HR: 1340h
AN: V33C-1528    [Abstracts]
TI: HREE Enrichment in Iron-rich Pyroxenes from Felsic Liquids
AU: * Olin, P H
EM: polin@mail.wsu.edu
AF: School of Earth and Environmental Science, Washington State University, Pullman, WA 99179, United States
AU: Wolff, J A
EM: jawolff@mail.wsu.edu
AB: Anomalous HREE enrichments in iron-rich pyroxenes from felsic liquids have been observed in a number of different systems (e.g., sodic salites in the phonolitic Fasnia Member, Tenerife; hedenbergites in the high-silica rhyolitic Bandelier Tuff, New Mexico, and aegirines and aegirine-augites in the Illimaussaq alkaline intrusion, Greenland). HREE enrichments are not predicted by the simple single-site lattice strain model for crystal-liquid partitioning of REE between pyroxenes and liquids, however HREE-enrichment can be modeled if the REE are present in both eight-fold and six-fold coordination states. Because ionic radii for REE in 8-coordination are very similar to the ionic radius of Ca in 8-coordination (1.16 to 0.977 angstrom and 1.12 angstrom, respectively), REE are typically modeled to substitute for Ca in the pyroxene M2 site. Considering that REE might also substitute for Fe (0.78 angstrom) and Mn (0.83 angstrom) in octahedral coordination, in either M2 or M1, creates a situation that favors the smaller ionic radii of the HREE over other REE (e.g. Lu(VI): 0.860 angstrom). This implies that measured partition coefficients for the REE are the sum of the different partition coefficients for the two coordination states. Observed HREE-enriched patterns of the Fasnia sodic salites are predicted using this modeling approach, as are HREE-enriched hedenbergites from the Bandelier Tuff and aegirines and aegirine- augites from Illimaussaq. Yttrium is interesting because its radius in 8-coordination in larger than that of Ho (1.019 vs. 1.015 angstrom), but in 6-coordination is effectively identical (0.900 vs. 0.901 angstrom, respectively). Predicted Y partition coefficients for iron-rich pyroxenes are most similar to Ho.
DE: 1065 Major and trace element geochemistry
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3640 Igneous petrology
DE: 8410 Geochemical modeling (1009, 3610)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting