17:15 〜 19:15
[HCG21-P02] 千枚岩を母材とする赤色土壌の生成過程－沖縄県国頭郡の白亜系を例に－
キーワード：国頭マージ、風化、土壌生成
Introduction: Studies on rock weathering have mainly focused on igneous rocks such as granite[1][2], demonstrating the loss of plagioclase and K-feldspar under tropical and subtropical climates[3]. However, research on metamorphic rock weathering is limited, and the effects of metamorphic rock structures on weathering remain unclear. The central and northern regions of Okinawa Island consist of steep mountains, hills, and plateaus covered with red-yellow acidic soil, known as "the Kunigami Mahji." The loss of red soil due to erosion has been recognized as an environmental issue[4], prompting studies on its physical and chemical properties[4][5]. However, the transformation of rock into the Kunigami Mahji and its relationship with local and artificial soils remain unexplored. This study aims to clarify the physical, mineralogical, and chemical changes in weathering of the Nago Formation phyllite in Okinawa and examine its relation to the Kunigami Mahji.
Methods: Field investigations were conducted at the Akaasasaki outcrop (Higashi Village) and the Shirahama outcrop (Ōgimi Village), Okinawa Prefecture. We performed outcrop descriptions, physical property tests (only at Akaasasaki), thin-section observations, X-ray diffraction (XRD), and X-ray fluorescence (XRF) analysis to examine changes associated with weathering.
Results: At the Akaasasaki outcrop, early-stage weathering caused brown discoloration along schistosity and fractures, while moderate weathering resulted in reddening. The upper part of the outcrop showed decreased bulk density and increased porosity and moisture content. Kaolinite formed in the early stage, while chlorite and plagioclase disappeared at the moderate stage. XRF analysis indicated a decrease in Na2O and CaO, with no significant changes in other elements.
At the Shirahama outcrop, the phyllite layer was unconformably overlain by a terrace sediment containing clasts of various shapes. The phyllite showed schistosity, whereas the sedimentary layer displayed no preferential orientation and exhibited soil structures near the surface. Muscovite showed lower-order interference colors in thin sections, suggesting clay mineralization. XRD analysis showed broadening and weakening of the 2θ=8.9° muscovite peak, while XRF results indicated leaching of alkali and alkaline earth elements, with a relative increase in Fe and Al. The plot of the A-CN-K diagram[6] suggested that weathering progresses through the loss of Na2O and CaO from plagioclase, followed by K leaching from K-feldspar and muscovite.
Discussion: The weathering of phyllite was confirmed to progress preferentially along cleavage and fractures, indicating its strong dependence on original structural characteristics. Kaolinite formed in the early stage, followed by the disappearance of plagioclase and chlorite, with vermiculite forming subsequently. In the sedimentary layer, goethite and gibbsite were identified, indicating stronger weathering than in the phyllite layer. The loss of K-feldspar and muscovite corresponded to their clay mineralization, confirmed by thin-section observations and XRD peak changes. The A-CN-K diagram clearly reflected the loss of plagioclase and K-feldspar due to weathering, indicating consistency between chemical and mineralogical changes. Previous studies identified mixed-layer chlorite/illite and chlorite-vermiculite intermediate minerals in artificial the Kunigami Mahji soil; however, these were not detected in our samples. This discrepancy may be attributed to differences in parent rock and weathering stages. Further studies are needed to investigate the physical, chemical, and mineralogical variations of the Kunigami Mahji comprehensively.
References: [1] Nesbitt H.W., Markovics G, 1997, Geochimica et Cosmochimica Acta, Vol.61, No.8, 1653-1670. [2] Braga MAS, Paquet H, Begonha A, 2002, Catena, Vol.49, Issues1-2, 41-56. [3] Mei H, Jian X, Zhang W, Fu H, Zhang S, 2020, Catena Vol.203, 105368. [4]Tokashiki Y, Moritaka S, Oya K, 1982, The science bulletin of the College of Agriculture, University of the Ryukyus, No, 29. [5] Onaga K, Yoshinaga A, Tokashiki Y, 1994, Journal of the Agricultural Engineering Society, Japan, Vol 62, No.4, 307-314. [6] Nesbitt H.W., Young G.M., 1982, Nature, Vol. 299, 715-717.
Methods: Field investigations were conducted at the Akaasasaki outcrop (Higashi Village) and the Shirahama outcrop (Ōgimi Village), Okinawa Prefecture. We performed outcrop descriptions, physical property tests (only at Akaasasaki), thin-section observations, X-ray diffraction (XRD), and X-ray fluorescence (XRF) analysis to examine changes associated with weathering.
Results: At the Akaasasaki outcrop, early-stage weathering caused brown discoloration along schistosity and fractures, while moderate weathering resulted in reddening. The upper part of the outcrop showed decreased bulk density and increased porosity and moisture content. Kaolinite formed in the early stage, while chlorite and plagioclase disappeared at the moderate stage. XRF analysis indicated a decrease in Na2O and CaO, with no significant changes in other elements.
At the Shirahama outcrop, the phyllite layer was unconformably overlain by a terrace sediment containing clasts of various shapes. The phyllite showed schistosity, whereas the sedimentary layer displayed no preferential orientation and exhibited soil structures near the surface. Muscovite showed lower-order interference colors in thin sections, suggesting clay mineralization. XRD analysis showed broadening and weakening of the 2θ=8.9° muscovite peak, while XRF results indicated leaching of alkali and alkaline earth elements, with a relative increase in Fe and Al. The plot of the A-CN-K diagram[6] suggested that weathering progresses through the loss of Na2O and CaO from plagioclase, followed by K leaching from K-feldspar and muscovite.
Discussion: The weathering of phyllite was confirmed to progress preferentially along cleavage and fractures, indicating its strong dependence on original structural characteristics. Kaolinite formed in the early stage, followed by the disappearance of plagioclase and chlorite, with vermiculite forming subsequently. In the sedimentary layer, goethite and gibbsite were identified, indicating stronger weathering than in the phyllite layer. The loss of K-feldspar and muscovite corresponded to their clay mineralization, confirmed by thin-section observations and XRD peak changes. The A-CN-K diagram clearly reflected the loss of plagioclase and K-feldspar due to weathering, indicating consistency between chemical and mineralogical changes. Previous studies identified mixed-layer chlorite/illite and chlorite-vermiculite intermediate minerals in artificial the Kunigami Mahji soil; however, these were not detected in our samples. This discrepancy may be attributed to differences in parent rock and weathering stages. Further studies are needed to investigate the physical, chemical, and mineralogical variations of the Kunigami Mahji comprehensively.
References: [1] Nesbitt H.W., Markovics G, 1997, Geochimica et Cosmochimica Acta, Vol.61, No.8, 1653-1670. [2] Braga MAS, Paquet H, Begonha A, 2002, Catena, Vol.49, Issues1-2, 41-56. [3] Mei H, Jian X, Zhang W, Fu H, Zhang S, 2020, Catena Vol.203, 105368. [4]Tokashiki Y, Moritaka S, Oya K, 1982, The science bulletin of the College of Agriculture, University of the Ryukyus, No, 29. [5] Onaga K, Yoshinaga A, Tokashiki Y, 1994, Journal of the Agricultural Engineering Society, Japan, Vol 62, No.4, 307-314. [6] Nesbitt H.W., Young G.M., 1982, Nature, Vol. 299, 715-717.