2:00 PM - 2:15 PM
[SGL23-02] Reanalyses of samples of Abukuma granitoids stored at the Geological Survey of Japan, and reexamination of their geochemical characteristics
Keywords:Abukuma granitoids, Archive sample, Technique of bulk chemical analyses, Trace element composition
The huge granitoid bodies distributed in the Japan arc from the inner SW Japan to NE Japan are often accompanied by gabbroic to intermediate plutonic rocks, and genetic relationships between them have attracted attention. The Abukuma granitoid is also accompanied by plutonic rocks with a very wide range of chemical and petrological characteristics, from gabbro to diorite and tonalite.
At the Geological Survey of Japan (GSJ), a group led by Dr. Shunso Ishihara investigated and chemically analyzed granite bodies within Japan from the 1960s through to the 2000s, and published the chemical data. Until the 1970s, whole-rock geochemical analysis was carried out using wet chemical analysis (Ishihara et al 1971, etc.), but in the 1980s X-ray fluorescence (XRF) analyses was introduced and has gradually replaced wet chemical analysis. In this context, around 2000, A polarized X-ray fluorescence (polarized XRF) analysis method has been developed that is effective for measuring trace elements (Heckel and Ryon 2002). And they have reanalyzed plutonic rock samples from various locations for which chemical data had been reported up until the 1970s, and are reporting new data. The rock samples collected by Dr. Ishihara are stored at the Geological Information Infrastructure Center (https://unit.aist.go.jp/gsc/ja/organization/index.html) within the GSJ and are accessible to the public. Meanwhile, chemical analysis has continued to advance since then, with trace element analysis being replaced by ICP-MS since the 2010s, and the introduction of LA-ICP-MS is also progressing in the 2020s. Amidst these advances in chemical analysis technology, this report focuses once again on the samples collected by Dr. Ishihara, and performs chemical analysis using the latest analytical techniques such as LA-ICP-MS to reexamine their chemical characteristics.
This time, the focus was on the Abukuma granitoids distributed in the Abukuma belt of NE Japan. Dr. Ishihara's chemical study of the Abukuma granitoids was published in the “Bulletin of the Geological Survey of Japan” in 1973. This research also has an aspect of uranium resource exploration, and is notable for analyzing U and Th in addition to the major elements. Subsequently, polarized XRF was introduced and new analytical values were reported in the “Bulletin of the Geological Survey of Japan” in 2003. Both reports used a series of groups with sample numbers beginning with "68A", with sampling points distributed between 37°00' N and 37°10' N, which corresponds to the southern part of the Abukuma belt. The analysis reported here is for the group with sample numbers beginning with "72A", with sampling points distributed between 37°20′N and 37°30′N, north of group 68A. Chemical analysis was performed using XRF at the Earthquake Research Institute, the University of Tokyo and LA-ICP-MS at the Faculty of Science, the University of Tokyo. The analysis results were very similar to those of the 68A group rocks reported in both of Dr. Ishihara's papers, and the tendency for radioactive element concentrations to be higher toward the east, as noted in both papers, was also observed. In addition, based on this new analytical data, we would like to examine the chemical characteristics of the Abukuma granitoids.
The samples used in this study were provided by Dr. Tomoaki Sumii and Dr. Norio Yanagisawa of GSJ, AIST. We would like to express our gratitude.
At the Geological Survey of Japan (GSJ), a group led by Dr. Shunso Ishihara investigated and chemically analyzed granite bodies within Japan from the 1960s through to the 2000s, and published the chemical data. Until the 1970s, whole-rock geochemical analysis was carried out using wet chemical analysis (Ishihara et al 1971, etc.), but in the 1980s X-ray fluorescence (XRF) analyses was introduced and has gradually replaced wet chemical analysis. In this context, around 2000, A polarized X-ray fluorescence (polarized XRF) analysis method has been developed that is effective for measuring trace elements (Heckel and Ryon 2002). And they have reanalyzed plutonic rock samples from various locations for which chemical data had been reported up until the 1970s, and are reporting new data. The rock samples collected by Dr. Ishihara are stored at the Geological Information Infrastructure Center (https://unit.aist.go.jp/gsc/ja/organization/index.html) within the GSJ and are accessible to the public. Meanwhile, chemical analysis has continued to advance since then, with trace element analysis being replaced by ICP-MS since the 2010s, and the introduction of LA-ICP-MS is also progressing in the 2020s. Amidst these advances in chemical analysis technology, this report focuses once again on the samples collected by Dr. Ishihara, and performs chemical analysis using the latest analytical techniques such as LA-ICP-MS to reexamine their chemical characteristics.
This time, the focus was on the Abukuma granitoids distributed in the Abukuma belt of NE Japan. Dr. Ishihara's chemical study of the Abukuma granitoids was published in the “Bulletin of the Geological Survey of Japan” in 1973. This research also has an aspect of uranium resource exploration, and is notable for analyzing U and Th in addition to the major elements. Subsequently, polarized XRF was introduced and new analytical values were reported in the “Bulletin of the Geological Survey of Japan” in 2003. Both reports used a series of groups with sample numbers beginning with "68A", with sampling points distributed between 37°00' N and 37°10' N, which corresponds to the southern part of the Abukuma belt. The analysis reported here is for the group with sample numbers beginning with "72A", with sampling points distributed between 37°20′N and 37°30′N, north of group 68A. Chemical analysis was performed using XRF at the Earthquake Research Institute, the University of Tokyo and LA-ICP-MS at the Faculty of Science, the University of Tokyo. The analysis results were very similar to those of the 68A group rocks reported in both of Dr. Ishihara's papers, and the tendency for radioactive element concentrations to be higher toward the east, as noted in both papers, was also observed. In addition, based on this new analytical data, we would like to examine the chemical characteristics of the Abukuma granitoids.
The samples used in this study were provided by Dr. Tomoaki Sumii and Dr. Norio Yanagisawa of GSJ, AIST. We would like to express our gratitude.