Abstract

The chemical and isotopic signatures of mud volcanic fluids were determined for 42 mud volcanoes of the Kerch–Taman region (Crimea–Caucasus district). The analysis of spatial geochemical trends of mud-volcanic waters and gases and estimates of formation temperatures using the Mg–Li geothermometer allowed us to attribute the mud volcanic fluids to sediments of the Maikop Formation, the thickness of which in the studied region varies from 1 to 5–6 km. It was shown that an increase of CO₂ and \({\text{HCO}}_{3}^{ - }\) concentrations and values of δ¹⁸O in H₂O and δ¹³C in CH₄, as well as a decrease of Cl^– concentrations and δ¹³C values in \({\text{HCO}}_{3}^{ - }\) and CO₂ correlate with increasing thickness of the Maikop sediments in the central part of the Kerch–Taman region. At the periphery of this region, in the foothills of the Caucasus and Crimea orogens, this correlation is absent. Active tectonic processes in these areas provided contribution of additional “higher temperature” sources of water and gas to fluids in the mud volcanic systems. Using the Kerch–Taman region gases as an example, it was shown for the first time that the enrichment of mud volcanic fluids in isotopically heavy CO₂ (δ¹³C CO₂ to +22.8‰; δ¹³C HCO₃ to +38.9‰) occurs within a narrow temperature range (40–80°C). At higher temperatures, the accumulation of isotopically heavy CO₂ is ceased. These trends suggest that the high-δ¹³C CO₂ is related to the hydrocarbon biodegradation. Using δ¹³C (in CH₄ and CO₂) and δ₂H (in CH₄) classification, it was shown that at least three methane sources could participate in the CH₄ balance: microbial (primary), connected with “biodegradation” (secondary), and thermogenic. The content of “secondary” methane in some cases can reach almost 100%. In the studied mud volcanic gases, the isotopic composition of molecular nitrogen was determined for the first time. The δ¹⁵N values vary from –5.2 to –0.1‰ (δ¹⁵Nav = –2.3 ± 0.9‰, n = 35) and indicate a significant admixture of the non-atmospheric nitrogen. A negative correlation between values of δ¹⁵N in N₂, δ¹³C in \({\text{HCO}}_{3}^{ - }\), and the calculated fluid generation temperatures (t(Mg-Li)) make it possible to attribute the origin of this nitrogen to the transformation of organic matter during the maturation of sediments.

中文翻译：

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刻赤-塔曼地区的泥火山流体：地球化学重建和区域趋势：交流 2. 泥火山气体的成因和区域地球化学趋势

摘要

为刻赤-塔曼地区（克里米亚-高加索地区）的 42 座泥火山确定了泥火山流体的化学和同位素特征。泥火山水和气体的空间地球化学趋势分析以及使用 Mg-Li 地温计估计地层温度使我们能够将泥火山流体归因于 Maikop 组的沉积物，其在研究区域的厚度从 1到 5-6 公里。结果表明，CO ₂和\({\text{HCO}}_{3}^{ - }\)浓度和H _{2 O 中的 δ} ¹⁸ O 和CH ₄中的δ ¹³ C 值的增加，以及随着 Cl ^–浓度和 δ ^13的降低\({\text{HCO}}_{3}^{ - }\)和 CO ₂中的 C值与刻赤-塔曼地区中部的 Maikop 沉积物厚度增加相关。在该地区的外围，在高加索和克里米亚造山带的山脚下，这种相关性是不存在的。这些地区的活动构造过程为泥火山系统中的流体提供了额外的“高温”水和气源。以刻赤-塔曼地区气体为例，首次表明泥质火山流体富集同位素重CO ₂（δ ¹³ C CO ₂至+22.8‰；δ ¹³ C HCO ₃到 +38.9‰）发生在狭窄的温度范围内（40–80°C）。在较高温度下，同位素重的CO ₂的积累停止。这些趋势表明，高δ ¹³ C CO ₂与烃类生物降解有关。使用δ ¹³ C（在CH ₄和CO ₂中）和δ ₂ H（在CH ₄中）分类，表明至少三个甲烷源可以参与CH ₄平衡：微生物（主要），与“生物降解”（次要）和产热有关。在某些情况下，“二次”甲烷的含量几乎可以达到 100%。在研究的泥质火山气体中，首次确定了分子氮的同位素组成。δ ¹⁵ N 值从 –5.2 到 –0.1‰ 不等（δ ¹⁵ Nav = –2.3 ± 0.9‰，n = 35），表明存在大量非大气氮。N _{2中的 δ} ^{15 N 值与}\({\text{HCO}}_{3}^{ - }\)中的δ ¹³ C值之间存在负相关, 和计算出的流体生成温度 (t(Mg-Li)) 可以将这种氮的来源归因于沉积物成熟过程中有机物的转化。