Effects of open and closed system oxidation on texture and magnetic response of remelted basaltic glass

Burgess, K, Cooper RF, Bowles JA, Gee JS, Cherniak DJ.  2010.  Effects of open and closed system oxidation on texture and magnetic response of remelted basaltic glass. Geochemistry Geophysics Geosystems. 11

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aluminosilicate glass, amorphous immiscibility, chemical diffusion, cooling rate, dynamic oxidation, ferrite chemistry, liquid immiscibility, magnetic properties, oxidation, paleointensity determination, relaxation geospeedometry, silicate melts, structural relaxation, submarine basaltic glass, tracer diffusion


As part of an experimental and observational study of the magnetic response of submarine basaltic glass (SBG), we have examined, using ion backscattering spectrometry (RBS), transmission and scanning electron microscopy, energy dispersive X-ray spectrometry, and surface X-ray diffraction, the textures wrought by the controlled, open and closed system oxidation of glasses prepared by the controlled environment remelting and quenching of natural SBG. Initial compositions with similar to 9 wt % FeO* were melted at 1430 degrees C with the oxygen fugacity buffered at fayalite-magnetite-quartz; melts were cooled at a rate of 200 degrees C min(-1) near the glass transition (T(g) = 680 degrees C). In open system experiments, where chemical exchange is allowed to occur with the surrounding atmosphere, polished pieces of glass were reheated to temperatures both below and above T(g) for times 1-5000 h; undercooled melts were oxidized at 900 degrees C and 1200 degrees C for 18 and 20 h, respectively. RBS demonstrates unequivocally that the dynamics of open system oxidation involves the outward motion of network-modifying cations. Oxidation results in formation of a Fe-, Ca-, and Mg-enriched surface layer that consists in part of Ti-free nanometer-scale ferrites; a divalentcation- depleted layer is observed at depths >1 mu m. Specimens annealed/oxidized above T(g) have magnetizations elevated by 1-2 orders of magnitude relative to the as-quenched material; this does not appear to be related to the surface oxidation. Quenched glass (closed system, i.e., no chemical exchange between sample and atmosphere) exhibits very fine scale chemical heterogeneities that coarsen with time under an electron beam; this metastable amorphous immiscibility is the potential source for the nucleation of ferrites with a wide range of Ti contents, ferrites not anticipated from an equilibrium analysis of the bulk basalt composition.






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