Limits...
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.


Related in: MedlinePlus

Compositions of melts formed by partial melting of mantle peridotite (spinel lherzolite) HK66 under hydrous conditions plotted on the pseudo-ternary diagram Ol (olivine)-Di (diopside)-Qz (silica).38) Opx, orthopyroxene. Solid circles, squares and triangle are melts formed at 1.2, 1.6 and 2.0 GPa, respectively. Italic numbers are H2O contents (wt.%) in the melts. Melts with no numbers contain >10 wt.% H2O. Thin curves indicate compositional trends of melts formed by partial melting of the same peridotite at 1.0, 2.0, and 3.0 GPa under anhydrous conditions.51)
© Copyright Policy - open-access
Related In: Results  -  Collection


getmorefigures.php?uid=PMC3756732&req=5

f6-83_001: Compositions of melts formed by partial melting of mantle peridotite (spinel lherzolite) HK66 under hydrous conditions plotted on the pseudo-ternary diagram Ol (olivine)-Di (diopside)-Qz (silica).38) Opx, orthopyroxene. Solid circles, squares and triangle are melts formed at 1.2, 1.6 and 2.0 GPa, respectively. Italic numbers are H2O contents (wt.%) in the melts. Melts with no numbers contain >10 wt.% H2O. Thin curves indicate compositional trends of melts formed by partial melting of the same peridotite at 1.0, 2.0, and 3.0 GPa under anhydrous conditions.51)

Mentions: To understand the changes in composition and H2O content of magmas formed under H2O-undersaturated conditions, melting phase-relations of mantle peridotite under hydrous conditions between the two extreme cases, H2O-saturated and anhydrous conditions, are examined. Fig. 4 shows the liquidi and solidi of mantle peridotite under both anhydrous and H2O-saturated conditions. The shaded area between the solidus under H2O-saturated conditions (PH2O=Ptotal) and the liquidus under anhydrous conditions marks the melting temperature range where H2O-undersaturated magmas with variable H2O contents are formed. In this range, the H2O content of magma decreases and the degree of melting increases with increasing temperature at constant pressure. Fig. 5A shows the changes of degree of melting (melt fraction) of mantle peridotite KLB1 as function of temperature at 1.0 GPa under hydrous conditions by Hirose and Kawamoto39) and Fig. 5B shows that of mantle peridotite PHN1611 at pressures between 0.5 and 3.0 GPa under anhydrous conditions by Kushiro.42) It is shown that the temperature range for 20% melting of mantle peridotite at 1.0 GPa is much wider under hydrous conditions than under anhydrous conditions (300° and 70°, respectively). The experiments of Gaetani and Grove 41) on a different mantle peridotite at 1.5 GPa under hydrous conditions also show a wide temperature range (i.e. >200° for 20% melting). In the wide melting temperature range between the H2O-saturated solidus and the anhydrous liquidus in Fig. 4, the composition of melt changes significantly. Contrarily, under anhydrous conditions, the melting temperature range is smaller and the compositional change of melt is relatively small. Such a large difference in melting temperature range is the fundamental difference between hydrous conditions such as island arcs or subduction zones and nearly anhydrous conditions such as mid-oceanic ridges. The SiO2 contents of melts increase with increasing H2O content at constant pressure in the experiments38)–40); however the absolute SiO2 contents of melts are different due to the difference in the composition of the starting materials. Fig. 6 shows the compositions of melts in equilibrium with mantle peridotite under both H2O-undersaturated, hydrous conditions and anhydrous conditions in the pseudo-ternary system olivine(Ol)-diopside (Di)-SiO2(Qz). As shown in the figure, the Qz/Ol ratio or SiO2/MgO ratio of melts are significantly large in the hydrous melts at pressures between 1.2–2.0 GPa and the melts with high H2O contents are silica-oversaturated (plotted in the Opx-Di-Qz area) even at 1.6 GPa. Under anhydrous conditions, melts remain silica-undersaturated at pressures higher than 1.0 GPa.


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

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

Compositions of melts formed by partial melting of mantle peridotite (spinel lherzolite) HK66 under hydrous conditions plotted on the pseudo-ternary diagram Ol (olivine)-Di (diopside)-Qz (silica).38) Opx, orthopyroxene. Solid circles, squares and triangle are melts formed at 1.2, 1.6 and 2.0 GPa, respectively. Italic numbers are H2O contents (wt.%) in the melts. Melts with no numbers contain >10 wt.% H2O. Thin curves indicate compositional trends of melts formed by partial melting of the same peridotite at 1.0, 2.0, and 3.0 GPa under anhydrous conditions.51)
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f6-83_001: Compositions of melts formed by partial melting of mantle peridotite (spinel lherzolite) HK66 under hydrous conditions plotted on the pseudo-ternary diagram Ol (olivine)-Di (diopside)-Qz (silica).38) Opx, orthopyroxene. Solid circles, squares and triangle are melts formed at 1.2, 1.6 and 2.0 GPa, respectively. Italic numbers are H2O contents (wt.%) in the melts. Melts with no numbers contain >10 wt.% H2O. Thin curves indicate compositional trends of melts formed by partial melting of the same peridotite at 1.0, 2.0, and 3.0 GPa under anhydrous conditions.51)
Mentions: To understand the changes in composition and H2O content of magmas formed under H2O-undersaturated conditions, melting phase-relations of mantle peridotite under hydrous conditions between the two extreme cases, H2O-saturated and anhydrous conditions, are examined. Fig. 4 shows the liquidi and solidi of mantle peridotite under both anhydrous and H2O-saturated conditions. The shaded area between the solidus under H2O-saturated conditions (PH2O=Ptotal) and the liquidus under anhydrous conditions marks the melting temperature range where H2O-undersaturated magmas with variable H2O contents are formed. In this range, the H2O content of magma decreases and the degree of melting increases with increasing temperature at constant pressure. Fig. 5A shows the changes of degree of melting (melt fraction) of mantle peridotite KLB1 as function of temperature at 1.0 GPa under hydrous conditions by Hirose and Kawamoto39) and Fig. 5B shows that of mantle peridotite PHN1611 at pressures between 0.5 and 3.0 GPa under anhydrous conditions by Kushiro.42) It is shown that the temperature range for 20% melting of mantle peridotite at 1.0 GPa is much wider under hydrous conditions than under anhydrous conditions (300° and 70°, respectively). The experiments of Gaetani and Grove 41) on a different mantle peridotite at 1.5 GPa under hydrous conditions also show a wide temperature range (i.e. >200° for 20% melting). In the wide melting temperature range between the H2O-saturated solidus and the anhydrous liquidus in Fig. 4, the composition of melt changes significantly. Contrarily, under anhydrous conditions, the melting temperature range is smaller and the compositional change of melt is relatively small. Such a large difference in melting temperature range is the fundamental difference between hydrous conditions such as island arcs or subduction zones and nearly anhydrous conditions such as mid-oceanic ridges. The SiO2 contents of melts increase with increasing H2O content at constant pressure in the experiments38)–40); however the absolute SiO2 contents of melts are different due to the difference in the composition of the starting materials. Fig. 6 shows the compositions of melts in equilibrium with mantle peridotite under both H2O-undersaturated, hydrous conditions and anhydrous conditions in the pseudo-ternary system olivine(Ol)-diopside (Di)-SiO2(Qz). As shown in the figure, the Qz/Ol ratio or SiO2/MgO ratio of melts are significantly large in the hydrous melts at pressures between 1.2–2.0 GPa and the melts with high H2O contents are silica-oversaturated (plotted in the Opx-Di-Qz area) even at 1.6 GPa. Under anhydrous conditions, melts remain silica-undersaturated at pressures higher than 1.0 GPa.

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.


Related in: MedlinePlus