The use of melt inclusions in studying the formation of magmatic-hydrothermal ore deposits
Melt inclusions are small (typically a few tens of micrometer in diameter) droplets of silicate melt that were trapped in minerals during their growth in crystallizing magmas. Their optical appearance depends on the magma cooling rate: melt inclusions in rapidly cooled samples are glassy, whereas ones in slowly cooled samples are crystalline. A major advantage of melt inclusions over whole-rock samples is that they if well preserved still contain all the volatiles and ore metals that normally are lost from magmas during their solidification. Furthermore, they allow more detailed insights into the magma evolution because they represent snapshots in time, whereas whole-rocks are the time-integrated end product. The trace element content of melt inclusions is most commonly measured by Laser-ablation ICP-MS, whereas volatiles are measured by electron microprobe, FTIR, Raman Spectroscopy, or SIMS. The former method works well also for unexposed, crystallized melt inclusions, but for measuring volatiles they need to be re-homogenized, quenched to glass, and exposed by polishing beforehand. Volatile concentrations provide information on the (minimum) depth of melt entrapment, on the major element composition of fluids that may have exsolved from the melt, and on the abundance of metal complexing agents such as F and S. Trace elements provide information on the behavior metals during magma crystallization ±fluid exsolution. In concert with analysis of coexisting fluid inclusions they can be used to constrain fluidmelt partition coefficients; in rhyolitic/granitic samples they can be used to constrain temperatures via zircon saturation thermometry; together with trace element abundances in host quartz they allow to constrain pressure; and in concert with magnetite inclusions they allow to reconstruct oxygen fugacity. In summary, melt inclusions are a very helpful and versatile tool not only in the study of formation of magmatic-hydrothermal ore deposits, but also in the study of volcanic and intrusive magma systems in general.
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The use of melt inclusions in studying the formation of magmatic-hydrothermal ore deposits
Abstract
Melt inclusions are small (typically a few tens of micrometer in diameter) droplets of silicate melt that were trapped in minerals during their growth in crystallizing magmas. Their optical appearance depends on the magma cooling rate: melt inclusions in rapidly cooled samples are glassy, whereas ones in slowly cooled samples are crystalline.
A major advantage of melt inclusions over whole-rock samples is that they if well preserved still contain all the volatiles and ore metals that normally are lost from magmas during their solidification. Furthermore, they allow more detailed insights into the magma evolution because they represent snapshots in time, whereas whole-rocks are the time-integrated end product.
The trace element content of melt inclusions is most commonly measured by Laser-ablation ICP-MS, whereas volatiles are measured by electron microprobe, FTIR, Raman Spectroscopy, or SIMS.
The former method works well also for unexposed, crystallized melt inclusions, but for measuring volatiles they need to be re-homogenized, quenched to glass, and exposed by polishing beforehand. Volatile concentrations provide information on the (minimum) depth of melt entrapment, on the major element composition of fluids that may have exsolved from the melt, and on the abundance of metal complexing agents such as F and S.
Trace elements provide information on the behavior metals during magma crystallization ±fluid exsolution. In concert with analysis of coexisting fluid inclusions they can be used to constrain fluidmelt partition coefficients; in rhyolitic/granitic samples they can be used to constrain temperatures via zircon saturation thermometry; together with trace element abundances in host quartz they allow to constrain pressure; and in concert with magnetite inclusions they allow to reconstruct oxygen fugacity. In summary, melt inclusions are a very helpful and versatile tool not only in the study of formation of magmatic-hydrothermal ore deposits, but also in the study of volcanic and intrusive magma systems in general.
