2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 3
Presentation Time: 2:00 PM

Geikielite and Coupled Spinel-Rutile Exsolution from Titanohematite in the Mesoproterozoic Granulite Facies, South Norway

ROBINSON, Peter1, LANGENHORST, Falko2, MCENROE, Suzanne A.1, FABIAN, Karl1, JERCINOVIC, Michael J.3 and BOFFA BALLARAN, Tiziana2, (1)Geol Survey of Norway, Trondheim, N-7491, Norway, (2)Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, D-95440, Germany, (3)Dept. of Geosciences, University of Massachusetts, Amherst, 611 North Pleasant Street, 233 Morrill Science Center, Amherst, MA 01003-9297, peter.robinson@ngu.no

Sizes and thermal evolution of exsolution in rhombohedral Fe-Ti oxides is a key to studies of remanence and lamellar magnetism of planetary crusts. Extensive negative aeromagnetic anomalies in the Modum area derive from rocks containing ilmenite with hematite exsolution or hematite with ilmenite exsolution, carrying strong/stable reversed remanence. Rocks include recrystallized gabbros, metamorphosed hydrothermally altered volcanics, and metamorphosed oxidized sediments, some possibly evaporitic, with Cl-scapolite. Magnetic properties at room-, high- and very-low-T aid phase identification before mineralogic study.

One rock contains a 3-cm-diameter titanohematite in a vein. Reflected-light and EMP analyses showed the titanohematite (8%R2+TiO3, 2%MgTiO3) contains three types of exsolution: spinel plates on (0001) of the host; rutile rod satellites on spinel parallel to host rhombohedral face edges; and lamellae 0.1-0.3 µm thick, also parallel to (0001), but too fine for EMP analyses. Overlap analyses showed enrichment in MgO, TiO2 and lack of Al2O3, indicating a mixture of titanohematite and geikielite-rich solid solution. Powder XRD gave a=5.0393, c=13.7687, V=302.81 for titanohematite (≈Ilm9), and unrefined reflections of rutile and geikielite.

Spinel and rutile, analyzed by EMP, formed earlier than geikielite. Spinel gave 96%MgAl2O4, 3%FeFe2O4, Mg/ total R2+ = 0.98. How did magnesian/aluminous spinel lacking Ti4+ ions exsolve from titanohematite? The answer is in coupled exsolution with ferrian rutile, where combined components were in solution in high-T titanohematite as corundum/geikielite. Phase separation lowered geikielite, and depleted the corundum component.

TEM-EDS analyses showed hematite is ≈6%R2+TiO3 (2%MgTiO3); geikielite is ≈100%R2+TiO3(70%MgTiO3). MgTiO3 is important because Mg2+ has no magnetic moment, but breaks up linkages between Fe atoms, lowers Néel temperatures, and produces unusual low-T properties. This sample shows hematite Néel T, 878K, geikielite, 34K (Fe2O3 953K, FeTiO3 57K), with possible spin-glass below 13K, and points toward magnetic study of the systems FeTiO3-MgTiO3 and Fe2O3-MgTiO3.