HR: 1330h
AN: V12C-0606    [PDF]
TI: Experimental Determination of the Liquid Line of Descent of Anhydrous Mantle Derived Tholeiitic Liquids by Fractional and Equilibrium Crystallisation at 1.0 GPa
AU: Villiger, S
EM: sam@erdw.ethz.ch
AF: Department of Earth Sciences, ETH-Zentrum Sonneggstr. 5, Zurich, CH-8092 Switzerland
AU: Ulmer, P
EM: peter.ulmer@erdw.ethz.ch
AF: Department of Earth Sciences, ETH-Zentrum Sonneggstr. 5, Zurich, CH-8092 Switzerland
AU: Thompson, A B
EM: alan.thompson@erdw.ethz.ch
AF: Department of Earth Sciences, ETH-Zentrum Sonneggstr. 5, Zurich, CH-8092 Switzerland
AU: * Muentener, O
EM: Othmar.Muentener@unine.ch
AF: Geological Institute, University of Neuchatel Rue Emile-Argand 11, Neuchatel, CH-2007 Switzerland
AB: To constrain phase equilibria as well as solid and liquid compositions along the liquid line of descent of primary basaltic magmas in reservoirs that are located at the base of the continental crust, two series of anhydrous experiments have been performed in end-loaded piston cylinder apparatus at 1.0 GPa pressure and temperatures in the range 1060 to 1330$\deg$C. As starting material an experimentally produced mantle-derived tholeiitic basalt that is in equilibrium with a lherzolitic residue at 1.5 GPa and 1300$\deg$C (Hirose \& Kushiro, 1993; EPSL 114, 477-489) was used. The first series are equilibrium crystallisation experiments on a single bulk composition. For the second series a step-wise approach was used to achieve near-perfect fractional crystallisation: Melting experiments where performed with temperature steps of 30$\deg$C and starting compositions corresponding to the liquid composition of the previous, higher temperature glass composition. Liquids of fractional crystallisation experiments evolve through constant silica increase from basalts to dacites, whereas liquids from equilibrium crystallisation experiments remain basaltic and display only a moderate SiO$_ {2}$ increase accompanied by more pronounced alumina enrichment. Responsible for these contrasting trends are suppression of the peritectic olivine + liquid = opx reaction and earlier plagioclase saturation in the fractionation experiments compared to the equilibrium experiments. At high pressure both processes form large volumes of ultramafic cumulates related to the suppression of plagioclase crystallisation relative to pyroxenes. This is in contrast to fractionation of tholeiitic liquids at low pressures, where silica is enriched in the latest stage of differentiation and early fractionation of plagioclase lead to the production of troctolites followed by (olivine-)gabbros. Compositional variation of pyroxenes and plagioclase are influenced by the fractionation process. In fractional crystallisation experiments, the Al content systematically decreases at lower temperature while in equilibrium crystallisation it does not. Thermodynamic calculations using the MELTS program indicates that the Al activity in the liquid exerts a fundamental control on Al partitioning between liquid and pyroxenes. Likewise different An contents in plagioclase at similar CaO/Na$_ {2}$O ratios in the liquid are controlled by Na and K activity in the residual liquid. This suggests that liquid composition potentially is an important factor in controlling crystal-liquid partitioning.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3655 Major element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting