Scottish 'B' megacrysts and associated xenoliths: highly fractionated residues in the vicinity of the crust-mantle boundary

Megacrysts, principally anorthoclase, Fe-rich biotite, clinopyroxene, magnetite, zircon and apatite, occur in alkali basaltic hosts at a number of Scottish localities. These minerals occur, not only as discrete, and composite megacrysts, but also as polycrystalline syenite (anorthoclasite) xenoliths. Composite xenoliths provide evidence that the anorthoclasites may occur as (pegmatitic) veins traversing pyroxenitic wall-rocks which may themselves be localized metasomatized peridotites within the shallow mantle. The anorthoclasites crystallized from highly trace element enriched melts which, in the case of the most geochemically extreme samples, were also peraluminous. Ion microprobe analyses show that the peraluminous (corundum-bearing) anorthoclasites comprise light rare earth element (LREE)-enriched alkali feldspars together with corundum and Nb-rich oxides (ilmenorutile, samarskite, yttro-niobate and columbite). The high contents of incompatible elements, together with oxygen isotope data, indicate crystallization of these syenitic facies from felsic melts, possibly originating through partial melting of metasomatized mantle lithologies. The aluminous character may be explained in terms of preferential loss of alkalis in fugitive carbonatitic fractions separated from the felsic melts.</p

Two phases of sulphide saturation in Reunion magmas: Evidence from cumulates

The behaviour of chalcophile elements in magmas is controlled both by degassing and by partitioning into a sulphide melt. While sulphide melts are rarely observed in erupted volcanic products, their high densities ensure that they should be concentrated in the plutonic roots of active volcanoes. The eruptions of the Bellecombe ash member on Réunion included plentiful sulphide-bearing ultramafic cumulate nodules and sulphides were also observed in oxide bearing gabbro nodules. However, Réunion. melt inclusions also show that S is affected by degassing. We have combined analyses of cumulates, sulphides and melt inclusions with an investigation of published data to determine the roles of sulphide saturation and magmatic degassing at Réunion. It is possible to identify two sulphide saturation events during magmatic evolution. The first relates to S becoming saturated during fractional crystallisation and formation of sulphide melts prior to partitioning into a gas phase. The second is more obvious in the more differentiated magmas of Piton des Neiges. and is prompted by the crystallisation of titanomagnetite which reduces the sulphur concentration at sulphide melt saturation. Sulphide melts saturation and segregation is likely to be a common feature at many volcanoes and should be taken into account when using trace metal concentrations to track gas transfer processes or when using the petrologic method to estimate loss of volatile or chalcophile elements such as S and Cu to the atmosphere. At Réunion. loss of sulphur by degassing could be over estimated by 20-90% when sulphide saturation is not taken into account. The effect of sulphide melt formation and immiscibility on gas compositions in volcanic systems has important implications for both ore exploration and atmospheric monitoring. © 2012

Evidence from episodic seamount volcanism for pulsing of the Iceland plume in the past 70 Myr

The North Atlantic volcanic province has been attributed to continental rifting about 60 Myr ago over an Iceland plume head with a diameter of 1,O00-2,000km (refs 1, 2). But evidence from a few igneous centre

Continental ice in Greenland during the Eocene and Oligocene

The Eocene and Oligocene epochs ( 55 to 23 million years ago) comprise a critical phase in Earth history. An array of geological records1–5 supported by climate modelling6 indicates a profound shift in global climate during this interval, from a state that was largely free of polar ice caps to one in which ice sheets on Antarctica approached their modern size. However, the early glaciation history of the Northern Hemisphere is a subject of controversy 3,7–9. Here we report stratigraphically extensive ice-rafted debris, including macroscopic dropstones, in late Eocene to early Oligocene sediments from the Norwegian–Greenland Sea that were deposited between about 38 and 30 million years ago. Our data indicate sediment rafting by glacial ice, rather than sea ice, and point to East Greenland as the likely source. Records of this type from one site alone cannot be used to determine the extent of ice involved. However, our data suggest the existence of (at least) isolated glaciers on Greenland about 20 million years earlier than previously documented10, at a time when temperatures and atmospheric carbon dioxide concentrations were substantially higher