How, when and what do geochronometers record in deformed metamorphic rocks V

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Geochronology fundamentally underpins our knowledge of how the continental crust forms and evolves by providing the rates and timescales of burial, metamorphism and deformation. High spatial resolution in-situ analyses (via laser ablation) allow for the precise and accurate measurement of isotope ratios from individual geochronometer minerals within thin sections. These isotope ratios provide tightly constrained ages that can be linked to petrographic observations and mineral chemical analyses, all of which underpin the modern field of 'petrochronology'. There is a still considerable debate about the importance and role of changing metamorphic conditions, bulk rock chemistry, deformation and fluid infiltration in determining when the geological clock starts ticking in deformed and metamorphosed rocks that have experienced a lengthy and protracted geological history. In-situ U-Th-Pb geochronology datasets from metamorphosed and deformed rocks commonly yield a range of dates that spans more time than the analytical uncertainty of a single "age" would suggest. This span of ages therefore suggests either that: (1) protracted crystallization took place over a range of pressure, temperature and deformation (P-T-d) conditions, (2) there was incomplete isotopic resetting during cooling and exhumation, or (3) there has been analytical mixing of mineral domains of different age. Recent studies have demonstrated that individual samples that have undergone similar P-T-d conditions, i.e. from the same outcrop, can yield strikingly varied mineral dates, indicating that the rock's bulk chemical composition exhibits a strong control on the reactions that allow the geochronometer minerals to crystallise or dissolve. It is also well known that different geochronometer minerals within the same rock respond differently to pressure, temperature and deformation.