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 [
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
(a) Geological and tectonic map showing the locations of major faults and Cenozoic basaltic volcanic fields in NE China (modified from Liu et al. [
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.
The Wudalianchi monogenetic volcanic field (WMVF) is an active K-rich volcanic region located in a continental rift system in NE China (Figure
Cenozoic volcanic activity has formed 800 km2 of lava flows, which includes the middle Pleistocene to Holocene volcanic activity in WMVF (Figure
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
Soil CO2 fluxes were measured
Gas samples from four cold bubbling springs (Figures
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
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
Caracausi et al. [
In this study, 50 sequential Gaussian simulations were performed over a grid of 108001 square cells (
CO2 flux map in the SE slope of the Laoheishan volcanic cone obtained by sequential Gaussian simulations.
The chemical and C-He isotopic compositions of gases from the WMVF cold springs are listed in Table
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 | 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)
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
Triangle plot of N2-He-Ar (a) and CO2-3He-4He (b) for the cold spring gases in WMVF (modified from Giggenbach et al. [
(a)
The isotopic composition of carbon and helium has been successfully used to quantify the carbonate vs. sediment contribution from subducted slab material recycling [
The
Measured 3He/4He ratios (
3He/4He (
Estimated carbon sources for the spring gas samples in WMVF.
Sample no. | Reference | 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 | [ |
−5.3 | 18.8 | 0.08 | 0.26 | 0.66 | 0.74 |
FH2 | [ |
−8.2 | 5.1 | 0.29 | 0.34 | 0.37 | 0.66 |
BY1 | [ |
−6.5 | 28.5 | 0.05 | 0.33 | 0.61 | 0.66 |
NYQ | [ |
−5.1 | 27.0 | 0.06 | 0.26 | 0.69 | 0.76 |
WBQ | [ |
−5.8 | 97.6 | 0.02 | 0.30 | 0.68 | 0.70 |
DZT | [ |
−7.4 | 2.3 | 0.65 | 0.17 | 0.18 | 0.83 |
SG2 | [ |
−6.6 | 1.7 | 0.88 | 0.05 | 0.07 | 0.95 |
Average | 0.30 | 0.24 | 0.46 | 0.76 |
Zhang et al. [
Samples with CO2/3He ratios lower than that of DMM are thought to result from the physical-chemical fractionation of CO2 to He [
Samples within the mixing trajectories have contributions from DMM, ORS, and CAR end members (Figure
The diagram of 3He/4He (
In the C-He isotope systematics based on Van Soest et al. [
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.
Reference values for associated parameters of DMM, CAR, ORS, and COS end members used in C-He isotope systematics.
End member | 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[
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 [
Melting of subducted carbonates (CAR) and/or organic sediments (ORS) would release volatiles, i.e., carbon dioxide [
The best-fit mixing trajectory with
As indicated by the C-He isotope systematics, EM-like 3He/4He and
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.
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 [
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
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 [
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.
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.