The Abukuma plateau, which is located along the Pacific coast of northeastern Japan, is composed mainly of Cretaceous granitic rocks and regional metamorphic rocks. In the central part of the plateau, the Gosaisho metamorphic rock series in the east overthrust onto the Takanuki metamorphic rock series in the west. The Gosaisho series is mostly composed of mafic and siliceous rocks, and the Takanuki series is mainly composed of pelitic-psammitic rocks. In the Gosaisho series, many small ultramafic bodies are present in the areas adjacent to the Takanuki series. These ultramafic rocks are affected by contact metamorphism of the Cretaceous granitic rocks in various degrees, but their protoliths are discerned as mantle peridotites and ultramafic cumulates based on their bulk rock chemistry. The ultramafic cumulates are accompanied by metagabbros. At Mount Otsube, mantle peridotites occupy the foot of the mountain and cumulates comprise its top part. The rock assemblage and geological relationship indicates that the ultramafic bodies in this area are fragments of the lower part of an ophiolite. Bulk rock chemistry of the mantle peridotite as well as its olivine and spinel chemistry suggest that the mantle peridotite is highly depleted and of island arc origin. In view of the spinel chemistry, the ultramafic rocks might be independent of the Gosaisho-Takanuki metamorphic rocks and be fragments of the early Paleozoic Hayachine-Miyamori ophiolite. However, the ultramafic cumulates are much richer in Fe (~ Fo70) than those in other ophiolites in Japan. This dismembered ophiolite, which we call Furudono ophiolite, may be a new type of the Paleozoic ophiolites in Japan.
Special Issue “Development of researches on rock-water interaction and Professor Naotatsu Shikazono” Part 1
This Special issue is devoted to the late professor Naotatsu Shikazono (1946-2014) for his great contribution to the development of the researches on rock-water interaction. The issue includes five review papers covering wide topics concerning rock-water interaction. They are Lithium resources, global carbon cycle modeling, reactive transport simulation, clay mineral-water systems and pottery stone deposits. The authors are colleagues or disciples of Professor Shikazono. The papers will signify his important role to the development of researches on rock-water interaction.
Lithium, one of the ‘rare metals’ defined by Japanese government, is industrially important, and Li compounds are used for many purposes (e.g., Li-ion batteries). The major types of lithium deposits are (1) brine, (2) pegmatite, and (3) sedimentary deposits. Because of the low production costs for Li in brine deposits, they account for approximately 60% of identified worldwide Li resources and for approximately 70% of worldwide Li production. Recent increases in Li production, an expected high demand for its use in eco-friendly cars, and uneven distribution of Li-producing countries underline the importance of maintaining a stable Li supply. Therefore, more brine deposits should be exploited, and the development of other types of Li deposits should be explored. Lithium carbonate is extracted from brine deposits in playas and salt crusts by exploiting the solubility differences of different ionic compounds. Li-rich brine deposits probably form by orographic/topographic effects and by local hydrothermal activity, because Li is a fluid-mobile element and its elution from solids into fluids is temperature dependent. Lithium-pegmatite deposits probably form by intermittent intrusions of pegmatite magma in which Li has become concentrated by the addition of Li-rich differentiates from felsic magma, because Li is a moderately incompatible element. Sedimentary-type Li deposits, which are composed of hectorite and jadarite, are still relatively undeveloped, but they are attracting a great deal of attention as possible new Li resources. In addition to these sources, methods to extract Li from seawater and to reclaim Li by urban mining of discarded products have also been examined. Lithium isotope analysis is a powerful tool for tracing water-rock interactions and for investigating various geochemical and geological processes. Moreover, the origin of Li and the history of Li accumulation in Li deposits can often be determined from its isotopic signature.
The global carbon cycle on a geological age consists of an inorganic carbon cycle (continental weathering and metamorphism-volcanism) and organic carbon cycle (oxidative weathering of organic carbon and organic carbon burial). The GEOCARB, one of the global carbon cycle models, calculates these geochemical carbon fluxes and atmospheric CO2 level in the period of Phanerozoic. Important parameters in the GEOCARB are those for the continental uplift, river runoff, evolution of vascular plants, weathering feedback, and CO2 degassings. Seawater strontium isotope ratio and sedimentation rate of terrigenous sediments can be used to estimate the continental uplift parameter. Regarding the vascular plant, more quantitative studies are necessary to elucidate its effect on the global carbon cycle. Concerning the degassing parameter, not only subduction volcanism but also hot spot volcanism and igneous activity in back arc basin should be considered. Runoff is dependent on continental positions and terrestrial temperature, but the interrelationship among them is not fully considered in the GEOCARB. In this respect, another type of the model, namely, the GEOCLIM, may be more appropriate. Volcanic rock weathering and climate sensitivity are also crucial in the global carbon cycle. Volcanic rock weathering might have controlled the atmospheric CO2 level in the Phanerozoic, although the value of the weathering flux has not been constrained. Regarding the climate sensitivity, a short-term feedback (Charney feedback) has been assumed in the GEOCARB. However, a long-term climate feedback, namely, the Earth System Sensitivity (ESS), should be incorporated. According to the recent version of GEOCARB (GEOCARBSULF), most influential parameters on the atmospheric CO2 are those for the climate sensitivity and vascular plants. In this model, atmospheric CO2 level in the Cenozoic is not well consistent with the results of geochemical proxies. This may be due to the insufficient estimation of degassing parameters, and/or a change in the climate sensitivity accompanied with formation of continental ice sheets in the Cenozoic.
Reactive transport simulation becomes essential tool for quantitative models of the reaction between flowing water and rock to predict the future evolution. It has been used in many investigations relevant to geothermal systems, hydrothermal ore deposits, CO2 sequestration, high level waste disposal, and groundwater pollution. Emergence of this method as a powerful tool in these years has a long background history which incorporates the progress in many disciplines. This article presents the historical review of the thermodynamic datasets, geochemical simulation codes, and reactive transport simulation codes and shows state-of-the-art of the selected reactive simulation codes. Limitations and future subjects of this method are also mentioned.
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April 11, 2015
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