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<p>Oscillatory zoning in blueschist and eclogitic garnets has been interpreted as indicative of fundamental processes that occur in subduction zones such as the infiltration of external fluids, fluctuations in the temperature and/or pressure, the accumulation and release of strain, or a change in the identities of reacting phases. Although these processes may and probably do occur, they are not necessarily required to produce oscillatory zoning in garnet from subduction zones. The present contribution describes a 2-D numerical model that generates oscillatory zoning of Mn in garnet (compensated by zoning in Fe and, to a lesser degree, Ca and Mg) based not on the invocation of external agents such as the influx of fluids but simply on the relative rates of garnet growth and elemental fluxes resulting from grain boundary diffusion. The number of oscillations is governed by the number of cycles in which the ratio of diffusion to reaction (D/R) changes. The magnitude of the compositional change is a function of the magnitude of the change in this ratio, and the width of the oscillations is a function of the duration of the change in the ratio. It is proposed that oscillatory zoning in blueschist and eclogitic garnet is common because the temperature of formation is appropriate for variations in the grain boundary diffusion of Mn to be manifested in the composition of garnet. At higher temperatures (e.g., Barrovian conditions), the grain boundary diffusivity of Mn and other major elements is sufficiently rapid to prevent the creation of major oscillations. However, several studies have shown that oscillations may still be preserved in more slowly diffusing trace elements even in the absence of major element oscillations. As a case study, we apply this model to evaluate the development of oscillatory zones in garnets from a retrogressed blueschist from the Tillotson Peak Complex, Vermont, USA. Results from flux calculations suggest that oscillatory zones can be formed on decadal timescales potentially consistent with paleo-seismic cycles.</p>

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A grain boundary diffusion model for the development of oscillatory zoning in garnet

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  • 发表日期：

    2026年8月4日

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  • Frank S. Spear,
  • Joseph P. Gonzalez

摘要

Oscillatory zoning in blueschist and eclogitic garnets has been interpreted as indicative of fundamental processes that occur in subduction zones such as the infiltration of external fluids, fluctuations in the temperature and/or pressure, the accumulation and release of strain, or a change in the identities of reacting phases. Although these processes may and probably do occur, they are not necessarily required to produce oscillatory zoning in garnet from subduction zones. The present contribution describes a 2-D numerical model that generates oscillatory zoning of Mn in garnet (compensated by zoning in Fe and, to a lesser degree, Ca and Mg) based not on the invocation of external agents such as the influx of fluids but simply on the relative rates of garnet growth and elemental fluxes resulting from grain boundary diffusion. The number of oscillations is governed by the number of cycles in which the ratio of diffusion to reaction (D/R) changes. The magnitude of the compositional change is a function of the magnitude of the change in this ratio, and the width of the oscillations is a function of the duration of the change in the ratio. It is proposed that oscillatory zoning in blueschist and eclogitic garnet is common because the temperature of formation is appropriate for variations in the grain boundary diffusion of Mn to be manifested in the composition of garnet. At higher temperatures (e.g., Barrovian conditions), the grain boundary diffusivity of Mn and other major elements is sufficiently rapid to prevent the creation of major oscillations. However, several studies have shown that oscillations may still be preserved in more slowly diffusing trace elements even in the absence of major element oscillations. As a case study, we apply this model to evaluate the development of oscillatory zones in garnets from a retrogressed blueschist from the Tillotson Peak Complex, Vermont, USA. Results from flux calculations suggest that oscillatory zones can be formed on decadal timescales potentially consistent with paleo-seismic cycles.