Limited underthrusting of India below Tibet: 3He/4He analysis of thermal springs locates the mantle suture in continental collision

Our regional-scale geochemical dataset (³He/⁴He) resolves the geometry of the continental collision between India and Asia. Geophysical images have led to contradictory interpretations that India directly underthrusts Tibet as a horizontal plate or India subducts steeply into the mantle. Helium transits from mantle depths to the surface within a few millennia, such that the ratio of mantle-derived ³He to dominantly crust-derived ⁴He provides a snapshot of the subsurface. ³He/⁴He data from 225 geothermal springs across a >1,000-km-wide region of southern Tibet define a sharp boundary subparallel to the surface suture between India and Asia, just north of the Himalaya, delineating the northern limit of India at ∼80-km depth. The India–Asia collision resembles oceanic subduction with an asthenospheric mantle wedge.

During continent–continent collision, does the downgoing continental plate underplate far inboard of the collisional boundary or does it subduct steeply into the mantle, and how is this geometry manifested in the mantle flow field? We test conflicting models for these questions for Earth’s archetypal continental collision forming the Himalaya and Tibetan Plateau. Air-corrected helium isotope data (³He/⁴He) from 225 geothermal springs (196 from our group, 29 from the literature) delineate a boundary separating a Himalayan domain of only crustal helium from a Tibetan domain with significant mantle helium. This 1,000-km-long boundary is located close to the Yarlung-Zangbo Suture (YZS) in southern Tibet from 80 to 92°E and is interpreted to overlie the “mantle suture” where cold underplated Indian lithosphere is juxtaposed at >80 km depth against a sub-Tibetan incipiently molten asthenospheric mantle wedge. In southeastern Tibet, the mantle suture lies 100 km south of the YZS, implying delamination of the mantle lithosphere from the Indian crust. This helium-isotopic boundary helps resolve multiple, mutually conflicting seismological interpretations. Our synthesis of the combined data locates the northern limit of Indian underplating beneath Tibet, where the Indian plate bends to steeper dips or breaks off beneath a (likely thin) asthenospheric wedge below Tibetan crust, thereby defining limited underthrusting for the Tibetan continental collision.

Mantle-Derived Helium Distribution and Tectonic Implications in the Sichuan-Yunnan Block, China

The geochemical characteristics of mantle degassing observed on the surface of the earth can indicate the origin and migration path of mantle fluids. Compared with the plate boundary tectonic environment, the intraplate tectonic environment does not have a large number of active volcanoes and active faults, and the observation of mantle volatiles in hot spring gas is relatively limited. We selected the Sichuan-Yunnan block to discuss mantle degassing based on the carbon and noble gas isotopes of the spring gases and previous studies on the fault slip rate and geophysical research. A total of five hot spring gas samples (including two free gases and three dissolved gases) were collected from the Sichuan-Yunnan block. Chemical and isotopic compositions were analyzed in N₂-dominant hot spring gases. The ³He/⁴He ratio (0.068-0.541 R _a) indicates the occurrence of mantle-derived helium throughout the Sichuan-Yunnan block, which has been diluted by a crustal radiogenic ⁴He component. The occurrence of mantle-derived helium in the study areas ranges from 0.74 to 5.67%. The lower proportion of mantle-derived helium in YNWQ and HGWQ than that in other spring gases near the Jinghe-Qinghe fault may be caused by the smaller scale of fault around YNWQ and HGWQ than the Jinghe-Qinghe fault. The correlation between ⁴He, ²⁰Ne, and N₂ concentrations implies a common trapping mechanism for ⁴He, ²⁰Ne, and N₂ in hot spring gases. The ⁴⁰Ar/³⁶Ar ratios and N₂/Ar ratios indicate that N₂ and Ar are mostly meteoric, and YNWQ and HGWQ have more crustal-derived Ar contribution (40.56 and 51.49%, respectively). The δ¹³C(CO₂)_o values calculated by Rayleigh fractionation and CO₂ concentration suggest that CO₂ has inorganic and organic origins. The plot of R _c/R _a versus δ¹³C(CO₂) indicates that the spring gas CO₂ origin in the Sichuan-Yunnan block is mainly derived from mixing of limestone and organic sediments with minor mantle CO₂. The δ¹³C(CH₄) versus CH₄/³He values indicate that the origin of methane is thermogenic and microbial oxidation. The low mantle-derived helium distribution pattern is most likely controlled by the weak fault activity rate, the small fault scale, and not obvious magmatic activity in the Sichuan-Yunnan block.

Linking deeply-sourced volatile emissions to plateau growth dynamics in southeastern Tibetan Plateau

The episodic growth of high-elevation orogenic plateaux is controlled by a series of geodynamic processes. However, determining the underlying mechanisms that drive plateau growth dynamics over geological history and constraining the depths at which growth originates, remains challenging. Here we present He-CO₂-N₂ systematics of hydrothermal fluids that reveal the existence of a lithospheric-scale fault system in the southeastern Tibetan Plateau, whereby multi-stage plateau growth occurred in the geological past and continues to the present. He isotopes provide unambiguous evidence for the involvement of mantle-scale dynamics in lateral expansion and localized surface uplift of the Tibetan Plateau. The excellent correlation between ³He/⁴He values and strain rates, along the strike of Indian indentation into Asia, suggests non-uniform distribution of stresses between the plateau boundary and interior, which modulate southeastward growth of the Tibetan Plateau within the context of India-Asia convergence. Our results demonstrate that deeply-sourced volatile geochemistry can be used to constrain deep dynamic processes involved in orogenic plateau growth.

In situ observation of helium and argon release during fluid-pressure-triggered rock deformation

Temporal changes in groundwater chemistry can reveal information about the evolution of flow path connectivity during crustal deformation. Here, we report transient helium and argon concentration anomalies monitored during a series of hydraulic reservoir stimulation experiments measured with an in situ gas equilibrium membrane inlet mass spectrometer. Geodetic and seismic analyses revealed that the applied stimulation treatments led to the formation of new fractures (hydraulic fracturing) and the reactivation of natural fractures (hydraulic shearing), both of which remobilized (He, Ar)-enriched fluids trapped in the rock mass. Our results demonstrate that integrating geochemical information with geodetic and seismic data provides critical insights to understanding dynamic changes in fracture network connectivity during reservoir stimulation. The results of this study also shed light on the linkages between fluid migration, rock deformation and seismicity at the decameter scale.

Continental degassing of helium in an active tectonic setting (northern Italy): the role of seismicity

In order to investigate the variability of helium degassing in continental regions, its release from rocks and emission into the atmosphere, here we studied the degassing of volatiles in a seismically active region of northern Italy (Mw_MAX = 6) at the Nirano-Regnano mud volcanic system. The emitted gases in the study area are CH₄-dominated and it is the carrier for helium (He) transfer through the crust. Carbon and He isotopes unequivocally indicate that crustal-derived fluids dominate these systems. An high-resolution 3-dimensional reconstruction of the gas reservoirs feeding the observed gas emissions at the surface permits to estimate the amount of He stored in the natural reservoirs. Our study demonstrated that the in-situ production of ⁴He in the crust and a long-lasting diffusion through the crust are not the main processes that rule the He degassing in the region. Furthermore, we demonstrated that micro-fracturation due to the field of stress that generates the local seismicity increases the release of He from the rocks and can sustain the excess of He in the natural reservoirs respect to the steady-state diffusive degassing. These results prove that (1) the transport of volatiles through the crust can be episodic as function of rock deformation and seismicity and (2) He can be used to highlight changes in the stress field and related earthquakes.

Mantle degassing along strike-slip faults in the Southeastern Korean Peninsula

On September 12, 2016, a M_L 5.8 earthquake hit Gyeongju in the southeastern part of the Korean Peninsula (SeKP), although the area is known to be far from the boundary of the active plate. A number of strike-slip faults are observed in heavily populated city areas (e.g., Busan, Ulsan, Pohang, and Gyeongju). However, dissolved gases related to the active faults have rarely been studied despite many groundwater wells and hot springs in the area. Here we report new results of gas compositions and isotope values of helium and carbon dioxide (CO₂) in fault-related fluids in the region. Based on gas geochemistry, the majority of gas samples are abundant in CO₂ (up to 99.91 vol.%). Measured ³He/⁴He ratios range from 0.07 to 5.66 Ra, showing that the mantle contribution is up to 71%. The range of carbon isotope compositions (δ¹³C) of CO₂ is from −8.25 to −24.92‰, showing mantle-derived CO₂ is observed coherently where high ³He/⁴He ratios appear. The weakening of faults seems to be related to enhanced pressures of fluids containing mantle-derived helium and CO₂ despite the ductile lower crust underneath the region. Thus, we suggest that the SeKP strike-slip faults penetrate into the mantle through ductile shearing.

Mantle fluids associated with crustal-scale faulting in a continental subduction setting, Taiwan

We report noble gas signatures of groundwater, hot springs, and bedrock samples from a major fault system that separates regional-scale blocks of accreted, continental materials in southern Taiwan. Despite the continental setting, the isotopic signatures argue for the presence of mantle derived fluids, suggesting that the active fault system is deep-seated. This is consistent with deep, non-volcanic tremors identified in the same area. We speculate that the mantle fluids are escaping along a crustal-scale fault marked by clusters of non-volcanic tremors directly beneath the southern Central Range. The evidence of these tremors and electrical conductivity anomalies along the strike of the fault recognized previously correlated up dip with the surface trace of a major active fault support the hypothesis.

0 °C is better?- Thawing temperature optimization study for cancer cryoablation in a mouse model with green fluorescent protein-labeled Lewis lung cancer

PURPOSE

There are two kinds of thawing temperatures commonly adopted in cancer cryosurgery. We attempted to compare their efficacy differences in this study to optimize the surgical method.

METHOD

Forty-five C57BL/6 J mice with GFP-labeled Lewis lung cancer were randomized into three groups (n = 15 for each): control group, T0 group (thawing temperature 0 °C), and T40 group (thawing temperature 40 °C). Cryoablation was performed using a combined surgical system. When the ice ball reached the border of the tumor, they were rewarmed to 0 °C and 40 °C, respectively, using a single freeze-thaw cycle. After the surgery, weight of these mice, length/width and the fluorescence intensity (FI) of the tumors were recorded. All mice were sacrificed on Day 14 after the procedures and their xenografts were excised and weighed immediately. We also checked for pulmonary metastasis, and examined tumor specimens using HE staining.

RESULTS

Body weights, tumor volumes and FI in the three groups did not differ significantly at baseline. On Day 14, 39% of the tumors in the T0 group decreased in volume, whereas only 17% in the T40 group did. The average FI in the control group increased by 60%, but declined by 72% in T0 mice and 69% in T40 mice. Tumor inhibition rates were 71.64% in the T0 group and 68.12% in the T40 group. Lung metastases rates and histological changes were compatible between the two intervention groups.

CONCLUSION

Using 0 °C as the thawing temperature may have more potential benefits in cryoablation efficacy.

Helium anomalies suggest a fluid pathway from mantle to trench during the 2011 Tohoku-Oki earthquake

Geophysical evidence suggests that fluids along fault planes have an important role in generating earthquakes; however, the nature of these fluids has not been well defined. The 2011 magnitude 9.0 Tohoku-Oki earthquake ruptured the interface between the subducting Pacific plate and the overlying Okhotsk plate. Here we report a sharp increase in mantle-derived helium in bottom seawater near the rupture zone 1 month after the earthquake. The timing and location indicate that fluids were released from the mantle on the seafloor along the plate interface. The movement of the fluids was rapid, with a velocity of ~4 km per day and an uncertainty factor of four. This rate is much faster than what would be expected from pressure-gradient propagation, suggesting that over-pressurized fluid is discharged along the plate interface.

Quality and age of shallow groundwater in the Bakken Formation production area, Williston Basin, Montana and North Dakota

The quality and age of shallow groundwater in the Bakken Formation production area were characterized using data from 30 randomly distributed domestic wells screened in the upper Fort Union Formation. Comparison of inorganic and organic chemical concentrations to health based drinking-water standards, correlation analysis of concentrations with oil and gas well locations, and isotopic data give no indication that energy-development activities affected groundwater quality. It is important, however, to consider these results in the context of groundwater age. Most samples were recharged before the early 1950s and had 14C ages ranging from <1000 to >30,000 years. Thus, domestic wells may not be as well suited for detecting contamination associated with recent surface spills as shallower wells screened near the water table. Old groundwater could be contaminated directly by recent subsurface leaks from imperfectly cemented oil and gas wells, but horizontal groundwater velocities calculated from 14C ages imply that the contaminants would still be less than 0.5 km from their source. For the wells sampled in this study, the median distance to the nearest oil and gas well was 4.6 km. Because of the slow velocities, a long-term commitment to groundwater monitoring in the upper Fort Union Formation is needed to assess the effects of energy development on groundwater quality. In conjunction with that effort, monitoring could be done closer to energy-development activities to increase the likelihood of early detection of groundwater contamination if it did occur.