The reaction kinetics of metamorphic minerals can be subdivided into interface-and diffusion-controlled kinetics. The discrimination of reaction kinetics is crucial for estimating reaction rates. Here, we propose a new and simple method for discriminating reaction kinetics. This method requires measuring only the initial and final grain sizes during growth. The reaction kinetics is inferred from different plotted arrays of initial vs. final grain sizes after the mineral growth. Using metamorphic zircon, we take detrital core sizes as the initial sizes and post-metamorphic grain sizes as the final sizes. The application of the method to the subduction-related highpressure Nagasaki metamorphic complex in Japan shows that this metamorphic zircon grew under interface-controlled kinetics even at the relatively low temperature of 440°C. This method is potentially applicable to other minerals that have timemarkers, such as chemical zoning or internal structures that are captured at a given point in time during growth.
| INTRODUCTIONThe kinetics of metamorphic reactions are subdivided into two regimes: interface-and diffusion-controlled. Interface-controlled kinetics imply that the rates of reaction at a mineral interface determine the net reaction rate, whereas the diffusion-controlled kinetics imply that the diffusion rate of the slowest chemical component controls the net reaction rate. Most metamorphic reactions involve dehydration or hydration. Therefore, the metamorphic reaction rates constrain the fluid flux. In addition, the growth rate of metamorphic minerals from progressive metamorphic reactions governs metamorphic rock texture. (Miyazaki, 1991, Miyazaki, 1996, diffusion-limited aggregation (DLA) (Miyazaki, 2001) and nucleation and growth (e.g. Carlson,
The Early Palæogene Silhouette/North Island volcano-plutonic complex was emplaced during the rifting of the Seychelles microcontinent from western India. The complex is thought to have been emplaced during magnetochron C28n. However, the magnetic polarities of the rocks are almost entirely reversed and inconsistent with a normal polarity. In this study we present new in situ zircon U/Pb geochronology of the different intrusive facies of the Silhouette/North Island complex in order to address the timing of emplacement and the apparent magnetic polarity dichotomy. The rocks from Silhouette yielded weighted mean 206Pb/238U ages from 62.4 ± 0.9 Ma to 63.1 ± 0.9 Ma whereas the rocks from North Island yielded slightly younger mean ages between 60.6 ± 0.7 Ma to 61.0 ± 0.8 Ma. The secular latitudinal variation from Silhouette to North Island is consistent with the anticlockwise rotation of the Seychelles microcontinent and the measured polarities. The rocks from Silhouette were emplaced across a polarity cycle (C26r-C27n-C27r) and the rocks from North Island were emplaced entirely within a magnetic reversal (C26r). Moreover, the rocks from North Island and those from the conjugate margin of India are contemporaneous and together mark the culmination of rift-related magmatism.
We applied interface-controlled kinetics, including interface-controlled Ostwald ripening and non-hydrostatic dissolutionprecipitation, to metamorphic zircon and phengite growth in a high-pressure (high-P) metamorphic complex. This kinetic modeling yields the growth duration of metamorphic zircon, based on the assumption that dissolution and precipitation of zircon is much slower than that of phengite. The model was applied to zircon and phengite growth in the Nishisonogi unit of the high-P Nagasaki metamorphic complex, western Kyushu, Japan. Given that detrital zircons that are tens of microns in size remain after metamorphism, our model assumption is justified. Our results show that the duration of metamorphic zircon growth in an individual rock ranges from 10 to 20 Myr. In general, the duration of metamorphic zircon growth in the whole Nishisonogi unit is ca. 30 Myr. This prolonged duration implies that high-P metamorphism is sustained by continuous subduction of hydrated oceanic crust and overlying trench-fill sediments. The continuous subduction and accretion of these materials may supply the metamorphic fluid into the high-P metamorphic domain in the deeper part of the accretionary prism, thereby contributing to interface-controlled kinetics in the metamorphic complex.
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