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

University dissertation from Department of Mineralogy and Petrology, Lund University

Abstract: 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 using the Rietveld method. The silica inclusion is a-quartz. The Grapesvare dolerite-eclogite occurrence is characterised by incomplete metamorphic transitions. C) A mineral-magnetic, microprobe and palaeomagnetic study on dark-clouded plagioclase-bearing dolerite dike at Bjärnum has revealed that the dolerite passed to pyroxene gneiss and subsequently retrogressed. The magnetic measurements show the highest magnetic intensity for the dark-clouded plagioclase-bearing site. The hysteresis and coercivity study of the dark-clouded plagioclase separates show magnetic grains of pseudo-single-domain. The thermal demagnetisation from hercynite-bearing site gives a blocking temperature in the range of 540-610°C. The dark clouding is lost due to the formation of scapolite, muscovite and biotite in the plagioclase. The obtained mean magnetic direction is (Decl.= 336°, Incl.= -72°, a95= 8.5°, k= 50.8) in agreement with previously obtained directions from the area and implies that the Bjärnum dolerite became remagnetised late in the Sveconorwegian orogeny. D) Topotaxy in oxide minerals is poorly understood. The red corundum, Al1.98Cr.02O3 from Froland is characterised structurally and compositionally. The crystal structure has been refined from X-ray (CuKa1) powder diffraction data. The substituting Cr-atoms depart 1.04Å from the sites of Al-atoms. A SEM study revealed microphases of diaspore; suggesting the reaction Al2O3 (Crn) + H2O <---> 2AlO(OH) (Dsp) occurred in the late stage of the metamorphic history of the Froland region.

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