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

• Published in: Sedimentology 2017-10, Vol.64 (6), p.1708-1730
• Main Authors: Olanipekun, Babatunde John, Azmy, Karem, Hollis, Cathy
• Format: Article
• Language: English
• Subjects: Boats; Carbonates; Cathodoluminescence; Composition effects; Crystals; Dissolution; Dolomite; Dolomitization; Dolostone; Electron microprobe; Electron microscopy; Electron probe microanalysis; Evaluation; Geochemistry; High resolution; Homogenization; Imaging techniques; Magnesium; major and trace elements; Major elements; Manganese; Mass spectrometry; Mass spectroscopy; Maturation; Ordovician; rare earth elements; Scanning electron microscopy; Secondary ion mass spectrometry; Sedimentary facies; SEM‐CL; SIMS; Solutes; Trace elements; Zoning
• ISSN: 0037-0746; 1365-3091
• DOI: 10.1111/sed.12371

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.