1:45 PM - 3:15 PM
[HRE11-P09] Discovery of serpentinite-bearing conglomerate outcrops consisted by magnesium silicate hydrate (M-S-H) in Yusuhara, Kochi, Japan
Keywords:Magnesium silicate hydrate, serpentinite-bearing conglomerate, serpentinite
Magnesium silicate hydrate cement is interesting these days because that is used in important conditions such as CO2 storage reservoirs and disposal of radioactive waste in underground repositories (eg; Nied et al., 2016). Therefore, several papers about synthetic M-S-H formation are reported (e.g.; Brew and Glasser, 2005; Nied et al., 2016). In nature, the conglomerate that is supported by M-S-H has been reported only reported in Norway as Tillite sediment. Kato (2021) reported that a part of the matrix in included serpentinite conglomerate is Deweylite (Coarse mixed materials that are low-crystallized talc and serpentine).
The purpose of this abstract is to presentation on new outcrops of conglomerate that are supported by M-S-H in western Shikoku and to constrain geologically the formation of M-S-H conglomerate.
The study area is in the Ochimen area, Yusuhara town, Kochi prefecture. This area is located at the southern foot of the Shikoku Karst, and a water system from the karst is well-developed. The geologic unit in this area is Northern Chichibu Unit. In this area, serpentinite bodies are observed near the southern boundary of the Onogahara Unit (Permian accretionary complex), the middle area of the Shimagawa Unit (Triassic metamorphic rocks), and the boundary of Cretaceous sedimentary rocks.
In this study area, we discovered six outcrops of serpentinite-bearing conglomerate formed by M-S-H. All these outcrops show the unconformity of conglomerate overlaying serpentinite. In other words, the distribution of the conglomerate is constrained within the serpentinite area. By microscopic observation of thin sections and compositional image, quartz and feldspar are confirmed as clastic materials in the M-S-H matrix and their solved structure was observed. Chemical composition analysis of the M-S-H matrix using EPMA showed that it is mainly composed of SiO2 and MgO, with some trace amounts of Al2O3, FeO, and MnO. The mean and median value Si/Mg ratios of the M-S-H matrix are 1.265 and 1.137. In the XRD analysis, the M-S-H matrix separated from the conglomerate with as few impurities as possible was analyzed as a bulk sample. The major peaks are quartz, feldspars, serpentines, muscovite, and illite. And a broad halo pattern was also observed. Water samples were taken from the Shikoku Karst and the study area, and preliminary water quality analysis was conducted using pH meter, EC meter, and ICP_OES. As a result, the pH and Mg values are relatively higher than those in the surrounding area at the intake point where the flow was low near the unconformity outcrop.
Based on the above, we discuss the present model of this conglomerate formation regarding Ruiter and Austrheim (2018) and others. After the formation of this area, cliff cone sediments are deposited on the serpentinite. In that stage, the Si component is supplied from the geological body in the hinterland and the Mg component is supplied from the serpentinite. Subsequently, the Mg component concentrates due to the reduction of water by evaporation, etc. Thus a high Mg / high pH environment is formed. In this high Mg/high pH environment, quartz and feldspar in the cliff cone sediments melt, and the supply of Si components is accelerated. This Si component and the Mg component from serpentinite form M-S-H, and conglomeration is considered to have been hastened.
Likely, conglomerates based on M-S-H are widely observed in the serpentinite area, including unconsolidated cliff cone sediments. Therefore, we would like to further comprehend the distribution of outcrops to elucidate the process of M-S-H formation.
The purpose of this abstract is to presentation on new outcrops of conglomerate that are supported by M-S-H in western Shikoku and to constrain geologically the formation of M-S-H conglomerate.
The study area is in the Ochimen area, Yusuhara town, Kochi prefecture. This area is located at the southern foot of the Shikoku Karst, and a water system from the karst is well-developed. The geologic unit in this area is Northern Chichibu Unit. In this area, serpentinite bodies are observed near the southern boundary of the Onogahara Unit (Permian accretionary complex), the middle area of the Shimagawa Unit (Triassic metamorphic rocks), and the boundary of Cretaceous sedimentary rocks.
In this study area, we discovered six outcrops of serpentinite-bearing conglomerate formed by M-S-H. All these outcrops show the unconformity of conglomerate overlaying serpentinite. In other words, the distribution of the conglomerate is constrained within the serpentinite area. By microscopic observation of thin sections and compositional image, quartz and feldspar are confirmed as clastic materials in the M-S-H matrix and their solved structure was observed. Chemical composition analysis of the M-S-H matrix using EPMA showed that it is mainly composed of SiO2 and MgO, with some trace amounts of Al2O3, FeO, and MnO. The mean and median value Si/Mg ratios of the M-S-H matrix are 1.265 and 1.137. In the XRD analysis, the M-S-H matrix separated from the conglomerate with as few impurities as possible was analyzed as a bulk sample. The major peaks are quartz, feldspars, serpentines, muscovite, and illite. And a broad halo pattern was also observed. Water samples were taken from the Shikoku Karst and the study area, and preliminary water quality analysis was conducted using pH meter, EC meter, and ICP_OES. As a result, the pH and Mg values are relatively higher than those in the surrounding area at the intake point where the flow was low near the unconformity outcrop.
Based on the above, we discuss the present model of this conglomerate formation regarding Ruiter and Austrheim (2018) and others. After the formation of this area, cliff cone sediments are deposited on the serpentinite. In that stage, the Si component is supplied from the geological body in the hinterland and the Mg component is supplied from the serpentinite. Subsequently, the Mg component concentrates due to the reduction of water by evaporation, etc. Thus a high Mg / high pH environment is formed. In this high Mg/high pH environment, quartz and feldspar in the cliff cone sediments melt, and the supply of Si components is accelerated. This Si component and the Mg component from serpentinite form M-S-H, and conglomeration is considered to have been hastened.
Likely, conglomerates based on M-S-H are widely observed in the serpentinite area, including unconsolidated cliff cone sediments. Therefore, we would like to further comprehend the distribution of outcrops to elucidate the process of M-S-H formation.