Clay Mineral Suites in Submarine Mud Volcanoes in the Kumano Forearc Basin, Nankai Trough: Constraints on the Origin of Mud Volcano Sediments

Akira Ijiri,Fumio Inagaki,Juichiro Ashi,Urumu Tsunogai,Koichi Iijima

doi:10.3390/geosciences8060220

Akira Ijiri, Fumio Inagaki + Show 3 more

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https://doi.org/10.3390/geosciences8060220

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Abstract

Clay mineralogy is an important characteristic of mud volcano sediments. This study determined the clay mineral compositions of sediment from two submarine mud volcanoes in the Kumano forearc basin, Nankai Trough, by X-ray diffraction analysis. Similar compositions dominated by smectite in the two mud volcanoes indicate that the mud volcanoes in the basin are rooted in the same source sequence. These clay mineral compositions differed from those in Pleistocene basin sediment, suggesting that the mud volcano sediment originated beneath the Pleistocene sediment. The illite content in the illite–smectite mixed layer averaged 32% in the mud volcano sediment, which implies that the sediment experienced temperatures above 60 °C that promoted the smectite-to-illite transformation. However, porewater extracted from the mud volcano sediment had Cl‒ concentrations roughly half that of seawater and proportional enrichment in 18O and depletion in D, indicating that dehydration reactions of clay minerals had previously occurred in a deeply buried sedimentary layer. The smectite and illite contents (<60%) in the clay-size fraction rule out in situ smectite dewatering as the cause of the dilution of Cl‒ in porewater. Thus, fluids derived from clay dewatering must have originated from deeper than the source of the mud volcano sediment.

Highlights

Submarine mud volcanoes are remarkable seafloor features that consist of mud breccia derived from sediment layers beneath the seafloor [1]
The similar compositions in two mud volcanoes in the Kumano basin indicate that these mud compositions in two mud volcanoes in the Kumano basin indicate that these mud volcanoes in the volcanoes in the basin are rooted in the same source sequence
Pleistocene basin sediment at Integrated Ocean Drilling Program Site C0002, in the southern part of basin sediment at Integrated Ocean Drilling Program Site C0002, in the southern part of the Kumano the Kumano basin, suggesting that the mud volcano sediment originated beneath the Pleistocene basin, suggesting that the mud volcano sediment originated beneath the Pleistocene sediment

Summary

Introduction

Submarine mud volcanoes are remarkable seafloor features that consist of mud breccia derived from sediment layers beneath the seafloor [1]. They form by expulsions of mud and associated deep-sourced fluids, predominantly methane gas [2,3]. The fluids contain hydrocarbon gases derived from the thermocatalytic decomposition of organic matter (e.g., [6,7,8]). These characteristics suggest that the fluids originate at depths where temperatures are high enough to promote these reactions. There are fewer lithological studies that include chemical and mineralogical analyses of mud volcano sediments than there are studies of mud volcano fluids (e.g., [9,10,11]),

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