HR: 0800h
AN: V41C-1406    [Abstracts]
TI: The Behavior of Pyroxenes During Partial Melting of Pyroxenite and Lherzolite in the Mantle: An Experimental and Numerical Study
AU: * Lo Cascio, M
EM: mauro@brown.edu
AF: Department of Geological Sciences, Brown University, Box 1846, Providence, RI 02912 United States
AU: Liang, Y
EM: Yan_Liang@brown.edu
AF: Department of Geological Sciences, Brown University, Box 1846, Providence, RI 02912 United States
AB: Pyroxenes are the primary phases involved in partial melting of peridotite and pyroxenite lithologies in the upper mantle. In order to better understand the grain-scale processes of pyroxene melting and their effects on major and trace element distributions during magma generation, we carried out a series of kinetic melting experiments using reaction couple method. Partial melting experiments were conducted at $1340\deg$C and 1.5 GPa for 72 hrs using reaction couples formed by juxtaposing pre-synthesized rods of orthopyroxenite (90% opx, 5% olivine, 5% melt) against clinopyroxenite (90% cpx + 10% melt). These laboratory experiments were supplemented by numerical simulations of partial melting in binary and ternary systems. Reaction between orthopyroxenite and clinopyroxenite at $1340\deg$C and 1.5GPa results in a reactive boundary layer (RBL, 240 $\mu$m after 72 hrs) that consists of euhedral olivine (35%), cpx (45%), and melt (20%). The RBL is located on the orthopyroxenite side of the original interface. The grain sizes of ol and cpx in the RBL are significantly larger than those in the orthopyroxenite and the clinopyroxenite. Clinopyroxene compositions vary systematically across the RBL, for example the Na$_{2}$O and TiO$_{2}$ content decrease from 1.05% and 0.35%, respectively, in the clinopyroxenite to 0.83% and 0.14% at the orthopyroxenite-RBL interface. The melt is concentrated in the RBL and appears to localize in those areas where olivine grains are more densely accumulated. The average melt in the RBL is a ne-normative alkali basalt (10.6% MgO, 49.2% SiO$_{2}$, 4.7% Na$_{2}$O, 0.28% K$_{2}$O) and has relatively high Al$_{2}$O$_{3}$ (15.5%) and CaO (10.7%). The characteristics of increasing grain size and varying cpx and melt compositions in the RBL are very similar to those observed in our clinopyroxenite-harzburgite partial melting experiments [1], consistent with the grain-scale melting processes that involve dissolution of opx at the orthopyroxenite-RBL interface, precipitation of ol and new cpx within the RBL, and production of Na$_{2}$O-Al$_{2}$O$_{3}$-SiO$_{2}$-rich melts in that region. This dissolution and reprecipitation process, due to two very different time-scales of crystal-melt interaction in partially molten systems, is reproduced in our numerical simulations of partial melting of bio-mineralic rocks in binary and ternary systems, and is likely to occur during lherzolite and pyroxenite partial melting in the laboratory and nature. One of the important consequences of cpx reprecipitation during peridotite and pyroxenite partial melting is the significant reduction in diffusive reequilibration time between the cpx and the surrounding melt, since diffusion coefficients of trace elements such as REE, U, and Th in cpx are much smaller than those of major elements in cpx and melt. The rate of cpx reprecipitation is dominated by the rate of chemical diffusion of the major components in cpx. This dissolution and reprecipitation process may help to explain the apparent equilibrium melting trends observed in some slab derived magmas that were produced at relatively low temperatures. The mechanisms outlined above can also be used to better understand the melting process of lherzolite and pyroxene-rich lithologies in the upper mantle and could partially explain the petrogenesis of high Na$_{2}$O-Al$_{2}$O$_{3}$-SiO$_{2}$ magmas, without the involvement of an eclogite component in the source region. [1] Lo Cascio et al., GRL, 31, L16605, doi: 10.1029/2004GL020602
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3630 Experimental mineralogy and petrology
DE: 3655 Major element composition
DE: 1065 Trace elements (3670)
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting