Crystal chemistry of Fe-Ti oxides in mafic rocks and metabasites: A study aimed at refinement of geological tools and interpretations

• Main Author: Estifanos, Benyam
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Department of Mineralogy and Petrology, Lund University 1997
• Subjects: Geology; Fe-Ti oxide; hercynite; siderite; stilpnomelane; olivine corona; Swedish Caledonides; hysteresis; PIXE; synchrotron XRD; Rietveld; microprobe; red corundum; palaeomagnetism; eclogite; plagioclase clouding; dolerite; pyroxene gneiss; physical geography; Geologi; fysisk geografi; Northern Sweden
• Online Access: https://lup.lub.lu.se/record/29700

This thesis consists of four subprojects: (A) ‘Clouding of plagioclase by Fe-Ti oxides’, (B) ‘Fe-Ti oxides during high-pressure metamorphism of mafic rocks’, (C) ‘Magnetic study of Fe-Ti oxides in mafic rocks and metabasites’, and (D) ‘Topotactic phase transitions in oxide minerals’. A) In southern Sweden dolerite dikes and gabbros with clouded plagioclase are common. The crystal structures of the clouded plagioclase —from Bjära, Bjärnum, Smålands Taberg, Värmland— have been refined from powder diffraction data using conventional and synchrotron sources. The result suggests that the crystal structure, hosting the inclusions/substitutions, preserves the conventional triclinic symmetry (Pbar1). A PIXE analysis on clouded plagioclase samples has shown undulated, stepwise, and homogeneous clouding at the elemental level. The dark-clouded plagioclase from Bjärnum contains microinclusions of Mg-hercynite and ilmenite. A synchrotron X-ray Rietveld study could identify and quantify the hercynite (a=8.1408(3) Å) inclusions. The result suggests that the dark clouding is induced by entrapped inclusions of hercynite and Fe-Ti-oxides. This study has shown two major groups (metamorphosed, unmetamorphosed) of plagioclase clouding. B) In the Grapesvare area, northern Sweden, coronitic dolerites pass partially into eclogites. Relics of olivine in the coronitic dolerite reveal Al-free stilpnomelane, siderite, and quartz. An electron probe micro study was performed on samples from a single boudin which displays a gradual transition of coronitic dolerite into retro-eclogite. The Fe-Ti oxide microassemblages could be related to coronitic dolerite, amphibolite, and eclogite stages: primary magnetite-ilmenite, and hypersthene (± magnetite vermicules) in the coronitic dolerite; rutile-sphene in the amphibolite; and ilmenite/rutile and few isolated grains of hematite in the retrograde eclogite. The crystal structure of garnet (with silica inclusions) of the retro-eclogite has been refined from synchrotron X-ray powder diffraction data ...