Abstract
The Kairei (KHF) and Yokoniwa hydrothermal fields (YHF) are hosted in mafic as well as ultramafic rocks distributed at an off-axis volcanic knoll of the Central Indian Ridge. Despite intensive investigations, their geological and geophysical background is still debated. Here, we show the results of near-seafloor magnetic anomaly surveys conducted using a submersible. We investigated the bulk magnetization of the hydrothermally altered zone and the surrounding lava flows and evaluated their intensities compared with previously reported values at axial areas of seafloor spreading environments. The KHF is characterized by low coherence between observed and modeled anomalies and low values of magnetization. This result suggests that magnetic minerals within basaltic lava flows were likely altered by hydrothermal fluid circulation. The variation pattern in the observed magnetic anomalies above the lava flows is in phase with that of the modeled magnetic anomalies for the simple assumption that the magnetization direction is parallel to the geomagnetic field. This result suggests that these lava flows preserve normal magnetic polarity corresponding to the Brunhes Chron. The estimated magnetic-anomaly-derived absolute magnetizations show a reasonable correlation with the natural remanent magnetizations of rock samples collected from the seafloor of the same region; their relationship is consistent with previously reported datasets from the Mariana Trough and Mid-Atlantic Ridge. The estimated magnetization intensity reaches 20 A/m in the study area, which is clearly greater than those of previously reported off-axis areas, suggesting that recent volcanic eruption may have occurred in these off-axis areas. The high magnetization distributions are commonly observed at the bottoms of the western slope from the KHF and YHF. This finding provides new insight into the distribution of highly magnetized lava flows in the off-axis areas and indicates the distribution of recent off-axis volcanic activities, which is potentially linked to the sub-seafloor hydrothermal circulation.
Highlights
Hydrothermal circulation within the oceanic lithosphere is a fundamental process in submarine volcanic systems such as mid-ocean ridges and essentially affects the solidEarth cooling, ocean temperature, and material cycles (e.g., Elderfield and Schultz 1996)
Variation in the observed magnetic anomalies reflects mainly the magnetization distribution of the uppermost crust, which geologically is the accumulation of lava flows and massive lavas covered by thin sediment (Fujii et al 2016a; Honsho et al 2009)
The integrated analysis of the magnetic anomaly and comparison with the rock magnetic properties led to the following conclusions: 1. The Kairei hydrothermal field is associated with a lack of magnetization and it extends several hundred meters toward the western slope
Summary
Hydrothermal circulation within the oceanic lithosphere is a fundamental process in submarine volcanic systems such as mid-ocean ridges and essentially affects the solidEarth cooling, ocean temperature, and material cycles (e.g., Elderfield and Schultz 1996). The Kairei hydrothermal field (KHF), known as the first confirmed active seafloor hydrothermal system of the Central Indian Ridge (CIR) since its discovery in 2000 (Gamo et al 2001), is situated in the southernmost segment of the CIR, namely CIR-S1, near the Rodrigues ridge–ridge–ridge triple junction (Fig. 1). This hydrothermal system is likely related to water–rock reactions of mafic as well as ultramafic host rocks because high H2 and CH4 gas content of. The YHF has been suggested to be an inactive mature hydrothermal system hosted by highly serpentinized ultramafic rocks based on near-seafloor magnetic anomalies and rock magnetic studies as well as visual observation of diminishing low-temperature venting and small dead chimneys (Fujii et al 2016a, b)
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