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I previously believed that continental crust owes its lower density to the partial melting of oceanic crust; the mantle would partially melt at mid-ocean ridges to produce basaltic crust, and when this oceanic crust eventually subducted, the basalt would then partially melt to produce an even lighter magma. This lighter magma would rise up and eventually erupt, building up a less dense continental crust above.

However, I have recently learned that the magma produced at subduction zones comes from the partial melting of the mantle above the subducting plate and that very little comes from the plate itself. This means that mid-ocean ridges, hot spots, and subduction zones all derive their magma directly from the mantle, yet they somehow produce magmas of differing compositions.

I have seen explanations which state that felsic magma forms due to fractional crystallization of magma while working its way through the thick continental crust, but this still wouldn't explain how it forms under thin oceanic crusts at island arcs.

I am trying to understand how a hot spot under oceanic crust produces shield volcanoes with basaltic rock while oceanic-oceanic subduction zones produce stratovolcanoes with felsic rock.

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    – f.thorpe
    Dec 28, 2022 at 2:29

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You wrote:

"I have recently learned that the magma produced at subduction zones comes from the partial melting of the mantle above the subducting plate and that very little comes from the plate itself."

It would be interesting if you could provide a source for this claim, as it is still unclear (Guo et al., 2022).

The island arcs and volcanic arcs are far from the mid-ocean ridge, and the plate that subducts has had time to accumulate a large stack of sediments. This source from the University of Dallas suggests that when the plate subducts, the melting of the oceanic crust with sediments becomes one of the sources of calc-alkaline and alkaline magmas that crystallize on the surface as felsic rocks. However, the volcanologist @Jean-Marie Prival doubts this assertion is true (see comments moved to chat).

Some studies suggest that andesites come from a mantle source and not the slab:

The sum of stratigraphic, geochemical, and isotopic evidence on Agrigan supports the derivation of calc-alkaline andesite by the removal of about 75% solids from a high-alumina basalt accompanied by a process of K and Rb enrichment, such as volatile-transfer. Considerations of 87Sr/86Sr, 143Nd/144Nd, and 3He/4He isotopic data indicate that the source region of these parental liquids lies in the mantle, not subducted crust. (Stern, R.J., 1979).

What is certain is that the sediments of the subducted plate are hydrated. When the subducted plate melts, the magma is enriched in H2O due to dehydration of the slab, contributing to the explosive eruptions that form stratovolcanoes in both island arcs and volcanic arcs.

H2O also affects the magma differentiation:

Subduction zones (arcs and back-arcs) are major sites for elemental cycling via slab dehydration and subsequent mantle metasomatism and melting; many models have been suggested by researchers to explain these processes. However, the influence of the overriding lithosphere, especially the lower crust, on the generation of back-arc magmas during the early spreading stage remains largely unknown (Guo,K. et al, 2022).

You also wrote:

"This means that mid-ocean ridges, hot spots, and subduction zones all derive their magma directly from the mantle, yet they somehow produce magmas of differing compositions."

Both hot spots and mid-ocean ridges produce basaltic magmas derived from the mantle. It is still debated whether the composition of the slab in subduction zones affects the magma that generates the resulting felsic rocks. It might affect them residually. What is certain is that "somehow" refers to the subducted slab, and more specifically, the H2O accumulated in the sediments that alters the mantle magma differentiation.

A final consideration is that the temperature of the mantle varies greatly, from 1000°C near its boundary with the crust to 3700°C near its boundary with the core (source). This raises the question of whether this temperature is enough to melt sediments.

  • The melting point of radiolaria SiO2 shells is 1.710 °C. The source of the Si enrichment can hardly be the sediments of the slag.
  • This paper sugest some CaCO3 shells are preserved even the trench is under the Carbonate Compensation Depth. They migth enrich the magma in Ca, as CaCO3 melting point is 825ºC.
  • Sedimentary clays have a melting point between 1250 °C to 1350 °C. They may enrich the magma in elements as Al or K.

-Eichelberger, J.C. Andesites in island arcs and continental margins: Relationship to crustal evolution. Bull Volcanol 41, 480–500 (1978). https://doi.org/10.1007/BF02597382

-Stern, R.J. On the origin of andesite in the northern Mariana Island Arc: Implications from Agrigan. Contr. Mineral. and Petrol. 68, 207–219 (1979). https://doi.org/10.1007/BF00371901

-Kun Guo, Xiaoyuan Wang, Shuai Chen, Luning Shang, Bingquan Liu, Xia Zhang, Zhiqing Lai (2022): "The delamination of lower crust in continental back-arc basin: Evidence from Sr isotope and elemental compositions of plagioclase and clinopyroxene in andesites from Kueishantao, north of Taiwan, China," Lithos, Volumes 416–417, ISSN 0024-4937, https://doi.org/10.1016/j.lithos.2022.106653.

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You are correct that arc magmas are produced by melting of the mantle wedge above the subducting slab. In the case of felsic rocks in island arcs, a model I have not seen debunked is that these volcanoes typically produce early melts that are no more felsic than andesite. These andesites are then re-melted as the source for more differentiated dacites and rhyolites. This is supported by their geochemistry, which is more enriched relative to continental arc and rift rhyolites in 'typical' mafic elements like Ca and Al.

https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1440-1738.2010.00746.x

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