Chinese scholars and overseas collaborators have made progress in using Zn isotopes to trace the earth's deep carbon cycle

Figure Formation of ZINC isotope tracer mantle HIMU components and their connection with the cycling process of surface matter

With the support of the National Natural Science Foundation of China (grant number: 42130310, 41973001), the team of Chen Lihui of Northwest University collaborated with researchers from Nanjing University, the Japan Agency for Marine Exploration (JAMSTEC), the Max Planck Institute of Chemistry and the University of Tokyo to make progress
in using the Zn isotopic composition of the ocean island basalt to trace the recycled carbonate components deep in the mantle 。 The findings, titled "Zinc isotopic evidence for recycled carbonate in the deep mantle," were published in Nature Communications on October 14
, 2022 。 Links to papers: _istranslated="1">.

Whether carbonates at the surface can subduct through plates into the lower mantle is a long-debated topic
.
The mantle plume originating from the lower mantle will melt when it rises to the shallow part of the upper mantle, and the resulting magma will erupt from the surface to form Ocean Island Basalt (OIB), so OIB is the main way to
understand the composition of the material in the lower mantle.
Since surface carbonates have a significantly heavier zinc isotopic composition relative to the normal mantle, the zinc isotopic composition of OIBs can be used to trace the presence of recycled surface carbonate components in the lower mantle
.
Based on this, the team selected classic HIMU-type OIB samples (206 Pb/²⁰⁴Pb>20.
5) from the Cook-Austral Islands in the South Pacific and the volcanic island of St.
Helena in the South Atlantic Ocean as the research object to carry out high-precision zinc isotope analysis
.
The study found that the zinc isotopic composition of these HIMU OIBs (δ 66 Zn = 0.
38±0.
03‰) was significantly more important than that of the normal mantle (δ 66 Zn = 0.
16±0.
06‰) and most other oceanic basalts (δ⁶⁶Zn = 0.
31±0.
10‰).

Simulation calculations show that the zinc isotopic composition characteristics of HIMU type OIB are obviously heavy cannot be generated
by partial melting and magma differentiation processes.
The correlation between zinc isotopes (δ⁶⁶Zn) and osmium isotopes (187 Os/¹⁸⁸Os) indicates that the heavy zinc isotopic composition is an inherent feature of the mantle source region of the HIMU component and is associated with
recycled substances.
Combined with other geochemical evidence from these basalts, the source area is carbonated peridotite
.
When surface carbonate sediments subduct into the mantle transition zone, melt occurs, and the resulting carbonate melt will account for the surrounding mantle peridotite, resulting in carbonated peridotite; Carbonated peridotite enters the lower mantle with subduction plates and persists for a long time at the core-mantle boundary, and its Sr-Nd-Pb isotopic composition evolves into HIMU characteristics.
This carbonated peridotite is eventually carried by the rising mantle plume to the shallow upper mantle and melts, and the resulting magma erupts the surface to form the HIMU type OIB.

This study not only clarifies the origin of HIMU-type basalt and mantle HIMU end elements, but also provides credible evidence that
surface carbonates are involved in the Earth's internal processes.