HR: 09:15h
AN: V11C-06    [Abstracts]
TI: Trace Element Partitioning Between Coexisting Silicate Melts: the Effect of Melt Composition
AU: * Schmidt, M W
EM: max.schmidt@erdw.ethz.ch
AF: Inst.Min.Petrol. ETH Z\"urich, Sonneggstr.5, Z\"urich, 8092 Switzerland
AU: Bogaerts, M
EM: michel.bogaerts@erdw.ethz.ch
AF: Inst.Min.Petrol. ETH Z\"urich, Sonneggstr.5, Z\"urich, 8092 Switzerland
AU: G\"unther, D
EM: guenther@inorg.chem.ethz.ch
AF: Dep.Chemistry, ETH Z\"urich, Z\"urich, 8093 Switzerland
AU: Pertermann, M
EM: maik.pertermann@erdw.ethz.ch
AF: Inst.Min.Petrol. ETH Z\"urich, Sonneggstr.5, Z\"urich, 8092 Switzerland
AB: Experiments on coexisting immiscible liquids allow to nail down the influence of melt structure/composition on the partitioning of trace elements without having to deal with crystal compositions changing with melt composition. Piston cylinder experiments at 0.3-0.7 GPa, 1050-1240 $\rm ^o$C in the K$_2$O-FeO-Al$_2$O$_3$-SiO$_2$ system yield a Fe-gabbroic coexisting with a Fe-rich granitic melt $\pm$ quartz and fayalite. The suitable temperature range is limited by massive crystallization of fayalite and quartz below 1120 $\rm ^o$C and by the abrupt closure of the miscibility gap above 1180 $\rm ^o$C. Typical textures yield droplets of gabbroic melt in granitic melt, in the presence of fayalite, these droplets coalesce with the crystals. In all cases, the gabbroic droplets have internal droplets of granitic melt, zones of quench-fayalite, and sometimes crystallize wuestite. In order to produce two melts measurable by LA-ICP-MS, the piston cylinder is mounted onto a centrifuge, after 48 hours of equilibration, we segregate the gabbroic melt completely from the granitic melt at typically 6-12 hours at 700-1000 {\em g}. Each of the melts was then free of droplets, the gabbroic melt has sometimes quench-fayalite but other quench textures are at the submicron scale. The resulting partition coefficients agree with the few previous studies (Watson 1976, CMP: Ryerson & Hess 1978, GCA) performed on few elements at microprobe concentration level, but deliver a very surprising element pattern. The alkalis and earth alkalis have D's (gabbro/granite) raising regularly with field strength or electronegativity from 0.1 to 5; only Cs, Rb, K, and Na partitioning into the granitic melt. A second group of elements (transition metals, REE, commonly used HFSE) partition into the gabbroic melt at fairly similar D's (e.g. 5-11) without correlation with r, Z/r, electronegativity or whatsoever. There is a faint trend to higher D's with higher oxygen coordination number and there are distinct small differences between elements (e.g. Nb/Ta) but the proposed dependence of D's with field strength or a similar parameter does at least not exist for these elements. A third group of elements with partition coefficients of 1-0.07 is comprised by the amphoteric elements which either substitute Si in the tetrahedra polymerizing the melt or form low ($\leq$ 4) coordination polyhedra with oxygen mostly due to lone electron pairs (e.g. Sn, Pb, As, Sb). The observed behavior can be best described by a 3 oxygen-type site model. The melt-effect on partition coefficients of commonly used trace elements in the investigated compositional range is thus up to 2 orders of magnitude, but apart from a few cations (e.g. alkalines and Ba,Sr,Pb,Al), the relative fractionation is less than a factor of 2. With a narrowing immiscibility gap, a straightforward correlation between gap width and (decreasing) D's is observed.
DE: 8439 Physics and chemistry of magma bodies
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 1020 Composition of the crust
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting