A comparative study of monazite reactivity, and its compositional and textural response to metamorphism and metasomatism in compositionally contrasting high-temperature natural fluid systems
Backus, Ethan L.
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Monazite is a light rare earth element orthophosphate with the general formula (LREE)PO4 that may incorporate Th (and U) in its crystalline structure. This had led to monazite’s application in geochronology, and more recently as a petrochronometer. Despite the wide and varied interest in monazite as a petrochronometer much work on the role of fluid composition on monazite stability has been experimental, whilst studies of natural systems have been restricted in terms of the diversity of fluid and rock compositions. The research in this study aims to apply and determine whether the experimentally derived constraints on monazite behavior as a function of fluid composition are applicable and representative geologic environments, as pertains to the monazite growth, composition and solubility, as well as mobility of the rare earth elements. In the Lamoille Canyon area much of the existing geochronology has been conducted on zircon. This proves to be problematic due to high U abundances and subsequently, metamictization of many zircon. This may be compensated for by chemical abrasion. Monazite on the other hand, is well known for its self-annealing properties and rarely shows evidence of metamictization. In the Ruby Mountains and East Humboldt Range, it allows for geochronological work to be conducted without the need for chemical abrasion and the potential pit-falls it introduces. Equi-granular gneiss samples from Lamoille Canyon in the Ruby Mountains-East Humboldt Range metamorphic core complex show three periods of thermal activity since the Cretaceous: 84-90 Ma, 65-80 Ma, and 30-40 Ma. The Ballachulish Igneous Complex and its thermal aureole are located in Western Scotland. The Ballachulish Igneous Complex and its metamorphic aureole provide a contrasting study area where a variety of contrasting metasedimentary lithologies have strikes oriented sub-perpendicular to the contact with the igneous complex. Analysis of monazite textures from this study and other suggest that there were differing fluid regimes in the thermal aureole. Trace element analysis of monazite led to the identification of four populations 467 ± 4 Ma, 450 ± 3 Ma, 431 ± 3 Ma, and 414 ± 2 Ma in samples from the thermal aureole.