Phase Relations in the System Olivine-Magnetite at High Pressures and Temperatures

Mario Koch (mkoch@classic.min.uni-heidelberg.de)¹, Alan B. Woodland (alan@classic.min.uni-heidelberg.de)¹ & Ross J. Angel (ross.angel@uni-bayreuth.de)²

¹ Mineralogisches Institut, Universität Heidelberg, Im Neuenheimer Feld 236, D-69120 Heidelberg, Germany

² Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany

The incorporation of Fe³⁺ in (Mg,Fe)₂SiO₄ silicate spinel and wadsleyite has important implications for the P-T stability of these phases, as well as for the oxidation state of the Earth's transition zone. Recent work on the Fe₂SiO₄-Fe₃O₄ binary has demonstrated complete spinel solid solution at high P and three stable intermediate phases at ca. 3.0-8.0 GPa. These intermediate phases are different spinelloid polytypes and are structural analogues to spinelloids II, III and V in the Ni-aluminosilicate system. Of particular petrologic interest is the stability of spinelloid III since it is also isostructural with Mg₂SiO₄-wadsleyite. To gain understanding of the influence of Fe³⁺ on more petrologically relevant compositions, we have begun experimentally investigating the phase relations in the Mg₂SiO₄-Fe₂SiO₄-Fe₃O₄ ternary system by equilibrating various molar proportions of olivine solid solutions with magnetite at 1100-1200°C and high P. With the fayalite-magnetite binary as a starting point, we are initially studying the Fe-rich portion of the ternary, using olivine solid solutions with 80 and 50 mol% fayalite.

Preliminary results document the stability of spinelloids II and III at 7.0-9.0 GPa and 1100°C-1200°C. Spinelloid V also appears to be stable at 9.0 GPa. The addition of Mg shifts the stability fields of the spinelloids to higher pressures compared to the Mg-free system. The spinelloids have intermediate compositions that always have a much larger Fe₂SiO₄ component compared to Mg₂SiO₄, which contrasts with that observed for wadsleyite on the Mg₂SiO₄-Fe₂SiO₄ binary (e.g. Katsura and Ito 1989). Pyroxene is present in most experiments and is the phase richest in Mg. Where olivine occurs, its Mg content is higher than that of the starting material. Spinel tends to contain little Si and Mg (each <0.16 cations pfu) and the amount of Si does not seem to increase with increasing pressure.

Katsura T & Ito E, J. Geophys. Res, 94, 15663-15670, (1989).

An Experimental Study on the Migmatization of Gabbros on Fuerteventura

Jürgen Koepke (koepke@mineralogie.uni-hannover.de)¹, Jasper Berndt¹, Stefan Graßmann¹, Francois Bussy² & Alice Hobson²

¹ Institut für Mineralogie, Universität Hannover, Welfengarten 1, 30167 Hannover, Germany

² Institute de Mineralogie et Petrographie, Université de Lausanne, BFSH2, 1015 Lausanne, Switzerland

The partial melting of gabbroic rocks at relatively low pressures (1 - 3 kbar) is discussed as a possible mechanism for generating dioritic to trondhjemitic-tonalitic magmas within the oceanic crust. Such processes can be observed in high heat flow regimes, e.g. in a high temperature shear zone in a spreading environment (Flagler and Spray, 1991) or by underplating of hot basaltic intrusions in a hot spot environment (Hobson et al., 1998).

On Fuerteventura the partial melting of gabbroic rocks at 2 - 3 kbar has occurred in the contact aureole of a mafic pluton. We started an experimental study to investigate the partial melting behaviour of these rocks. The experiments were performed in internally heated pressure vessels at 2 kbar, 900 - 1200°C. To prevent quench crystals a rapid quench device was used. Starting material was a powdered ferrogabbro consisting of plagioclase (An 54), clinopyroxene, amphibole (kersutite), and Fe-Ti oxides (assumed protolith of the migmatites). Small amounts of water were added to the charge.

At 1000°C, the temperature of migmatization according to Hobson et al. (1998), the observed experimental phases were plagioclase (An 76), clinopyroxene (higher in Al, Ti, Fe, and lower in Na, Mg than the starting clinopyroxene), amphibole (titanomagnesiohastingsite, higher in Mg, Al and lower in Fe, K, Ti), Fe-Ti oxides, and a basaltic, K-poor melt. The melt contains a high amount of normative plagioclase (70%). This is consistent with the natural observation that some melanosomes of the migmatites are strongly depleted in plagioclase which was incorporated into the melt phase. Our results imply that the widespread K-rich leucosomes within the migmatites cannot be produced by an in-situ process within a closed system without the presence of K-rich fluids.

Flagler PA & Spray JG, Geology, 19, 70-73, (1991).

Hobson A, Bussy F & Hernandez J, J. Petrol, 39, 1025-1037, (1998).

Cordierite II: The Incorporation and Orientation of CO₂ and H₂O

Boris A. Kolesov¹ & Charles A. Geiger (chg@min.uni-kiel.de)²

¹ Institute of Inorganic Chemistry, Lavrentiev prosp. 3, Novosibirsk 630090, Russia

² Institut für Geowissenschaften der Universität Kiel, Olshausenstr. 40, D-24098 Kiel, Germany

Polarized single-crystal Raman spectra at room temperature and 5 K and polarized infrared spectra at room temperature have been recorded on four natural cordierites of different compositions (two-Mg rich, one Fe-rich and one Na-rich sample) in the energy region of the CO₂ and H₂O (3500-4000 cm^-1) stretching vibrations. It can be shown that the largest fraction (~90%) of the CO₂ molecules is aligned parallel to the X axis, which is consistent with results from X-ray diffraction and optical studies. The CO₂ contents of six natural cordierites previously studied by powder IR methods (Vry et al., 1990) were measured by Raman spectroscopy. They show a linear relationship between their CO₂ content and the intensity ratio of their CO₂ stretching mode against a Si-O stretching mode.

The spectroscopic data permit an assignment of all the stretching modes of H₂O occurring in the channel cavity to be made. Three different orientations of H₂O molecules that have no interactions with alkali cations located in the six-membered rings are described. They are classified as Class I H₂O molecules. The H...H vector for two of them is directed along [001], and their molecular planes lie in the XZ and YZ crystal planes. The third type has its H...H vector directed along the X axis and the molecule lies in the XZ plane. The spectra also give evidence for the existence of two other types of H₂O that have interactions with the alkali cations. They are classified as Class II H₂O. They distinguish themselves in the number of water molecules bonded to the alkali atoms found in the six-membered rings. The formation of weak hydrogen bonds at low temperatures can explain the presence of some stretching bands in the low-temperature spectra which are absent in room-temperature spectra. H₂O in cordierite has some hydrogen bonding at low temperatures, but it is clearly less strong than in liquid water or ice.

Vry JK, Brown, PE & Valley, JW, Am. Min., 75, 71-88, (1990).