Physics
Scientific paper
Aug 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993gecoa..57.4095s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 57, Issue 16, pp.4095-4102
Physics
4
Scientific paper
Rare earth element (REE) abundances in four garnet phenocrysts from two monazite-bearing leucogranites from southern British Columbia have been measured with an ion microprobe. This study examines the effect of monazite crystallization on the LREEs by using garnet as a monitor of elemental variations in the liquid. This approach virtually eliminates uncertainties associated with source-induced LREE fractionation. The garnets examined are spessartine-rich (Alm 70-75 Sps 18-22 Pyp 3-4 Grs 3-4 ) phenocrysts that show primary igneous zoning and lack monazite inclusions. Garnet rims have zoning profiles (i.e., Eu to Ce) that are depleted by factors of five to ten relative to cores. Of the three models investigated to constrain processes controlling LREE zoning in garnet-- 1. (1) variations in the garnet/melt distribution coefficients, 2. (2) liquid diffusion controlled partitioning and 3. (3) crystal diffusion controlled partitioning--the latter model best explains the observed zoning. This model is consistent with experimental studies suggesting that monazite has a low solubility in granitic melts. These results suggest that Nd model ages (calculated using measured 147 Sm / 144 Nd ) in leucogranites should be viewed with caution, as is demonstrated by neodymium isotopic data from southern British Columbia and the central Nepalese Himalaya.
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