Abstract
The elements Rb, Sr, Sm, Nd, U and Pb are all redistributed during the movement of fluids associated with regional metamorphism. The effect of this redistribution on the isotopic systems commonly used for age determination depends on several factors. Rb, U and Pb are lost on a regional scale during prograde metamorphism to granulite facies assemblages and are regained during retrogression. Sr is redistributed on a cm scale during both prograde and retrograde metamorphism and is mobile on a scale of tens of meters during retrogression associated with shearing. As Rb is markedly more mobile than Sr, Rb/Sr ratios of individual samples can be modified without homogenisation of Sr on outcrop scale. Addition of Rb to a sample suite will displace an original Rb-Sr isochron to the right, giving rise to an anomalously low intercept 87Sr/86Sr ratio. Loss of Rb has the opposite effect. A high amount of Rb movement is required before isochron relationships are totally destroyed. Redistribution of Sm and Nd takes place on outcrop scale in response to the formation of partial melts and new mineral assemblages. This can lead to local resetting of the Sm-Nd whole rock isotopic system at the time of high grade metamorphism. The effect of regional metamorphism on the redistribution of the REE on a larger scale is not well constrained by bulk rock analyses. Addition of LREE is recorded in tonalitic gneisses up to tens of meters from adjacent granite sheets and pegmatites. This results in both changes in bulk REE patterns and in changes in the Nd isotopic signatures depending on the isotopic composition of the Nd introduced. Basic rocks and ironstones with low primary REE contents from the 3.8 Ga Isua supracrustal belt show variable amounts of LREE addition during late Archaean and Proterozoic metamorphism. Estimates of crustal residence times or early Archaean mantle compositions based on initial end values from individual samples of these polymetamorphic rocks must be evaluated using field constraints. U loss during granulite facies metamorphism will arrest the radiogenic evolution of Pb. Straightforward interpretation of the resulting “palaeoisochrons” without allowing for secondary U loss leads to spuriously high ages and µ1 values. U loss during granulite facies metamorphism is commonly accompanied by Pb loss, and in some examples by partial isotopic homogenisation of the remaining Pb. Addition of U to rocks with approximately isotopically homogenous Pb (for example during retrogression from granulite facies) will yield ages close to that of U addition. The effect of Pb addition is more complex as it depends on the isotopic composition of the introduced Pb itself depending on the source of the Pb. Under certain circumstances mixing of Pb from different sources can give rise to Pb-Pb pseudoisochrons.
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Bridgwater, D., Rosing, M., Schiøtte, L., Austrheim, H. (1989). The Effect of Fluid-Controlled Element Mobility During Metamorphism on Whole Rock Isotope Systems, Some Theoretical Aspects and Possible Examples. In: Bridgwater, D. (eds) Fluid Movements — Element Transport and the Composition of the Deep Crust. NATO ASI Series, vol 281. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-0991-5_24
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