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Advent of Continents: A New Hypothesis

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ABSTRACT

The straightforward but unexpected relationship presented here relates crustal thickness to magma type in the Izu-Ogasawara (Bonin) and Aleutian oceanic arcs. Volcanoes along the southern segment of the Izu-Ogasawara arc and the western Aleutian arc (west of Adak) are underlain by thin crust (10–20 km). In contrast those along the northern segment of the Izu-Ogasawara arc and eastern Aleutian arc are underlain by crust ~35 km thick. Interestingly, andesite magmas dominate eruptive products from the former volcanoes and mostly basaltic lavas erupt from the latter. According to the hypothesis presented here, rising mantle diapirs stall near the base of the oceanic crust at depths controlled by the thickness of the overlying crust. Where the crust is thin, melting occurs at relatively low pressures in the mantle wedge producing andesitic magmas. Where the crust is thick, melting pressures are higher and only basaltic magmas tend to be produced. The implications of this hypothesis are: (1) the rate of continental crust accumulation, which is andesitic in composition, would have been greatest soon after subduction initiated on Earth, when most crust was thin; and (2) most andesite magmas erupted on continental crust could be recycled from “primary” andesite originally produced in oceanic arcs.

No MeSH data available.


Related in: MedlinePlus

Schematic illustration of the effect of pressure on forsterite-enstatite equilibria in the hydrous environment.Primary magmas in equilibrium with magnesian olivine and magnesian orthopyroxene become progressively more silica-rich with decreasing depth53. (a) At high pressure and in hydrous conditions, congruent melting of magnesian orthopyroxene results in primary basalt melt. (b) At lower pressure and in hydrous conditions the liquidus field of forsterite expands relative to that of enstatite, with the result that, at some point, enstatite melts incongruently to produce primary andesite melt. (c) Schematic illustration showing primary basalt and andesite magmas and their possible fractionation trends.
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f5: Schematic illustration of the effect of pressure on forsterite-enstatite equilibria in the hydrous environment.Primary magmas in equilibrium with magnesian olivine and magnesian orthopyroxene become progressively more silica-rich with decreasing depth53. (a) At high pressure and in hydrous conditions, congruent melting of magnesian orthopyroxene results in primary basalt melt. (b) At lower pressure and in hydrous conditions the liquidus field of forsterite expands relative to that of enstatite, with the result that, at some point, enstatite melts incongruently to produce primary andesite melt. (c) Schematic illustration showing primary basalt and andesite magmas and their possible fractionation trends.

Mentions: Figure 5 shows the schematic effect of pressure on forsterite-enstatite equilibria in hydrous conditions. Similar diagrams can be seen in many petrology textbooks, based on high-pressure experiments under dry conditions53. Primary magmas in equilibrium with magnesian olivine and orthopyroxene become progressively more silica-rich or silica-poor with decreasing and increasing depths, respectively. A) At high pressure (>~1 GPa) and in hydrous conditions, congruent melting of magnesian orthopyroxene results in only primary basalt melt. B) At lower pressure (<~1 GPa) and in hydrous conditions, the liquidus field of forsterite expands relative to that of enstatite with the consequence that at some point, enstatite melts incongruently to produce primary andesite melt.


Advent of Continents: A New Hypothesis
Schematic illustration of the effect of pressure on forsterite-enstatite equilibria in the hydrous environment.Primary magmas in equilibrium with magnesian olivine and magnesian orthopyroxene become progressively more silica-rich with decreasing depth53. (a) At high pressure and in hydrous conditions, congruent melting of magnesian orthopyroxene results in primary basalt melt. (b) At lower pressure and in hydrous conditions the liquidus field of forsterite expands relative to that of enstatite, with the result that, at some point, enstatite melts incongruently to produce primary andesite melt. (c) Schematic illustration showing primary basalt and andesite magmas and their possible fractionation trends.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5037404&req=5

f5: Schematic illustration of the effect of pressure on forsterite-enstatite equilibria in the hydrous environment.Primary magmas in equilibrium with magnesian olivine and magnesian orthopyroxene become progressively more silica-rich with decreasing depth53. (a) At high pressure and in hydrous conditions, congruent melting of magnesian orthopyroxene results in primary basalt melt. (b) At lower pressure and in hydrous conditions the liquidus field of forsterite expands relative to that of enstatite, with the result that, at some point, enstatite melts incongruently to produce primary andesite melt. (c) Schematic illustration showing primary basalt and andesite magmas and their possible fractionation trends.
Mentions: Figure 5 shows the schematic effect of pressure on forsterite-enstatite equilibria in hydrous conditions. Similar diagrams can be seen in many petrology textbooks, based on high-pressure experiments under dry conditions53. Primary magmas in equilibrium with magnesian olivine and orthopyroxene become progressively more silica-rich or silica-poor with decreasing and increasing depths, respectively. A) At high pressure (>~1 GPa) and in hydrous conditions, congruent melting of magnesian orthopyroxene results in only primary basalt melt. B) At lower pressure (<~1 GPa) and in hydrous conditions, the liquidus field of forsterite expands relative to that of enstatite with the consequence that at some point, enstatite melts incongruently to produce primary andesite melt.

View Article: PubMed Central - PubMed

ABSTRACT

The straightforward but unexpected relationship presented here relates crustal thickness to magma type in the Izu-Ogasawara (Bonin) and Aleutian oceanic arcs. Volcanoes along the southern segment of the Izu-Ogasawara arc and the western Aleutian arc (west of Adak) are underlain by thin crust (10&ndash;20&thinsp;km). In contrast those along the northern segment of the Izu-Ogasawara arc and eastern Aleutian arc are underlain by crust ~35&thinsp;km thick. Interestingly, andesite magmas dominate eruptive products from the former volcanoes and mostly basaltic lavas erupt from the latter. According to the hypothesis presented here, rising mantle diapirs stall near the base of the oceanic crust at depths controlled by the thickness of the overlying crust. Where the crust is thin, melting occurs at relatively low pressures in the mantle wedge producing andesitic magmas. Where the crust is thick, melting pressures are higher and only basaltic magmas tend to be produced. The implications of this hypothesis are: (1) the rate of continental crust accumulation, which is andesitic in composition, would have been greatest soon after subduction initiated on Earth, when most crust was thin; and (2) most andesite magmas erupted on continental crust could be recycled from &ldquo;primary&rdquo; andesite originally produced in oceanic arcs.

No MeSH data available.


Related in: MedlinePlus