Hexbyte Glen Cove Zinc isotopes of arc-related lavas reveal recycling of forearc serpentinites into subarc mantle

Hexbyte Glen Cove

Credit: Unsplash/CC0 Public Domain

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 forearc mantle wedge above the subduction slab (mantle wedge serpentinite) in the subduction zone. 

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

A research team led by Zeng Zhigang from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS), in collaboration Prof. Chen Jiubin from Tianjin University, investigated zinc isotopes of subduction-related lavas from the Western Pacific and implications for crust-mantle recycling.

Their study, published in Journal of Geophysical Research: Solid Earth, provided an effective means to distinguish the contributions of slab and mantle wedge serpentinite-derived fluids to arc magmas, which is significant to understand the role of serpentinite in material recycling in subduction zones.

The researchers found that the arc-related lavas had lower δ66Zn values than those of the mid-ocean ridge basalts (MORB), whereas back-arc lavas displayed MORB-like δ66Zn values. Moreover, δ66Zn has a good correlation with proxies for fluid addition (87Sr/86Sr and Ba/La) and slab depths. 

Since mantle melting and magmatic differentiation induces heavy Zn isotope enrichment in primary and evolved magmas, respectively, while melt extraction yields the limited Zn isotope fractionation in the mantle, lavas with low δ66Zn values thus potentially indicate the involvement of isotopically light fluids in their mantle sources.

In contrast to the heavy Zn isotope of the slab serpentinites, the forearc serpentinites are typically characterized by extremely light Zn isotope. Correspondingly, fluids released by forearc serpentinite dehydration have a significantly lower Zn isotopic composition relative to the mantle wedge.

Therefore, such forearc materials were likely dragged downward to subarc depths and released isotopically light Zn in fluids to modify the overlying mantle wedge, thereby producing low δ66Zn values in arc-related magmas. Beyond subarc depths, forearc serpentinites were broken down completely, so light Zn isotope fluids were absent.

Accordingly, the lavas from the back-arc basin displayed MORB-like δ66Zn values. It provided conclusive evidence for the hypothesis that forearc mantle wedge serpentinites could be involved in the subduction channel and transported into the subarc depth, and then dehydrate and modify the subarc mantle wedge.

More information:
Zuxing Chen et al, Zinc Isotopes of the Mariana and Ryukyu Arc‐Related Lavas Reveal Recycling of Forearc Serpentinites Into the Subarc Mantle, Journal of Geophysical Research: Solid Earth (2021). DOI: 10.1029/2021JB022261

Zinc isotopes of arc-related lavas reveal recycling of forearc serpentinites into subarc mantle (2021, December 23)
retrieved 25 December 2021

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Hexbyte Glen Cove Newly discovered Miocene biome sheds light on rainforest evolution thumbnail

Hexbyte Glen Cove Newly discovered Miocene biome sheds light on rainforest evolution

Hexbyte Glen Cove

Selected plant taxa from the Zhangpu biota. Credit: NIGPAS

An international research group led by Prof. Wang Bo and Prof. Shi Gongle from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences (NIGPAS) has collected approximately 25,000 fossil-containing amber samples and about 5,000 fossil plants in Zhangpu County, Fujian Province, southeast China from 2010 to 2019.

Their findings were published in Science Advances on April 30.

The Zhangpu biota, including amber biota and co-occurring megafossils, is the richest tropical seasonal biota discovered so far. It reveals that extraordinary species diversity existed within a 14.7 million-year-old tropical rainforest and sheds light on the evolution of the rainforest.

Diverse winged fruits of Dipterocarpaceae and legumes, as well as leaves of 78 different broadleaf trees show that tropical seasonal rainforests extended further north than today, offering an insight into what changes might take place in a future warmer world if ecosystems are able to adapt.

The Zhangpu amber biota contains a diverse, exquisitely preserved fossil arthropod fauna and abundant botanical and other inclusions such as fungi, snails, and even feathers. Botanical inclusions include bryophytes (liverworts and mosses) and flowering plants.

Arthropod inclusions cover an impressive array of more than 250 families including various spiders, mites, millipedes, and at least 200 families of insects in 20 orders. The extremely high variety of arthropods renders the Zhangpu amber biota one of the world’s four richest, along with the widely known Cretaceous Burmese amber biota (> 568 families), Eocene Baltic amber biota (> 550 families), and Miocene Dominican amber biota (205 families).

Representative inclusions in Zhangpu amber. Credit: NIGPAS

The insect fauna in Zhangpu amber include many ants, bees, lacewings, stick insects, termites, and grasshoppers that are today restricted to tropical Southeast Asia and/or New Guinea.

“The most unexpected finding is that the high diversity of ants and springtails all belong to living genera. In addition, the vast majority of previously identified insects in Zhangpu amber, such as bark lice, grasshoppers, beetles, and bees, also belong to living genera,” said Prof. Wang.

These results suggest that Asian rainforest insect communities have remained stable since the middle Miocene (at least 15 million years ago). It also highlights that tropical rainforests act as museums of biological diversity at the generic level. The relative ecological stability of such “megathermal” environments facilitates the continued accumulation of species diversity and makes them even more precious than previously realized.

The Zhangpu amber biota is unique because the samples are not commercially extracted and consequently the species census is minimally skewed by human selective bias. Moreover, its precise age is well-constrained by radioisotopic dating and the associated plant compression/impression fossils allow quantitative reconstruction of the ancient climate.

Ecological reconstruction of the Zhangpu biota. Credit: NIGPAS

Compared to the modern climate of Zhangpu, the most notable difference is that the middle Miocene climate had a warmer winter, leading to a relatively stable temperature throughout the year.

In scenarios of global warming, winter warming is commonly more pronounced than summer warming, and has larger and more widespread effects on terrestrial and marine ecosystems. It reduces “winterkills” and is beneficial for reproduction and growth of tropical animals and plants.

“Winter warming is likely to have been a major driver of the northern expansion of the megathermal biota in South China during the Mid-Miocene Climatic Optimum,” said Prof. Shi.

More information:
“The mid-Miocene Zhangpu biota reveals an outstandingly rich rainforest biome in East Asia” Science Advances (2021). DOI: 10.1126/sciadv.abg0625

Newly discovered Miocene biome sheds light on rainforest evolution (2021, April 30)
retrieved 3 May 2021
from https://phys.org/news/2021-04-newly-miocene-biome-rainforest-evolution.html

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