10:45 〜 11:00
[SIT15-06] 高温高圧実験から予測されるマントル遷移層におけるマグマの含水量プロファイル
キーワード：含水マグマ、マントル遷移層
Introduction
Water affects various properties of the Earth’s interior materials. Especially the effect on the melting point drop of minerals is a particularly important factor. The presence of water in the Earth’s deep interior is evidenced by the discovery of hydrous ringwoodite, in natural diamonds inclusions, at least locally in the mantle transition zone. Such minerals dehydration and decomposition reactions promote the formation of magma. The seismologically observed low-velocity seismic wave regions are interpreted to be magma as described above. However, the properties of magmas formed under mantle transition zone conditions are still poorly understood, and accurate determination of water content, which affects density and other properties, is essential. This study determined the water content in the magma produced in the mantle transition zone by experiment.
Methods
Experiments were conducted using Kawai-type multi-anvil presses installed at Hiroshima Univ. and Ehime Univ. . Experimental conditions were 13-25 GPa pressure and 1000-1800℃, assuming a mantle transition zone. Two starting materials were prepared by adding 8.3 & 14.8 wt% of H2O to the five main components of pyrolite (SiO2, MgO, FeO, Al2O3, & CaO), which is the model mantle composition. In order to avoid leakage of H2O during the experiment, the sample was encapsulated using an AuPd capsule. The recovered sample was mounted in resin, mirror polished, and analyzed using SEM & EPMA. The chemical composition values of each phases, normalized by the five components except H2O, were used to determine the proportion of experimental product. The calculation is called a mass balance calculation. In this experiment, the sample was encapsulated in a closed system, and the law of conservation of mass was in effect. Therefore, the sum of the substances is equal between the starting materials and the experimental products, and the following equation is valid.
(Compositions of starting materials) = (Compositions of products) × (Proportion of products)
Using the obtained proportion of magma, in other words, the degree of melting and the water content of the starting materials, the water content of the magma was determined by the following equation.
Water content of magma = (Water content of starting material – Water content of hydrous mineral × Proprotion of hydrous mineral) / Degree of melting
This equation calculates the amount of H2O concentrated in the magma by dividing the amount of H2O in the starting material by the degree of melting. There is a term for hydrous minerals in the numerator, which is a correction term for the presence of hydrous minerals in the experimental products, subtracting the amount of H2O fixed by the minerals from the H2O content of the starting material.
Result & Discussion
Quenched crystals were observed in the recovered samples under all experimental conditions, confirming the formation of magma. This indicates a significant melting point drop due to the influence of water. It is clear that the degree of melting increases with increasing temperature. On the other hand, the water content of the magma decreased with increasing temperature. The figure is a graph of contour lines about water content of magma as a function of temperature and pressure. This graph makes it possible to constrain the water content of magma originating from the mantle transition zone, as shown in the introduction.
Water affects various properties of the Earth’s interior materials. Especially the effect on the melting point drop of minerals is a particularly important factor. The presence of water in the Earth’s deep interior is evidenced by the discovery of hydrous ringwoodite, in natural diamonds inclusions, at least locally in the mantle transition zone. Such minerals dehydration and decomposition reactions promote the formation of magma. The seismologically observed low-velocity seismic wave regions are interpreted to be magma as described above. However, the properties of magmas formed under mantle transition zone conditions are still poorly understood, and accurate determination of water content, which affects density and other properties, is essential. This study determined the water content in the magma produced in the mantle transition zone by experiment.
Methods
Experiments were conducted using Kawai-type multi-anvil presses installed at Hiroshima Univ. and Ehime Univ. . Experimental conditions were 13-25 GPa pressure and 1000-1800℃, assuming a mantle transition zone. Two starting materials were prepared by adding 8.3 & 14.8 wt% of H2O to the five main components of pyrolite (SiO2, MgO, FeO, Al2O3, & CaO), which is the model mantle composition. In order to avoid leakage of H2O during the experiment, the sample was encapsulated using an AuPd capsule. The recovered sample was mounted in resin, mirror polished, and analyzed using SEM & EPMA. The chemical composition values of each phases, normalized by the five components except H2O, were used to determine the proportion of experimental product. The calculation is called a mass balance calculation. In this experiment, the sample was encapsulated in a closed system, and the law of conservation of mass was in effect. Therefore, the sum of the substances is equal between the starting materials and the experimental products, and the following equation is valid.
(Compositions of starting materials) = (Compositions of products) × (Proportion of products)
Using the obtained proportion of magma, in other words, the degree of melting and the water content of the starting materials, the water content of the magma was determined by the following equation.
Water content of magma = (Water content of starting material – Water content of hydrous mineral × Proprotion of hydrous mineral) / Degree of melting
This equation calculates the amount of H2O concentrated in the magma by dividing the amount of H2O in the starting material by the degree of melting. There is a term for hydrous minerals in the numerator, which is a correction term for the presence of hydrous minerals in the experimental products, subtracting the amount of H2O fixed by the minerals from the H2O content of the starting material.
Result & Discussion
Quenched crystals were observed in the recovered samples under all experimental conditions, confirming the formation of magma. This indicates a significant melting point drop due to the influence of water. It is clear that the degree of melting increases with increasing temperature. On the other hand, the water content of the magma decreased with increasing temperature. The figure is a graph of contour lines about water content of magma as a function of temperature and pressure. This graph makes it possible to constrain the water content of magma originating from the mantle transition zone, as shown in the introduction.