JpGU-AGU Joint Meeting 2017

講演情報

[EE]Eveningポスター発表

セッション記号 S (固体地球科学) » S-IT 地球内部科学・地球惑星テクトニクス

[S-IT26] [EE] Fluid-mediated processes and properties near convergent plate boundaries

2017年5月20日(土) 17:15 〜 18:30 ポスター会場 (国際展示場 7ホール)

[SIT26-P01] Noble gas and major element composition of deep groundwater in the fore-arc region of southwest Japan; widespread distribution of fluids dehydrated from the Philippine Sea Plate

*森川 徳敏1風早 康平1高橋 正明1稲村 明彦1高橋 浩1大和田 道子1安原 正也1,2佐藤 努1 (1.産業技術総合研究所 活断層・火山研究部門、2.立正大学)

キーワード:noble gas, groundwater, Philippine Sea Plate, helium, Kii Peninsula

Chemical and isotopic studies including analyses of noble gases were conducted on the groundwater in the fore-arc region of southwest Japan (Kii Peninsula and Shikoku Island) where the Philippine Sea Plate is subducting. High 3He/4He ratios relative to the atmospheric value (up to 6.7 Ra) were observed throughout the studied area, covering a wider area than documented previously. From the wide distribution of high 3He/4He values and the associated 20Ne and Cl concentrations, Morikawa et al. (2016) infer that aqueous fluids derived from dehydration of the subducting slab are present at depth beneath the entire peninsula. These aqueous fluids may ascend along the major north-dipping boundary faults. The variety of water types documented may be due to water–gas separation and the subsequent incorporation of gaseous species into shallow meteoric groundwater. The observed high 3He/4He ratios in the absence of a mantle wedge below the southern part of the Kii Peninsula may reflect the oblique ascent of these fluids along north-dipping boundary faults.
As already reported by Dogan et al. (2006) and Umeda et al (2006), moderately high-3He/4He groundwater has been observed on Shikoku Island, west of the Kii Peninsula, although no sampling point exceeded 4 Ra. By analogy with the Kii Peninsula, incorporation of the ascending fluids along faults accounts for the groundwater of this area, but this moderately high 3He/4He ratio (< 4 Ra) is possible to relate the slab configuration. The depth of the slab surface is relatively shallow, and therefore the thickness of the mantle wedge beneath Shikoku Island is less than that below the Kii Peninsula, resulting in a low mantle-He budget around the pathway of the fluids.

References: Dogan et al. (2006) Chem. Geol., 233, 235-248, Morikawa et al. (2016) Geochim. Cosmochim. Acta, 182, 173-196, Umeda et al. (2006) Geochem. Geophys. Geosyst. 7, Q04009, doi:10.1029/2005GC001210