66 surface water samples were analyzed for their Fe²⁺, total Fe, total V, SO ₄ ²⁻ , Cd²⁺, and Pb²⁺ contents. Pb and Cd were found in high concentrations averaging 150 μg/l and 77 μg/l, respectively, and exhibit a marked positive correlation with each other (r=+0.94). Vehicular emissions and industrial waste matter contribute largely to the Pb and Cd contents of the lake, the anthropogenic influence outweighing the contributions made by geological materials. All field observations and laboratory experiments indicate a tendency of the Kandy Lake towards eutrophicity.     

Fluids in granulites of the Southern Marginal Zone of the Limpopo Belt, South Africa

The Southern Marginal Zone of the Limpopo Belt in South Africa is characterised by a granulite and retrograde hydrated granulite terrane. The Southern Marginal Zone is, therefore, perfectly suitable to study fluids during and after granulite facies metamorphism by means of fluid inclusions and equilibrium calculations. Isolated and clustered high-salinity aqueous and CO₂(-CH₄) fluid inclusions within quartz inclusions in garnet in metapelites demonstrate that these immiscible low H₂O activity fluids were present under peak metamorphic conditions (800-850 °C, 7.5-8.5 kbar). The absence of widespread high-temperature metasomatic alteration indicates that the brine fluid was probably only locally present in small quantities. Thermocalc calculations demonstrate that the peak metamorphic mineral assemblage in mafic granulites was in equilibrium with a fluid with a low H₂O activity (0.2-0.3). The absence of water in CO₂-rich fluid inclusions is due to either observation difficulties or selective water leakage. The density of CO₂ inclusions in trails suggests a retrograde P-T path dominated by decompression at T<600 °C. Re-evaluation of previously published data demonstrates that retrograde hydration of the granulites at 600 °C occurred in the presence of H₂O and CO₂-rich fluids under P-T conditions of 5-6 kbar and ~600 °C. The different compositions of the hydrating fluid suggest more than one fluid source.     

Compositional Zoning in Pyroxenes from Lunar Rock 12021, Oceanus Procellarum

Electron probe study of the pyroxene phenocrysts in rock 12021of the Apollo 12 sample reveals an extraordinary variety ofzoning phenomena. These phenocrysts have cores of relativelyhomogeneous pigeonite which are mantled by Ca-rich pyroxenes;the mantles are zoned outward from about 20 mol per cent FeSiO₃at the core-mantle boundaries to 70 mol per cent FeSiO₃ at themargins of the phenocrysts. The character of the normal zoningdiffers in each growth sector. Four different trends characterizinggrowth on (110), (100), (010), and parallel to c are described.Pronounced but erratic oscillatory effects are impressed onthe normal zoning in all sectors. Normal zoning in lunar pyroxenes is generally more extensivethan in terrestrial pyroxenes, probably because the lunar lavaswere an order of magnitude more fluid than terrestrial basalts.Turbulent flow in lunar lavas may have caused some of the oscillatoryeffects. The cyclic nature of augite and pigeonite crystallizationin these pyroxenes is probably kinetic in origin; conceivablyit could reflect the presence of an augite-pigeonite peritectic.It is likely that further knowledge of the crystal structureof pigeonite at high temperatures will be required before astructural model can be formulated that will satisfactorilyexplain the details of the sector zoning. Coarse-grained pyroxferroite and subcalcic ferroaugite havecrystallized in apparent ‘equilibrium’ in the finalstages of formation of the groundmass in rock 12021. Nevertheless,the crystallization of the pyroxferroite was probably metastablerelative to the assemblage ferroaugite+ fayalite+ tridymite.Data on the effect of Mg substitution on the stability of pyroxferroitewill be needed to establish this point. We suggest that the pigeonite cores of the phenocrysts in rock12021 crystallized under quiescent conditions, probably in amagma chamber below the lunar surface, and that the stronglyzoned augite mantles crystallized during emplacement of thelava on the surface.     

Alpine olivine- and titanian clinohumite-bearing assemblages in the Erro-Tobbio peridotite (Voltri Massif, NW Italy)

The microstructures in the Erro-Tobbio peridotite indicate several stages of recrystallization of olivine + titanian clinohumite-bearing assemblages. The development of these assemblages is closely associated with serpentinite mylonites, in which they occur in shear bands and foliations and are inferred to have grown synkinematically, in veins, and as post-kinematic radial aggregates. In the peridotite wall-rock adjacent to these mylonites, the same assemblages have recrystallized statically at the expense of original olivine and pyroxenes, mesh-textured chrysolite and antigorite veins. In addition, the olivine-bearing assemblage occurs in widespread vein systems. The brittle deformation of the peridotite resulting in the development of these vein systems is closely related to ductile deformation of metagabbroic dykes in the peridotite. Although early metasomatism resulted in extensive rodingitization of the gabbros, some dykes show an eclogitic assemblage of Na-clinopyroxene + garnet + chloritoid + chlorite ± talc. These observations, the microstructures and the mineral chemistry all suggest that the assemblages in the ultramafic rocks and metagabbros developed during a prograde evolution towards high pressures (>13–16 kbar, 450–550° C), and during subsequent decompression. This metamorphic evolution is considered to be related to Late Cretaceous intraoceanic subduction in the Alps-Apennine system and closure of the Piedmont-Ligurian basin.