HR: 1330h
AN: V12C-0605    [PDF]
TI: Grain-Scale Processes during Isobaric and Isothermal Melting of Lherzolite
AU: * Lo Cascio, M
EM: mauro@brown.edu
AF: Brown University, Department of Geological Sciences, Box 1846, Providence, RI 02912 United States
AU: Liang, Y
EM: Yan_Liang@brown.edu
AF: Brown University, Department of Geological Sciences, Box 1846, Providence, RI 02912 United States
AU: Hess, P
EM: Paul_Hess@brown.edu
AF: Brown University, Department of Geological Sciences, Box 1846, Providence, RI 02912 United States
AB: During partial melting of spinel lherzolite at moderate pressures olivine and melt are produced at the expense of pyroxenes and spinel via the reaction cpx + opx + sp $\rightarrow$ ol + melt. In detail, however, the grain-scale processes through which this melting reaction takes place are still not well understood. For example, the zonation in residual pyroxenes of most lherzolite melting experiments is attributed to simple solid-state diffusion during re-equilibration. However, recent theoretical studies have shown that more complicated processes such as dissolution and re-precipitation are likely to take place during isothermal and isobaric melting of solid solution forming minerals. To better understand the grain-scale processes of lherzolite melting in the laboratory, we conducted a series of partial melting experiments at $1340\deg$C and 1.5 GPa for 48 to 80 hours using reaction couples formed by juxtaposing pre-synthesized harzburgite against pre-synthesized melt-bearing clinopyroxenite. Starting minerals were from a fertile spinel lherzolite xenolith. A typical quenched experimental charge consists of three zones separated by nearly planar interfaces: 1) ol + opx (harzburgite) + melt, 2) a reaction zone made of ol + cpx + melt, 3) cpx + melt. The reaction zone is opx-free and its thickness increases with time (up to 250 $\mu$m after 80 hours) at the expense of the harzburgite. Modal abundance of cpx decreases in the reaction zone whereas the mode of olivine and the melt increases with time. Olivine grain size is also significantly larger in the reaction zone than in the harzburgite. Detailed microprobe traverses across the charge and X-ray intensity maps of selected elements of the reaction zone, show that individual olivine grains are not zoned, whereas individual cpx grains are zoned and vary in composition within the reaction zone. Clinopyroxenes are rich in MgO, FeO, and SiO$_{2}$ near the harburgite, and rich in CaO, and jadeite component near the clinopyroxenite. When plotted in an oxide or 6-oxygen based cation correlation diagram, such as Al$^{3+}$ vs. Ca$^{2+}$, the variations in composition exhibited by our pyroxenes are virtually identical to the pyroxene core-to-rim variations reported in recent lherzolite partial melting studies that were conducted under similar run conditions. Pyroxenes and olivine Mg\# are 91.8 and nearly constant through out the charge except in the reaction zone where cpx Mg\# decreases down to 90.5. The systematic variations in mineralogy and mineral chemistry of our experiments are consistent with a process where three fundamental mechanism of dissolution and re-precipitation are operating at the same time: 1) complete dissolution of orthopyroxene due to the reaction with the melt-bearing clinopyroxenite, 2) diffusive mixing of the dissolved opx and cpx component in the melt, 3) precipitation in the reaction zone of cpx with low jadeite component and olivine around pre-existing olivine grains. Thus, the cpx of varying composition in the reaction zone cannot be produced by diffusion in the solid alone because cation diffusion rates in cpx are too slow. The dominant transport mechanism here is diffusion in the melt. It is possible that the same process is producing the pyroxene zonation observed in other lherzolite melting studies. One of the objectives of this study will be to determine whether local equilibrium is achieved during partial melting of lherzolite and its implications for trace element fractionation.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3630 Experimental mineralogy and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting