HR: 17:15h
AN: V54A-06    [Abstracts]
TI: Early Silicate Liquid Immiscibility in the Skaergaard Intrusion: Evidence from high Temperature Centrifugation Experiments
AU: Veksler, I
AF: GeoForschungsZentrum Potsdam, Department 4.1, Telegrafenberg, Potsdam, 14473, Germany
AU: Dorfman, A M
AF: Earth and Environment, LMU-University of Munich, Theresienstr. 41/III, Munich, 80333, Germany
AU: Borisov, A A
AF: Institute of Geology and Mineralogy, University of Cologne, Cologne, 50923, Germany
AU: Wirth, R
AU: * Dingwell, D B
EM: Dingwell@lmu.de
AF: Earth and Environment, LMU-University of Munich, Theresienstr. 41/III, Munich, 80333, Germany
AB: Immiscible droplet textures are common in groundmass glasses and plagioclase-hosted melt inclusions of tholeiitic basalts (Philpotts, 1982). Our experiments on synthetic analogues of natural immiscible basaltic-rhyolitic glasses showed that conventional quenching experiments in 1-atm gas mixing furnaces were in most cases unable to reproduce unmixing yielding instead either turbid, opalescent glasses, or crystallization of tridymite and pyroxenes. In contrast, experiments involving in situ high-temperature centrifugation at 1000g on some of the liquids did yield macroscopic unmixing and phase separation. It appears that experimental reproduction of immiscibility in complex ferrobabsaltic aluminosilicate melts is hampered by nucleation barrier, metastable crystallization, and sluggish phase separation kinetics. Three-four hours of centrifugation were insufficient to complete phase segregation, and resulted in sub-micron immiscible emulsions in quenched glasses. For a model liquid composition of the Middle Zone of the Skaergaard intrusion obtained from experiments by Toplis and Carroll (1995) centrifugation at super-liquidus temperatures of 1110-1120 degrees C, produced a thin, silicic layer (64.5 wt.% SiO2 and 7.4 wt.% FeO) at the top of the main Fe-rich glass (46 wt.% SiO2 and 21 wt.% FeO). Transmission electron microscopy of the quenched products revealed silica-rich immiscible globules of about 20—30 nm in diameter suspended in the Fe-rich glass. The globules are however not a quench feature because they moved during centrifugation over a few millimeters of the sample length and eventually accumulated in the thin (0.2 mm) silicic liquid layer at the top. The divergent compositions of the top and at the bottom were shown in a series of static runs to crystallize very similar crystal assemblages of plagioclase, pyroxene, olivine, and Fe-Ti oxides. In light of our centrifuge experiments, immiscibility in the Skaergaard intrusion may have started already at the transition from the Lower to the Middle Zone. Thus, magma unmixing may be an important factor of the Fe enrichment documented in cumulates of the Skaergaard Layered Series. Philpotts A.R. (1982) Contrib. Mineral. Petrol. 80, 201-218. Toplis M.J. and Carroll M.R. (1995) J. Petrol. 36, 1137–1171.
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting