1:45 PM - 3:15 PM
[SVC36-P09] Plagioclase diffusion chronometry as a constraint on pluton formation history: numerical modeling
Keywords:petrology, diffusion chronometry, plagioclase
The generation and transport of magma is the dominant process responsible for transfer of mass and heat energy from the mantle to the crust; it has also been the prime cause of continental crust growth through the Earth’s history. Such igneous activity is focused in subduction or rift settings. It is well established that magma erupted at volcanic centers is drawn from an underlying reservoir or magma chamber. However, there is no good consensus about the way in which magma chambers grow including time scales and whether growth is continuous or punctuated and any variability that may be displayed. These aspects of magma chamber growth are closely linked to the temperature evolution of the domain of magma and hence also the amount of melt, the viscosity of the magma and also its potential for eruption. Plutons represent frozen magma chambers and studies of these bodies are important in assessing the evolution of magma chambers. These magma chambers have traditionally been treated as melt-rich domains that progressively solidify inward by transport of heat through the boundaries. This model is strongly supported by geological and chemical structure of ultramafic to mafic layered intrusions such as the Skaergaard of Greenland. However, geophysical observations of active volcanic arcs have not identified equivalent kilometer-scale liquid-dominated magma chambers beneath the volcanic edifices. A consensus is growing that the main magmatic reservoir contains quite small amount of melt (<~30%). In addition, recent high-precision dating within some plutons with intermediate to felsic compositions has revealed growth of plutons by incremental sill accretion of units of tens to hundreds of meters in events separated by tens to hundreds of thousands of years. Thermal modelling suggests such a growth history should result in a crystal-dominated mush state.
In this model, magma emplacement rate and frequency of intrusions are important factors governing the melt fraction and how it changes in the magma reservoir. However, neither of these are well constrained. There is also little understanding of how they might change with composition of the magma, tectonic setting or geological time. In this study, we focused on the thermal history of plutons as a means to distinguish possible emplacement processes. The thermal history of a pluton depends on a variety of factors including magma emplacement rate, geometry of the pluton and intrusion pulses, temperature of the magma, temperature of the surrounding rock, composition of the magma which affects the latent heat released during crystallization. Many of these aspects can be constrained by measurements of the pluton and constituent igneous rocks and petrological studies of the country rock opening the possibility that the growth history maybe revealed by a study that can reveal information about the thermal history of the magma as it solidifies. Here, we focus on the chemical diffusion of trace elements in plagioclase crystals. Since chemical diffusivity strongly depends on temperature, this method has the potential to decode the duration of heating. The abrupt decrease in diffusion rate below some critical temperature enables us to extract information about the duration of high-temperature conditions. Diffusion chronometry of plagioclase is commonly used to investigate time scales in volcanic rocks. However, the same approach has been rarely if ever been applied to plutonic rocks. Two reasons for this are likely to be the assumption that time scales of heating in plutons were too long to be easily assessed by diffusion in plagioclase and complex thermal histories in pluton could result in complex profiles that are not easy to model. As a first step, in this study we conducted numerical modeling of chemical diffusion in plagioclase combined with a variety of different thermal models for distinct growth histories of a model pluton. Various conditions for the intruded magma, such as temperature, frequency, geometry, and whether magma is lost from the system by eruption, were considered. As a practical example we looked at samples from the Shinshiro pluton, which is thought to have had a longer thermal activity than many surrounding plutons and is a good example to test the method applicability for long-lived magma chambers. The diffusion modeling in plagioclase crystal is based on known diffusion constants in plagioclase for different elements. Diffusion is allowed to occur for the different thermal conditions obtained from the thermal modeling. Results of the modeling, the conditions in which the emplacement process can be distinguished and some suggestion for practical application will be reported.
In this model, magma emplacement rate and frequency of intrusions are important factors governing the melt fraction and how it changes in the magma reservoir. However, neither of these are well constrained. There is also little understanding of how they might change with composition of the magma, tectonic setting or geological time. In this study, we focused on the thermal history of plutons as a means to distinguish possible emplacement processes. The thermal history of a pluton depends on a variety of factors including magma emplacement rate, geometry of the pluton and intrusion pulses, temperature of the magma, temperature of the surrounding rock, composition of the magma which affects the latent heat released during crystallization. Many of these aspects can be constrained by measurements of the pluton and constituent igneous rocks and petrological studies of the country rock opening the possibility that the growth history maybe revealed by a study that can reveal information about the thermal history of the magma as it solidifies. Here, we focus on the chemical diffusion of trace elements in plagioclase crystals. Since chemical diffusivity strongly depends on temperature, this method has the potential to decode the duration of heating. The abrupt decrease in diffusion rate below some critical temperature enables us to extract information about the duration of high-temperature conditions. Diffusion chronometry of plagioclase is commonly used to investigate time scales in volcanic rocks. However, the same approach has been rarely if ever been applied to plutonic rocks. Two reasons for this are likely to be the assumption that time scales of heating in plutons were too long to be easily assessed by diffusion in plagioclase and complex thermal histories in pluton could result in complex profiles that are not easy to model. As a first step, in this study we conducted numerical modeling of chemical diffusion in plagioclase combined with a variety of different thermal models for distinct growth histories of a model pluton. Various conditions for the intruded magma, such as temperature, frequency, geometry, and whether magma is lost from the system by eruption, were considered. As a practical example we looked at samples from the Shinshiro pluton, which is thought to have had a longer thermal activity than many surrounding plutons and is a good example to test the method applicability for long-lived magma chambers. The diffusion modeling in plagioclase crystal is based on known diffusion constants in plagioclase for different elements. Diffusion is allowed to occur for the different thermal conditions obtained from the thermal modeling. Results of the modeling, the conditions in which the emplacement process can be distinguished and some suggestion for practical application will be reported.