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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.


Possible temperature distributions in the hot fingers with two different H2O contents to generate subduction zone magmas. a: H2O content 0.1∼0.2 wt.%; b: 0.2∼0.5 wt.%.
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f12-83_001: Possible temperature distributions in the hot fingers with two different H2O contents to generate subduction zone magmas. a: H2O content 0.1∼0.2 wt.%; b: 0.2∼0.5 wt.%.

Mentions: As discussed above, primary tholeiite and high-alumina basalt magmas are generated by as much as 15 and 25 wt.% melting of mantle peridotite similar to relatively fertile peridotites PHN1611 and KLB1. Therefore, if the mantle wedge of north-east Honshu consists of such relatively fertile peridotites, primary tholeiite and high-alumina basalt magmas cannot be formed with the velocity structures of Fig. 10 in which 2–5 wt.% melting likely occurs. If the mantle wedge of northeast Honshu consists of more depleted peridotites, those magmas may be formed with the velocity structures of Fig. 10. To resolve this problem, seismic velocity structures with higher precisions are needed. In any case, primary tholeiite and high-alumina basalt magmas would be formed in higher temperature parts of the mantle, and such higher-temperature parts (blocks) of the mantle would ascend from deeper parts of the mantle wedge intermittently. Tamura et al.59) recognized that Quaternary volcanoes in northeast Honshu form 10 clusters separated by 20–50 km spaces, and that each cluster extends along an east-west direction. Beneath the volcano clusters, seismic low-velocity regions exist, which Tamura et al.59) called ‘hot fingers’. Fig. 10 is the seismic velocity structure of one hot finger. The high-temperature blocks of the mantle or diapirs, in which primary tholeiite and high-alumina basalt magmas are generated, may ascend within the hot fingers or ascend nearly vertically from the mantle below the hot fingers. In the former case, hot blocks ascend obliquely from the deeper part of the hot fingers on the Japan Sea side toward the shallower part on the Pacific Ocean side within or along the hot fingers and generate magmas. Fig. 12 shows possible temperature distributions in the hot fingers with two different H2O contents to generate representative subduction zone magmas. Fig. 12a and b are for H2O contents 0.1∼0.2 wt.% and 0.2∼0.5 wt.%, respectively. In the former case, tholeiitic basalt, high-alumina basalt and alkali basalt magmas are formed, whereas in the latter case, boninite and high-magnesia andesite magmas are formed.


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

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

Possible temperature distributions in the hot fingers with two different H2O contents to generate subduction zone magmas. a: H2O content 0.1∼0.2 wt.%; b: 0.2∼0.5 wt.%.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f12-83_001: Possible temperature distributions in the hot fingers with two different H2O contents to generate subduction zone magmas. a: H2O content 0.1∼0.2 wt.%; b: 0.2∼0.5 wt.%.
Mentions: As discussed above, primary tholeiite and high-alumina basalt magmas are generated by as much as 15 and 25 wt.% melting of mantle peridotite similar to relatively fertile peridotites PHN1611 and KLB1. Therefore, if the mantle wedge of north-east Honshu consists of such relatively fertile peridotites, primary tholeiite and high-alumina basalt magmas cannot be formed with the velocity structures of Fig. 10 in which 2–5 wt.% melting likely occurs. If the mantle wedge of northeast Honshu consists of more depleted peridotites, those magmas may be formed with the velocity structures of Fig. 10. To resolve this problem, seismic velocity structures with higher precisions are needed. In any case, primary tholeiite and high-alumina basalt magmas would be formed in higher temperature parts of the mantle, and such higher-temperature parts (blocks) of the mantle would ascend from deeper parts of the mantle wedge intermittently. Tamura et al.59) recognized that Quaternary volcanoes in northeast Honshu form 10 clusters separated by 20–50 km spaces, and that each cluster extends along an east-west direction. Beneath the volcano clusters, seismic low-velocity regions exist, which Tamura et al.59) called ‘hot fingers’. Fig. 10 is the seismic velocity structure of one hot finger. The high-temperature blocks of the mantle or diapirs, in which primary tholeiite and high-alumina basalt magmas are generated, may ascend within the hot fingers or ascend nearly vertically from the mantle below the hot fingers. In the former case, hot blocks ascend obliquely from the deeper part of the hot fingers on the Japan Sea side toward the shallower part on the Pacific Ocean side within or along the hot fingers and generate magmas. Fig. 12 shows possible temperature distributions in the hot fingers with two different H2O contents to generate representative subduction zone magmas. Fig. 12a and b are for H2O contents 0.1∼0.2 wt.% and 0.2∼0.5 wt.%, respectively. In the former case, tholeiitic basalt, high-alumina basalt and alkali basalt magmas are formed, whereas in the latter case, boninite and high-magnesia andesite magmas are formed.

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.