HR: 0800h
AN: V21B-0602    [Abstracts]
TI: Making metamorphic monazite - major silicates are not major players
AU: * Corrie, S L
EM: scorrie@geol.sc.edu
AF: University of South Carolina, 701 Sumter St., EWS 617, Columbia, SC 29208 United States
AU: Kohn, M J
EM: mjk@geol.sc.edu
AF: University of South Carolina, 701 Sumter St., EWS 617, Columbia, SC 29208 United States
AB: Monazite is commonly assumed to derive its LREEs from the breakdown of LREE-rich accessory phases such as allanite. However, it is also possible that significant LREEs are scavenged from the major silicates. In this study, we tested the latter hypothesis via high-precision ($\pm$3-5 ppb), {\it in situ} LA-ICP-MS analysis of LREE zoning patterns and concentrations in all major silicates from garnet and amphibolite facies pelitic rocks of the Great Smoky Mountains, North Carolina. Previous research on schists from the Smokies suggests that monazite formed during the staurolite-in reaction (Kohn and Malloy, 2004, Geochim. Cosmochim. Acta). Garnets from below the staurolite- and monazite-in isograds contain 100s of ppb LREEs in the core, with a gradual decrease rimward to a concentration that is about one half that in the core. Plagioclase from garnet-zone rocks shows an increase in LREE contents from 1-2 ppm in the core to about 4 ppm at the rim. Above staurolite-in and monazite-in, LREEs in garnet typically show core-rim zoning patterns characterized by 1) a gradual decrease outward from a core value of 10-150 ppb, 2) a spike to approximately 1.5x the core concentration, coincident with a high-Y annulus, 3) a decrease by at least an order of magnitude towards the rim, and 4) a very slight rise at the rim. Monazite occurs external to the Y annulus, indicating monazite formation was accompanied by a large drop in the LREE content of garnet and presumably all other major silicates. This inference is consistent with zoning in plagioclase from st-zone rocks, which shows a decrease from 3-6 ppm in the cores to about 1 ppm on the rims. LREE contents in muscovite and biotite average less than 50 ppb each, and staurolite and kyanite contain about 10 ppb for all rocks. The large and immediate decreases in LREEs in the garnet and plagioclase at the onset of monazite formation show that monazite indeed scavenges LREEs from major silicates; nonetheless, the absolute concentration of LREEs in the silicates (no more than a few ppm) is not sufficient to produce monazite in the abundance witnessed by these rocks. A LREE-rich accessory phase must have been present prior to monazite formation to account for the quantity of monazite formed. Single allanite grains in three samples (<5% of samples investigated), and possible pseudomorphs of plagioclase after allanite indicate that allanite must have been a common source of LREEs, despite the lack of direct evidence in a vast majority of the samples.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3660 Metamorphic petrology
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting