HR: 1330h
AN: V22B-0581    [PDF]
TI: Significance of Ilmenite for REE and P Distribution in Garnet From Metapelitic Schists
AU: * Whitney, D L
EM: dwhitney@umn.edu
AF: University of Minnesota, Dept. of Geology and Geophysics, Minneapolis, MN 55455 United States
AU: Pertermann, M
EM: maik.pertermann@erdw.ethz.ch
AF: Swiss Federal Institute of Technology, Institute for Mineralogy and Petrography, ETH-Zentrum, Zurich, CH-8092 Switzerland
AB: The distribution of trace elements in metamorphic minerals such as garnet and monazite is of interest in studies of the timing and conditions of petrologic and tectonic events. We have analyzed major element concentrations and trace element zoning in garnet and ilmenite in metapelitic rocks from the garnet, lower staurolite, and upper staurolite zones of the Barrovian sequence in Dutchess County, New York. This study was in part motivated by the observation that ilmenite in these rocks contains trace elements of interest, such as Ce, P, and possibly Y. In the garnet zone sample, minute grains of phosphate minerals (apatite, monazite) are ubiquitous as inclusions in garnet and as matrix phases, and occur in both the trace element-rich, Ca-poor garnet core and the trace element-poorer rest of the garnet. Garnet cores are relatively enriched in Y, Yb, and Sc; the Sc-enriched core region is larger than the Y-rich region, which is in turn larger than the Yb-rich core region. Ilmenite inclusions occur only outside these trace element-enriched regions. Staurolite zone samples show a similar distribution of phosphate minerals, but Y zoning in garnet is reversed, with Y-poor cores and Y-richer rims. Unlike in the garnet zone, the garnet core is enriched in Ca, and ilmenite occurs as inclusions in the Ca- and Yb-rich core. Both inclusion and matrix ilmenite from garnet and staurolite zone samples can be seen in X-ray maps to contain elevated Ce, P, and Yb relative to the host garnet outer core/rim. In all samples, some inclusion ilmenite is significantly higher in Mn than matrix ilmenite, and negatively correlated with Fe. Mn partitioning between ilmenite and garnet is well behaved in the staurolite zone, but more erratic in the garnet zone. With our preliminary data, we have documented the potential importance of ilmenite as a carrier of trace elements such as Ce and P in metapelitic rocks, indicating that ilmenite must be taken into account when considering metamorphic reactions that account for REE and other trace element zoning in garnet, and in particular equilibria involving garnet and phosphate minerals.
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting