Noritoshi Morikawa

Arima hot spring waters as a deep-seated brine from subducting slab

Non-volcanic hot springs are generally believed to originate through circulation of meteoric or buried sea water heated at depth. In this study, we report the geochemical characteristics of the Arima and Takarazuka hot spring waters, known as Arima-type deep brine, in a forearc region of southwestern Japan. We examine 14 water samples to determine the levels of 12 solute elements or components and the isotopic ratios of H, He, C, O, and Sr, and we perform correlation analysis of the data to deduce the source materials and origin of the deep brine. Moreover, we perform numerical modeling of oxygen and hydrogen isotopic fractionation along subducting slabs to examine the composition of slab-derived fluid as a possible candidate of the deep brine. The results suggest that the high salinity and solute concentrations with characteristic oxygen, hydrogen, carbon, and strontium isotope compositions, as well as high 3He/4He ratios, can be explained by a dehydrated component of the subducted Philippine Sea slab. Hence, this study may provide an invaluable understanding of geofluid processes over a significant depth range.

Biogeochemical Signals from Deep Microbial Life in Terrestrial Crust

In contrast to the deep subseafloor biosphere, a volumetrically vast and stable habitat for microbial life in the terrestrial crust remains poorly explored. For the long-term sustainability of a crustal biome, high-energy fluxes derived from hydrothermal circulation and water radiolysis in uranium-enriched rocks are seemingly essential. However, the crustal habitability depending on a low supply of energy is unknown. We present multi-isotopic evidence of microbially mediated sulfate reduction in a granitic aquifer, a representative of the terrestrial crust habitat. Deep meteoric groundwater was collected from underground boreholes drilled into Cretaceous Toki granite (central Japan). A large sulfur isotopic fractionation of 20–60‰ diagnostic to microbial sulfate reduction is associated with the investigated groundwater containing sulfate below 0.2 mM. In contrast, a small carbon isotopic fractionation (<30‰) is not indicative of methanogenesis. Except for 2011, the concentrations of H2 ranged mostly from 1 to 5 nM, which is also consistent with an aquifer where a terminal electron accepting process is dominantly controlled by ongoing sulfate reduction. High isotopic ratios of mantle-derived 3He relative to radiogenic 4He in groundwater and the flux of H2 along adjacent faults suggest that, in addition to low concentrations of organic matter (<70 µM), H2 from deeper sources might partly fuel metabolic activities. Our results demonstrate that the deep biosphere in the terrestrial crust is metabolically active and playing a crucial role in the formation of reducing groundwater even under low-energy fluxes.

Origin of saline waters distributed along the Median Tectonic Line in southwest Japan: Hydrogeochemical investigation on possibility of derivation of metamorphic dehydrated fluid from subducting oceanic plate

To identify of metamorphic dehydrated fluid as source fluid of hot spring water, we conducted chemical and isotopic analyses of water and accompanied gas samples collected from hot-spring wells along the Median Tectonic Line (MTL) in the forearc region of the southwestern part of Japan. As a result, we found the hot spring waters having anomalous δD and δ18O compositions as compared with modern seawater and shallow groundwater in Wakayama and Shikoku regions. Judging from data in relative B–Li–Cl composition and He isotopic systematics, the source fluid of the hot springs in Shikoku could be identified to be one of diagenetic fluids. On the other hand, the source fluid of the hot springs of Wakayama had different B–Li–Cl composition and higher 3He/4He ratio in comparison with diagenetic dehydrated fluids and then the fluid was thought to be originated from metamorphic dehydrated fluid as well as Oita plain. There was another striking contrast between the source fluid of Wakayama and Oita and that of Shikoku and Miyazaki; accompanied gases by the former were rich in CO2, whereas those with the latter were rich in CH4, and CO2 in the accompanied gases of Wakayama and Oita is mostly derived from marine carbonate like volcanic gases in subduction zones. Moreover, the Li–B–Cl compositions of them showed transitive values between the relative composition of diagenetic fluids and those of volcanic thermal waters. Consequently, the source fluid of hot springs in Wakayama and Oita was likely to be dehydrated metamorphic fluids released from the subducting Philippine-Sea plate.

Origin of the Arima-type and Associated Spring Waters in the Kinki District, Southwest Japan

Rare earth elements (REEs) of the spring waters upwelling in the non-volcanic fore-arc region of the Kinki district in southwest Japan were investigated to assess their upwelling processes and deep-seated origins. A principal component analysis of the REE data identified three principal components (PCs) that cover 89% of the entire sample variance: (1) PC-01, which corresponds to a dilution process by which fluids are introduced at low concentrations, previously represented by major solute binary trends, including δ18O–δD systematics; (2) PC-02, which is a precipitation process of REEs from the brine; and (3) PC-03, which is an incorporation of REEs from country rock by carbonic acidity, although the types of country rocks may also have a significant impact on the spring water compositions. Based on these three PCs, together with the major solute concentrations and hydrogen, oxygen, and helium isotopic compositions determined in previous studies, five distinct types of spring waters in the Arima and Kii areas were identified: (i) “Tansansen”, (ii) “Kinsen”, (iii) “Ordinary Arima”, (iv) “Ginsen”, and (v) “Eastern Kii”. These five types probably represent (ii) a deep brine, (iii) an evolved deep brine that precipitated REE-bearing minerals, (iv) a mixture of (iii) and meteoric water, (v) a meteoric water carbonated by deep gas derived from (ii), and (i) a spring water similar to (v) with a more significant influence of the country rock constituting the aquifer. A comparison of the spring waters in the Arima and Kii areas revealed systematic geographic distributions. The “Ordinary Arima”-type occurs along the Median Tectonic Line, and the “Eastern Kii”-type occurs in the eastern part of the Kii area. The latter seems to upwell in the restricted region where deep low-frequency tremors are observed. We suggest that the geographical distributions are linked to the tectonic setting and/or temporal evolution of fluid upwelling.