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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.


Liquidus diagram of the system forsterite (Mg2SiO4)-nepheline (NaAlSiO4)-silica. Thin curves are liquidus boundaries at 1 atm (Fo, forsterite; Pr, protoenstatite; Cr, cristobalite; Tr, tridymite; Ab, albite; Ne, nepheline; Cg, carnegieite; Sp, spinel). L1: forsterite-protoenstatite liquidus boundary at 1 atm; Thick curves L2, L3 and L4: forsterite-enstatite liquidus boundary at 1.0, 2.0 and 3.0 GPa, respectively.13), 14)
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f2-83_001: Liquidus diagram of the system forsterite (Mg2SiO4)-nepheline (NaAlSiO4)-silica. Thin curves are liquidus boundaries at 1 atm (Fo, forsterite; Pr, protoenstatite; Cr, cristobalite; Tr, tridymite; Ab, albite; Ne, nepheline; Cg, carnegieite; Sp, spinel). L1: forsterite-protoenstatite liquidus boundary at 1 atm; Thick curves L2, L3 and L4: forsterite-enstatite liquidus boundary at 1.0, 2.0 and 3.0 GPa, respectively.13), 14)

Mentions: From the beginning of 1960s, high-pressure experimental studies on the origin of basalt magmas started intensively, partly due to the development of the piston-cylinder high-pressure apparatus11) and partly because of the vigorous debates on genetic relationships between tholeiite and alkali basalt magmas, including the above-mentioned model of Kuno.1) The most noteworthy study was made by Yoder and Tilley,12) who conducted a series of experiments on basaltic compositions at 1 atmosphere and at high pressures. These workers showed that tholeiite-type magma is formed at relatively low pressures or at shallow depths in the mantle and alkali basalt-type magma at higher pressures or at greater depths from the same primary source rock in the mantle. Their discussion is consistent with Kuno’s model; however, the proposed mechanisms of generation of two basalt magmas were different (i.e., direct partial melting of source mantle peridotite vs. fractional crystallization of a more primitive magma). In order to examine the Kuno’s model, Kushiro13), 14) conducted high-pressure experiments in the systems Mg2SiO4-NaAlSiO4-SiO2, Mg2SiO4-CaMgSi2O6-SiO2 and Mg2SiO4-CaAl2SiO6-SiO2 which include the major components of mantle peridotites. Fig. 2 shows the shift of the forsterite-enstatite liquidus boundary with pressure in the system Mg2SiO4-NaAlSiO4-SiO2. As shown in the figure, melts in equilibrium with forsterite and enstatite, which lie on the forsterite-enstatite liquidus boundary, become more silica-depleted and Na-enriched with increasing pressure. At 2.0 and 3.0 GPa, the low-temperature parts (upper parts) of the liquidus boundaries lie in the nepheline normative compositional area (Mg2SiO4-NaAlSi3O8-NaAlSiO4 triangular area). The results imply that magmas formed by partial melting of mantle peridotite have relatively silica-rich tholeiitic compositions at low pressures, whereas they have silica-poor alkali basaltic compositions at high pressures, thus supporting Kuno’s model. However, the pressure ranges where two magmas are formed are much lower than those in Kuno’s model. The results of Kushiro’s experiments indicate that even alkali basaltic magmas can be formed at pressures less than 2.0 GPa (depths<60 km), compared to > 200 km in the Kuno’s model. Green and Ringwood15) and Green et al.16) also discussed, based on their high-pressure experiments on natural basalt compositions, that the depth ranges for generation of silica-saturated tholeiite, high-alumina basalt, and alkali basalt magmas are < 15 km, 15–35 km, and 35–60 km, respectively. These depth ranges are much shallower than those in Kuno’s model; however, they suggested that the deep earthquake foci may be related to the initiation of upward movement of mantle materials, which eventually partially melt at shallower depths and finally undergo magma segregation.


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

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

Liquidus diagram of the system forsterite (Mg2SiO4)-nepheline (NaAlSiO4)-silica. Thin curves are liquidus boundaries at 1 atm (Fo, forsterite; Pr, protoenstatite; Cr, cristobalite; Tr, tridymite; Ab, albite; Ne, nepheline; Cg, carnegieite; Sp, spinel). L1: forsterite-protoenstatite liquidus boundary at 1 atm; Thick curves L2, L3 and L4: forsterite-enstatite liquidus boundary at 1.0, 2.0 and 3.0 GPa, respectively.13), 14)
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f2-83_001: Liquidus diagram of the system forsterite (Mg2SiO4)-nepheline (NaAlSiO4)-silica. Thin curves are liquidus boundaries at 1 atm (Fo, forsterite; Pr, protoenstatite; Cr, cristobalite; Tr, tridymite; Ab, albite; Ne, nepheline; Cg, carnegieite; Sp, spinel). L1: forsterite-protoenstatite liquidus boundary at 1 atm; Thick curves L2, L3 and L4: forsterite-enstatite liquidus boundary at 1.0, 2.0 and 3.0 GPa, respectively.13), 14)
Mentions: From the beginning of 1960s, high-pressure experimental studies on the origin of basalt magmas started intensively, partly due to the development of the piston-cylinder high-pressure apparatus11) and partly because of the vigorous debates on genetic relationships between tholeiite and alkali basalt magmas, including the above-mentioned model of Kuno.1) The most noteworthy study was made by Yoder and Tilley,12) who conducted a series of experiments on basaltic compositions at 1 atmosphere and at high pressures. These workers showed that tholeiite-type magma is formed at relatively low pressures or at shallow depths in the mantle and alkali basalt-type magma at higher pressures or at greater depths from the same primary source rock in the mantle. Their discussion is consistent with Kuno’s model; however, the proposed mechanisms of generation of two basalt magmas were different (i.e., direct partial melting of source mantle peridotite vs. fractional crystallization of a more primitive magma). In order to examine the Kuno’s model, Kushiro13), 14) conducted high-pressure experiments in the systems Mg2SiO4-NaAlSiO4-SiO2, Mg2SiO4-CaMgSi2O6-SiO2 and Mg2SiO4-CaAl2SiO6-SiO2 which include the major components of mantle peridotites. Fig. 2 shows the shift of the forsterite-enstatite liquidus boundary with pressure in the system Mg2SiO4-NaAlSiO4-SiO2. As shown in the figure, melts in equilibrium with forsterite and enstatite, which lie on the forsterite-enstatite liquidus boundary, become more silica-depleted and Na-enriched with increasing pressure. At 2.0 and 3.0 GPa, the low-temperature parts (upper parts) of the liquidus boundaries lie in the nepheline normative compositional area (Mg2SiO4-NaAlSi3O8-NaAlSiO4 triangular area). The results imply that magmas formed by partial melting of mantle peridotite have relatively silica-rich tholeiitic compositions at low pressures, whereas they have silica-poor alkali basaltic compositions at high pressures, thus supporting Kuno’s model. However, the pressure ranges where two magmas are formed are much lower than those in Kuno’s model. The results of Kushiro’s experiments indicate that even alkali basaltic magmas can be formed at pressures less than 2.0 GPa (depths<60 km), compared to > 200 km in the Kuno’s model. Green and Ringwood15) and Green et al.16) also discussed, based on their high-pressure experiments on natural basalt compositions, that the depth ranges for generation of silica-saturated tholeiite, high-alumina basalt, and alkali basalt magmas are < 15 km, 15–35 km, and 35–60 km, respectively. These depth ranges are much shallower than those in Kuno’s model; however, they suggested that the deep earthquake foci may be related to the initiation of upward movement of mantle materials, which eventually partially melt at shallower depths and finally undergo magma segregation.

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.