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

Prograde suites of pelitic rocks were examined with electron microprobe INTRODUCTION and laser ablation inductively coupled plasma mass spectrometry to Monazite [(Ce,La,Th)PO4] plays an important role in determine the systematics of element partitioning between coexisting studies of igneous and metamorphic petrogenesis. Foremonazite, xenotime, and garnet. Monazite grains that grew in most, monazite is used to date specific events in a equilibrium with xenotime are enriched in Y and Dy compared with petrogenetic sequence (e.g. Parrish, 1990; Harrison et al., monazite that grew in xenotime-absent assemblages. Y and heavy 1997; Hawkins & Bowring, 1999; Foster et al., 2000). rare earth element contents of monazite coexisting with xenotime Monazite may contain a large percentage of the sample increase with rising temperature. Monazite–xenotime Y–Gd and rare earth element (REE) budget and, thus, exert an Y–Dy partitioning is systematic within a metamorphic grade, and important influence on the evolution of melt composition increases slightly with increasing metamorphic grade, suggesting (Wark & Miller, 1993; Bea, 1996). The thermobarometric that monazite–xenotime pairs approached partitioning equilibrium. potential of monazite (coexisting with xenotime) has been Garnet and monazite in both xenotime-bearing and xenotime-absent recognized, leading to calibration of monazite geoassemblages show a strong ( R = 0·94) systematic relationship thermometers (Gratz & Heinrich, 1997, 1998; Heinrich between inverse temperature and ln( KEq ) for the net-transfer equiet al., 1997; Andrehs & Heinrich, 1998). In addition, the librium YAG+OH-Ap+ (25/4)Qtz= (5/4)Grs+ (5/4)An recognition that nearly all lead in monazite is radiogenic + 3YPO4-Mnz + 1/2H2O, suggesting that garnet and monazite (Parrish, 1990; Montel et al., 1994) has led to development crystallized in compositional equilibrium. The following temand application of electron microprobe monazite dating perature–KEq relationship for the equilibrium above has been derived: techniques (Suzuki & Adachi, 1991; Montel et al., 1996; Williams et al., 1999). T(°C)= −1·45P(bars)+447772(±32052) 567(±40)−Rln( KEq ) −273·15 Full realization of monazite as a thermochronometer requires detailed understanding of the specific reactions responsible for its formation. Previous studies of monazite with a precision of some ±30°C for temperature estimates. Our petrogenesis have focused on monazite compositional observations suggest that major and accessory phases interact in a zonation (Zhu & O’Nions, 1999a, 1999b), monazite coupled fashion during metamorphism, and also approach a state growth kinetics (Ayers et al., 1999), behavior of monazite of compositional equilibrium as reactions proceed. during hydrothermal alteration (Poitrasson et al., 1996; Crowley & Ghent, 1999) and melting events (Watt, 1995), or, to a limited extent, the influence of major-phase mineral assemblage in monazite reactivity (Zhu &