Detailed distribution of mantle helium in the fore-arc region of northeastern Japan
Abstract
Recently, the behavior and origin of crustal fluid are intensively discussed beyond individual research fields. The helium isotope ratio (3He/4He) is one of effective indicators to identify the origin of fluid, which can act as a carrier of helium. The spatial distribution of 3He/4He ratios in northeastern Japan is characterized by low values less than 1 Ra (where Ra is the atmospheric 3He/4He ratio) in the fore-arc region and high values ranging 4-8 Ra along the volcanic front and in the back-arc region (e.g., Sano and Wakita, 1985; Horiguchi et al., 2010). This trend has simply been interpreted that helium in the back-arc region originates from the mantle and that in the fore-arc region is from the crust. The 3He/4He ratios along the volcanic front and in the back-arc region were particularly focused in the previous studies because many hot springs exists in these regions. At the same time, an increasing number of hot spring wells have been constructed in the fore-arc region in recent years. We collected water samples from hot springs, wells, and mineral springs in the fore-arc region of northeastern Japan, and carried out the analysis of helium isotope ratio. We focus on detailed distribution of 3He/4He ratios from hot spring water/gases in the fore-arc region of northeastern Japan. Main features of our results for the fore-arc region of northeastern Japan are as follows: 1) All of the 3He/4He ratios in the fore-arc region are below 1 Ra indicating the predominance of crustal helium in the fore-arc region as previously pointed out. 2) Detailed distribution of the 3He/4He ratios reveals some regional contrast which shows subtle but clear difference. We plan to compare these geochemical results with the geophysical data in this region.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T21C2579H
- Keywords:
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- 1031 GEOCHEMISTRY / Subduction zone processes