Experimental study of Ni and Mn partitioning between olivine and siliceous melt: Implications for high Ni-olivines in Hawaiian lavas ZHENGRONG WANG GLENN A. GAETANI,STANLEY HART Woods Hole Oceanographic Institution, USA Olivine is abundant in the Earth’s upper mantle and ubiqui- tous in basaltic lavas, but rarely occurs in eclogite. Partial melts of eclogite are therefore not in equilibrium with olivine, but will react with peridotite until achieving olivine saturation. We inves- tigated experimentally the reaction between olivine and siliceous eclogite partial melt, and the partitioning of Ni and Mn between olivine and the melt produced by this reaction. Our results show that high-Ni olivines similar to those in Hawaiian lavas can crys- tallize from magmas with moderate Ni contents depending on the major element composition of these melts. Therefore, an olivine- free mantle source is not required to explain high-Ni olivine phe- nocrysts from Hawaii. Experiments were carried out at 1200–1350 °C, 1 bar, QFM f O 2 buffer using San Carlos olivine (SCO) crucibles and a mixture of eclogite melt (SiO 2 = 63%, AK200 doped with Ni, Perter- mann and Hirschmann, 2003) and SCO powder (<15 lm) as start- ing material. Our results for Ni and Mn partitioning between the melt (4.6–14.5 wt% MgO) and the equilibrated olivine (Fo 88.3–91.4 ) are in excellent agreement with previous studies (Watson, 1977; Hart and Davis, 1978). More importantly, melts in equilibrium with these forsteritic olivines retain their high SiO 2 (57.9–62.8%) and low MgO characters. Therefore, these forsteritic olivines have significantly higher NiO content than those in equilibrium with typical basic melts (e.g., MgO > 16%). Our results have direct implications for the origin of high-Ni olivines in Hawaiian lavas. A previous study calls upon an oliv- ine-free mantle source to produce high-Ni parent melt of Koolau lavas that have high SiO 2 content, out of which high-Ni forsteritic olivines precipitate (Sobolev et al., 2005). Alternatively, theoreti- cal modelling calibrated by our experimental data demonstrates that high-NiO olivines precipitate from a mixture of siliceous melt (e.g., eclogite/pyroxenite melt) and peridotite melt due to the sig- nificant increase in Ni partition coefficient with decreasing MgO, even though the total NiO content of the melt decreases during mixing. Thus, an olivine-free mantle source is not required beneath Hawaiian volcanoes. References Hart, S.R., Davis, K.E., 1978. Earth Planet. Sci. Lett. 40, 203. Pertermann, M., Hirschmann, M.M., 2003. J. Petrol. 44, 2173. Sobolev, A.V., Hofmann, A.W., Sobolev, S.V., Nikogosian, I.K., 2005. Nature 434, 590. Watson, E.B., 1977. Geochim. Cosmochim. Acta 41, 1363. doi:10.1016/j.gca.2006.06.379 Insights into magma evolution in Capraia Volcano (Italy) from plagioclase chemical and isotopic zonation3 D. GAGNEVIN 1 , J.S. DALY 1 , T.E. WAIGHT 2 , G. POLI 3 , S. CONTICELLI 4 1 UCD, School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland (Damien.Gagnevin@ucd.ie; TODW@geol.ku.dk) 2 Geological Institute, Øster Voldgade 10, 1350 Copenhagen K, Denmark (TODW@geol.ku.dk) 3 Department of Earth-Sciences, Piazza Universita´ , 06100 Perugia, Italy (polig@unipg.it) 4 Dipartimento di Scienze della Terra, Universita degli Studi di Firenze, Firenze, Italy (sandro.conticelli@unifi.it) Chemical and isotopic zoning in plagioclase phenocrysts has proved to be a powerful tool to decipher petrogenesis in ‘open’ volcanic (Davidson and Tepley, 1997) and plutonic (Gagnevin et al., 2005) systems. While single-grain isotopic variations con- strain the nature of the melt involved in magma mixing, the prom- ising aspect of using trace elements in plagioclase is their sensitivity to intensive parameters (P, T, XH 2 O) during growth. Capraia Volcano (7.2–7.6 Ma) belongs to the Tuscan Magmat- ic Province. Most lavas are andesitic and dacitic in composition, with a predominant high-K calc-alkaline signature. Dacites that display the best evidence of plagioclase zoning were selected for this study. The aim was to correlate the zoning patterns to eluci- date the evolution of the Capraia magma chamber. Phenocrysts display complex textures (see accumulated BSE image below) con- sisting of growth zones separated by resorption surfaces, suggest- ing repeated disequilibrium conditions during crystallisation. By taking into consideration mol% An (as a temperature proxy), Fe (as an oxygen fugacity proxy) and Sr melt (as a melt composition proxy), we show that the phenocrysts have shared part of their growth history and resorption events, and postulate the existence of a long-lived magma chamber periodically replen- ished by andesitic magmas. This model has been tested using Sr and Pb isotope data on microdrilled samples extracted from the various growth zones. References Davidson, J.P., Tepley III, F.J., 1997. Science 275, 826–829. Gagnevin, D., Daly, J.S., Poli, G., Morgan, D., 2005. J. Petrol. 46, 1689– 1724. doi:10.1016/j.gca.2006.06.380 A188 Goldschmidt Conference Abstracts 2006