The origin of methane in serpentinite-hosted hyperalkaline hot spring at Hakuba Happo, Japan: Radiocarbon, methane isotopologue and noble gas isotope approaches
The origin of methane in serpentinite-hosted hyperalkaline hot spring at Hakuba Happo, Japan: Radiocarbon, methane isotopologue and noble gas isotope approaches
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DOI：
10.1016/j.epsl.2022.117510
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发表时间：
2022-05
影响因子：
5.3
通讯作者：
K. Suda;T. Aze;Y. Miyairi;Y. Yokoyama;Y. Matsui;Hisahiro Ueda;T. Saito;Tomohiko Sato;Y. Sawaki;R. Nakai;H. Tamaki;H. Takahashi;N. Morikawa;S. Ono
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作者：
K. Suda;T. Aze;Y. Miyairi;Y. Yokoyama;Y. Matsui;Hisahiro Ueda;T. Saito;Tomohiko Sato;Y. Sawaki;R. Nakai;H. Tamaki;H. Takahashi;N. Morikawa;S. Ono
与海洋和陆地蛇纹石化遗址相关的甲烷（ch4）被认为是非生物起源的。然而，碳的来源以及甲烷合成的深度和温度往往尚无定论。通过测量日本白波哈波2口高碱性地热井（Happo# 1和Happo# 3）天然气样品中甲烷的放射性碳（14c）、总体稳定同位素比值（δ 13c和δD）和同位素丰度（Δ 13ch3 D），以及稀有气体同位素组成，以限定碳源和甲烷生成过程。两个地点的甲烷样品几乎不含14c，而Happo# 1井内的碳酸盐沉淀物含有14c，相当于51%至62%的现代碳，这表明大部分甲烷不是由温泉水中溶解的碳酸盐的还原产生的。来自Happo# 1和Happo# 3的甲烷样品产生Δ 13基于甲烷的表观温度分别为206 - 40+ 52°C和323 - 85+ 143°C，远高于测量的井水温度（~ 50°C）。因此，14c和块状同位素温度表明，白波Happo ch4产生于浅层大气水循环深度以下。Happo# 1和Happo# 3的3 He/4 He比值分别为4.10和4.47 R / atm。3 He/4 He和4 He/20 Ne的比值表明，约50%的He来自地幔，表明白波哈波温泉从地幔吸收了挥发物，包括co2。然而，观测到的δ 13 C CH4值（约为−35‰）明显低于典型的幔源碳（-5‰），表明幔源CO 2不是CH4形成的主要碳源。与与蛇纹石化无关的地区温泉相比，白波Happo流体中较高的ch4浓度表明，ch4是由13 - C-贫碳化石碳源和蛇纹石化产生的h2在高温（bb0 ~ 200℃）下产生的，随后被带入较冷的循环大气水系统。
Methane (CH 4) associated with marine and terrestrial sites of serpentinization has been proposed to be abiotic in origin. However, the source of carbon and the depth and temperature of CH 4 synthesis often remain inconclusive. We measured the radiocarbon (14 C), bulk stable isotope ratios (δ 13 C and δD) and isotopologue abundance (Δ 13 CH 3 D) of CH 4, and noble gas isotope composition of gas samples from two hyperalkaline geothermal wells (Happo# 1 and Happo# 3) at Hakuba Happo, Japan, to constrain the source of carbon and the CH 4 generation processes. The CH 4 samples from both sites were nearly 14 C-free, whereas the carbonate precipitates inside the Happo# 1 well contained 14 C corresponding to 51 to 62 percent modern carbon, indicating that the majority of CH 4 was not generated from the reduction of dissolved carbonate in the hot spring water. CH 4 samples from Happo# 1 and Happo# 3 yielded Δ 13 CH 3 D-based apparent temperatures of 206− 40+ 52° C and 323− 85+ 143° C, respectively, which are much higher than the measured well water temperatures (∼ 50° C). Therefore, 14 C and clumped isotopologue temperatures suggest that Hakuba Happo CH 4 was generated below the depth where the shallow meteoric water circulated. The 3 He/4 He ratios were 4.10 and 4.47 R atm for Happo# 1 and Happo# 3, respectively. The 3 He/4 He and 4 He/20 Ne ratios revealed that approximately 50% of He was of mantle origin, suggesting that the Hakuba Happo hot spring received volatiles, including CO 2, from the mantle. However, the observed δ 13 C CH4 values (approximately− 35‰) were significantly lower than those of typical mantle-derived carbon (–5‰), implying that mantle-derived CO 2 is not the major carbon source for CH 4 formation. High CH 4 concentrations in the Hakuba Happo fluids, compared to those in hot springs in the area not associated with serpentinization, suggest that CH 4 was generated from 13 C-depleted fossil carbon sources and serpentinization-derived H 2 at high temperatures (> 200° C), and subsequently entrained into the cooler circulating meteoric water system.