Chemical Geology, 110 (1993) 127-146 127 Elsevier Science Publishers B.V., Amsterdam A model for monazite/melt equilibrium and application to the generation of granitic magmas Jean-Marc Montel CNRS URA 10, Dkpartement des Sciences de la Terre, Universitk Blaise Pascal, 5 rue Kessler, F-63038 Clermont-Ferrand, France (Received May 10, 1992; revised and accepted July 6, 1992) ABSTRACT In order to model the behaviour ofZr, Hf, Ti, P, U, Th, Y and the rare-earth elements (REE) in the continental crust, the parameters which control the equilibria between accessory minerals and granitic melts must be determined. This paper proposes equations for the equilibrium between monazite and Ca-poor felsic melts. In a first step, all the light REE are considered as a single component and an equation describing monazite solubility is deduced from the available experi- mental data. In a second step, the fractionation of REE by monazite is investigated on the basis of a natural example of coexisting monazite and obsidian from Macusani, Peru. The solubility of monazite can be used as a thermometer, based on the total REE content of natural magmatic rocks. Application of REE thermometry to various peraluminous rock series suggests that it is a reliable tool for determining temperatures of magmatic processes. The equation governing REE frac- tionation by monazite is used to discuss genetic relationships between a melt and a presumed parental rock, using the composition of the monazite in the parental rocks and the REE pattern of the daughter rock. This method has been applied successfully to High Himalayan Manaslu and Tibetan Slab granites. It is suggested that detailed petrographical work on monazite including electron microprobe analyses and SEM imaging of its internal structures, combined with the model presented here, can be a useful tool for deciphering the genesis and the differentiation of peraluminous granitic magmas in the continental crust. 1. Introduction In contrast with mafic or intermediate suites, for which the evolution of trace-element con- centrations is governed by relatively simple laws involving the major minerals (All6gre and Minster, 1978 ), it is now well established that accessory minerals play the dominant role in felsic suites. Zircon, apatite, monazite, sphene, allanite and xenotime, for example, control the behaviour of trace elements such as Zr, Hf, Ti, P, U, Th, Y and the rare-earth elements (REE). As a consequence, the evolution of most iso- topic systems, and most of the commonly used geochronometers (Sm/Nd, U/Pb, Th/Pb, Lu/ Hf) are dominated by accessories. A compre- hensive knowledge of the behaviour of these minerals during the main magmatic processes is therefore required in order to interpret cor- rectly the isotopic characteristics and ages of felsic rocks. Such data will become even more important as in situ isotopic analyses [ second- ary ion mass spectrometry (SIMS), laser abla- tion-inductively coupled plasma-mass spec- trometry (LA-ICP-MS) ] become more routine and their application more widespread in geological laboratories. The ubiquitous distribution of monazite in high-grade metasediments and in peralumi- nous granites suggests that it controls the be- haviour of REE, U and Th during the forma- tion and differentiation of these magmas (Overstreet, 1967; Miller and Mittlefehldt, 1982; Montel, 1986; Rapp and Watson, 1986; Cuney and Friedrich, 1987 ). It is supported by the available experimental data (Montel, 1986; Rapp and Watson, 1986) which demonstrate that for the range of temperature and melt 0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.