Textural, chemical and isotopic insights into the nature and behaviour of metamorphic monazite

Abstract

Monazite is a mineral of choice for dating metamorphism in amphibolite- and granulite-grade metapelites. However, there exist a number of difficulties that complicate the interpretation of monazite geochronological data and prevent its application to many geological problems. The two main obstacles addressed in this contribution are firstly, the minor but significant (e.g. 1–30 Ma) dispersal in duplicate isotope dilution thermal ionisation mass spectrometry (ID-TIMS) U–Pb age data commonly recorded from a single rock, and secondly, the difficulty of attaching monazite age data to pressure and temperature information. Through a multidisciplinary approach utilising TIMS and laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) isotope data, quantitative and qualitative EMP chemical analyses of monazite, and textural studies, we assess the significance of Pb loss, older components, and continuous and episodic monazite growth in the generation of dispersed age data. Three samples from the Canadian Cordillera and one sample from the Himalaya of Pakistan are examined. Each sample exhibits an age dispersion of between 1 and 12 Ma for single crystal and multi-grain TIMS U–Pb monazite age determinations. Consideration of the closure temperature for Pb diffusion in monazite and the metamorphic temperatures experienced by these samples suggests diffusive Pb loss did not play a significant part in generating this age dispersal. The LA-MC-ICPMS study indicates that an older component (<100 Ma older than the TIMS ages) contributed to the age dispersal in three of the four samples. In all the samples however, chemical analyses identified that the majority of monazites examined exhibited significant intra-crystalline zoning in Y content. The LA-MC-ICPMS analysis of one sample that was constrained to zones of distinct Y content indicates that these zones are of distinct age. We suggest that monazite grown before the appearance of garnet and during garnet breakdown is relatively rich in Y, whereas monazite grown after garnet is relatively poor in Y. A combination of these chemical data with textural observations suggests that once monazite had entered the mineral assemblage it grew or recrystallised episodically throughout the prograde and retrograde paths of the metamorphic event. This behaviour contributes to, and in one of the samples controls, the observed age dispersal. This recognition allows the generation of pressure–temperature–time points by combining textural and chemical information of monazite with in situ age determinations, and pressure–temperature information from garnet. Thus, the episodic growth of compositionally distinct monazite throughout a metamorphic event provides the geochronologist with a very valuable chronological tool.