Geofluids
Volume 2019 (2019), Article ID 8053579, 14 pages
https://doi.org/10.1155/2019/8053579

Research Article

Volcanogenic CO₂ Degassing in the Songliao Continental Rift System, NE China

Wenbin Zhao,1,2,3 Zhengfu Guo,1,2,3 Ming Lei,1,2,3 Maoliang Zhang,1,2,3 Lin Ma,1,2,3 Danielle Fortin,⁴ and Guodong Zheng5,6

¹Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
²CAS Center for Excellence in Life and Paleoenvironment, Beijing 100044, China
³College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
⁴Department of Earth and Environmental Sciences, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5
⁵Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China
⁶Key Laboratory of Petroleum Resources, Gansu Province, Lanzhou 730000, China

Correspondence should be addressed to Zhengfu Guo; zfguo@mail.iggcas.ac.cn

Received 26 January 2019; Revised 14 May 2019; Accepted 17 June 2019; Published 19 August 2019

Academic Editor: Giovanni Martinelli

Copyright © 2019 Wenbin Zhao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

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.

1. Introduction

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 [6]. 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 [4]. Intraplate volcanism along continental rift systems is primarily derived from the metasomatized mantle [7]. 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)) [10]. 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 [22] hypothesized partial melting of a subcontinental lithospheric mantle (SCLM) metasomatized by ancient fluids.

(a)

(b)

(c)

Figure 1: (a) Geological and tectonic map showing the locations of major faults and Cenozoic basaltic volcanic fields in NE China (modified from Liu et al. [10]). The inset shows the location of Eastern China. Abbreviations of Cenozoic volcanic fields in NE China are as follows: WMVF: Wudalianchi monogenetic volcanic field; XK: Xunke; KL: Keluo; NMH: Nuominhe; XGLH: Xiaogulihe; AES: Aershan; JPH: Jingpohu; LG: Longgang; CBS: Changbaishan; KD: Kuandian; SLB: Songliao basin; NCC: North China craton. (b) Geological map of WMVF (modified from Zhao et al. [23]). (c) Geomorphologic map showing the location of monogenetic volcanoes and cold springs in WMVF. Red dots represent the location of the soil CO₂ flux survey, and the stars represent the sample locations.

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 [25]. 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 [10], which constitute the eastern part of the Asian tectono-magmatic province that extends from Lake Baikal, Siberia, to Eastern China [28].

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 [23], which finally led to the unique volcanic landscapes seen nowadays in WMVF [24], 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 [32].

(a)

(b)

(c)

(d)

(e)

Figure 2: Photos showing the Fanhua spring (a), Hualin spring in August 2017 (b) and April 2019 (c), basaltic lava flow under the thin pumiceous deposits beside the Laoheishan volcanic cone (d), and dominant tectonic weakness direction enlarged by erosion in the Huoshaoshan volcanic cone (e).

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 [33]. 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. [34]. The edge of the inverted cylindrical chamber was sealed with damp soil to diminish air contamination of the soil gases in the chamber [35]. 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 [16]. 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. [36] and Zhang et al. [37].

4. Results

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 [38]: 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. [39]: (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)).

(a)

(b)

Figure 3: Cumulative probability plot of calculated soil CO₂ fluxes in the whole WMVF (a) and SE slope of the Laoheishan volcanic cone (b). Black solid lines represent the partition components of Groups A, B, and C and D, E, and F.

Caracausi et al. [40] 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) [41].

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.

Figure 4: CO₂ flux map in the SE slope of the Laoheishan volcanic cone obtained by sequential Gaussian simulations.

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.

Table 1: Chemical and C-He isotopic compositions of the spring gases in the WMVF.

Sample no.
T
N₂
O₂
Ar
CO₂
CH₄
He
He/Ar
N₂/Ar
⁴He/²⁰Ne

δ¹³C (‰)
CO₂/³He (×10⁹)

(°C)
(%)
(%)
(%)
(%)
(%)
(ppm)

NYQ17
9.2
8.0
0.35
0.16
91.1
0.07
129
0.08
50
182
2.64
713
2.64
−7.2
1.9

NYQ18
7.7
8.7
0.55
0.18
90.5
0.14
278
0.15
49
153
3.10
597
3.10
−5.6
0.8

BYQ17
7.8
3.4
0.32
0.06
95.8
0.10
379
0.64
58
685
2.26
2675
2.26
−7.3
0.8

BYQ18
6.5
3.2
0.25
0.07
96.5
0.38
18
0.03
46
40
3.16
155
3.17
−6.0
12.1

HLQ17
19.8
4.6
0.88
0.08
94.1
0.02
10
0.01
58
38
2.34
150
2.35
−5.0
29.5

HLQ18
24.0
4.9
1.29
0.09
93.7
0.04
433
0.48
54
72
2.97
281
2.98
−2.6
0.5

FHQ17
12.7
15.8
3.66
0.25
80.2
0.08
718
0.28
62
145
2.81
568
2.81
−4.3
0.3

FHQ18
16.0
22.6
0.21
0.43
76.7
0.89
1845
0.43
53
359
3.18
1401
3.18
−2.5
0.1

(1) is the observed ³He/⁴He ratio divided by the ³He/⁴He ratio in the air (, [42]). (2) , where β represents the Bunsen coefficients assuming a groundwater recharge temperature of 10°C (βNe/βHe = 1.25, [43]), (⁴He/²⁰Ne)M is the measured ratio of samples, and (⁴He/²⁰Ne)Air is the ratio of the air (0.32, [42]). (3) is the air-corrected Helium isotope ratio by applying the following formula [44]: .

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 (, [46]) and depleted mantle (, [46]), indicating CO₂ addition and/or loss during volatile ascending processes (Figure 5(b)).

(a)

(b)

Figure 5: Triangle plot of N₂-He-Ar (a) and CO₂-³He-⁴He (b) for the cold spring gases in WMVF (modified from Giggenbach et al. [47]). Filled and open symbols represent, respectively, data in this study and published data in [18–22, 48]. The compiled data in previous studies are available in Supplementary Table S3. Abbreviations: NYQ: South spring; BYQ: North spring; FHQ: Fanhua spring; HLQ: Hualin spring; ASW: air-saturated water; Arc average: arc-related volatiles (, [49]; , [46]); SCLM: subcontinental lithospheric mantle (, [50]; , [51]); DMM: depleted MORB-source mantle (, [52]; , [46]).

(a)

(b)

Figure 6: (a) δ¹³C (‰) versus CO₂/³He ratios for cold spring gases from WMVF. The yellow arrow denotes the effects of He-CO₂ fractionation on the samples outside the mixing envelope. (b) ³He/⁴He (RA) versus CO₂/³He ratios for cold spring gases from WMVF. Abbreviations and reference values for end members are as follows: CAR: carbonate rocks (, [53]); ORS: organic sediments (, [35]). The ³He/⁴He () ratios of CAR and ORS end members are assumed to be [66] and the CO₂/³He values ranging from 10¹² to 10¹⁴ [62]. The CO₂/³He ratios of DMM are ranging from to , with an average of [46]. The continental crustal source has a wide range of CO₂/³He ratios based on [62]. Data source and symbols are as in Figure 5.

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 [46] and the mantle vs. crustal contribution [44], 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‰, [46]). Some samples show relatively heavier carbon isotope compositions when compared to upper mantle values (, [46]), suggesting a possible contribution from inorganic carbon-rich components (carbonate rocks, , [53]).

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, , [52]) and the subcontinental lithospheric mantle (SCLM, , [50]), which point to a mixed origin of volatiles from the upper mantle and an end member with high radiogenic ⁴He (e.g., crustal materials, , [42]) (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 (, [42]; , [54]), which indicate that the high N₂ and low CO₂ contents do not result from the air contamination in the Fanhua spring gases.

Figure 7: ³He/⁴He () versus (He/Ne)M/(He/Ne)ASW for cold spring gases in WMVF. The calculated binary mixing curves between the air-saturated water (ASW) and crust-mantle mixtures with variable proportions of mantle helium (e.g., 100%, 35%, 20%, 10%, 5%, 1%, and 0%) are shown. Reference values for end members are as follows: ASW: [55], crust: [42] and [56], and DMM: [57]. Data source and symbols are as in Figure 5.

Table 2: Estimated carbon sources for the spring gas samples in WMVF.

Sample no.
Reference
δ¹³C (‰)
CO₂/³He (×10⁹)
Carbon inventory

DMM
ORS
CAR
M+C

HLQ17
This study
−5.0
29.5
0.05
0.25
0.70
0.75

NYQ17
This study
−7.2
1.9
0.79
0.11
0.10
0.89

BYQ18
This study
−6.0
12.1
0.12
0.28
0.60
0.72

HL2
[21]
−5.3
18.8
0.08
0.26
0.66
0.74

FH2
[21]
−8.2
5.1
0.29
0.34
0.37
0.66

BY1
[21]
−6.5
28.5
0.05
0.33
0.61
0.66

NYQ
[18]
−5.1
27.0
0.06
0.26
0.69
0.76

WBQ
[18]
−5.8
97.6
0.02
0.30
0.68
0.70

DZT
[18]
−7.4
2.3
0.65
0.17
0.18
0.83

SG2
[20]
−6.6
1.7
0.88
0.05
0.07
0.95

Average
0.30
0.24
0.46
0.76

5. Discussion

5.1. Carbon Provenance of Volatiles in WMVF

Zhang et al. [16] 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 [61]. 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 [62]. 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. [64] 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) [67]. 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)) [62].

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. [59], 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.

Figure 8: C-He isotope systematics showing a two-stage model for cold spring gases in WMVF. Stage 1: interactions between slab-derived melts (CAR and ORS) in MTZ and the depleted upper mantle (DMM); stage 2: the different proportions of crustal contribution (COS and CAR) during the ascending process of EM-derived volatiles. COS represents the continental organic metasediments.

Table 3: Reference values for associated parameters of DMM, CAR, ORS, and COS end members used in C-He isotope systematics.

End member
δ¹³CCO2 (‰)
C contents (ppm)
³He/⁴He ()
He contents (ppm)
CO₂/³He (×10⁹)

DMM
−6.5a
1920b
8a
0.0288b
1.5a

CAR
0c
11400d
0.05e
0.00023d
1000-100000f

ORS
−18.5g
12618j
0.05e
0.0667j
1000-100000f

COS
−11h
Unknown
0.02i
Unknown
0.1-100000f

Data sources: a[46], b[37], c[53], d[59], e[68], f[62], g[65], h[69], and i[66]. jCalculated based on U-Th decay.

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 [13]. The COS end member was assumed to have a helium ratio equal to that of the bulk continental crust (, [66]). 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 [70], and generate hydrous-carbonated plumes (HCPs, [71]) 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. [59]. 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 [62], 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)).

(a)

(b)

Figure 9: Genetic model of the cold spring gases in WMVF. (a) Crustal contribution to the EM-derived volatiles. (b) A schematic map shows the interactions between the depleted mantle and the slab materials in the MTZ. Abbreviations are as follows: MTZ: mantle transition zone; Moho: Mohorovičić discontinuity; SCLM: subcontinental lithospheric mantle; CC: continental crust.

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].

6. Conclusions

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.

Data Availability

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.

Acknowledgments

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.

Supplementary Materials

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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