Determination of reaction kinetics using grain size: An application for metamorphic zircon growth

Abstract

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,

Peak metamorphic temperature of the Nishisonogi unit of the Nagasaki Metamorphic Rocks, western Kyushu, Japan

“…Although the estimated peak metamorphic temperature is associated with a large error, significant variation is observed among the samples. Within the accuracy of the measurements, the results of this study are consistent with those of Uda, (1995, 1997) and Miyazaki et al (2018) (Fig. 2b).…”

Timescale of material circulation in subduction zone: U–Pb zircon and K–Ar phengite double‐dating of the Sanbagawa metamorphic complex in the Ikeda district, central Shikoku, southwest Japan

“…CL images show that most of the zircons have typical oscillatory zoning.Cores and metamorphic rims are readily identifiable in CL images of zircons from samples IK-15 and IK-17 (Figure 5). Fine-grained inclusions of graphite occur in the metamorphic rims of the zircons, and this is typical of zircons from high-P/low-T pelitic schists(Miyazaki, Mori, Nishiyama, Suga, & Shigeno, 2018). The metamorphic rims of zircons from IK-15 and IK-17 are usually <10 μm thick, but in IK-15 some are as thick as 20 μm.…”

Microdiamond in a low-grade metapelite from a Cretaceous subduction complex, western Kyushu, Japan

“…temperature condition estimated by Raman microspectroscopy of graphite. The peak metamorphic temperature of the metapelite was estimated to be 450 °C according to Raman microspectroscopy of graphite 31,34 . Graphite shows two kinds of occurrence.…”

“…S1). The empirical phengite barometer 33 gives 1.95 to 1.99 GPa for T = 450 o C 31,34 . The pressure is apparently lower than that for the graphite-diamond transition, probably because of the retrograde re-equilibration during exhumation.…”