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[HDS07-P11] Differential weathering mechanism accompanying the formation of halloysite-rich layer in the Quaternary volcanic fall deposits
Keywords:Halloysite, Iron, Differential weathering, Redox condition
Halloysite is one of the weathering products in the volcanic region. The secondary mineral could be the cause of slope failures because the halloysite-including soil layer could be the sliding surface during heavy rain and earthquake. Many studies have been conducted in terms of crystallographic aspect and synthesis experiment of halloysite in vitro scale, but few studies were conducted in which weathering processes have been discussed by taking into account hydrological processes in the real weathering profile. The aim of our study is to clarify the formation mechanism of halloysite-rich layer in the region which experienced earthquake-induced landslide, with a special focus on the behavior of iron in the weathering processes.
We divided a tephra, Tarumae volcanic fall deposit-d2 (Ta-d2) which was erupted from Tarumae volcano 30 km east from the study site at 9 ka, into the 3 weathering conditions. White weathered pumice (WP) contained much larger amount of halloysite than the other two with tri-dimensionally concave upward cone-like shape. Reddish weathered pumice (RP) distributes with the vertically downward tongue-like morphology, the pumice contained no halloysite, but did larger amount of iron oxides such as goethite and ferrihydrite, amorphous secondary mineral like allophane and soil organic carbon. Greyish weathered pumice (GP), which was located inside of the cone-like WP, contained the largest amount of silicon and small but detectable amount of halloysite.
The framework element, Si, Al and Fe, in soil water collected by centrifuge separation (10000 rpm = pF 4.3) presented the highest concentration in the GP and a bit lower concentration in the RP, but Si concentrations in almost all the solution were higher than 10 ppm and Al concentrations were higher than 10 ppb. All the solution composition plotted on the stability diagram in Si-Al system satisfied the condition of halloysite stability field. Although the pore water in the RP and WP contained very few amount of iron, that in the GP represented much higher concentration of iron that might have exist as divalent cation easy to dissolve compared to the trivalent state.
Mossbauer spectroscopic analysis of solid powder samples showed that the contents of ferrous iron decreased in the order, GP > RP >WP. In addition, the lowest amount of ferrous iron were found in the 2μfraction separated from WP samples. However, even though the RP and WP contained the same oxidized form of iron, ferric iron, the values of QS, quadrupole splitting, showed that the bonding states of iron were completely different, implying that although both of RP and WP have been under oxidizing condition, those weathering processes would be very different when the GP have altered into RP or WP. The mechanism of different processes can be explained from the hydrological aspect.
A series of hydraulic experiments represented that the WP were much lower conductivity and higher water-holding capacity than the RP. It is well known that iron takes divalent anion under submerged condition like paddy field, because microbial respiration consumes dissolved oxygen during the decomposition of organic matter, in turn leading to reducing condition. In our study, because the residence time of subsurface water were much longer, 102~103 sec/cm, in the WP than RP, total microbial respiration was larger in the WP compared to RP, leading to the reducing condition in the WP domain. On the other hand, because the RP showed much shorter residence time of water, oxidizing condition have been dominant in the RP domain. The hydrologically-controlled redox discrepancy mediated by microbial activity would be the driving force for the different alteration of each weathering conditions.
We divided a tephra, Tarumae volcanic fall deposit-d2 (Ta-d2) which was erupted from Tarumae volcano 30 km east from the study site at 9 ka, into the 3 weathering conditions. White weathered pumice (WP) contained much larger amount of halloysite than the other two with tri-dimensionally concave upward cone-like shape. Reddish weathered pumice (RP) distributes with the vertically downward tongue-like morphology, the pumice contained no halloysite, but did larger amount of iron oxides such as goethite and ferrihydrite, amorphous secondary mineral like allophane and soil organic carbon. Greyish weathered pumice (GP), which was located inside of the cone-like WP, contained the largest amount of silicon and small but detectable amount of halloysite.
The framework element, Si, Al and Fe, in soil water collected by centrifuge separation (10000 rpm = pF 4.3) presented the highest concentration in the GP and a bit lower concentration in the RP, but Si concentrations in almost all the solution were higher than 10 ppm and Al concentrations were higher than 10 ppb. All the solution composition plotted on the stability diagram in Si-Al system satisfied the condition of halloysite stability field. Although the pore water in the RP and WP contained very few amount of iron, that in the GP represented much higher concentration of iron that might have exist as divalent cation easy to dissolve compared to the trivalent state.
Mossbauer spectroscopic analysis of solid powder samples showed that the contents of ferrous iron decreased in the order, GP > RP >WP. In addition, the lowest amount of ferrous iron were found in the 2μfraction separated from WP samples. However, even though the RP and WP contained the same oxidized form of iron, ferric iron, the values of QS, quadrupole splitting, showed that the bonding states of iron were completely different, implying that although both of RP and WP have been under oxidizing condition, those weathering processes would be very different when the GP have altered into RP or WP. The mechanism of different processes can be explained from the hydrological aspect.
A series of hydraulic experiments represented that the WP were much lower conductivity and higher water-holding capacity than the RP. It is well known that iron takes divalent anion under submerged condition like paddy field, because microbial respiration consumes dissolved oxygen during the decomposition of organic matter, in turn leading to reducing condition. In our study, because the residence time of subsurface water were much longer, 102~103 sec/cm, in the WP than RP, total microbial respiration was larger in the WP compared to RP, leading to the reducing condition in the WP domain. On the other hand, because the RP showed much shorter residence time of water, oxidizing condition have been dominant in the RP domain. The hydrologically-controlled redox discrepancy mediated by microbial activity would be the driving force for the different alteration of each weathering conditions.