Monazite-Xenotime-Garnet Equilibrium in Metapelites and a New Monazite-Garnet Thermometer

Prograde suites of pelitic rocks were examined with electron microprobe and laser ablation inductively coupled plasma mass spectrometry to determine the systematics of element partitioning between coexisting monazite, xenotime, and garnet. Monazite grains that grew in equilibrium with xenotime are enriched in Y and Dy compared with monazite that grew in xenotime-absent assemblages. Y and heavy rare earth element contents of monazite coexisting with xenotime increase with rising temperature. Monazite–xenotime Y–Gd and Y–Dy partitioning is systematic within a metamorphic grade, and increases slightly with increasing metamorphic grade, suggesting that monazite–xenotime pairs approached partitioning equilibrium. Garnet and monazite in both xenotime-bearing and xenotime-absent assemblages show a strong ( R2 = 0·94) systematic relationship between inverse temperature 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 compositional equilibrium. The following temperature–KEq relationship for the equilibrium above has been derived: \[ {\rm T}(^{\circ}\hbox{C})\left[{-1\cdot 45{\rm P}(\hbox{bars})+447772(\pm 32052)\over 567(\pm 40) - {\rm R}\ln({\rm K}_{Eq})}\right]-273\cdot 15 \] with a precision of some ±30°C for temperature estimates. Our observations suggest that major and accessory phases interact in a coupled fashion during metamorphism, and also approach a state of compositional equilibrium as reactions proceed.