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在岛弧长英质的岩浆是怎么产生的吗?

我以前认为大陆地壳欠其低密度海洋地壳的部分熔融;地幔部分熔融在大洋中脊玄武岩地壳,这最终海洋地壳俯冲,然后,玄武岩部分熔融产生一个更轻的岩浆。这个打火机岩浆上升并最终爆发,建立密度较低的大陆地壳以上。

不过,我最近了解到,在俯冲带岩浆产生来自地幔的部分熔融上方俯冲板块,很少来自板本身。这意味着大洋中脊、热点和俯冲带所有获得直接从地幔岩浆,但他们不知何故产生岩浆的成分不同。

我所看到的解释哪个州,长英质的岩浆形式由于岩浆分离结晶工作时通过厚大陆地壳,但这仍不能解释下它形成薄的海洋地壳在岛弧。

我想了解下一个热点海洋地壳产生盾火山玄武岩岩石而oceanic-oceanic俯冲带生产与基性岩成层火山。

回答

你写:“我最近了解到,在俯冲带岩浆产生>来自俯冲带之上的地幔的部分熔融>板块,很少来自于板块本身。”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][1] and [volcanic arcs][2] 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][3] 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][4] 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][5] 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][6] and [mid-ocean ridges][7] produce basaltic magmas derived from the mantle. It is still debated whether the composition of the [slab][8] 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][9]). 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][10] sugest some CaCO3 shells are preserved even the trench is under the [Carbonate Compensation Depth][11]. 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. [1]: https://en.wikipedia.org/wiki/Island_arc [2]: https://en.wikipedia.org/wiki/Volcanic_arc [3]: https://en.wikipedia.org/wiki/Radiolaria [4]: https://en.wikipedia.org/wiki/Felsic [5]: https://en.wikipedia.org/wiki/Stratovolcano [6]: https://en.wikipedia.org/wiki/Hotspot_(geology) [7]: https://en.wikipedia.org/wiki/Mid-ocean_ridge [8]: https://en.wikipedia.org/wiki/Slab_(geology) [9]: https://education.nationalgeographic.org/resource/mantle [10]: https://pubs.geoscienceworld.org/books/book/1144/chapter-abstract/10571036/Preservation-of-Foraminifera?redirectedFrom=PDF [11]: https://en.wikipedia.org/wiki/Carbonate_compensation_depth

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