Arc-bound lava zinc isotopes reveal the recycling of fore-arc serpentinites into the subarc mantle


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Serpentinite, formed by low temperature hydrothermal alteration of mantle peridotite, is distributed in the lithospheric mantle at the bottom of the subduction slab (slab-serpentinite) and the corner of the fore-arc mantle above the subduction slab (mantle serpentinite) in the subduction zone.

Since they usually contain a large amount of water, mobile fluid elements (Cs, Rb, Sr, Ba, Pb, Li, etc.) and heavy B isotopes, using traditional geochemical means to distinguish the two different sources of serpentinite fluids in the genesis of arc magmas is difficult.

A research team led by Zeng Zhigang of the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS), in collaboration with Professor Chen Jiubin of Tianjin University, studied the isotopes of zinc in lavas linked to Western Pacific subduction and their implications for crust-mantle recycling.

Their study, published in Journal of Geophysical Research: Solid Earth, provided an effective way to distinguish the contributions of fluids derived from slab and mantle serpentinite to arc magmas, which is important for understanding the role of serpentinite in material recycling in areas of subduction.

The researchers found that the arc-related lavas had an inferior66values ​​of Zn than those of basalts of the mid-oceanic ridge (MORB), while the lavas of the back-arc presented values ​​of the MORB type δ66Zn values. What’s more,66Zn has a good correlation with the proxies for the addition of fluid (87Sr /86Sr and Ba / La) and the depths of the slabs.

Since mantle fusion and magmatic differentiation induce high enrichment of Zn isotopes in primary and evolved magmas, respectively, while melt extraction results in limited fractionation of Zn isotopes in the mantle, low lavas66Zn values ​​therefore potentially indicate the involvement of isotopically light fluids in their mantle sources.

Unlike the heavy Zn isotope of plate serpentinites, fore-arc serpentinites are typically characterized by an extremely light Zn isotope. As a result, the fluids released by the dehydration of the fore-arc serpentinite have a significantly lower Zn isotopic composition relative to the mantle corner.

Therefore, these pre-arc materials were likely dragged down to subarctic depths and released isotopically light Zn in the fluids to modify the wedge of the overlying mantle, thus producing a weak δ66Zn values ​​in arc-related magmas. Beyond the depths of the sub-arc, the serpentinites of the fore-arc were completely decomposed, so that the light isotopic fluids of Zn were absent.

Consequently, the lavas of the back-arc basin presented a MORB type66Zn values. He provided conclusive evidence for the hypothesis that serpentinites from the mantle corner of the fore-arc could be involved in the subduction channel and transported to the depth of the sub-arc, then dehydrate and modify the mantle corner. of the sub-arc.

Recycling of tectonic plates a key factor in Earth’s oxygen budget

More information:
Zuxing Chen et al, the zinc isotopes of the arc-bound Mariana and Ryukyu lavas reveal the recycling of fore-arc serpentinites in the subarc mantle, Journal of Geophysical Research: Solid Earth (2021). DOI: 10.1029 / 2021JB022261

Provided by Chinese Academy of Sciences

Quote: Zinc isotopes from arc-related lavas reveal the recycling of fore-arc serpentinites in the sub-arc mantle (2021, December 23) retrieved December 26, 2021 from news / 2021-12-zinc-isotopes-arc-related-lavas -reveal.html

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