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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

Variation diagrams of SiO2 (wt. %) vs FeO*/MgO ratios for lavas from (a,b) the northern segment of the present Izu-Ogasawara arcs, (c,d) Torishima, (e,f) the southern segment of the present Izu-Ogasawara arcs, and (g,h) the Oligocene Izu-Ogasawara-Mariana arcs. FeO*; total iron as FeO.
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f4: Variation diagrams of SiO2 (wt. %) vs FeO*/MgO ratios for lavas from (a,b) the northern segment of the present Izu-Ogasawara arcs, (c,d) Torishima, (e,f) the southern segment of the present Izu-Ogasawara arcs, and (g,h) the Oligocene Izu-Ogasawara-Mariana arcs. FeO*; total iron as FeO.

Mentions: FeO*(total iron as FeO)/MgO vs. SiO2 diagrams show the contrast between lava compositions from the northern and southern segments of the Izu-Ogasawara arcs (Fig. 4a–f), and between the northern segment and the Oligocene IOM system of arcs (Fig. 4g,h). FeO*/MgO and the corresponding values of molar Mg# [100Mg/(Mg + Fe)] are shown on the vertical axes. Primary basaltic magmas from the Mariana arc2526 and the bulk continental crust27 are also plotted. Figure 4 shows that lavas from the northern segment of the Izu-Ogasawara arcs have significantly lower SiO2 contents than those from the southern segment and the Oligocene IOM arcs at similar FeO*/MgO and molar Mg#. In other words, silica contents are different at the same differentiation indices reflecting crystal fractionation of olivines and pyroxenes. According to the Miyashiro definition28, most lavas from the northern segment are defined as tholeiitic basalts (lower SiO2 at a given FeO*/MgO and Mg# or molar Mg/(Mg + Fe)), whereas those from the southern segment are calc-alkaline andesites (higher SiO2 at the same FeO*/MgO or Mg#). Torishima, located between the segments, has features of both, containing tholeiitic and strongly calc-alkaline lavas.


Advent of Continents: A New Hypothesis
Variation diagrams of SiO2 (wt. %) vs FeO*/MgO ratios for lavas from (a,b) the northern segment of the present Izu-Ogasawara arcs, (c,d) Torishima, (e,f) the southern segment of the present Izu-Ogasawara arcs, and (g,h) the Oligocene Izu-Ogasawara-Mariana arcs. FeO*; total iron as FeO.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: Variation diagrams of SiO2 (wt. %) vs FeO*/MgO ratios for lavas from (a,b) the northern segment of the present Izu-Ogasawara arcs, (c,d) Torishima, (e,f) the southern segment of the present Izu-Ogasawara arcs, and (g,h) the Oligocene Izu-Ogasawara-Mariana arcs. FeO*; total iron as FeO.
Mentions: FeO*(total iron as FeO)/MgO vs. SiO2 diagrams show the contrast between lava compositions from the northern and southern segments of the Izu-Ogasawara arcs (Fig. 4a–f), and between the northern segment and the Oligocene IOM system of arcs (Fig. 4g,h). FeO*/MgO and the corresponding values of molar Mg# [100Mg/(Mg + Fe)] are shown on the vertical axes. Primary basaltic magmas from the Mariana arc2526 and the bulk continental crust27 are also plotted. Figure 4 shows that lavas from the northern segment of the Izu-Ogasawara arcs have significantly lower SiO2 contents than those from the southern segment and the Oligocene IOM arcs at similar FeO*/MgO and molar Mg#. In other words, silica contents are different at the same differentiation indices reflecting crystal fractionation of olivines and pyroxenes. According to the Miyashiro definition28, most lavas from the northern segment are defined as tholeiitic basalts (lower SiO2 at a given FeO*/MgO and Mg# or molar Mg/(Mg + Fe)), whereas those from the southern segment are calc-alkaline andesites (higher SiO2 at the same FeO*/MgO or Mg#). Torishima, located between the segments, has features of both, containing tholeiitic and strongly calc-alkaline lavas.

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 (10–20 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