HR: 13:40h
AN: V12F-01 INVITED     [PDF]
TI: Computer Simulation of Thermodynamic Mixing Properties and Trace Element Incorporation in Garnet Solid Solutions
AU: * van Westrenen, W
EM: willem.vanwestrenen@erdw.ethz.ch
AF: Institut f\"{u}r Mineralogie und Petrographie, ETH Z\"{u}rich, Z\"{u}rich, 8092 Switzerland
AU: Lavrentiev, M Y
EM: m.lavrentiev@bris.ac.uk
AF: School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS United Kingdom
AU: Allan, N L
EM: n.l.allan@bris.ac.uk
AF: School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS United Kingdom
AU: Freeman, C L
EM: c.l.freeman@bris.ac.uk
AF: School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS United Kingdom
AB: Garnet solid solutions are present in many igneous and metamorphic environments. Thermodynamic mixing properties of these solid solutions must be known accurately in order to retrieve the pressure-temperature ($P-T$) conditions of garnet growth, and hence constrain part of a rock's $P-T$ evolution. Over the past 25+ years, many experimental determinations of garnet thermodynamic mixing properties have been made, but doubts remain about the accuracy of our knowledge of garnet thermodynamics. Recently developed techniques (e.g., Lavrentiev et al., J. Phys. Chem. B, 2001) allow computational studies of mixing properties in geologically relevant solid solutions to be made for the first time. We present results of simultaneous high-$P$, high-$T$ computer experiments, aimed at determining garnet mixing properties, using modified Monte Carlo and quantum mechanical methods. We derive excess volume and enthalpy data for garnets with compositions along the pyrope-almandine and pyrope-grossular joins at $P$ up to 20 GPa and $T$ exceeding 2273 K. These are compared with existing laboratory experiments and commonly used thermodynamic data bases. As seen in the experimental data, simulations show virtually ideal mixing behaviour in garnets on the pyrope-almandine join, while large excess volumes and enthalpies of mixing are predicted for garnets along the pyrope-grossular join. Simulations shed additional light on the link between microscopic structural behaviour and macroscopic thermodynamic properties: the avoidance of certain Ca-Mg contacts in the solid solutions (see Bosenick et al., Phys. Chem. Min., 2000) at high temperature and pressure manifests itself in dips in the excess enthalpies, which disappear at higher temperatures and lower pressures. Computer simulation of trace element incorporation into garnet solid solutions shows this avoidance may also be responsible for some remarkable features of rare earth element garnet-melt partitioning behaviour (e.g., van Westrenen et al., Phys. Chem. Min., 2003).
DE: 1065 Trace elements (3670)
DE: 3630 Experimental mineralogy and petrology
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting