ژئوشیمی و منشا عناصر تشکیل‌دهندۀ نهشته‌های تراورتن آذرشهر (آذربایجان شرقی)

رسوبات تراورتن در جنوب‌غرب آذرشهر (استان آذربایجان شرقی) در منطقۀ وسیعی گسترش دارند و در حال حاضر نیز چشمه‌های فعال امروزی در بخش‌هایی در‌حال ته‌نشست تراورتن هستند. با‌توجه‌به مشاهده‌های صحرایی، ریخت‌شناسی رایج نهشته‌های تراورتنی آذرشهر از نوع برجا و شامل تپه‌ای و مخروطی، شکاف پشته‌ای و آبشاری است. مطالعه‌های ژئوشیمیایی انجام‌شده روی نهشته‌های قدیمی و امروزی تراورتن در امتداد دو چشمۀ تاپتاپان و قزل‌داغ نشان می‌دهند مقدار عنصر آهن در این نهشته‌ها به‌طور مشخص بر درون‌زاد (ترموژن)‌بودن و مقادیر استرانسیم، باریم و بریلیم بر سنگ منشا آهکی، تبخیری یا دولومیتی آنها دلالت دارد. مقادیر تهی‌شدۀ ایزوتوپ اکسیژن (δ18o) و غنی‌شدۀ ایزوتوپ کربن (δ13c) نیز ترموژن‌بودن و سنگ منشا کربناتۀ این نهشته‌ها را نشان می‌دهد. بر اساس مطالعه‌های انجام‌شده می‌توان نتیجه گرفت سیالات گرمابی ابتدا گاز co2 منشاگرفته از ماگما را با خود حمل کرده‌اند و هنگام حرکت رو به بالا، کربن‌زدایی ضمن برخورد با سنگ‌های کربناته انجام شده است. این سیالات، co2 حاصل از هیدرولیز کربنات‌ها را با خود حمل می‌کنند و به‌شکل چشمه‌های آب گرم در سطح ظاهر می‌شوند، و در اثر خروج سریع co2 به‌شکل تراورتن رسوب می‌کنند.

Geochemistry and origin of elements in the Azarshahr travertine deposits (Eastern Azarbaijan)

Abstract: Travertine deposits cover a vast area in the southwest of Azarshahr (East Azarbaijan province) and in some parts there are active springs which currently deposit these types of sediments. Regarding the field observations in the Azarshahr area, the typical morphology of Azarshahr travertine deposits is in situ and consists of the mounds, fissureridge and cascades. Geochemical studies on the travertine deposits of the Taptapan and Qizildagh springs revealed that the amount of iron in travertine deposits clearly emphasizes their thermogenic nature. The amounts of strontium, barium and beryllium also indicate the thermogenesis and calcareous, evaporative or dolomite origin. The depleted oxygen isotope (δ18O) and enriched carbon isotope (δ13C) is also indicator of thermogenic source, and carbonate source rocks for these deposits. Based on geochemical studies, it can be concluded that hydrothermal fluids may have initially taken the CO2derived from magma, and during the upward movement with carbonate rocks, decarbonization has been occurred. These fluids transport CO2 from carbonates, and then they appear as spring’s water at the surface and precipitate travertine due to the rapid release of CO2. Keywords: Geochemistry, Travertine, Thermogene, Azarshahr   Introduction: Continental carbonates comprise a wide range of lithologies including speleothem, calcrete, palustrine, travertine and tufa. Travertine and tufa deposits frequently associated with limestone dissolution in superficial (epigean) or deep (hypogean) hydrogeological reservoirs (Pentecost and Viles 1994; Pentecost 2005; Jones and Renaut 2010; Capezzuoli et al. 2014; Gandin and Capezzuoli 2014). However, a large variety of other rocks can occur as substratum and source of elements building these carbonates. Many studies have shown that igneous rocks (basalts, rhyolites, ultramafics, granites) and sedimentary rocks  such as dolostones, evaporates and marls may constitute derivative sources for calcium and other elements required for travertine and tufa buildup and this can be determined by elemental and isotopic geochemical studies. Travertine deposits cover a vast area in the southwest of Azarshahr (East Azarbaijan province) and in some parts there are active springs. The area is located on the western margin of Sahand volcanic complex and eastern margin of Lake Urmia. Given the importance of geochemical studies mentioned in travertine studies, the main objective of this study is to emphasize the use of geochemical data in travertine classification, determination of origion of elements building travertines, CO2 origin and comparison of Azarshahr region samples with global thermogene and metheogene travertines.   Material & Methods: For petrographic studies on travertine samples deposited around Azarshahr springs, 20 samples of travertine sediments and old travertine rocks around two modern travertine springs called the Taptapan and Ghezelagh are collected. In order to compare the geochemical chatactristic of samples, ten samples of travertine rock samples in the quarry near Qizildagh spring are also colleted. Fifteen samples were examined for mineralogical composition using XRD analysis. For geochemical analysis (ICPMS), seven travertine samples around Taptapan and Qizildagh springs and three travertines samples of quarry near Qizildagh spring, has been analysed at the Zarazma Laboratory, Zanjan. Moreover, for the purpose of isotopic analysis of δ18O and δ13C, four travertines samples around the Taptapan spring were selected and analysed at the Isotopes Research Laboratory of the Arak University.   Discussion of Results & Conclusions: According to the geochemical studies of the major elements on the travertine of the Azarshahr, iron, sodium and potassium elements clearly emphasize the thermogenesis of these travertines, but other trace elements cannot definitely determine the type of travertine. The concentration of rare earth elements in Taptapan spring samples is lower than that of Qizildagh samples, indicating a lower reaction rate of this spring with source rocks and its rapid ascent to Earth’s surface compared to Qizildagh spring. Due to high concentrations of strontium and low concentration of barium, the Azarshahr travertine deposits plotted in the range of hypogean Calcite and Aragonite Travertine and Tufa (CATT) originated from limestone, evaporaites and dolostones. The origins of the samples using beryllium values indicate that the Azarshahr travertine was similar to the values of this element in hypogean travertines with carbonate sources. The depleted δ18O and enriched δ13C is also indicator of thermogenic source, and carbonate or igneous source rocks for these deposits. Based on this study, it seems that hydrothermal fluids may have initially taken the CO2derived from magma, and during the upward movement with carbonate rocks, decarbonization has been occurred. These fluids transport CO2 from carbonates, and then they appear as spring’s water at the surface and precipitate travertine due to the rapid release of CO2.