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Journal of Petrology | Volume 44 | Number 12 | Pages 2173-2201 | 2003
© Oxford University Press 2003; all rights reserved
Anhydrous Partial Melting Experiments on MORB-like Eclogite: Phase Relations, Phase Compositions and MineralMelt Partitioning of Major Elements at 23 GPa
1 EIDGENÖSSISCHE TECHNISCHE HOCHSCHULE, INSTITUT FÜR MINERALOGIE UND PETROGRAPHIE, ETH-ZENTRUM, CH-8092 ZÜRICH, SWITZERLAND
2 DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF MINNESOTA, 310 PILLSBURY DRIVE SE, MINNEAPOLIS, MN 55455, USA
* Corresponding author. Telephone: +41 1 632 75 92. Fax: +41 1 632 10 88. E-mail: maik.pertermann{at}erdw.ethz.ch
We present melt and mineral compositions from nominally anhydrous partial melting experiments at 23 GPa on a quartz eclogite composition (G2) similar to average oceanic crust. Near-solidus partial melts at 3 GPa, determined with melt traps of vitreous carbon spheres, have 5557 wt % SiO2, rather less silica than the dacitic compositions that are generally assumed for near-solidus eclogite partial melts. At 2 GPa, equivalent near-solidus partial melts are less silicic (
52 wt % SiO2). The 3 GPa near-solidus partial melts (up to melt fractions of
3%) are saturated in rutile and have 5·76·7 wt % TiO2. The G2 composition is K2O-poor (0·03 wt %), but a modified composition with 0·26 wt % K2O (G2K) produces dacitic near-solidus melts with 6164 wt % SiO2. Rutile saturation for G2K extends to higher melt fraction (
13%) and occurs at lower TiO2 melt contents (3·3 wt %) than for G2. These results can be understood in terms of a simplified thermodynamic model in which alkalis increase the SiO2 content of liquids saturated in quartz, which in turn diminishes the TiO2 concentrations required to maintain rutile saturation. Additionally, the mode of residual garnet and generation of silicic liquids by partial melting of anhydrous eclogite are linked, as garnet is required to mass-balance formation of appreciable SiO2-rich melt. Partitioning of Na between clinopyroxene and melt shows significant increases with pressure, but only modest shifts with changing temperature. In contrast, partitioning of Ti between cpx and melt, as well as between cpx and garnet, shows pronounced dependence on temperature for compositions relevant to anhydrous partial melting of eclogite. Mixtures between partial melts of eclogite and primitive picritic Hawaiian magmas are similar to magnesian, SiO2-rich compositions inferred from melt inclusions from the Koolau volcano. However, in detail, no eclogitic partial melt has been identified that is capable of explaining all of the compositional features of the exotic Koolau component. Based on phase compositions in our experiments, the calculated density of near-solidus eclogite is 3440 kg/m3, notably less than commonly assumed. Therefore, the excess temperature required for a plume to support a given proportion of eclogite in the upper mantle may be less than previously assumed.
KEY WORDS: eclogite; experimental petrology; mantle melting; partial melt; pyroxenite
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