In situ characterization of dolomite crystals: Evaluation of dolomitization process and its effect on zoning

Abstract

Dolomitization is a dissolution–re‐precipitation process which proceeds via micrometre‐scale interface reaction horizons. However, the effect of this fundamental local medium of replacement process on precipitated dolomite crystals is rarely investigated. For this purpose, dolomite samples of Ordovician Boat Harbour Formation carbonates (Newfoundland, Canada) from Main Brook and Daniel's Harbour (about 150 km apart) were studied. The investigation used high‐resolution approaches to carry out imaging and elemental analyses of individual dolomite crystals, including scanning electron microscopy, scanning electron microscopy‐based cathodoluminescence, secondary ion mass spectrometry and electron microprobe analysis. The purpose was to better understand geochemical variations across the crystal traverses and their association with dolomite replacement processes and conditions. The scanning electron microscopy‐based cathodoluminescence‐zoning character reveals three major crystal facies. Distinctly (planar boundaries) zoned core and indistinctly zoned core are zoned dolomite crystal facies in burial dolomite from Main Brook. Relatively unzoned crystal facies was identified in that of Daniel's Harbour. Compositionally zoned dolomite crystals are characterized by decreasing Sr, Na, Y and ∑REE but increasing Mn and Fe from core to rim sections. ‘Core to rim’ zoning was not found for major (Mg and Ca) elements. The zoning exhibited by the replacive dolomite crystals is interpreted to be, principally, a result of limited exchange between solutes of the bulk pore fluid and those of dissolving precursor marine carbonate. In tandem with petrographic features, the systematic compositional zoning character demonstrates that the effectiveness of the dolomitization process grades from indistinctly zoned core to relatively unzoned crystal facies via interface reaction horizons. Homogenization of major elements of core and rim sections, seen in distinctly zoned core crystal facies and to a much greater extent in relatively unzoned crystal facies, via a secondary reaction interface, is the first recognized stage of the ‘maturation process’ followed by homogenization of the trace elements seen largely in relatively unzoned crystal facies.

Towards <i>in-situ</i> U–Pb dating of dolomites

“…Dolomite cores of MU-1 appear much brighter in BSE compared to their concentric enclosing rims, probably due to higher Mg/Ca ratio and minor concentration of Fe (Fig. 5A; Olanipekun and Azmy, 2017). Dolomite crystals from sample KM-1 display mainly a concentric zoning pattern with a very thin lamina separating core from rim.…”

“…5B). Such signature is likely to be associated with the mechanism of epigenetic dolomitization governed by diagenetic replacement of pore fluids and re-precipitation of dolomite (Putnis & Putnis, 2007;Olanipekun & Azmy, 2017).Within the large fragments of breccia sample EFN-1, a mixture of different zoning patterns can be seen: dolomite crystals that lack distinctive core to rim zones and crystals with two zones of bright cores and dark rims (Fig. 5C).…”

“…Intracrystalline porosity may include other phases beside dolomite, such as k-feldspar, pyrite, oxides and bituminous minerals (Fig. 5A; Olanipekun and Azmy, 2017) and is an important feature to be considered. When selecting locations for analyses pores should be avoided as much as possible.…”

Geochemistry of Late Ordovician dalmanelloid brachiopods from Laurentia: testing the effects of paleolatitudinal gradient

“…D r a f t SEM (FEI, Hilsboro, OR, USA), which is equipped with an energy dispersive X-ray (EDS) microanalytical system (Bruker AXS, XFlash Detector 4010; Bruker Corporation, Billerica, MA, USA). The CL system is equipped with three detectors that correspond to red, green and blue colors, thus showing image colors that are very close to those of real luminescence emitted from the studied samples (e.g., Olanipekun and Azmy 2017). The images were obtained at 15 kV acceleration voltage and 13 nA beam current.…”

Tracing fault‐related fluids using in situ trace elements: A case study in Upper Cambrian dolostone in Gucheng area, Tarim Basin, NW China

“…hamper to decipher the geochemical records by only using bulk samples. Therefore, high-resolution in situ trace elemental analyses can provide much better constraints on tracing the origins of diagenetic fluids in burial diagenesis (Olanipekun, Azmy, & Hollis, 2017;Wang et al, 2009;Zhang, Guan, Jian, Feng, & Zou, 2014).…”