Date of Award
Earth and Environmental Sciences
carbonate, dolomite, fractionation, LA-ICP-MS, Laser ablation, magnesium isotopes
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This study evaluates the potential of conventional nanosecond laser ablation inductively coupled plasma mass spectrometry (ns-LA-ICP-MS) analysis in conjunction with advanced data reduction techniques based on integrated counts per second (ICPS) summation for the in situ microanalysis of the Mg isotopic composition of dolomitized rocks from hydrocarbon reservoirs. Chemical composition of these carbonates is affected by fluid/rock interactions between infiltrating dolomitizing fluid(s) and carbonate host rock(s). Diagenetic fluids can impart a unique chemical composition, causing zonation, each zone potentially having characteristic 25Mg/24Mg and 26Mg/24Mg ratios. Three standard reference materials (SRMs), both matrix-matched and pure, were compared by solution nebulization (SN)- and LA-ICP-MS methods. LA-ICP-QMS methods were then applied to dolomites from the Deep Geologic Repository (DGR) site at Bruce (Tiverton, Ontario) and Western Canada (Alberta and British Columbia). The precision of Mg isotope ratios derived by summation of outlier removed ICPS values increased by 27.6% and 72.3% for δ25MgDSM-3 and 48% and 13.9% for δ26MgDSM-3 on JDo-1 and NIST SRM 980, respectively, over mean ICPS ratios. Both accuracy and precision were approximately 2 times higher for SN over LA, indicating the presence of matrix-related interferences (standard deviation of δ25MgDSM-3 of 3.98 and 7.54 for δ26MgDSM-3 compared to 11.6 and 4.02, respectively, for data, processed the same way on solution). All 3 SRMs were found to be unsuitable for in situ microanalysis due to isotopic and elemental heterogeneity at micrometer scales, as well as multiple interferences on all mass numbers. JDo-1 showed significant loss of precision over SN due to heterogeneity of vacuum impregnated epoxy dolomite pellets (SN > LA STD dev δ25MgDSM-3: 1.88 > 56.16) and (SN > LA STD dev δ26MgDSM-3: 4.10 > 60.39). Natural samples analyzed in situ lacking this significant precision loss is evidence of this effect.
Al-Aasm, Namir, "Microanalysis of Mg isotope abundances using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS): application to hydrocarbon reservoirs" (2019). Electronic Theses and Dissertations. 7796.