HR: 14:20h
AN: V12F-03    [PDF]
TI: REE Zoning in Garnet as a Record of Pressure Changes During Growth
AU: * Lane, E D
EM: edlane@mail.utexas.edu
AF: Department of Geological Sciences, University of Texas at Austin, Austin, TX 78712 United States
AU: Carlson, W D
EM: wcarlson@mail.utexas.edu
AF: Department of Geological Sciences, University of Texas at Austin, Austin, TX 78712 United States
AB: Radial variations in the concentrations of rare-earth elements (REE) in garnets from metamorphic rocks of the Llano Uplift of central Texas display systematic patterns that indicate that the REE may be sensitive recorders of changes in pressure during growth. LA-ICP-MS analysis of REE concentrations was performed at interior and rim positions of centered sections through garnets from garnet-clinopyroxenites at Whitt Ranch (WR) and from garnet-clinopyroxenites (PHM) and pelitic gneisses (PHP) at Purdy Hill. REE-abundance curves are flatter for PHM garnets than for WR garnets. Curves for WR garnets flatten towards rims; curves for PHM garnets steepen towards rims; and curves for PHP garnets first steepen in HREE toward rims, and then flatten, with negative slopes in HREE for some rim analyses. WR garnets contain negative Eu anomalies in interiors that disappear towards rims; PHM garnets contain small positive Eu anomalies throughout; and PHP garnets contain small negative Eu anomalies in both interiors and rims. Garnet-inclusion suites document that PH garnets formed at higher pressures than WR garnets, which suggests that flatter REE-abundance curves reflect higher pressures, corroborating the observations of Bea et al. (1997, Geostds Newsltr 21:253). Flattening of curves from interiors to rims suggests that WR garnets grew as pressure increased, which is supported by the distribution of inclusions in garnet: interiors contain plagioclase inclusions but rims do not, accounting for the rimward loss of negative Eu anomalies. Conversely, steepening of curves in PHM garnets may imply that they grew during decompression, although positive Eu anomalies suggest breakdown of plagioclase during garnet growth. Relative depletion of HREE in the rims of PHP garnets may record the growth of xenotime. Application of the Gd/Dy-in-garnet barometer of Bea et al. to PHP samples yielded pressures of 0.4-0.5 GPa, inconsistent with their eclogite-facies origin and with prior grt-opx barometry at PH of 1.5-2.1 GPa. Thus, although the shapes of the REE-abundance curves for the Llano garnets change systematically with pressure in a manner similar to the pattern described by Bea et al., the ratio of Gd/Dy alone fails to capture the complex behavior of the REE over the full range of pressure commonly experienced by crustal metamorphic rocks.
DE: 3660 Metamorphic petrology
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting