Mid-ocean ridge basalts (MORBs), located far from subduction zones, are typically thought to be unaffected by subduction processes. However, some MORBs display arc-like geochemical signatures—including negative Nb anomalies and elevated H₂O/Ce and Ba/Th ratios—referred to as "ghost-arc signatures."

The origins of these signatures remain controversial. Factors such as mantle plume material, recycled sediments, ancient cratonic mantle, and subducted altered mantle complicate the identification of their source using traditional radiogenic isotopes.

A recent study led by Prof. ZENG Zhigang from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS), along with collaborators, has shown that molybdenum (Mo) isotopes can provide new insights into the origin of ghost-arc signatures in MORBs. This research was published in Earth and Planetary Science Letters.

The team analyzed Mo isotopes in fresh basalts from the South Atlantic mid-ocean ridge (15–24°S). They discovered heavy Mo signatures similar to those found in arc basalts influenced by subduction fluids, alongside low (La/Sm)N, Nb/Zr, and Ce/Pb ratios, as well as non-enriched Sr-Nd isotopes. These characteristics cannot be attributed to interactions with recycled sediments, the lithospheric mantle, or the St. Helena plume.

By integrating seismic tomography and plate reconstructions, the researchers linked these geochemical signatures to the subduction-modified mantle beneath the southwestern margin of Gondwana during the Mesozoic era. This mantle component was transported towards the upper mantle beneath Gondwana by a process known as supersized flat subduction and ultimately served as the source for the studied MORBs.

"This study not only provides key evidence for the role of the ancient subduction-modified mantle in shaping the current heterogeneity of MORB mantle but also offers a new perspective on the origin of ghost-arc signatures in global MORBs," said Dr. CHEN, the first author of the study.

Fig. 1 Global Mo isotopic compositions of mid-ocean ridge and arc basalts, and seismic tomography of the deep structure beneath the St. Helena plume–South Atlantic ridge system. (Image by IOCAS)

Fig. 2 Conceptual model for the origin of arc-like heavy Mo isotope signals in South Atlantic MORBs. (Image by IOCAS)