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Origin of magmas in subduction zones: a review of experimental studies.

Kushiro I - Proc. Jpn. Acad., Ser. B, Phys. Biol. Sci. (2007)

Bottom Line: Studies of the origin of magmas in subduction zones, particularly in the Japanese island arc, have been significantly advanced by petrological, geochemical, geophysical and experimental studies during last 50 years.Based on experimental studies, it is suggested that the compositions of primary magmas depend mainly on the H2O content and degree of melting in the melting zones, and that primary tholeiite magmas are formed by 10-25% melting of the source mantle containing less than 0.2 wt.% H2O.High-alumina basalt and alkali basalt magmas are formed by smaller degrees of melting of similar mantle, whereas primary boninite magmas are formed by more than 20% melting of the source mantle with more than 0.2 wt.% H2O, and finally, high-magnesia andesite magmas are formed by smaller degrees of melting of similar mantle.

View Article: PubMed Central - PubMed

ABSTRACT
Studies of the origin of magmas in subduction zones, particularly in the Japanese island arc, have been significantly advanced by petrological, geochemical, geophysical and experimental studies during last 50 years. Kuno's original model(1)) for magma generation in the Japanese island arc, that tholeiite magmas are formed at relatively shallow levels in the mantle on the Pacific Ocean side whereas alkali basalt magmas are formed in deeper levels on the Japan Sea side, stimulated subsequent studies, particularly high-pressure experimental studies in which the author participated. Recent seismic tomographic studies of regions beneath the Japanese island arc demonstrate that seismic low-velocity zones where primary magmas are formed have finger-like shapes and rise obliquely from the Japan Sea side toward the Pacific Ocean side. Based on experimental studies, it is suggested that the compositions of primary magmas depend mainly on the H2O content and degree of melting in the melting zones, and that primary tholeiite magmas are formed by 10-25% melting of the source mantle containing less than 0.2 wt.% H2O. High-alumina basalt and alkali basalt magmas are formed by smaller degrees of melting of similar mantle, whereas primary boninite magmas are formed by more than 20% melting of the source mantle with more than 0.2 wt.% H2O, and finally, high-magnesia andesite magmas are formed by smaller degrees of melting of similar mantle.

No MeSH data available.


The system Mg2SiO4-CaMgSi2O6-SiO2-H2O at 2.0 GPa.29), 43) The right-hand figure is the shaded plane of the left-hand figure projected from the H2O apex. Abbreviations as in Fig.3.
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f7-83_001: The system Mg2SiO4-CaMgSi2O6-SiO2-H2O at 2.0 GPa.29), 43) The right-hand figure is the shaded plane of the left-hand figure projected from the H2O apex. Abbreviations as in Fig.3.

Mentions: These compositional changes of melts can be explained by the phase equilibrium relations in the system Mg2SiO4-CaMgSi2O6-SiO2-H2O at 2.0 GPa as established by Kushiro29), 43) (Fig. 7). Points A and B are isobaric invariant points where melts are in equilibrium with major mineral components of mantle peridotite (i.e. forsterite, diopside and enstatite) under H2O-saturated and anhydrous conditions, respectively. Point A is silica-oversaturated whereas point B is silica-undersaturated as noted in Fig. 3. The shaded area shows a plane where melts are H2O-undersaturated and coexist with forsterite and enstatite or diopside. The right-hand figure shows the same plane projected from the H2O apex onto the Mg2SiO4-CaMgSi2O6-SiO2 plane. When melting of mantle peridotite occurs in the presence of relatively small amounts of H2O, the first melt produced is A, which is silica-oversaturated and also H2O-saturated. As melting progresses, the composition of melt leaves A and changes along the univariant curve A-B in the H2O-undersaturated region and becomes less silicic. At a certain point on the univariant curve A-B, diopside disappears and melt leaves the curve A–B and changes toward MgSiO3 on the shaded plane. The point where the melt leaves the curve A–B depends on the initial H2O content—the larger the initial H2O content, the closer this is to A. Melts are, therefore, more silica-rich and even silica-oversaturated through the melting range, when the initial H2O contents are relatively large. This is shown by a dashed arrow in the right-hand figure, whereas they cross the join CaMgSi2O6-MgSiO3 and become silica-undersaturated as shown by a thick arrow when the initial H2O contents are relatively small. The former case would correspond to the generation of high-magnesia andesite and boninite magmas, and the latter case would correspond to high-alumina basalt and some island arc tholeiite magmas.


Origin of magmas in subduction zones: a review of experimental studies.

Kushiro I - Proc. Jpn. Acad., Ser. B, Phys. Biol. Sci. (2007)

The system Mg2SiO4-CaMgSi2O6-SiO2-H2O at 2.0 GPa.29), 43) The right-hand figure is the shaded plane of the left-hand figure projected from the H2O apex. Abbreviations as in Fig.3.
© Copyright Policy - open-access
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3756732&req=5

f7-83_001: The system Mg2SiO4-CaMgSi2O6-SiO2-H2O at 2.0 GPa.29), 43) The right-hand figure is the shaded plane of the left-hand figure projected from the H2O apex. Abbreviations as in Fig.3.
Mentions: These compositional changes of melts can be explained by the phase equilibrium relations in the system Mg2SiO4-CaMgSi2O6-SiO2-H2O at 2.0 GPa as established by Kushiro29), 43) (Fig. 7). Points A and B are isobaric invariant points where melts are in equilibrium with major mineral components of mantle peridotite (i.e. forsterite, diopside and enstatite) under H2O-saturated and anhydrous conditions, respectively. Point A is silica-oversaturated whereas point B is silica-undersaturated as noted in Fig. 3. The shaded area shows a plane where melts are H2O-undersaturated and coexist with forsterite and enstatite or diopside. The right-hand figure shows the same plane projected from the H2O apex onto the Mg2SiO4-CaMgSi2O6-SiO2 plane. When melting of mantle peridotite occurs in the presence of relatively small amounts of H2O, the first melt produced is A, which is silica-oversaturated and also H2O-saturated. As melting progresses, the composition of melt leaves A and changes along the univariant curve A-B in the H2O-undersaturated region and becomes less silicic. At a certain point on the univariant curve A-B, diopside disappears and melt leaves the curve A–B and changes toward MgSiO3 on the shaded plane. The point where the melt leaves the curve A–B depends on the initial H2O content—the larger the initial H2O content, the closer this is to A. Melts are, therefore, more silica-rich and even silica-oversaturated through the melting range, when the initial H2O contents are relatively large. This is shown by a dashed arrow in the right-hand figure, whereas they cross the join CaMgSi2O6-MgSiO3 and become silica-undersaturated as shown by a thick arrow when the initial H2O contents are relatively small. The former case would correspond to the generation of high-magnesia andesite and boninite magmas, and the latter case would correspond to high-alumina basalt and some island arc tholeiite magmas.

Bottom Line: Studies of the origin of magmas in subduction zones, particularly in the Japanese island arc, have been significantly advanced by petrological, geochemical, geophysical and experimental studies during last 50 years.Based on experimental studies, it is suggested that the compositions of primary magmas depend mainly on the H2O content and degree of melting in the melting zones, and that primary tholeiite magmas are formed by 10-25% melting of the source mantle containing less than 0.2 wt.% H2O.High-alumina basalt and alkali basalt magmas are formed by smaller degrees of melting of similar mantle, whereas primary boninite magmas are formed by more than 20% melting of the source mantle with more than 0.2 wt.% H2O, and finally, high-magnesia andesite magmas are formed by smaller degrees of melting of similar mantle.

View Article: PubMed Central - PubMed

ABSTRACT
Studies of the origin of magmas in subduction zones, particularly in the Japanese island arc, have been significantly advanced by petrological, geochemical, geophysical and experimental studies during last 50 years. Kuno's original model(1)) for magma generation in the Japanese island arc, that tholeiite magmas are formed at relatively shallow levels in the mantle on the Pacific Ocean side whereas alkali basalt magmas are formed in deeper levels on the Japan Sea side, stimulated subsequent studies, particularly high-pressure experimental studies in which the author participated. Recent seismic tomographic studies of regions beneath the Japanese island arc demonstrate that seismic low-velocity zones where primary magmas are formed have finger-like shapes and rise obliquely from the Japan Sea side toward the Pacific Ocean side. Based on experimental studies, it is suggested that the compositions of primary magmas depend mainly on the H2O content and degree of melting in the melting zones, and that primary tholeiite magmas are formed by 10-25% melting of the source mantle containing less than 0.2 wt.% H2O. High-alumina basalt and alkali basalt magmas are formed by smaller degrees of melting of similar mantle, whereas primary boninite magmas are formed by more than 20% melting of the source mantle with more than 0.2 wt.% H2O, and finally, high-magnesia andesite magmas are formed by smaller degrees of melting of similar mantle.

No MeSH data available.