Origin of dolomites and associated porosities in lower ordovician boat harbour formation carbonates, western Newfoundland, Canada

Field: Earth and Planetary Sciences (Physical Sciences)

The Boat Harbour Formation constitutes the upper part of the Lower Ordovician St. George Group on the Northern Peninsula, western Newfoundland. It varies in thickness from 140 m at Main Brook to 96 m at Daniel’s Harbour (about 200km). Dolomitization of the carbonate sequence is more pervasive in the lower 30–40 m at Main Brook, whereas at Daniel’s Harbour the section is entirely dolomitized. \nPetrography suggests that the formation has been affected by three phases of dolomitization: earliest (near-surface/syn-sedimentary) phase is D1 dolomicrite, mid–burial phase D2 dolomite, and late stage D3 dolomite. The earliest (near-surface/syn-sedimentary) phase is D1 dolomicrite. The geochemical composition suggests that D1 was developed from fluids of a mixture of meteoric and marine waters at near-surface conditions. \nThe mid–burial phase D2 dolomite consists of coarse planar sub–euhedral crystals that precipitated from hot fluids that circulated through crustal rocks with progressive burial. The late stage D3 dolomite has large and coarse non–planar crystals that exhibit sweeping extinction. In addition to its micro-thermometric data these factors suggest that it likely precipitated during deeper burial in pulses and from hot fluids. \nFor porosity the data suggest that it is mainly associated with the mid–burial D2 dolomite. Intercrystalline porosity is the dominant type and it varies in the formation from <1 to 8 % at Main Brook and from 7 to 12 % at Daniel’s Harbour. \nRecrystallization to more stoichiometric dolomite is usually accompanied by characteristic textural and geochemical signatures. These signatures are primarily studied using multiple populations of crystals, comparison of modern and ancient dolomites, or from results of high temperature dolomite formation experiments. This approach is inadequate. \nTherefore study was done using multi proxy high resolution approaches to carry out imaging and ion microprobe elemental analyses of individual dolomite (burial) crystals viz: Scanning Electron Microscopy (SEM), SEM based cathodoluminescence (SEM-CL), Secondary Ion Mass Spectrometer (SIMS). This is to better understand geochemical variations across the crystal traverses and to constrain possible conditions of crystal growth. \nThe study reveals multiple mechanisms of dolomite crystal growth within constrained diagenetic settings and also shows that recrystallization and episodes of dolomitization evidenced by multi-crystal population, are also apparent within dolomite crystals. Further to this, photo-luminescence (PhL) features of the dolomite crystals were obtained using epifluorescence microscope. Combined with SEM-CL, SIMS measured Mn, Fe and REE-Y content of zoned and unzoned dolomite crystal facies are correlated with their luminescence characters (Chapter Four). \nThe study affirms that, at low concentrations (<200 ppm), the correlation of activators (Mn and REE) and quenchers (Fe) with occurrence and intensity of luminescence remains speculative. Broadly speaking, REE-Y content was too low to control luminescence and Fe could not be conclusively demonstrated to have caused quenching, using the employed methods. \nMorphology and genesis of nanopores and micro-inclusions hosted in intracrystalline areas of dolomites and their association/s with dolomite formation was investigated (Chapter Five). Burial dolomite samples of the Boat Harbour Formation were subjected to Broad Ion Beam (BIB) argon milling. Thereafter, Scanning Electron Microscope (SEM) was used to examine, at high resolution, micrometer to nanometer scale pores hosted in the crystals of the dolomites. Intracrystalline nanopores are abundant within the burial dolomite crystals. They are shown to have developed as a result of imperfection associated with the alignment process of crystallites’ agglomeration. This occurred during the formation of the dolomite’s major crystal face. Furthermore, the origin of mineral inclusions, ‘which are accidentals’ in intracrystalline nanopores is related to the mechanism of dolomitization that involves dissolution of precursor carbonate mineral (calcite or early dolomite) and precipitation of dolomite.