Nucleation and stabilization of Eocene dolomite in evaporative lacustrine deposits from central Tibetan plateau

Abstract

In past decades, the formation of dolomite at low temperature has been widely studied in both natural systems and cultured experiments, yet the mechanism(s) involved in the nucleation and precipitation of dolomite remains unresolved. Late Eocene dolomitic deposits from core in the upper Niubao Formation (Lunpola Basin, central Tibetan Plateau, China) are selected as a case study to understand the dolomitization process(es) in the geological record. Dolomite formation in Lunpola Basin can be ascribed to a different mechanism forming the large quantities of replacive dolostones in the geological record; and provides a potential fossil analogue for primary dolomite precipitation at low temperature. This analogue consists of an alternation of laminated dolomitic beds, organic‐rich and siliciclastic layers; formed in response to intense evaporation interpreted to take place in a continental shallow lake environment. Mineralogical, textural and stable isotopic evaluations suggest that the dolomite from those dense‐clotted laminated beds is a primary precipitate. At the nanoscale, these dolomitic beds are composed of Ca–Mg carbonate globular nanocrystals (diameter 80 to 100 nm) embedded in an organic matrix and attached to clay flakes. Micro‐infrared spectroscopy analyses have revealed the presence of aliphatic compounds in the organic matrix. Microscopic and elemental compositional studies suggest that clay surfaces may facilitate the nucleation of dolomite at low temperature in the same way as the organic matrix does. The dolomite laminae show values for δ18OVPDB from −3.2 to −1.76‰ and for δ13CVPDB from −2.62 to −3.78‰. Inferred δ18OSMOW values of the lake water reveal typical evaporitic hydrological conditions. These findings provide a potential link to primary dolomite formation in ancient and modern sedimentary environments; and shed new light on the palaeoenvironmental conditions in central Tibet during the Eocene.

Multielement Imaging Reveals the Diagenetic Features and Varied Water Redox Conditions of a Lacustrine Dolomite Nodule

“…Thirdly, the close attachment of dolomite (especially the ankerite crystals) to chlorite flakes in this study is another potent evidence supporting the primary genesis of dolomitization. Highly negative-charged clay minerals manage to catalyze the abiotic precipitation of dolomite at low temperatures [51]. The clay surface could facilitate the process of nucleation, by forming an electrostatic binding with Mg 2+ and Ca 2+ ions [52], a possible route for dolomitization in this study.…”

Constraints on dolomite formation in a Late Palaeozoic saline alkaline lake deposit, Junggar Basin, north‐west China

“…This phenomenon is in accordance with the scarcity of dolomite in modern marine deposits although seawaters are supersaturated with respect to ordered dolomite (Raudsepp et al ., 2022). The difficulty for low‐T dolomite formation mainly resides in the strong hydration of Mg 2+ ions (Zhang et al ., 2012; Shen et al ., 2014; Kaczmarek et al ., 2017; Wen et al ., 2020; Fang & Xu, 2022), which, however, seemed not to be a big constraint for the formation of other Mg‐rich minerals in alkaline environments. The direct precipitation of anhydrous Na‐Mg‐carbonate (eitelite) from lake waters, as well as the displacive diagenetic formation of northupite near the water–sediment interface, suggests that there were enough free Mg 2+ ions in the saline alkaline lakes of the Fengcheng Formation available for Mg‐Na‐carbonate formation.…”

“…Isolated dispersed dolomite is a common mineral component in saline lake siliciclastic sediments (Last, 1990) and it can be present in various lithofacies, such as deep‐water laminates and marginal‐playa mudstones. Genesis of such dolomite is generally controversial and suggested hypotheses include primary dolomite (Smith & Stuiver, 1979; Wen et al ., 2020), penecontemporaneous dolomite (Wolfbauer & Surdam, 1974), organogenic dolomite (Slaughter & Hill, 1991; Mazzullo, 2000), detrital dolomite (Eugster & Surdam, 1973) and hydrothermal primary dolomite (Wen et al ., 2013; Yang et al ., 2020). In the Fengcheng Formation, the isolated dispersed dolomite crystals show zoned cathodoluminescence (Fig.…”

“…Large, isolated dolomite crystals (20 to 70 μm) are also the major dolomite type in the saline alkaline lacustrine Wilkins Peak Member of the Green River Formation, which are much larger in size than the dolomite crystals of the overlying meromictic brackish lacustrine Laney Member (5 to 10 μm) (Murphy et al ., 2014). Since most reported primary dolomite (Ca‐rich dolomite or very high Mg‐calcite) crystals, that generated in microbial mats or on Mg‐clay mineral surfaces in environmental depositional settings (Casado et al ., 2014; Liu et al ., 2019; Wen et al ., 2020) occur at the nanometre scale (Sánchez‐Román et al ., 2008), the large dolomite crystals of the Wilkins Peak Member and Fengcheng Formation were interpreted to have undergone overgrowth on tiny primary nuclei. The zoned dark red to olive green cathode‐luminescence and zoned bright fluorescence suggest an anoxic, organic‐rich environment for dolomite growth in the shallow subsurface, a specific subsurface milieu that could continue supplying Mg 2+ , Ca 2+ and .…”

“…(Tosca & Wright, 2015; del Buey et al ., 2018; Pozo & Calvo, 2018; Tutolo & Tosca, 2018; Table 1). Diagenetic relationships between Mg‐clays and Ca‐Mg‐carbonates can be classified into the following. The existence of clays facilitates proto‐dolomite (non‐ordered Ca‐rich dolomite and/or very high Mg‐calcite) formation, by providing templates for dolomite nucleation or by keeping high alkalinity water in its clay structures (Díaz‐Hernández et al ., 2013; Casado et al ., 2014; Liu et al ., 2019; Wen et al ., 2020). At an early diagenetic stage, the formation of Mg‐rich clays via transformation of reactive detrital dioctahedral smectite or from authigenesis provides a sink for Mg 2+ in porewaters, limiting authigenic dolomite precipitation (Bristow et al ., 2012; Chase et al ., 2021).…”