Journal
CHEMICAL GEOLOGY
Volume 544, Issue -, Pages -Publisher
ELSEVIER
DOI: 10.1016/j.chemgeo.2020.119602
Keywords
Monazite; Experimental petrology; U-Th-Pb; Coupled dissolution-precipitation; Metamorphism
Categories
Funding
- UniSA New Adventures Fund
- Australian Research Council [DP160103449]
- Australian Government Research Training Program (RTP) Scholarship
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Monazite U-Th-Pb geochronology of high-temperature and melt-bearing metamorphic rocks sometimes produces a continuum of concordant ages ('smear') without well-defined age populations. In combination with textural and geochemical criteria, these datasets are typically interpreted to infer either variable Pb-loss between two discrete events or protracted metamorphism lasting >50 Myr. We experimentally react monazite with granitic melt (0.7 GPa and 750 degrees C) to produce a range of complex monazite textures and age patterns similar to those found in natural high-temperature rock samples. Our observations show that within a short duration (<= 24 h experiment time), variable REE, Y, Th, U and Pb mobility is achieved via coupled dissolution-precipitation in a melt-bearing system. An ultra-local interface melt is saturated in dissolved components, including the U-Th-Pb isotopic character of the original monazite, such that newly precipitated monazite may copy or partly inherit the age of a precursor grain. This implies that monazite grown via coupled dissolution-precipitation may not faithfully record the age or duration of metamorphism in melt-bearing rocks typical of a variety of geodynamic settings.
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