Microcrystalline dolomite in a middle Permian volcanic lake: Insights on primary dolomite formation in a non‐evaporitic environment
Microcrystalline dolomite in a middle Permian volcanic lake: Insights on primary dolomite formation in a non‐evaporitic environment
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DOI：
10.1111/sed.13031
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发表时间：
2022-08
期刊：
影响因子：
3.5
通讯作者：
X. Jiao;Yiqun Liu;Wan Yang;Hong Li;Z. Meng;Minru Zhao;Zhexuan Li
中科院分区：
文献类型：
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作者：
X. Jiao;Yiqun Liu;Wan Yang;Hong Li;Z. Meng;Minru Zhao;Zhexuan Li
湖相白云石成核通常发生在现代和新近纪蒸发碱性湖泊中。因此，古代湖相微晶白云石通常被解释为在蒸发环境中形成。然而，这项研究表明，白云石的非蒸发起源是在火山热液湖中沉淀的，其中热液和火山过程相互作用。白云岩产于中国西北三塘湖陆内裂谷盆地中二叠统芦草沟组湖相细粒沉积岩中。白云岩主要由纳米级至微米级白云石组成，形状为自形至亚面形，阳离子有序度较低，与凝灰质页岩互层夹。非白云石矿物，包括石英、碱性长石、蒙脱石和菱镁矿以不同比例与白云石混合。白云岩的 87Sr/86Sr 比值（0.704528 ~ 0.705372，平均值 = 0.705004）和 δ26Mg 值（-0.89 ~ -0.24‰，平均值 = -0.55‰）与地幔岩石相似，表明沉积物主要来源于从地幔向上运移并受到影响的流体。 深层的水-岩反应。白云岩的δ18O值（−3.1至−22.7‰，平均值=−14.0‰）表明热液影响。痕量和稀土元素浓度表明存在盐水、缺氧和火山热液影响的水下环境。在芦草沟湖的水下环境中，局部高温和火山热液活动引起的深部丰富的Mg2+供应，为原生白云石的直接沉淀形成了有利的化学条件。这项研究的结果加深了对湖相原生白云石形成的起源和过程的理解，并为古代白云岩的环境解释提供了另一种可能性。
Lacustrine dolomite nucleation commonly occurs in modern and Neogene evaporitic alkaline lakes. As a result, ancient lacustrine microcrystalline dolomite has been conventionally interpreted to be formed in evaporitic environments. This study, however, suggests a non‐evaporitic origin of dolomite precipitated in a volcanic–hydrothermal lake, where hydrothermal and volcanic processes interacted. The dolomite occurs in lacustrine fine‐grained sedimentary rocks in the middle Permian Lucaogou Formation in the Santanghu intracontinental rift basin, north‐west China. Dolostones are composed mainly of nano‐sized to micron‐sized dolomite with a euhedral to subhedral shape and a low degree of cation ordering, and are interlaminated and intercalated with tuffaceous shale. Non‐dolomite minerals, including quartz, alkaline feldspars, smectite and magnesite mix with the dolomite in various proportions. The 87Sr/86Sr ratios (0.704528 to 0.705372, average = 0.705004) and δ26Mg values (−0.89 to −0.24‰, average = −0.55‰) of dolostones are similar to those of mantle rocks, indicating that the precipitates mainly originated from fluids that migrated upward from the mantle and were subject to water–rock reactions at a great depth. The δ18O values (−3.1 to −22.7‰, average = −14.0‰) of the dolostones indicate hydrothermal influence. The trace and rare earth element concentrations suggest a saline, anoxic and volcanic–hydrothermally‐influenced subaqueous environment. In this subaqueous environment of Lucaogou lake, locally high temperatures and a supply of abundant Mg2+ from a deep source induced by volcanic–hydrothermal activity formed favourable chemical conditions for direct precipitation of primary dolomite. This study's findings deepen the understanding of the origin and processes of lacustrine primary dolomite formation and provide an alternative possibility for environmental interpretations of ancient dolostones.