HR: 0830h
AN: V31D-0963    [PDF]
TI: Fluid and Hydrous Melt Compositions in Equilibrium With K-Free Eclogite at 6 GPa and Temperatures of $900-1400\deg$C
AU: * Kessel, R
EM: kessel@erdw.ethz.ch
AF: Institut f\"{u}r Mineralogie und Petrographie, ETH-Zentrum, Sonneggstrasse 5, Z\"{u}rich, CH-8092 Switzerland
AU: Ulmer, P
EM: ulmer@erdw.ethz.ch
AF: Institut f\"{u}r Mineralogie und Petrographie, ETH-Zentrum, Sonneggstrasse 5, Z\"{u}rich, CH-8092 Switzerland
AU: Pettke, T
EM: thomas.pettke@erdw.ethz.ch
AF: Institut f\"{u}r Isotopengeologie und Mineralische Rohstoffe, ETH-Zentrum, Sonneggstrasse 5, Z\"{u}rich, CH-8092 Switzerland
AU: Schmidt, M W
EM: max.schmidt@erdw.ethz.ch
AF: Institut f\"{u}r Mineralogie und Petrographie, ETH-Zentrum, Sonneggstrasse 5, Z\"{u}rich, CH-8092 Switzerland
AU: Thompson, A B
EM: thompson@erdw.ethz.ch
AF: Institut f\"{u}r Mineralogie und Petrographie, ETH-Zentrum, Sonneggstrasse 5, Z\"{u}rich, CH-8092 Switzerland
AB: Fluid or hydrous melt compositions in equilibrium with K-free basaltic eclogite were determined experimentally at 6 GPa and $900-1400\deg$C. Each capsule contained 15-26 wt% H$_{2}$O, the eclogitic starting material mixed with CsOH solution, and a thin layer of diamond aggregates as a fluid/melt trap. The liquid (fluid or melt) compositions were determined using a novel analytical technique, the freezing approach: the capsule was frozen prior to opening and kept frozen during analyses to prevent any fraction of the fluid to be lost prior to analysis. The liquid composition was thus analyzed as solid ice using a laser-ablation ICP-MS employing Cs as an internal standard (which partitions completely into the liquid). Parallel set of experiments was performed and analyzed following a conventional, water evaporation approach, whereby the exsolved water/fluid is dried out of the capsule prior to analysis. The trapped quench solute was analyzed using the laser-ablation ICP-MS and the residual eclogitic mineralogy was analyzed using an electron microprobe. The liquid compositions were calculated using mass-balance constraints. The two analytical approaches are in good agreement, suggesting that the freezing approach can be applied to experimentally problematic systems, e.g., K-bearing, in which a significant portion of K still resides in the exsolved water/fluid after quench and is then lost while drying the capsule in the water evaporation approach. At 6 GPa, the liquid phase shows a smooth change in composition from an aqueous fluid to hydrous melt with increasing temperature. The liquid contains 85 wt% H$_{2}$O in equilibrium with residual eclogite at $900\deg$C that gradually changes into a hydrous melt with 5 wt% H$_{2}$O at $1400\deg$C, indicating that at this pressure, a K-free eclogite is above its second critical endpoint. Similarly, Na$_{2}$O content increases from 1 wt% at $900\deg$C to 5 wt% at $1400\deg$C. Subtle changes in the smooth trends correlate with phase boundaries: TiO$_{2}$ concentration increases with increasing temperature from $900\deg$C to $1100\deg$C and remains constant with further increase in temperature. These trends concur with the eclogitic mineral assemblages and modal proportions reflecting an increase in the relative mode of garnet while clinopyroxene and rutile are consumed with increasing temperature.
DE: 1025 Composition of the mantle
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting