Study on the Origin of Near-Ridge Seamount Chains in the South Pacific

Academic Background

Seamount chains in the ocean have long been considered products of mantle plume activity, such as the famous Hawaiian-Emperor seamount chain. However, most seamounts in the Pacific Ocean do not exhibit age progression and are relatively small in volume, suggesting that they may not be formed by mantle plumes. Recent studies have shown that only about 18% of intraplate volcanism in the Pacific is related to mantle plumes. Therefore, scientists have begun to explore other possible mechanisms to explain the formation of these non-plume seamount chains.

The Pukapuka Ridge (PPR) in the South Pacific is a typical near-ridge seamount chain, and its formation mechanism remains unclear. This seamount chain is located in the South Pacific Superswell (SPS), extending approximately 3,000 kilometers from north of the Tuamotu Plateau to the East Pacific Rise (EPR). The volcanic activity of the Pukapuka Ridge ranges from about 27.5 million years ago to 5.6 million years ago, and the age progression is not linear, indicating that it was not formed by a fixed mantle plume. Therefore, studying the origin of the Pukapuka Ridge is of great significance for understanding the formation mechanisms of non-plume seamount chains.

Source of the Paper

This paper was co-authored by Pengyuan Guo, Fangyu Shen, Dongyong Li, and other scientists from the Institute of Oceanology of the Chinese Academy of Sciences, the Pilot National Laboratory for Marine Science and Technology (Qingdao), and Ocean University of China. The paper was published online on October 3, 2024, in the journal Geology, titled “Ridgeward flow of compositionally heterogeneous mantle produces near-ridge seamount chains in the South Pacific.” The research was funded by the National Natural Science Foundation of China and the Science and Technology Department of Shandong Province.

Research Process and Results

Research Process

1. Sample Collection and Analysis
   The research team conducted detailed geochemical analyses of basalt samples from the Pukapuka Ridge and the East Pacific Rise (EPR) segment between 13°S and 23°S. The samples included nine basalt samples from the Pukapuka Ridge and 29 mid-ocean ridge basalt (MORB) samples from the EPR. Additionally, seven basalt samples from the Garrett Transform Fault (GTF) were analyzed.

2. Geochemical Analysis
   The research team analyzed the samples for major elements, trace elements, Sr-Nd-Pb-Hf radiogenic isotopes, and Fe stable isotopes. The Fe isotope analysis was a highlight of this study, as Fe is a major element in mantle minerals (such as olivine, clinopyroxene, and orthopyroxene), and its isotopic fractionation is limited during major melting processes. Therefore, variations in Fe isotopes can reflect contributions from different mantle lithologies.

3. Data Processing and Model Construction
   By comparing the geochemical data of the Pukapuka Ridge and EPR basalts, the research team constructed a mantle flow model to explain the formation mechanism of the Pukapuka Ridge. The model assumes the presence of compositionally heterogeneous materials in the mantle, which flow toward the ridge under ridge suction and undergo decompression melting during the process, forming the seamount chain.

Key Findings

1. Geochemical Variations in the Pukapuka Ridge
   The study found that the basalt samples from the Pukapuka Ridge exhibit significant geochemical variations, with a gradual decrease in enriched components from the East Pacific Rise axis westward. This spatial variation was interpreted as the result of decompression melting of compositionally heterogeneous mantle materials flowing toward the ridge.

2. Fe Isotope Variations
   The basalt samples from the Pukapuka Ridge show higher δ56Fe values (+0.14‰ to +0.26‰), while the MORB samples from the EPR and the basalt samples from the GTF have lower δ56Fe values (+0.05‰ to +0.16‰). These Fe isotope variations further support the presence of compositionally heterogeneous materials in the mantle, which are gradually consumed during the flow process.

3. Mantle Flow Model
   The research team proposed a mantle flow model to explain the formation mechanism of the Pukapuka Ridge. The model suggests that the mantle contains low-melting-point enriched materials (such as metasomatic pyroxenites) and refractory peridotite matrices. Far from the ridge, the enriched materials preferentially melt under thicker lithosphere, forming enriched melts. As the mantle materials flow toward the ridge, the remaining refractory materials undergo further melting under thinner lithosphere, forming progressively depleted melts. This process leads to the geochemical variations observed in the Pukapuka Ridge basalts.

Conclusions and Significance

1. Scientific Value
   This study, through detailed geochemical analysis, reveals the formation mechanism of the Pukapuka Ridge and proposes a mantle flow model to explain the origin of non-plume seamount chains. This discovery not only deepens our understanding of the formation mechanisms of near-ridge seamount chains but also provides new insights into explaining the widespread non-plume seamount chains in the Pacific Ocean.

2. Practical Value
   The results of this study are significant for understanding mantle dynamics, plate tectonics, and volcanic activity near ridges. Additionally, the study provides new geochemical indicators for future marine geological exploration, particularly the application of Fe isotopes, which offers a new tool for identifying different lithologies in the mantle.

Research Highlights

1. Novel Geochemical Analysis Methods
   This study is the first to apply Fe isotope analysis to the study of near-ridge seamount chains, revealing the potential of Fe isotopes in identifying mantle lithologies.

2. Innovative Mantle Flow Model
   The mantle flow model proposed by the research team not only explains the formation mechanism of the Pukapuka Ridge but also provides a new theoretical framework for understanding the origin of other non-plume seamount chains.

3. Important Scientific Discovery
   The study reveals the flow and melting processes of compositionally heterogeneous mantle materials under ridge suction, offering a new perspective for understanding volcanic activity near ridges.

Summary

Through detailed geochemical analysis and an innovative mantle flow model, this study reveals the formation mechanism of the Pukapuka Ridge in the South Pacific, providing new insights into the origin of non-plume seamount chains. This research not only has significant scientific value but also offers new geochemical indicators for future marine geological exploration.