Formation of ¹⁸O‐depleted dolomite within a marine evaporitic sequence, Triassic Reichenhall Formation, Austria

Abstract

Dark grey, bituminous dolostones interbedded with marine‐derived anhydrite horizons occur in the Triassic Reichenhall Formation of western Austria. Fossils are rare and indicate a hostile, hypersaline depositional environment. The dolomites are finely crystalline, fairly stoichiometric, well ordered and non‐ferroan. Closely spaced samples (94 in total) of individual dolomite units have been analysed for their carbon and oxygen isotopic composition. The data indicate surprisingly low δ18O values (‐5.7 to ‐2.1%0 PDB), whereas the δ13C values are comparable to the contemporary Triassic seawater (+0.2 to +2.6%0 PDB). Sedimentological evidence, including (i) lack of any evidence for extensive dissolution, (ii) distinct oxygen and carbon isotope ratios of individual dolomite units, (iii) covariance of carbon and oxygen isotopes within some dolomite layers and (iv) inclusions of celestite in dolomite, indicates a nearly closed system after early diagenesis. Combining this information with water‐rock interaction calculations suggests that the lightest oxygen isotope compositions are the result of freshwater influx into the basin during very early dolomite formation. A secondary factor may be dolomite recrystallization at elevated temperatures during burial.

Hydrothermal dolomitization in a complex geodynamic setting (Lower Palaeozoic, northern Spain)

“…These cements might have preserved the isotopic signature of early diagenesis operated by fresh water fluids. Triassic evaporitic sabkha dolomites were formed under a comparable setting and the lightest oxygen isotopes measured are interpreted as due to a fresh water influx (Spötl & Burns, ).…”

The Kilmar Magnesite Deposits: Evaporitic Metasediments in the Grenville Supergroup, Morin Terrane, Quebec

“…The lower oxygen isotope ratios of some magnesite deposits (<10-15% ) often correlates with higher metamorphic grade of orogenic host rocks, suggesting high-temperature fluid interaction [43]. In some magnesite deposits hydrothermal alteration by meteoric water may have lowered oxygen isotope ratios (e.g., [47]), an effect which is not uncommon in marine evaporitic dolomites [48], but is not observed at Kilmar. The magnesite and dolomitic marble host rock from the Dobbie mine are more consistent with a marine or marine diagenetic origin than a later metasomatic genesis.…”

川东北地区三叠系飞仙关组白云化流体温度

“…不过, 由于川东 北地区飞仙关组白云化流体更多被认为是与川东 北孤立碳酸盐岩蒸发台地的蒸发海水或海源流体有 关 [2,4~6,14] , 飞仙关组中大量出现和广泛分布的蒸发盐 类矿物、膏盐层是这一推断的典型证据, 因而飞仙关 组白云化流体的氧同位素组成显然高于三叠纪正常 海 水 的 氧 同 位 素 组 成 (0‰(SMOW) [38] ). Spötl 和 Burns [39] 曾提出弱蒸发三叠纪海水的氧同位素组成为 1‰ (SMOW), 但对于川东北地区飞仙关组中出现的 较强蒸发海水很可能具有更高的氧同位素组成, 故我 们选取 Spötl 和 Burns [39] 建议值的两倍-2‰(SMOW) 作为川东北地区飞仙关组白云化流体的氧同位素组 成, 该取值也与四川盆地(包括川东北地区)飞仙关组 邻近的上覆地层三叠系嘉陵江组和雷口坡组、 下伏地 层二叠系的气田卤水氧同位素组成主要分布范围 (2‰~5‰ (SMOW) [40] )的初始值一致(图 5). [1,8,16,20,21,44,45] 的主要烃源层(上二叠统烃源岩)的有机质在晚三叠世 晚期-早侏罗世进入大量成油期 [1,8,16,20,21,44,45] , 大规模 侵位的液态烃已经基本替换了原有空间的多数白云 化流体, 占据有限空间的残余或新进白云化流体可 能更多只在液态烃充注间隙期或减弱期发生作用, 因而这些高温白云化流体的作用强度远弱于低温白 云化流体, 直接造成了它们虽具有更高的温度, 但其 形成的白云石数量远少于低温白云化流体.…”