09:00 〜 10:30
[SVC30-P08] Boron and strontium isotope geochemistry in Hatchobaru-Otake geothermal field, Kyushu, Japan
キーワード：岩石ｰ水反応、流体の混合
Hatchobaru – Otake geothermal field is located in the western part of Kuju volcanic area. Host rock of the geothermal reservoir is divided into three major lithologic groups in descending order; 1) Hohi volcanic rocks (pyroxene andesite lava) 2) Usa group (altered andesite lava flows and tuff breccias) and 3) basement rocks (granite rocks and metamorphic rocks) (Fujino and Yamasaki, 1985). This study presents boron and strontium isotopic compositions of both geothermal fluid and geological samples collected from Hatchobaru-Otake geothermal field. Purpose of this study is to understand water-rock interactions and mixing processes which fluids have experienced, based on isotope geochemistry, as demonstrated in previous studies (e.g. Millot et al., 2007, 2012, and Battistel et al., 2016).
In total, 14 geothermal fluids were collected from production wells in Hatchobaru geothermal power plant and in Otake geothermal power plant. As geological samples, 9 cuttings samples were collected from some drilled wells; 6 samples were andesitic lava (Hohi volcanic rocks and Usa group) and,3 samples were basement rocks (granite rocks and metamorphic rocks).
Strontium isotope ratio of the geological samples showed difference reflecting the lithology. The basement rocks showed 87Sr/86Sr as high as 0.706, whereas the overlaid volcanic rocks (Hohi and Usa groups) showed 87Sr/86Sr in a range from 0.704 to 0.705. The fluid samples from the wells that have a feeder point within the basement rocks showed similarly high 87Sr/86Sr ratio around 0.706. Whereas, fluid samples from shallower wells with the feeder zone within the volcanic layers showed 87Sr/86Sr in a range from 0.704 to 0.705. This good accordance suggests strontium isotope ratio of the geothermal fluid is useful to identify lithology where dominant water-rock interactions occur. In contrast,δ11B values of the geothermal fluids were a narrow range from +2 to -5 per mill, whereas the geological samples showed δ11B value in a wide range from -3 to -20 per mill. Although some previous studies demonstrated that boron isotope ratio in geothermal fluids are controlled by that of host rock (e.g. Millot et al., 2007), boron isotope ratio of the Hatchobaru-Otake geothermal fluids would be attributed controlled by other factors such as magmatic contribution.
Acknowledgement: Analysis of boron and strontium isotope is supported by Japan Organization for Metals and Energy Security (JOGMEC) and we are grateful to Kyushu Electric Power Co., for their permission of presentation.
In total, 14 geothermal fluids were collected from production wells in Hatchobaru geothermal power plant and in Otake geothermal power plant. As geological samples, 9 cuttings samples were collected from some drilled wells; 6 samples were andesitic lava (Hohi volcanic rocks and Usa group) and,3 samples were basement rocks (granite rocks and metamorphic rocks).
Strontium isotope ratio of the geological samples showed difference reflecting the lithology. The basement rocks showed 87Sr/86Sr as high as 0.706, whereas the overlaid volcanic rocks (Hohi and Usa groups) showed 87Sr/86Sr in a range from 0.704 to 0.705. The fluid samples from the wells that have a feeder point within the basement rocks showed similarly high 87Sr/86Sr ratio around 0.706. Whereas, fluid samples from shallower wells with the feeder zone within the volcanic layers showed 87Sr/86Sr in a range from 0.704 to 0.705. This good accordance suggests strontium isotope ratio of the geothermal fluid is useful to identify lithology where dominant water-rock interactions occur. In contrast,δ11B values of the geothermal fluids were a narrow range from +2 to -5 per mill, whereas the geological samples showed δ11B value in a wide range from -3 to -20 per mill. Although some previous studies demonstrated that boron isotope ratio in geothermal fluids are controlled by that of host rock (e.g. Millot et al., 2007), boron isotope ratio of the Hatchobaru-Otake geothermal fluids would be attributed controlled by other factors such as magmatic contribution.
Acknowledgement: Analysis of boron and strontium isotope is supported by Japan Organization for Metals and Energy Security (JOGMEC) and we are grateful to Kyushu Electric Power Co., for their permission of presentation.