5:15 PM - 6:45 PM
[SVC30-P13] Rates and styles of pluton formation recorded in plagioclase diffusion profiles: numerical assessments and application in Busetsu granite in Mikawa region, Japan
Keywords:petrology, magma flux, diffusion, plagioclase
Magma transportation from the lower to the upper crust represents vertical transfer of mass and heat energy and is a first-order lithospheric process responsible for the formation of continents and volcanic activity. The rate of magma transport, or magma flux helps determine the frequency and magnitude of volcanic eruptions and plays a part on ore genesis. Obtaining reliable estimates of magma flux through geological time can help develop our understanding of the dynamics in magma plumbing systems. Some magma erupts on the surface of the Earth, but most freezes before it reaches the surface and forms intrusions of varying sizes including large kilometer-scale plutons. Estimating flux rates from these plutons is a key research topic. Several methods have been proposed to estimate magma fluxes based on (i) pluton size and time scale of formation, (ii) the thermal profiles preserved in contact metamorphic aureoles (CMAs) (Yamaoka et al., 2023), (iii) and zircon crystallization age spectra (Caricchi et al., 2014). However, these methods require either detailed information on the thermal structure of the CMA or the common presence of datable zircon. Here, we propose a new and complementary approach to estimating magma flux based on chemical diffusion in plagioclase crystals and thermal modeling. Our new method is applicable to a wide range of plutons where other methods may not be suitable.
Recent studies demonstrate that magma flux controls the thermal history of plutons as they grow (e.g. Annen et al., 2023). This suggests the inverse may also be possible where the magma flux associated with pluton formation can be estimated from thermal histories recorded in natural samples of plutons. Chemical diffusion in crystals is one possible record of the thermal history of igneous rocks, but this approach was generally thought to be unsuitable for plutonic rocks. One of the main reasons was that plutons were thought to have remained at high T for long periods of time and suitable diffusive chemical elements would have reached equilibrium. However, the prevailing view of pluton formation has changed from rapid intrusion of large volumes of magma to "cold storage models" in which plutons form by incremental sill accretions and potentially remain below the solidus for most of the history of the pluton after an initial quick cooling. Plagioclase was selected as the target mineral and strontium was selected as the diffusing element. During crystallization from a magma, Sr in plagioclase shows a positive correlation with #An. However, solid state diffusion works in the opposite sense tending to form a negative correlation with #An due to the chemical potential dependency on #An. These contrasting behaviors are important when trying to distinguish the effects of diffusion from growth zoning and are special to Sr in plagioclase. We used forward numerical modeling to assess the utility of Sr diffusion in plagioclase as an indicator of magma flux.
Our model shows that the diffusion is sufficiently sensitive to discuss the magma flux in pluton formation, and does not reach equilibrium for a wide range of plausible magma flux rates. The calculations also show that the results are strongly affected by the thickness of the sills, the geometry of the pluton, and position within the pluton. These results show that Sr diffusion in plagioclase has good potential as a marker of magma flux when supported by good constraints on the geometry of the intrusions from geological field surveys.
As a field example we examined the Busetsu granite in Mikawa region. After geological survey, sampling and plagioclase in-situ analysis, magma flux in formation was estimated from diffusion data and thermal modeling. The flux was estimated to be 10-1.9~10-2.2 [km3/yr] consistent with other estimates in smaller plutons (de Saint Blanquat et al., 2011). This result shows the utility of our new method for estimating magma flux rates based on Sr diffusion in plagioclase.
Recent studies demonstrate that magma flux controls the thermal history of plutons as they grow (e.g. Annen et al., 2023). This suggests the inverse may also be possible where the magma flux associated with pluton formation can be estimated from thermal histories recorded in natural samples of plutons. Chemical diffusion in crystals is one possible record of the thermal history of igneous rocks, but this approach was generally thought to be unsuitable for plutonic rocks. One of the main reasons was that plutons were thought to have remained at high T for long periods of time and suitable diffusive chemical elements would have reached equilibrium. However, the prevailing view of pluton formation has changed from rapid intrusion of large volumes of magma to "cold storage models" in which plutons form by incremental sill accretions and potentially remain below the solidus for most of the history of the pluton after an initial quick cooling. Plagioclase was selected as the target mineral and strontium was selected as the diffusing element. During crystallization from a magma, Sr in plagioclase shows a positive correlation with #An. However, solid state diffusion works in the opposite sense tending to form a negative correlation with #An due to the chemical potential dependency on #An. These contrasting behaviors are important when trying to distinguish the effects of diffusion from growth zoning and are special to Sr in plagioclase. We used forward numerical modeling to assess the utility of Sr diffusion in plagioclase as an indicator of magma flux.
Our model shows that the diffusion is sufficiently sensitive to discuss the magma flux in pluton formation, and does not reach equilibrium for a wide range of plausible magma flux rates. The calculations also show that the results are strongly affected by the thickness of the sills, the geometry of the pluton, and position within the pluton. These results show that Sr diffusion in plagioclase has good potential as a marker of magma flux when supported by good constraints on the geometry of the intrusions from geological field surveys.
As a field example we examined the Busetsu granite in Mikawa region. After geological survey, sampling and plagioclase in-situ analysis, magma flux in formation was estimated from diffusion data and thermal modeling. The flux was estimated to be 10-1.9~10-2.2 [km3/yr] consistent with other estimates in smaller plutons (de Saint Blanquat et al., 2011). This result shows the utility of our new method for estimating magma flux rates based on Sr diffusion in plagioclase.