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Volcanogenic CO2 Degassing in the Songliao Continental Rift System, NE China
The Wudalianchi monogenetic volcanic field (WMVF) is located in the Songliao basin within a major continental rift system in NE China. Bubbling springs and diffuse degassing from soils are typical features of the WMVF. Chemical compositions and C-He isotope analyses revealed that the cold spring gases might originate from the enriched upper mantle (EM), which resulted from the mixing between slab materials (subducted organic sediments and carbonates) in the mantle transition zone (MTZ) and the ambient depleted mantle. These EM-derived volatiles experienced variable degrees of crustal input, including both continental organic metasediments and crustal carbonates during their ascending path to the surface. The estimated results of the degassing CO2 fluxes, combined with previous geophysical evidence, suggest that the CO2 degassing activities become weaker from early to late in Quaternary.
Continental rift systems, together with the related intraplate volcanism, have been regarded as a possible trigger of deep-derived CO2 degassing into the atmosphere and long-term climate change [1–6]. Positive spatial correlation between CO2 discharges and the extensional tectonic regimes confirms that the continental rift systems are critical pathways for deep carbon degassing from the Earth’s interior to the exosphere . Research on continental rift lengths and paleoatmospheric CO2 concentrations over the last 200 million years also indicates that continental fragmentation may control the atmospheric CO2 levels via massive CO2 degassing in rift systems . Intraplate volcanism along continental rift systems is primarily derived from the metasomatized mantle . Under these conditions, the reduction of the peridotite solidus due to the presence of volatiles allows for partial melting of the upper mantle at depth and thus for the release of large amounts of CO2 into the atmosphere via extensive volcanism [4, 5, 8, 9].
The Songliao basin in NE China has experienced long-term extension-induced continental rifting since the Late Mesozoic as indicated by many intraplate volcanoes (Figure 1(a)) . Based on both petrogenesis and geophysical evidences, these Cenozoic volcanic activities have been considered to be linked to the stagnant Pacific slab materials in the mantle transition zone (MTZ) [11–15]. However, the origin and evolution of magma degassing in the continental rift system involving the deep subduction of oceanic slab are poorly understood [16, 17]. Located at the northern margin of the Songliao continental rift system in East Asia (Figure 1(a)), the Wudalianchi monogenetic volcanic field (WMVF) is characterized by extensive CO2 degassing, i.e., cold bubbling springs, diffused CO2 emissions from soils, and volcanogenic fault systems (Figure 1(b)). CO2 is supposed to be of mainly magmatic origin, but the mechanisms of deep CO2 formation are still debated. Some authors [18–21] indicated a binary mixing between the depleted mantle- and upper crust-derived volatiles. Other authors  hypothesized partial melting of a subcontinental lithospheric mantle (SCLM) metasomatized by ancient fluids.
In this paper, we report the first soil CO2 flux measurements in the WMVF. Additionally, new data on the chemical and C-He isotopic composition of gases associated with four cold springs located in the area are presented and used to gain insights into the mechanisms responsible for CO2 formation at depth.
2. Geological Setting
The Wudalianchi monogenetic volcanic field (WMVF) is an active K-rich volcanic region located in a continental rift system in NE China (Figure 1(a)). Its latest eruption dates back to 1721 AD (e.g., Laoheishan and Huoshaoshan volcanoes, Figures 1(b) and 1(c)) [23, 24]. The WMVF basement consists of Archean granites, Mesozoic andesite, and granite intrusions . Limestones and marine carbonate sediments have been present in outcrops in the ambient Songliao basin since the Mesozoic age [26, 27]. During the Cenozoic period, dispersed calc-alkaline, tholeiitic, and alkali basaltic rocks were formed in Northeastern China , which constitute the eastern part of the Asian tectono-magmatic province that extends from Lake Baikal, Siberia, to Eastern China .
Cenozoic volcanic activity has formed 800 km2 of lava flows, which includes the middle Pleistocene to Holocene volcanic activity in WMVF (Figure 1(b)) [23, 24]. These fissure-central-type eruptions are controlled by NE- to NNW-striking fractures or deep faults [23, 29], which are linked to the continental rift systems in East Asia [30, 31]. Deep buried faults provided potential rising channels for the magma , which finally led to the unique volcanic landscapes seen nowadays in WMVF , such as linear rows of scoria cones (Figures 1(b) and 1(c)), dominant tectonic weakness direction enlarged by erosion (e.g., Huoshaoshan volcanic cone, Figure 2(e)), and overlapping scoria cones (e.g., Wohushan volcanic cone, Figure 1(c)). All of these features were used in the past to locate feeding fissures beneath the monogenetic volcanoes in continental rift systems .
Seismic tomography studies have shown the presence of a magma reservoir under the Weishan volcano in WMVF (Figures 1(b) and 1(c)), which is considered as evidence for partial melting in a shallow magma chamber at 7~13 km depth . Surface manifestation of such activity is mainly expressed as cold-mineral bubbling springs with high pCO2 and low water temperature (generally between 4 and 7°C), prevalently distributed on the flanks or at the margin of the active volcanic cones (Figure 1(c)) [18, 21], and located roughly at the same elevation (about 300 to 320 m.a.s.l., Figure 1(c)). The Hualin spring (HLQ) is located at the eastern slope of the Huoshaoshan volcanic cone (Figures 1(c) and 2(b)) and shows the highest water temperature (24°C) in the WMVF. The North, South, and Fanhua cold springs are located east of the Yaoquanshan volcano and west of the Shilong river (Figure 1(c)) and are characterized by water temperatures lower than 10°C.
3. Sampling and Analytical Methods
3.1. Soil CO2 Flux Measurements
Soil CO2 fluxes were measured in situ using soil diffuse gas flux meters based on the accumulation chamber method (Figure 2(d)), which was firstly reported by Chiodini et al. . The edge of the inverted cylindrical chamber was sealed with damp soil to diminish air contamination of the soil gases in the chamber . The measurement sites were located near the main volcano-structural features of the region and preferentially placed on uncovered ground to minimize the influence of the vegetation on the measured fluxes (Figure 1(c)). The temperature was measured in situ at a depth of 10 cm, and the location was recorded (measured with a Garmin GPSMAP 60SCx) at each measurement point (see details in Supplementary Material). 92 measurement points were made during stable suitable weather and similar atmospheric conditions in August and September 2017. In order to compare the CO2 emission rate of the new erupted region with that of the whole WMVF, additional 38 points were considered on the southeastern slope of the Laoheishan volcanic cone (Figures 1(c) and 2(d)), whose last activity occurred about 300 years ago.
3.2. Spring Gas Sampling and Composition Analysis
Gas samples from four cold bubbling springs (Figures 1(c) and 2) were collected by means of the drainage method using lead-bearing glass bottles . The temperature of the spring water was measured in situ using a portable thermometer. Chemical and isotopic compositions of the gas samples were measured in the Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), Lanzhou, China. The chemical composition of the gas samples was determined using a MAT 271 mass spectrometer. The δ13CCO2 was analyzed with a Delta Plus XP mass spectrometer. All samples were analyzed for the helium isotope composition (3He/4He) using a Noblesse noble gas mass spectrometer. Detailed analytical procedures are described in Luo et al.  and Zhang et al. .
4.1. Degassing Flux of Soil CO2
We obtained a broad range of soil CO2 fluxes (1.1 to 161.5 g m−2 d−1) and soil temperatures (18.1°C to 33.3°C; Supplementary Table S1), which demonstrate high variability of soil CO2 emissions in WMVF. In the log probability plot (Figure 3(a)), our analytical data could be basically divided into the following three groups by two inflection points on the skewed curve : less than 5 g m−2 d−1 (Group A), 5-60 g m−2 d−1 (Group B), and more than 60 g m−2 d−1 (Group C), characterized by average soil CO2 flux values of 3.5 g m−2 d−1, 17.6 g m−2 d−1, and 92.1 g m−2 d−1, respectively (Figure 3(a)). These groups may represent different sources of soil CO2 based on Chiodini et al. : (i) biological source with low soil CO2 flux (e.g., soil respiration or pedochemical process, Group A) and (ii) geological source with high soil CO2 flux (e.g., magmatic or hydrothermal related carbon degassing, Group C). In particular, Group B with intermediate soil CO2 fluxes may represent the mixture of biological and geological sources (Figure 3(a)). The average soil CO2 flux of the whole WMVF was calculated using the weighted mean method, which is 18.7 g m−2 d−1. Using the same statistical method, an average soil CO2 flux of 11.8 g m−2 d−1 was obtained for the SE slope of the Laoheishan volcanic cone (Figure 3(b)).
Caracausi et al.  proposed that size-normalized CO2 fluxes inversely correlate with the ages from the last eruptions, which means that active outgassing of volatiles from magmatic bodies would occur for a long time after the last volcanic activity. However, the average soil CO2 flux in the SE slope of the Laoheishan volcanic cone, which experienced the latest eruption, is lower than that in the whole WMVF (Figure 3). Considering the dispersed cold springs, and the decoupling of soil temperatures and CO2 fluxes (with a correlation coefficient of 0.0714) in WMVF, we suggest that the resulting weak degassing is due to the solidification and/or cooling down of the underlying magma body beneath the Laoheishan in about 300 years. Our proposal is in good agreement with crustal electric conductivity studies that a rivet-shaped block with high resistivity, which was considered as the result of solidified magma chamber, is distributed beneath the Laoheishan-Huoshaoshan volcanic chain (Figure 1) .
In this study, 50 sequential Gaussian simulations were performed over a grid of 108001 square cells () covering an area of 0.11 km2 in the southeastern slope of the Laoheishan volcano (Figure 4). Total CO2 output in the selected area of the Laoheishan volcano (ca. 0.11 km2) is 536.3 t/yr (), which is a preliminary survey or first-order estimation of diffusive CO2 degassing in WMVF.
4.2. Chemical and C-He Isotopic Compositions of Cold Spring Gases
The chemical and C-He isotopic compositions of gases from the WMVF cold springs are listed in Table 1. Our analytical results are in excellent agreement with previous studies (Supplementary Table S3) [18–22], which suggest no distinct changes in chemical and isotopic compositions of the spring gases during the last 20 years.
(1) is the observed 3He/4He ratio divided by the 3He/4He ratio in the air (, ). (2) , where β represents the Bunsen coefficients assuming a groundwater recharge temperature of 10°C (βNe/βHe = 1.25, ), (4He/20Ne)M is the measured ratio of samples, and (4He/20Ne)Air is the ratio of the air (0.32, ). (3) is the air-corrected Helium isotope ratio by applying the following formula : .
4.2.1. Chemical Composition
Our results show that most of the gas samples from the Wudalianchi cold springs are characterized by high CO2 content (higher than 90%) and low O2 content (lower than 1.29%) and N2 content (3.2%-8.7%) (Table 1), with the exception of the samples from the Fanhua spring (FHQ17 and FHQ18), which are characterized by low CO2 content (76.7%-80.2%) and relatively high O2 (~3.66%) and N2 content (15.8%-22.6%). N2/Ar ratios of the WMVF samples in this study range from 27 to 75, suggesting an interaction between helium-rich components, air-saturated water (ASW, , temperature dependent), and air () [44, 45], as shown by the linear trend from the high He/Ar to ASW and air endpoints (Figure 5(a)). CO2/3He ratios (; Table 1 and Figure 6) were calculated using CO2/He and 3He/4He ratios, which overlap those of typical arc-related volatiles (, ) and depleted mantle (, ), indicating CO2 addition and/or loss during volatile ascending processes (Figure 5(b)).
4.2.2. C-He Isotopic Compositions
The isotopic composition of carbon and helium has been successfully used to quantify the carbonate vs. sediment contribution from subducted slab material recycling  and the mantle vs. crustal contribution , respectively, in hydrothermal and volcanic gases worldwide.
The δ13C-CO2 values of the spring gas samples from the WMVF fall between −7.3‰ and −2.5‰ (versus VPDB, Table 1), overlapping with the compositional fields reported for arc-related volatiles (−9.1‰ to −1.3‰, ). Some samples show relatively heavier carbon isotope compositions when compared to upper mantle values (, ), suggesting a possible contribution from inorganic carbon-rich components (carbonate rocks, , ).
Measured 3He/4He ratios () of the WMVF gas samples range between and (Table 1), suggesting an obvious contribution from the mantle-derived 3He (Figures 5–7). Such intermediate helium ratios are lower than those of both the depleted MORB-source mantle (DMM, , ) and the subcontinental lithospheric mantle (SCLM, , ), which point to a mixed origin of volatiles from the upper mantle and an end member with high radiogenic 4He (e.g., crustal materials, , ) (Table 2 and Figure 7). As the values are high (>150) for all the samples of this study (Table 1), there is little difference between measured () and air-corrected () helium ratios (Table 1), which indicates little influence from air contamination. Gas samples from the Fanhua spring show higher 4He/20Ne (145-359) and He/Ar ratios (0.28-0.43) (Table 1) than those of the air (, ; , ), which indicate that the high N2 and low CO2 contents do not result from the air contamination in the Fanhua spring gases.
5.1. Carbon Provenance of Volatiles in WMVF
Zhang et al.  proposed that the upper mantle (DMM) and slab-derived components related to the deep subduction of the Pacific plate, including the slab carbonate (CAR) and subducted organic sediments (ORS), are involved in the CO2 inventory of the volatiles in the Changbaishan volcanic fields in NE China, which are similar to those from arc volcanism [46, 58–60]. The CO2/3He ratio remains constant as the gas phase separates from molten basalt, because of the similar solubilities of CO2 and He in melts . Figure 6(a) shows the relationship between CO2/3He ratios and δ13C values of cold spring gases in the WMVF, whereas the CO2/3He ratios of the dataset show marked disparity (Table 1; Figures 5(b) and 6).
Samples with CO2/3He ratios lower than that of DMM are thought to result from the physical-chemical fractionation of CO2 to He [46, 61] or variable CO2/3He ratios of continental crustal contribution . However, these samples with low CO2/3He ratios in the WMVF basically have constant 3He/4He () ratios when compared to those within the three end members (Figures 7 and 6(b)), which eliminate the additional contribution from the crustal origin. Phase separation within the shallow aquifer can potentially fractionate elemental CO2/3He ratios due to the greater solubility of CO2 in the aqueous solution relative to that of the helium [43, 63], especially in low-temperature systems. For example, Sano et al.  suggested that a positive correlation between observed CO2/3He ratios and temperatures of fumaroles and springs results from the differences in solubility. We thus suggest that the He-CO2 fractionation in the cold shallow aquifer would act as the first-order controlling factor on these low CO2/3He samples, including gas samples from the Fanhua spring, which are characterized by relatively high N2 contents, low CO2 contents, and low CO2/3He ratios (Table 1; Figures 5 and 6). Considering this process, the original CO2/3He ratios of gas samples may be higher than the observed values, as shown by the black arrow in Figure 6(a).
Samples within the mixing trajectories have contributions from DMM, ORS, and CAR end members (Figure 6(a)) [16, 46] and are used to calculate the proportion of their contributions to the total carbon inventory in the WMVF, as shown in Table 2. In this calculation, elemental fractionation and its effect on C-He isotope systematics are not considered. We took the average δ13C (−18.5‰) of metamorphosed reduced carbon as that of the ORS end member when considering the potential fractionation during the subduction [16, 65]. If the contribution from continental crustal materials is not taken into account, an average proportion of subducted organic sediments (ORS) would be 24% (Table 2). The upper mantle and slab carbonate-derived carbon are the principal contributors to the carbon budget, with an average total contribution of 76% (Table 2), which is analogous to carbon inventories of worldwide arc magma-related volatiles [46, 58–60].
The diagram of 3He/4He () versus CO2/3He ratios of volcanic or hydrothermal volatiles has provided important constraints on the origin of carbon [35, 61]. In Figure 6(b), the 3He/4He values of the studied samples within the mixing curves (Figure 6(a)) are negatively correlated with CO2/3He ratios (), further supporting the mixing between DMM or EM end member (both with high- and low-CO2/3He ratios) and subducted slab materials (CAR/ORS end member with low- and high-CO2/3He ratios) . However, these samples are obviously located away from the above mixing line (Figure 5(b)), which shows a binary mixing trend between EM-derived volatiles and continental crustal materials with variable CO2/3He ratios (Figure 6(b)) .
5.2. Origin and Evolution of the Volatiles in the WMVF
5.2.1. C-He Isotope Systematics and End-Member Parameters
In the C-He isotope systematics based on Van Soest et al. , 3He/4He ratios coupled with δ13CCO2 values were applied to quantitatively constrain the evolution of volatiles (Figure 8). Following the deep subduction scenario, upper mantle (DMM), slab carbonate (CAR), and organic sediments (ORS) are involved in the origin of volatiles in the WMVF, which exhibited variable contribution of continental crustal components, including organic metasediments (COS) and crustal carbonate (CAR) during their ascending process (Figures 6(b) and 8). Reference values for associated parameters of end members used in the modelling calculation are listed in Table 3.
Helium concentration of the ORS end member was calculated based on the U-Th decay of the global subducting sediments (GLOSS with high SiO2 content; see details in the Supplementary Material) with reservoir accumulation age of 2.2 Ga in the MTZ constrained by the lead isotope . The COS end member was assumed to have a helium ratio equal to that of the bulk continental crust (, ). Considering the potential fractionation effects of the He-CO2 system, the samples with lower CO2/3He ratios than DMM (Figure 6) were not considered for further discussion when calculating the C-He isotope systematics (Figure 8). The results of the C-He isotope mixing calculations between different end members are listed in Supplementary Tables S5 and S6.
5.2.2. Nature of the Upper Mantle Enriched by CAR and ORS with respect to Melts beneath the WMVF
Melting of subducted carbonates (CAR) and/or organic sediments (ORS) would release volatiles, i.e., carbon dioxide , and generate hydrous-carbonated plumes (HCPs, ) and ORS-related silicate melts under the P-T conditions of the MTZ. Based on the C-He isotope systematics, binary mixing between carbonate and ambient depleted upper mantle was first considered to produce the carbonated peridotite (CP) (Figure 8). Interaction between CP and ORS-derived silicate melts would produce enriched upper mantle (EM), which is considered as the primary source of WMVF volatiles (Figures 6(b) and 8).
The best-fit mixing trajectory with for mixing between ORS and CP was estimated based on an optimal CO2/3He ratio of 1013 due to the lack of carbon contents in the assumed ORS end member, which lies above all the WMVF samples (Figure 8). represents the ratio of He/C values between CP and ORS according to Van Soest et al. . The carbon content (12618 ppm) in the ORS end member is obtained by calculation of the optimal coefficient (1.71). The reasonable upper and lower limits for the CP-ORS mixing are marked by mixing trajectories with values of 17.1 and 0.171, which yield CO2/3He ratios of 1014 and 1012 , respectively.
As indicated by the C-He isotope systematics, EM-like 3He/4He and δ13C ratios beneath WMVF are marked by and −9.7‰ (, −10.3 to −9.1‰; Figures 8 and 9(b)), respectively. Such C-He isotopes indicate an involvement of 10% (in average; ranging from 8% to 13%) carbonate and 13% (in average; ranging from 11% to 15%) ORS-derived silicate melts in the depleted mantle source, along with the Nd-Mg isotope information from potassic basalts in the WMVF (Supplementary Material). Partial melting of the enriched upper mantle (EM) source would produce basaltic magmas and concomitant initial volatiles in the WMVF (Figures 8 and 9(b)).
5.2.3. Crustal Contribution of the Ascending EM-Derived Volatiles
Previous studies have shown that up to 6 km of sediments piled up and underwent significant heating subsidence associated with the early Cretaceous rifting in the Songliao basin [29, 31]. Buried materials consisted of carbonate sediments (CAR) and shallow organic metasediments (COS) [26, 27]. CO2/3He ratios and C-He isotope values of the gas samples in the WMVF display obvious mixing evolution trends between the EM source and crustal components which consists of different proportions of COS and CAR (Figures 6(b) and 8). As indicated by the C-He isotope mixing calculation, the proportion of crustal components (including COS and CAR) ranges from 30% to 60% of the overall carbon budget of the EM source-derived volatiles (Figure 8). In this binary mixing model, continental carbonate sediments (CAR) represent 45% to 82% of the crustal components (Figure 8).
5.3. Genetic Model of Deep CO2 Emissions in the Songliao Continental Rift System
C-He isotope systematics provide constraint on the source region of volatiles collected at the surface in the WMVF. Following the Pacific oceanic crust deep subduction scenario, we proposed a two-stage model to explain the evolution process of cold spring gases in the WMVF. Firstly, the interactions between the depleted mantle and carbonated silicate melts (CAR and ORS) derived from a stagnant Pacific oceanic slab in the mantle transition zone (MTZ) produce an enriched upper mantle beneath WMVF (Figure 9(b)); secondly, EM-derived initial volatiles underwent different proportions of crustal input, including carbonates (CAR) and organic metasediments (COS) in the continental crust during their arising process (Figures 6(b), 8, and 9(a)). Deep-fed CO2 is emitted into the atmosphere through spring bubbles and diffusive soil CO2 emissions in the adjacent areas (Figures 9(a)).
This model provides an integrated constraint on the source region of cold spring gases (e.g., He and C isotopes) for further understanding the carbon cycling processes in the Songliao continental rift system beneath East Asia, which is supported by petrogenesis of potassic basalts erupted in 1721 AD [11–13] and evidence from seismic tomographic studies [14, 72]. The interactions between the MTZ-derived melts and the ambient upper mantle provide a plausible hypothesis to explain the lithospheric thinning, extensive continental rifting, Cenozoic intraplate basaltic volcanism, and magma-related degassing in East Asia (Figure 9(b)) [14, 16, 71].
Our modeling calculated results indicate that the average soil CO2 flux in the Laoheishan volcano (11.8 g m−2 d−1) is lower than that of the whole WMVF (18.7 g m−2 d−1), which suggest weak degassing of the solidified underlying magma body beneath the Laoheishan volcano. On the basis of the C-He isotope mixing simulation results, we propose a two-stage model to constrain the provenance and evolution of volatiles in the WMVF. The first stage is concerned with the interactions between the depleted upper mantle (DMM) and the accumulated Pacific oceanic slab materials (CAR and ORS) in the MTZ and finally results in the formation of the enriched mantle source region (EM), which is considered as the primary source of cold spring gases in the WMVF. The second stage is related to the crustal contamination (including continental organic metasediments and carbonates) when the CO2-dominated gases rise to the surface.
The chemical and C-He isotope data of spring gases used to support the findings of this study are included within the article and the supplementary material.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This work was funded by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB26000000), the Key Research Project of Frontier Sciences of the Chinese Academy of Sciences (Grant No. QYZDY-SSW-DQC030), and the National Science Foundation of China (Grant Nos. 41572321 and 41702361). We thank Drs. Lihong Zhang and Yutao Sun for interpreting the soil flux data. Drs. Liwu Li, Zhongping Li, Li Du, Lantian Xing, and Hengliang Gao are acknowledged for help in analyzing the samples.
The Supplementary Materials file provides additional tables/figures and data mentioned in the revised manuscript, including (1) measured soil CO2 fluxes of whole WMVF (Table S1) and SE slope of the Laoheishan volcanic cone (Table S2), (2) chemical and C-He isotopic compositions of spring gases in this and previous studies (Table S3), (3) detailed reference values for associated parameters (Table S4) and results (Table S5 and S6) in C-He isotope systematics, and (4) detailed reference values for associated parameters (Table S7) and results (Table S8 and Figure S1) in the Nd-Mg isotope coupling calculation. (Supplementary Materials)
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