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Utökad sökning > Hålenius mat:art lärosäte:nrm språk:eng > (2010-2014)

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  • Nyquist, Susanne, et al. (författare)
  • An EELS study of near edge structures of the oxygen K-edge in spinels
  • 2014
  • Ingår i: Physics and chemistry of minerals. - 0342-1791. ; 41:4, s. 255-265
  • Tidskriftsartikel (refereegranskat)abstract
    • Spectra of seven aluminate, ferrite and chromate spinels were collected at the oxygen K-edge in order to examine the effect of octahedral ion composition on the near edge structures and explore whether these may shed some light on previously reported major variations in the optical absorption coefficient (epsilon) for the spin-allowed d-d band transitions due to tetrahedrally coordinated Fe2+ in optical spectra of oxide spinels. Interpretation of the energy loss near edge structure (ELNES) spectra was aided using the multiple scattering code FEFF8.40. For the chromate samples spin polarised calculations were included. Gold's iterative deconvolution method was utilized to improve on energy resolution, resulting in spectra equal in merit to those recorded by technically superior instruments. Results include absolute energy positions and interpretation of ELNES features in terms of transitions to available states. We conclude that the major causes of differences in the optical absorption coefficient mentioned above are hybridization between oxygen p- and octahedral metal orbitals. Our study does not support the idea presented by other authors that there exists a local antiferromagnetic ordering in MgCr2O4 at room temperature.
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  • Johansson, Åke, et al. (författare)
  • Petrogenesis and geotectonic setting of early Svecofennian arc cumulates in the Roslagen area, east-central Sweden
  • 2012
  • Ingår i: Geological Journal. - 0072-1050 .- 1099-1034. ; 47:6, s. 557-593
  • Tidskriftsartikel (refereegranskat)abstract
    • Several intrusions of ultrabasic to basic composition occur in the Roslagen area of east-central Sweden in close spatial and temporal association with the surrounding 1.90-1.87Ga old early orogenic Svecofennian granitoids. An imprecise Sm-Nd WR errorchron yields an age of 1895±71Ma. In spite of the penetrative deformation in the granitoids, the basic-ultrabasic rocks mostly appear undeformed and largely preserve magmatic textures with plagioclase, olivine (in some rock types), orthopyroxene and clinopyroxene, and amphibole as major constituents. The plagioclase is typically very anorthitic (ca. An 90). The Roslagen intrusions range in composition from primitive to evolved (Mg# 80 to 49) but contain only 40-50wt% SiO 2. Many samples are highly elevated in Al 2O 3 (up to 30wt%), CaO (up to 16wt%) and Sr (up to 800ppm), with strongly positive Eu and Sr anomalies, in line with being plagioclase cumulates. Although masked by cumulus effects, the relative trace element contents indicate a volcanic arc signature. The initial Nd isotope composition is homogeneously 'mildly depleted', with ε Nd of +0.3 to +1.1, and the initial Sr isotope composition 'mildly enriched', with ε Sr of +8 to +15. Non-cumulus rocks with small Eu and Sr anomalies can be used to deduce the composition of the parental magma. This LILE- and LREE-enriched and HFSE-depleted high-alumina basalt magma, with Mg# of ca. 50-60 and Ca# of ca. 80, most likely formed by partial melting of mantle material, enriched by fluids in a subduction environment, at 1.9Ga. The cumulate rocks apparently crystallized from a somewhat more evolved water-rich magma with Mg# of ca. 40. Crystallization was followed by the development of late-magmatic to post-magmatic coronas between olivine and plagioclase in the presence of H 2O-rich fluids. The subduction-related setting would make these intrusions Palaeoproterozoic counterparts of Alaskan-type ultramafic intrusions, but they differ from those in being plagioclase enriched, possibly reflecting different levels of exposure.
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