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

Abstract

The genesis of dolostone has long been puzzling for more than two centuries. Although much work has been done on investigating the process of dolomitization, little emphasis has been put on examining the diagenetic water redox condition with the wealthy geochemical information preserved in primary dolomite, which is believed to archive the aqueous environment as well as biotic and/or abiotic effects during formation. In situ interpretation with high resolution is a prerequisite in refined research of dolomite. Here, we reported the multielement imaging results of a lacustrine dolomite nodule with the host black shale from the Songliao Basin, northeast of China. Micro X-ray fluorescence (μ-XRF) with a spatial resolution down to 10 μm was used for in situ scanning. Two key parameter settings of the μ-XRF, including single-point exposure time and spatial resolution, were optimized to achieve a better result in a reasonable scanning time scale. The final imaging data graphically revealed dynamic variation of elemental distributions, including elements enriched in dolomite (e.g., Ca, Mg, Fe, and Mn), clastic quartz (Si), and clay minerals (e.g., Al and K) and redox-sensitive trace elements (e.g., Cr, Mo, V, and U). The well-preserved laminated structures inside the nodule and the features with a magnesium-rich core wrapped with an iron-concentric outer layer and a manganese-concentric shell together indicated its primary form as dolomite and a gradual transformation into ankerite as well as manganese-ankerite. The elemental variation indicates a varied bottom water redox condition, which involved from sulfidic to ferruginous and manganous zones. Here, we propose that the intermittent supplies of sulfate and Fe-/Mn-oxidized minerals interrupting the black shale deposition while favoring dolomitization might be brought by the oxidized and salted seawater. And this lacustrine dolomite is expected to be a potential fingerprint mineral in tracking the seawater intrusions to the Songliao Basin which happened 91 million years ago.

Early Diagenesis in the Lacustrine Ostracods from the Songliao Basin 91.35 Million Years Ago and Its Geological Implications

“…However, it should be noted that the ankerite found in ostracods might be different from the dolomite nodules and layers found in the K2qn 1 , which were formed in a dynamically varied diagenetic water redox conditions [21]. The dolomite nodule was suggested to have a primary form as dolomite and a gradual transformation into ankerite as well as manganese-ankerite [21]. Rare earth elements and Sr isotope values also provide further evidence for the formation of dolomite nodules and layers resulting from the seawater intrusion events of the Songliao Basin [53,54].…”

“…This also can be used to explain the massive 13 C-enriched carbonate rock deposits in Precambrian without large-scale black shale deposits [52,53]. However, it should be noted that the ankerite found in ostracods might be different from the dolomite nodules and layers found in the K2qn 1 , which were formed in a dynamically varied diagenetic water redox conditions [21]. The dolomite nodule was suggested to have a primary form as dolomite and a gradual transformation into ankerite as well as manganese-ankerite [21].…”

“…However, during the burial and early diagenetic stage, a series of heterotrophic microbial processes occurs to degrade organic matters and generate secondary carbonate minerals, such as dolomite, calcite, ankerite, and siderite [18][19][20][21]. As the bicarbonate (HCO3 − ) forming these secondary minerals is mainly from pore water rather than water column [19,22], the minerals are encased in ostracod shells and may be difficult to effectively remove, thereby causing geochemical contamination.…”

Constraints on Diagenetic Fluid Source and Genesis in Tight Dolostone Reservoir of Submember Ma55 of Ordovician Majiagou Formation in Northwestern Ordos Basin, China: Evidence from Petrology and Geochemistry

“…However, in most cases, carbonate reservoirs have strong heterogeneity in terms of their lithologies and pore structures [2][3][4][5][6]. The influence of diagenesis on reservoir quality has been reported in numerous studies [7][8][9][10][11][12], among which the differences of diagenetic fluids is one of the most significant controlling factors affecting the formation of effective dolostone reservoirs [13].…”