ویژگی‌های ژئوشیمیایی و ایزوتوپی گازهای گسیل شده از گل‌فشان پیرگل، جنوب شرق ایران

نوع مقاله : علمی -پژوهشی

نویسندگان

1 گروه ژئوشیمی، دانشکده علوم زمین، دانشگاه خوارزمی، تهران، ایران

2 گروه ژئوشیمی، دانشکده علوم زمین، دانشگاه فلورانس، ایتالیا

3 موسسه کانی‌شناسی تجربی، آکادمی علوم، روسیه

4 موسسه زمین‌شناسی کانسارهای معدنی، پتروگرافی، کانی‌شناسی و ژئوشیمی، آکادمی علوم، روسیه

چکیده

گل­فشان­ها مجراهای مهاجرت رو به بالای سیالات عمیق پرفشار در حوضه­های رسوبی در نتیجه نیروهای عمدتاً فشارشی هستند. گل­فشان پیرگل به عنوان بزرگ­ترین گل­فشان ایران بین دو آتشفشان تفتان و بزمان در جنوب شرق ایران واقع شده ­است. مطالعات ژئوشیمیایی و ایزوتوپی بر روی گازهای منتشر شده از مکان­های تراوش فعال این گل فشان برای تعیین منشأ گازهای هیدروکربوری و غیرهیدروکربوری انجام شد. داده­های ترکیب شیمیایی گاز نشان می­دهد که دی اکسیدکربن گاز خروجی غالب و متان دومین گاز خروجی در این گل­فشان است. مقادیر کمی از گازهای هیدروکربوری سنگین­تر مانند اتان، پروپان، ایزوبوتان و ان- بوتان هم مشاهده شد. منشأ ترموژنیک گازهای هیدروکربوری توسط نسبت متان به مشتقات هیدروکربوری سنگین­تر (C1/C2+) پایین و مقادیر δ13C متان  از ۷/۴۰- تا ‰ ۲/۴۲- آشکار می­شود که ممکن است مرتبط با شکستن حرارتی موادآلی ناشی از وجود فرآیندهای حرارتی یا منابع گرمایی باشد. حضور گازهای هیدروکربنی ترموژنیک در گل­فشان پیرگل می­تواند بیانگر احتمال حضور سیستم هیدروکربوری در این منطقه باشد. مقادیر ایزوتوپی کربن دی اکسیدکربن  از ۹/۱۱- تا ‰۸/۱۳- در گازهای تجزیه شده نیز حدواسط بین دی اکسیدکربن تولید شده در طی فرآیند ترموژنیک موادآلی و دی اکسیدکربن ناشی از سیستم­های آتشفشانی است. نقش گوشته بالایی به عنوان یکی از منشأهای احتمالی گازهای خروجی از گل­فشان پیرگل توسط مقادیر ایزوتوپی هلیم 3He/4He از ۵۸/۱ تا  R/Ra۶۱/۱ مشخص می­شود که ممکن است مرتبط با حضور سیالات سیستم­های زمین گرمایی منطقه باشد.

کلیدواژه‌ها


عنوان مقاله [English]

Geochemical and isotopic characteristic of emitted fluids from Pirgel mud volcano, SE Iran

نویسندگان [English]

  • Mahin Farhadian Babadi 1
  • Behzad Mehrabi 1
  • Franco Tassi 2
  • Michael Zelenski 3
  • Ilya Chaplygin 4
  • Ata Shakeri 1
  • Stefania Venturi 2
1 Department of Geochemistry, Faculty of Earth Sciences, Kharazmi University, Tehran, Iran
2 Department of Geochemistry, Faculty of Earth Sciences, Florence University, Italy
3 Institute of Experimental Mineralogy, Academy of Science, Russia
4 Institute of Geology of Ore Deposits, Petrology, Mineralogy and Geochemistry, Academy of Science, Russia
چکیده [English]

IntroductionMud volcanoes or sedimentary volcanoes, represent one of the most intriguing phenomena of the Earth's crust. They are important for energy resource exploration, seismicity, geo-hazard and atmospheric budget of greenhouse gases. These structures are surface expressions of fluids including gas, water and mud inside hydrocarbon-bearing sedimentary basins. The discharged gas is typically dominated by methane, however where hydrocarbon systems are located close to subducting slabs, relatively high geothermal gradient environments or are related to the final stages of thermogenic gas generation, the gas can be mainly CO2 or N2. The Pirgel mud volcanoes, are the largest Iranian mud volcanos, located between Taftan and Bazman volcanoes.Materials and methodsDuring this study, twelve gas samples were collected from all gas emitting points including pool, gryphon and cone using Giggenbach-alkaline solution in order to evaluate the hydrocarbon and non-hydrocarbon gas sources. The molecular composition of gas consisting methane and heavier hydrocarbon derivatives, CO2, O2, N2, He, Ar and He, carbon isotopic composition of methane and CO2 and He isotopic signature were determined by using GC and GC-MS methods, respectively.Results and discussionThe gases emitted by Pirgel mud volcano are CO2-rich ranging from 83.9 to 88.7 vol. %. The other gas components are CH4 (9.31-12.90 vol. %), N2 (1.28-2.08 vol. %), C2H6 (0.43-0.54 vol. %), C3H8 (0.051-0.098 vol. %), iC4H10 (0.027-0.052 vol. %), nC4H10 (0.016-0.024 vol. %), O2 (0.088-0.180 vol. %), Ar (0.028-0.050 vol. %), He (77.76-111.77 ppm) and Ne (0.44-5.41 ppm).Carbon dioxide in CO2-richgas reservoirs is produced by a variety of organic and inorganic processes.Different CO2 sources can be recognized based on the 13C/12C ratios: 0‰ vs. V-PDB, <−20‰ vs. V-PDB and −6.5 ± 2.5‰ vs. V-PDB for marine limestone, alteration of organic material and Mid-Oceanic Ridge Basalts (MORB), respectively.At a first approximation, the δ13C-CO2 values measured in the CO2-rich Pirgel gases (from −11.9 to −13.8‰ vs. V-PDB) are intermediate between those typical of mantle and a biogenic source. The R/Ra values of these gases are ∼1.6 R/Ra, with low air contamination as indicated by the high He/Ne ratios (14–257). The contribution of upper mantle, as a probable source for discharged gas from Pirgel mud volcano is likely linked to geothermal fluids of nearby volcanic systems which is documented by 3He/4He isotopic ratio of ∼1.61 R/Ra. The CO2/3He ratios, ranging from 20×109 to 30×109, i.e. slightly higher than that typically measured for MORB (2×109–1×1010), coupled with the above mentioned δ13C-CO2 values, suggest that the origin of the CO2 discharged from Pirgel mud volcano is related to both biogenic and volcanic sources. The hydrocarbon gases are thermogenic in origin, evidenced by low methane to heavier hydrocarbon components (C1/C2+) ratio and δ13CCH4 valuesfrom -40.7 to -42.2 ‰. The presence of thermogenic gases are likely related to thermal degradation of organic matters in response to the existence of thermal processes or heat sources.ConclusionPirgel mud volcano emits gases dominated by CO2 and shows relatively high R/Ra values (∼1.6). Gas geochemistry shows that the CO2-rich gases characterizing this mud volcano probably originated from both thermal degradation of sedimentary organics and hydrothermal fluids from the neighboring volcanic complex. The area probably hosts the hydrocarbon systems testified by the presence of thermogenic gases and dark oil appearance slicks in Pirgel mud volcano.

کلیدواژه‌ها [English]

  • Helium isotopes
  • Carbon Dioxide
  • Hydrocarbon and non-hydrocarbon gases
  • Pirgel mud volcano
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