As an important subduction component, altered oceanic crust (AOC) is widely distributed on the oceanic subducting slab and potentially has significant contributions to the chemistry of arc magmas.
However, identifying this contribution in arc magmas is challenging, because AOC is not as enriched in incompatible elements as sediments nor has as high H2O concentrations as do serpentinites. Thus, it is necessary to find a sensitive tracer for subducted AOC.
Recently, Dr. ZHANG Yuxiang of Prof. ZENG Zhigang's team from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) worked with Prof. TURNER Simon from Macquarie University and Prof. HUANG fang from University of Science and Technology of China and put that a combination of Ba-Sr-Nd isotopes could effectively identify the presence of recycled AOC in arc magmas.
The study was published in Journal of Geophysical Research: Solid Earth on Mar. 4.
A characteristic geochemical feature of AOC is the decoupling of its Sr-Nd isotopes, i.e., compared with unaltered oceanic crustal rocks, AOC has higher 87Sr/86Sr but similar 143Nd/144Nd. Accordingly, AOC overall deviates from the mantle array in the Sr-Nd isotope plot.
Interestingly, this Sr-Nd isotope decoupling feature has been commonly observed in intra-oceanic arc rocks. Thus, many researchers consider the Sr-Nd isotope decoupling as an indicator of contribution of recycled AOC. However, before drawing this conclusion, the influence of sediment-derived melt or fluid is required to be eliminated.
In this study, researchers compiled the Sr-Nd isotopes and concentrations of Ba, Th, Yb for global arc rocks. They found that the magnitude of Sr-Nd isotope decoupling of arc rocks is positively correlated with Ba/Th, whereas negatively correlated with Th/Yb, which implies that the Sr-Nd isotope decoupling is not related to contributions of sediments.
In addition, they found that the arc rocks with more pronounced Sr-Nd isotope decoupling tend to have higher Ba isotope ratios, which is also a feature of AOC-derived fluid; more importantly, this phenomenon occurs in many subduction zones such as Mariana, Tonga, and Kermadec. Therefore, an intimate link can be established between subducted AOC, heavy Ba isotope compositions, and Sr-Nd isotope decoupling signature in island arcs, which provides a powerful tool to trace the AOC recycling in subduction zones.
Furthermore, researchers found that on a global scale, the magnitude of Sr-Nd isotope decoupling is positively correlated with B isotope ratio, which is an effective tracer for subducted serpentinite.
"It implies that subducted AOC and serpentinite might have coupled behaviors during slab dehydration, which has great significance for understanding the material transfer in subduction zones," said Dr. ZHANG Yuxiang, first author of the study.
Fig. 1 Correlations of Sr-Nd isotope decoupling with Ba/Th and Th/Yb for arc rocks.
Fig. 2 Correlations of Sr-Nd isotope decoupling with Ba, B isotopes for arc rocks.
Zhang, Y., Shu, Y., Turner, S., Chen, Z., Zeng, Z., & Huang, F. (2024). Deciphering contribution of recycled altered oceanic crust to arc magmas using Ba-Sr-Nd isotopes. Journal of Geophysical Research: Solid Earth, 129, e2023JB028407.
(Text by ZHANG Yuxiang)
Media Contact:
ZHANG Yiyi
Institute of Oceanology
E-mail: zhangyiyi@qdio.ac.cn
(Editor: ZHANG Yiyi)