5:15 PM - 7:15 PM
[HGM04-P18] Relationship between weathering and 10Be ages of granitic rocks in the Sanriku Coast, Japan
Keywords:chemical weathering, granitic rock, 10Be dating, Sanriku, age determination
Chemical weathering occurs primarily through reactions between rock and water and takes place in a wide range of environments. It has been reported that the longer the weathering duration, the thicker the weathering layer (the zone affected by weathering). Based on this, methods for utilizing weathering layer thickness for dating have been explored. However, a standardized method has not been established yet, as the relationship between weathering processes and time varies depending on environmental conditions and rock type. Therefore, this study aims to elucidate the chemical weathering process of granitic rocks and clarify the relationship between chemical weathering and time.
The rock sampling sites are located in Samuraihama -cho, Kuji City, and Sanriku-cho, Ofunato City, Iwate Prefecture, Japan. Previous studies using TCN (Terrestrial in situ Cosmogenic Nuclides) dating have estimated the geomorphic ages of marine and fluvial terraces in this region. In this study, rock samples were collected from both outcrops on terrace surfaces and underground at sites with estimated geomorphic ages of approximately 25 kyr, 96 kyr, and 193 kyr. A thin, discolored layer was consistently observed visually on the surfaces of all collected samples.
In this study, X-ray powder diffraction (XRD) and X-ray fluorescence (XRF) were performed to analyze the petrological characteristics and degree of weathering of the rock samples. The results suggested that the collected rock samples are granodiorite. XRD analysis detected clay minerals in all samples, with particularly high concentration observed in the rocks collected from underground. The weathering index was calculated from the XRF results and related to the 10Be dating results.
To further investigate the degree of weathering at the nanoscale, thin sections were prepared and observed using polarizing microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). Polarizing microscopy observations revealed that weathering extends beyond the discoloration layer, and that the degree of weathering for each mineral tends to weaken from the surface to the interior of the rock. In addition, the SEM analysis provided detailed observations of mineral surface conditions, while SEM-EDS results suggested the possible leaching of certain materials along grain boundaries.
In the future, we aim to establish a dating method based on weathering by conducting a more quantitative analysis of weathering intensity and comparing the results with sample ages. To achieve this, it will be necessary to increase the number of samples and explore quantitative evaluation methods.
The rock sampling sites are located in Samuraihama -cho, Kuji City, and Sanriku-cho, Ofunato City, Iwate Prefecture, Japan. Previous studies using TCN (Terrestrial in situ Cosmogenic Nuclides) dating have estimated the geomorphic ages of marine and fluvial terraces in this region. In this study, rock samples were collected from both outcrops on terrace surfaces and underground at sites with estimated geomorphic ages of approximately 25 kyr, 96 kyr, and 193 kyr. A thin, discolored layer was consistently observed visually on the surfaces of all collected samples.
In this study, X-ray powder diffraction (XRD) and X-ray fluorescence (XRF) were performed to analyze the petrological characteristics and degree of weathering of the rock samples. The results suggested that the collected rock samples are granodiorite. XRD analysis detected clay minerals in all samples, with particularly high concentration observed in the rocks collected from underground. The weathering index was calculated from the XRF results and related to the 10Be dating results.
To further investigate the degree of weathering at the nanoscale, thin sections were prepared and observed using polarizing microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). Polarizing microscopy observations revealed that weathering extends beyond the discoloration layer, and that the degree of weathering for each mineral tends to weaken from the surface to the interior of the rock. In addition, the SEM analysis provided detailed observations of mineral surface conditions, while SEM-EDS results suggested the possible leaching of certain materials along grain boundaries.
In the future, we aim to establish a dating method based on weathering by conducting a more quantitative analysis of weathering intensity and comparing the results with sample ages. To achieve this, it will be necessary to increase the number of samples and explore quantitative evaluation methods.