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Brine samples from the wells in the Kashio mineral spring (an "Arima-type" hot spring at Ooshika-Mura, central Japan) were analyzed to determine the original chemical and isotopic compositions of the deep brine end-member before its dilution by meteoric water and to elucidate the origin of the end-member. The trends of variation between Cl, δD, and δ18O indicated the existence of a two-component mixing system and a systematic variation in the mixing ratio, which were mentioned in previous studies. By carefully tracking the variation in tritium (3H) and atmospheric noble gas in the brine, the Cl concentration in the end-member was determined to be 24,000 mg/L. This value is consistent with the result of previous studies. Based on the estimated composition and other related data, we inferred that the end-member originated from slab-derived fluid, which in turn may have undergone oxygen isotope exchange reactions with minerals. Although both the Arima and Kashio brines are considered to be derived from fluid dehydrated from the Philippine Sea slab, the chemical and isotopic compositions of the Kashio end-member are different from those of the Arima end-member. In particular, the Kashio end-member is characterized by low Cl concentration (~ 40% lower than that in the Arima end-member), low hydrogen isotope ratio, and low 3He/4He ratio (1.4 Ra). These results indicate that the chemical and isotopic compositions of the slab-derived fluid are different for each location. The significant difference in δD could reflect the difference in the dehydration depth. Finally, the low temperature and relatively low 3He/4He ratio of the brine end-member could be explained by its long residence time within the crust. [ABSTRACT FROM AUTHOR]

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) specifies that an overall network of at least 40 International Monitoring System (IMS) stations should monitor the presence of radioxenon in the atmosphere upon its entry into force. The measurement of radioxenon concentrations in the air is one of the major techniques to detect underground nuclear explosions. It is, together with radionuclide particulate monitoring, the only component of the network able to confirm whether an event originates from a nuclear test, leaving the final proof to on-site inspection. Correct and accurate interpretation of radioxenon detections by State Signatories is a key parameter of the verification regime of the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). In this context, the discrimination between the highly variable radioxenon background generated by normal operations of nuclear facilities and CTBT-relevant events is a challenging, but critical, task. To this end, the radioxenon background that can be expected at IMS noble gas systems must be sufficiently characterized and understood. All activities conducted to study the global radioxenon background are focused on the calibration and performance of the verification system as described in the Treaty. The unique CTBTO noble gas system network is designed to optimally covering the globe. By the end of 2019, 31 systems were put in operation, 25 of which being already certified. It took two decades from the first experimental setup of noble gas system in the field to reach this stage of maturity. In the meantime, it was an urgent need to gain empirical evidence of atmospheric radioxenon concentrations with the full spectrum of characteristics that IMS noble gas systems may be observing. This experience was significantly advanced through temporary measurement campaigns. Their objective was to gain the additional necessary knowledge for a correct understanding and categorization of radioxenon detections. The site selection for these campaigns put emphasis on regions with low coverage by the initially few experimental noble gas systems at IMS locations or where potential interferences with normal background might be observed. Short-term measurements were first initiated in 2008. Sites of potential interest were identified, and campaigns up to few weeks were performed. Based on the findings of these short campaigns, transportable systems were procured by the CTBTO. Longer temporary measurement campaigns were started afterwards and operated by local hosts in different regions of the globe. Site selections were based on purely scientific criteria. Objectives of the measurement campaigns were continually reassessed, and projects were designed to meet the scientific needs for radioxenon background understanding as required for nuclear explosion monitoring. As of today, several thousands of samples have been collected and measured. Spectra of temporary measurement campaigns were (and are still) analysed in the International Data Centre (IDC). As they are not part of the CTBT monitoring system, no IDC product is generated. Analysis results are stored in a non-operational database of the CTBTO and made available, together with raw data, to authorized users of States Signatories through a Secure Web Portal (SWP) and to scientific institutions for approved research projects through a virtual Data Exploitation Centre (vDEC) after signing a cost-free confidentiality agreement (https://www.ctbto.org/specials/vdec). This paper aims at providing an overview of the temporary measurement campaigns conducted by the CTBTO since the very first field measurements. It lays out scientific results in a systematic approach. This overview demonstrates the asset of radioxenon background measurement data that have been collected with a wide variety of characteristics that may be observed at IMS stations. It bears a tremendous opportunity for development, enhancement and validation of methodologies for CTBT monitoring. In 2018, a campaign started in Japan with transportable noble gas systems in the vicinity of the IMS station RN38 in Takasaki. It will be described separately once the measurements are completed. • Monitoring 131mXe, 133Xe, 133mXe and 135Xe as part of the CTBTO verification regime. • Overview of 10 years worldwide radioxenon background measurement campaigns. • Characterization of the background away from/at proximity to nuclear facilities. • Variability of radioxenon observations from routine releases of nuclear facilities. • Challenges in discriminating CTBT-relevant observations from normal releases. [ABSTRACT FROM AUTHOR]

Using particulate and noble gas raw data analysed from Comprehensive Nuclear Test-Ban-Treaty (CTBT) monitoring sites, Canadian National Surveillance networks and atmospheric transport modelling, it is possible to associate some of the main events in the Fukushima accident timeline with observations, and perturbations in isotopic ratios in Japan, through the Pacific and into North America. Additional sources identified during the accident period will be commented upon in the context of CTBT treaty verification. [ABSTRACT FROM AUTHOR]

Abstract: We report on the first measurements of short-lived gaseous fission products detected outside of Japan following the Fukushima nuclear releases, which occurred after a 9.0 magnitude earthquake and tsunami on March 11, 2011. The measurements were conducted at the Pacific Northwest National Laboratory (PNNL), (46°16′47″N, 119°16′53″W) located more than 7000 km from the emission point in Fukushima Japan (37°25′17″N, 141°1′57″E). First detections of 133Xe were made starting early March 16, only four days following the earthquake. Maximum concentrations of 133Xe were in excess of 40 Bq/m3, which is more than ×40,000 the average concentration of this isotope is this part of the United States. [Copyright &y& Elsevier]

Abstract: The relation of magma and crustal activity has been studied from spatial distribution of 3He/4He ratios of gas and/or water samples over the Izu Peninsula, where significant crustal deformation associated with seismic swarm activities has been observed since 1970s. The air-corrected values of 3He/4He ratios ranged from 3.5 to 8.2 R A, where R A is the atmospheric 3He/4He ratio=1.4×10−6, indicating that helium is mostly of magmatic origin. Among the three pressure sources proposed to explain the crustal deformation, two inflation sources beneath the inland of northeast and the mid east coast of the Izu Peninsula locate in the broad distribution of high 3He/4He ratios, which supports relation of magma to the crustal uplift. In contrast, the distribution of 3He/4He ratios around the tensile fault assumed in the area of seismic swarms appears not to indicate existence of significant amount of magma below the tensile fault. Alternatively, the results suggest magma below a point several kilometers south of the tensile fault. The seismic swarms are explained either by fluid pressurization of thermal water heated by this magma or by intrusion of magma to the tensile fault moved obliquely from the deep magma reservoir. [ABSTRACT FROM AUTHOR]

Iwojima volcano, located on the southernmost part of the Izu–Ogasawara arc, is characterized by a magma chemistry, which differs from that of the other volcanoes in the arc, suggesting a larger contribution of subducted component. In order to elucidate the behavior of volatile elements during the subduction process in generating this unusual magma, noble gas and carbon isotope ratios of volcanic volatiles were measured. The noble gas and carbon isotopic characteristics of Iwojima magma are quite distinct from other volcanoes in the Izu–Ogasawara–Mariana arc system: (1) a low 3He/4He ratio of 5.6RA; (2) enrichment in heavy noble gases; and (3) a heavy carbon isotope ratio (+1.5‰) almost equivalent to the highest value for marine carbonates. These unusual features strongly suggest the involvement of slab-derived volatiles in the magma, although the possibility of a shallow origin for the low 3He/4He ratio still remains. Based on a mixing calculation using helium–carbon isotope systematics, an extremely large contribution of slab-derived components (31% for 4He and more than 95% for 12C) is required to explain the unusual features of helium and carbon isotopes of Iwojima. This is in agreement with other petrological and geochemical features which imply a greater role of subducted materials, although the precise relative contributions of each source materials are somewhat different for several of the tracers. This difference is probably caused by different provenance in subducting materials carrying them to melt generation depth in the mantle wedge. The elevated carbon/helium ratio of the slab-derived component also suggests decoupling during the subduction process probably resulting from the preferential loss of helium from the subducting slab beneath the forearc region. Enhanced outputs of slab-derived carbon and helium (3.5×109 and 4.0×103 mol/year) indicate a large input of slab component to the mantle wedge and/or effective devolatilization of slab materials. As for its origin, we believe that the unusual tectonic setting around Iwojima may be responsible: (1) subduction of the Ogasawara Plateau, which brings a significant volume of thick crustal blocks into the mantle wedge beneath Iwojima; (2) flattened subduction as a consequence of the buoyancy of the subducting Ogasawara Plateau; and (3) propagation of the back-arc basin rift axes in the Mariana Trough. In contrast to large-scale subduction of overthickened oceanic crust associated with slab melting, the subduction of a relatively small oceanic plateau may be responsible for the characteristics of the Iwojima magma, in which slab material itself does not play an important role in magma genesis but slab-derived volatiles make a significant contribution. [Copyright &y& Elsevier]

A total of 27 well gas samples collected from SW Japan were subjected to precise analysis of He, Ne and Ar isotopic compositions. The sampling area covers localities where mantle-derived helium emanates in the fore-arc of the Kii Peninsula, and is known as the ‘Kinki spot’ [e.g., Sano and Wakita, J. Geophys. Res. 90 (1985) 8728–8741]. The Kinki spot apparently is located within the fore-arc with no obvious magmatic activity, which contrasts to the occurrence of similarly high 3He/4He ratios along the volcanic front of NE Japan. Our high density sampling revealed more clearly that the high 3He/4He ratio wells are distributed in the Kii Peninsula above the area where the young and hot Philippine Sea Plate is subducting with a relatively steeper dip than in the adjacent area; this situation favors the near-trench side of the subducting slab entering eclogite facies conditions involving slab dehydration, without inducing melting of the mantle wedge. The aqueous fluids derived from the slab acquire mantle He during their passage through the mantle wedge. Thus, the observed association of mantle He and the fore-arc dehydration of the Philippine Sea Plate indicates that fluid derived from the subducting slab is contributing to the occurrence of mantle He in well gases of the Kii Peninsula. The high 3He/4He region of Kii Peninsula also coincides with the occurrence of long-period tremors. These are caused by the movement of fluids derived from the subducting slab. Localized stresses cause significant uplift in the area, interpreted as indicating that intra-crustal fractures provide pathways through the crust for the fluids derived from depth. Thus, magma is not necessarily required as the carrier of mantle-derived helium to the surface of the fore-arc region of Kii Peninsula. [Copyright &y& Elsevier]

• Overpressured fluid near downdip of seismogenic zone were mainly derived from the mantle. • Serpentinized mantle-derived fluid is a previously unseen part for generating fluid overpressure. • Serpentinized mantle-derived fluids are required to cause brittle failure. Subduction plate boundaries are commonly weak owing to fluid overpressure. The generation of fluid overpressure has been invoked for compaction disequilibrium and mineral dehydration in subducting sediments and altered oceanic crust. The subduction mélange in the Shimanto accretionary complex, southwest Japan, preserves the quartz-calcite-filled shear veins and sigmoidal extension veins formed under fluid overpressure near the downdip limit of the thermally-controlled seismogenic zone. The shear veins record the repeated low-angle brittle thrusting under fluid overpressure, while the extension veins constitute the Riedel shear zones developed during subduction under vertical maximum principle stress. To examine the origin of overpressured fluids, we conducted fluid inclusion and noble gas analyses on the shear and extension veins. The result of fluid inclusion analysis shows various fluid pressures between lithostatic to hydrostatic pressures during the vein formation, which were associated with warm fluid infiltration. 3He/4He values of shear veins and extension veins range 1.6–2.5 Ra and 1.6–2.3 Ra, respectively, indicating the presence of mantle helium. 40Ar/36Ar values and 84Kr/36Ar and 130Xe/36Ar ratios of the veins are similar to those of serpentinized mantle, suggesting that mantle helium originated from serpentinized mantle. Because the mantle-derived rocks such as peridotite and serpentinite are absent in the mélange, the results of noble gas analyses represent the infiltration of fluids from the serpentinized mantle into the mélange shear zone. Our results show that an infiltration of fluids derived from serpentinized mantle contributed the fluid overpressure in the mélange shear zone near the downdip limit of the seismogenic zone. [ABSTRACT FROM AUTHOR]