17:15 〜 18:30
[SCG44-P09] 現生蒸発および熱水環境におけるホウ酸濃集プロセス：前生物界におけるホウ素濃集環境形成への制約
Boron-rich environments are favored places for the evolution of prebiotic nucleotides. Modern evaporitic borate basins are often considered as modern analogs of ancient boron-rich environments. Despite this, the (1) source of highly concentrated boron, (2) factors in accumulation of boron, and (3) post-deposition modification of initial boron-bearing minerals in modern evaporitic borate basins are not yet decided.
In order to approach the above problems, the borate mine in Boron, California (= ‘Boron mine’, hereafter), and boron-enriched (up to 980 ppm) hot spring waters of Shin-appi hot spring, Iwate prefecture were studied. The Boron mine in California is one of the biggest boron-ore mines in the world. A huge borax occurs together with Miocene lake sediments. At Shin-appi hot spring in Iwate prefecture, which has the highest boron concentration in the region, aragonite and calcite precipitates rapidly around the discharging areas. Geochemical and mineralogical studies were performed on samples from Boron mine and Shin-appi hot spring which include mineral precipitate and water samples.
Shin-appi hot spring was abnormally enriched in bicarbonate, sodium, LILEs, REEs and radioactive elements. Measured d¹³C values of dissolved inorganic carbon ranged from -5 to -3 per mil. Such geochemical characteristics suggest that the source of boron-rich waters at Ninohe are deep magmatic brines evolved from felsic plutons. No boron-containing minerals were found in the precipitates, although the chemistry of the water seemingly allows for the formation of tourmalines and other boro-silicates. Instead, borax was found precipitating from an actively evaporating area with minor halite. This suggests the difficulty to precipitate boron-bearing minerals in aquatic environments, even with the high availability of boron.
Chemical analyses of borax and kernite samples from the Boron mine indicated that both were enriched in water-mobile heavy elements. These heavy metals were likely supplied by hydrothermal activity through the constraining faults of the mine. Magmatic brines formed by ancient felsic magma were considered as a source of boron-rich hydrothermal fluids at Boron mine, the same as in Shin-appi hot spring. Ancient boron-rich hydrothermal fluids were discharged into the evaporitic sedimentary basin at the Boron mine, where boron minerals initially precipitated with other chemical and clastic components. The texture of borax and kernite indicate that initial boron minerals were dissolved and reprecipitate as borax and kernite during later diagenesis, by separating other chemical and clastic components. Such later processes could further concentrate boron in a specific environment.
From the above, we found that evaporitic borate deposits may require (1) brine evolved from felsic plutons, (2) rapid separation of components other than borate, and (3) reprecipitation by diagenesis. This environment may have occurred on the surface of the proto-arc on the early Earth, which was likely ideal for the evolution of prebiotic nucleotides.
In order to approach the above problems, the borate mine in Boron, California (= ‘Boron mine’, hereafter), and boron-enriched (up to 980 ppm) hot spring waters of Shin-appi hot spring, Iwate prefecture were studied. The Boron mine in California is one of the biggest boron-ore mines in the world. A huge borax occurs together with Miocene lake sediments. At Shin-appi hot spring in Iwate prefecture, which has the highest boron concentration in the region, aragonite and calcite precipitates rapidly around the discharging areas. Geochemical and mineralogical studies were performed on samples from Boron mine and Shin-appi hot spring which include mineral precipitate and water samples.
Shin-appi hot spring was abnormally enriched in bicarbonate, sodium, LILEs, REEs and radioactive elements. Measured d¹³C values of dissolved inorganic carbon ranged from -5 to -3 per mil. Such geochemical characteristics suggest that the source of boron-rich waters at Ninohe are deep magmatic brines evolved from felsic plutons. No boron-containing minerals were found in the precipitates, although the chemistry of the water seemingly allows for the formation of tourmalines and other boro-silicates. Instead, borax was found precipitating from an actively evaporating area with minor halite. This suggests the difficulty to precipitate boron-bearing minerals in aquatic environments, even with the high availability of boron.
Chemical analyses of borax and kernite samples from the Boron mine indicated that both were enriched in water-mobile heavy elements. These heavy metals were likely supplied by hydrothermal activity through the constraining faults of the mine. Magmatic brines formed by ancient felsic magma were considered as a source of boron-rich hydrothermal fluids at Boron mine, the same as in Shin-appi hot spring. Ancient boron-rich hydrothermal fluids were discharged into the evaporitic sedimentary basin at the Boron mine, where boron minerals initially precipitated with other chemical and clastic components. The texture of borax and kernite indicate that initial boron minerals were dissolved and reprecipitate as borax and kernite during later diagenesis, by separating other chemical and clastic components. Such later processes could further concentrate boron in a specific environment.
From the above, we found that evaporitic borate deposits may require (1) brine evolved from felsic plutons, (2) rapid separation of components other than borate, and (3) reprecipitation by diagenesis. This environment may have occurred on the surface of the proto-arc on the early Earth, which was likely ideal for the evolution of prebiotic nucleotides.