Physics
Scientific paper
Jun 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981gecoa..45..827a&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 45, Issue 6, pp.827-837
Physics
3
Scientific paper
High-pressure equilibrium studies were conducted in piston-cylinder apparatus to determine rare earth element (REE) partitioning between garnet and H 2 O-vapor-saturated liquidus, from 20 kbar/980°C to 30 kbar/1060 °C. Ag capsules were employed to eliminate loss of Fe. Partition coefficients ( K D ' s ) were determinined with autoradiographic techniques employing beta-active isotopes of Ce, Sm, and Tm. Major elements in garnet were determined by microprobe analysis. Synthesis and reversal runs of 24 hr or greater duration were used to bracket values of K D ' s within analytical uncertainty. The K D values for all three REE are constant over the radiogenic concentration range of 1 to 350 ppm, suggesting that the high abundance of natural REE in the starting materials may suppress possible deviations from Henry's Law behavior reported in similar autoradiographic studies of synthetic systems with no natural REE. Changes in K D with increasing pressure and temperature at near-liquidus conditions suggest that the dominant control of K D is the average size of cations occupying the 8-fold sites in garnet. Specifically, the substitution of 8-fold Ca for Mg and Fe causes an increase in K D values greater than that attributable to the coincident effects of pressure and temperature. The inverse correlation of increasing K D with REE ionic radius supports the interpretation that the average size of the 8-fold cation controls the relative variation of K D among REE. These experimental K D values produce less relative fractionation in melts between light REE and heavy REE than do previous K D ' s derived from data on coexisting natural garnet phenocryst/rockmatrix pairs. Models for the derivation of orogenic andesites from partial melting of subducted basaltic eclogite are qualitatively improved by these new K D ' s . Existing calculations of K D values necessary for the viability of the eclogite fractionation are also in good agreement with these experimental K D values.
Apted Michael J.
Boettcher Arthur L.
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