Monazite-Xenotime-Garnet Equilibrium in Metapelites and a New Monazite-Garnet Thermometer |
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| Authors: | PYLE JOSEPH M; SPEAR FRANK S; RUDNICK ROBERTA L; McDONOUGH WILLIAM F |
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| Institution: | 1DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, RENSSELAER POLYTECHNIC INSTITUTE, TROY, NY 12180, USA
2DEPARTMENT OF EARTH AND PLANETARY SCIENCES, HARVARD UNIVERSITY, CAMBRIDGE, MA 01238, USA |
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| Abstract: | Prograde suites of pelitic rocks were examined with electronmicroprobe and laser ablation inductively coupled plasma massspectrometry to determine the systematics of element partitioningbetween coexisting monazite, xenotime, and garnet. Monazitegrains that grew in equilibrium with xenotime are enriched inY and Dy compared with monazite that grew in xenotime-absentassemblages. Y and heavy rare earth element contents of monazitecoexisting with xenotime increase with rising temperature. Monazite–xenotimeY–Gd and Y–Dy partitioning is systematic withina metamorphic grade, and increases slightly with increasingmetamorphic grade, suggesting that monazite–xenotime pairsapproached partitioning equilibrium. Garnet and monazite inboth xenotime-bearing and xenotime-absent assemblages show astrong ( R2 = 0·94) systematic relationship between inversetemperature and ln(KEq) for the net-transfer equilibrium YAG+ OH-Ap + (25/4)Qtz = (5/4)Grs + (5/4)An + 3YPO4-Mnz + 1/2H2O,suggesting that garnet and monazite crystallized in compositionalequilibrium. The following temperature–KEq relationshipfor the equilibrium above has been derived:
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