HR: 15:10h
AN: V23B-07    [Abstracts]
TI: Experimental Quantification of the Plagioclase-Liquid Europium Anomaly as a Function of Oxygen Fugacity: Application as an in-situ redox sensor
AU: * Toplis, M J
EM: toplis@pontos.cst.cnes.fr
AF: CRPG-CNRS, BP20, Vandoeuvre, 54501 France
AU: * Toplis, M J
EM: toplis@pontos.cst.cnes.fr
AF: DTP (UMR5562), 14, Av. E. Belin, Toulouse, 31400 France
AB: It is well known that plagioclase fractionates europium (Eu) from the other rare earth elements (REE) during magmatic differentiation. This is because Eu occurs in divalent and trivalent forms in geologic liquids and because Eu²⁺ is significantly more compatible in plagioclase than Eu³⁺ (and the other REE). The bulk partitioning behaviour of Eu is therefore a function of the ratio of Eu²⁺ and Eu³⁺ in the liquid, which in turn is a function of oxygen fugacity. The partitioning behaviour of Eu in a magmatic rock is therefore a potential in-situ sensor of the redox conditions at the time of crystallization. To quantify the link between europium partitioning and oxygen fugacity, an experimental study has been initiated using natural basaltic glasses dredged along the South West Indian Ridge (EDUL cruise). The results concern primarily a glass of Mg# = 0.61 (DR20). In addition to the glass with 'natural' concentration levels of REE, a sample was prepared doped with 100ppm of Ce, Sm, Eu, Gd and Yb, and another doped with the same REE, plus 100ppm Ba. Experiments were performed at 1 bar in a vertical drop-quench furnace (CRPG, Nancy). Groups of three samples were heated at 1234° C (above the liquidus) for 9 hours to ensure redox equilibrium, cooled at 3° C/hour to 1200° C, left to equilibrate for 72 hours, then quenched. Oxygen fugacities in the range 10^-3 to 10^-12 were studied using CO-CO₂ gas mixtures. All samples contained plagioclase and abundant liquid, whose compositions was determined using a combination of electron microprobe and ion microprobe (Cameca ims-3f, CRPG). The results are identical for the three samples studied at a given fO₂. The anomaly in the partitioning behaviour of Eu relative to the other REE is a smooth function of oxygen fugacity and may be rationalised in terms of the expected behaviour of the Eu²⁺/Eu³⁺ of the liquid. The derived values for the partition coefficient of Eu²⁺ are close to those observed for Sr, consistent with the similar ionic radius of these two cations. Modelling of the data infers that oxygen fugacity of an unknown sample may be estimated with an uncertainty on the order of 0.05 log units, a significant improvement on previous models.
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005