Iain M. Samson

Quantitative analysis of silicate certified reference materials by LA-ICPMS with and without an internal standard

Quantitative analysis of silicate minerals by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) typically has relied on the use of an internal standard estimated from mineral stoichiometry or obtained using an alternative analytical technique (e.g., electron microprobe (EMP)). Major, minor and trace element analysis of silicate certified reference materials (CRMs) can be conducted by LA-ICPMS without using an internal standard by analyzing for a comprehensive list of elements, converting the elements to equivalent oxide concentrations, scaling the oxides to 100%, and converting the scaled oxide concentrations back into element concentrations. This method demonstrates that quantitative element concentrations can be obtained from silicate CRMs having a wide range of compositions without using an internal standard and, in general, results obtained using this method are equally or more accurate than those obtained using any one of three elements (Si, Ca or Fe) as internal standards. The approach expands the application of LA-ICPMS to the analysis of unknown silicate minerals.

Precise isotope ratio determination of common Pb using quadrupole LA-ICP-MS with optimized laser sampling conditions and a robust mixed-gas plasma

This work presents a method for the precise (0.2% RSE) determination of common (i.e. non-radiogenic) Pb isotope ratios using quadrupole Nd:YAG (266 nm) LA-ICP-MS at low (∼2 ppm) Pb concentrations. Laser sampling conditions significantly influence the precision of Pb isotope ratio measurements. This paper presents a set of optimum sampling conditions for the described system. Setting the laser focus above the sample surface significantly improves the precision of ratio measurements due to increased count rates and a reduction in the heterogeneity of particulate matter produced by fracturing at the site of ablation. In addition, using a more robust, mixed-gas (Ar–N2) plasma significantly increases sensitivity and reduces mass bias. With a mixed Ar–N2 plasma and optimized laser sampling conditions, single collector quadrupole LA-ICP-MS can be superior, for some applications (e.g., where micrometre-scale spatial resolution is important), to TIMS, and at low Pb concentrations is a cost-effective alternative to LA-MC-ICP-MS. The application of single detector, quadrupole LA-ICP-MS to the precise determination of common Pb isotope ratios in minerals has not been previously documented. This method is a powerful tool for use in isotope tracer studies of ore deposits and has potential applications to a range of environmental problems.

The nature of orthomagmatic hydrothermal fluids in the Oka carbonatite, Quebec, Canada: Evidence from fluid inclusions

Fluid inclusions in apatite, monticellite, and calcite from the Oka carbonatite complex generally comprise either aqueous liquid + vapour or aqueous liquid + vapour + halite. Some inclusions also contain a variety of trapped solids, including calcite, strontianite, nahcolite, and mirabilite. Microthermometry and Raman spectroscopy indicate that CO2 is not an important constituent of these fluids. Microthermometric data also indicate that the hydrothermal fluids liberated from the carbonatitic magma had a wide range of salinities (4.2 to 49 eq. wt% NaCl) and densities (0.77 to 1.26 g/cm3). Most of the fluids had relatively low concentrations of SO42− and CO32− (<6 wt%); however, the nature of the trapped minerals and the phase behaviour of some inclusions indicate that some fluids may have had higher concentrations of SO42−, CO32− and/or HCO3− Textural relationships indicate that the crystallization sequence for the host minerals was monticellite → apatite → calcite, which, when combined with fluid inclusion data from the three minerals, indicates that fluid salinity decreased and fluid density increased during progressive crystallization of the magma. The density increase is thought to have been mainly caused by decreasing temperature. Isochoric projections indicate that the fluids were exsolved at minimum pressures of 4 to 8 kb. These data provide a basis with which to assess the involvement of orthomagmatic aqueous fluids in the formation of hydrothermal rare element mineralization associated with carbonatites.

A Devonian hydrothermal chert reservoir: the 225 bcf Parkland field, British Columbia, Canada

The 225 bcf (original gas in place [OGIP]) Parkland A pool was discovered in 1956 with the drilling of Imperial Pacific 6-29-81-15w6. This well initially production tested at 19 mmcf gas/day and has produced more than 95 bcf of sweet gas. The Parkland play type (as defined by the Geological Survey of Canada) consists principally of fracture-associated hydrothermally karsted and dolomitized reservoirs hosted in the Upper Devonian (Famennian) Wabamun Group medial ramp carbonates resting on the axis of the Peace River arch and includes major fields such as Tangent, Teepee, and Gold Creek. Ironically Parkland itself owes little of its production to hydrothermally dolomitized carbonates; instead, most of the reservoir pore volume can be attributed to microintercrystalline porosity within a pervasive replacement microquartz (chert) that occurs at the dolomite-limestone interface. The thickness of this chert zone in the 6-29 well is 35 m and occurs near the top of the Wabamun. The origin of the chert is problematic. It postdates matrix dolomitization and crosscuts the early Tournaisian (Mississippian) Exshaw Ash but is in turn truncated by later saddle dolomite and calcite veins. The replacement chert is composed of a microporous (up to 30%) meshwork of microquartz crystals averaging 5-10 µm in size and is texturally distinct from early diagenetic chert nodules also found in the Wabamun. The two cherts are also distinguishable isotopically, with the replacement microquartz showing significantly more depleted oxygen values (22 vs. 25 d18O standard mean ocean water [SMOW]), consistent with precipitation from hot fluids having a temperature range from about 140 to 200°C. Silicification has had the greatest impact on slightly dolomitized limestone at the outer margin of the hydrothermal dolostone pod. Entrapped floating euhedral crystals of dolomite within the replacement chert show little evidence of microdissolution, suggesting that the silicic (Begin page 52) acid-charged fluid, although undersaturated with respect to the microspar lime matrix, was saturated with respect to dolomite. We postulate that hydrothermal fluids of common parentage, only marginally evolved in composition--if at all, were responsible for both dolomitization and silicification. Both replacement processes occurred in relatively rapid succession at shallow burial depths and were related to an early Tournaisian period of hydrothermal activity associated with both wrench and minor extension structuring linked with the nascent development of the Fort St. John graben. The source of the silica is thought to be the immediately subjacent Granite Wash or possibly the Precambrian basement. Stratigraphic proximity to the Granite Wash is likely a prerequisite condition for chert reservoir development within the Wabamun and serves to explain, along with significantly different hydrothermal fluid temperatures (much hotter to the west), why Parkland is such a distinct field compared to Tangent and Teepee. Hydrocarbon reservoirs that are dominantly chert-hosted are relatively uncommon, but where recognized (e.g., Monterey Formation, California) they are related to the redistribution and transformation of biogenic opal-A silica derived from diatoms, spicules, and/or radiolarians in deeper water sediments. To our knowledge, Parkland is the only hydrothermal chert reservoir that has been reported anywhere in the world.

The chemistry of hydrothermal fluids in carbonatites: Evidence from leachate and SEM-decrepitate analysis of fluid inclusions from Oka, Quebec, Canada

Apatite and calcite from calcite carbonatites at Oka, Quebec, Canada, contain primary, liquid-vapour and liquid-vapour-halite fluid inclusions. Leachate analysis of these inclusions indicate that they contain significant concentrations of Na, K, and Cl and lower concentrations of Fe, Mg, and probably Ca. Energy-dispersive X-ray (SEM) analysis of decrepitates from these inclusions confirm that Na, K, and Cl are abundant. In addition, significant S was detected in the decrepitates. A correction was applied to the spectra from decrepitates on apatite in order to subtract the contribution from the substrate (apatite). The reliability of a decrepitate analysis depends on the concentration of an element in the decrepitate relative to that in the substrate and to the relative contributions to the spectrum of decrepitate and substrate. Each elemental analysis was screened for its reliability based on the concentration of the element in the mixed analysis and its concentration in the substrate. The only reliable results were for Na, K, Cl, and S. Both the leachate and decrepitate analyses indicate that Na > K. KNa + K ratios fall within a narrow range, from 0.01-0.11. Potassium was a more important constituent in the LV than in LVS inclusions. Coupled with high salinities indicated from microthermometic data, these data confirm that aqueous fluids with high concentrations of alkalis may have evolved from low-alkali carbonatitic magmas. In contrast to the KNa + K ratios, ClCl + S ratios cover a wide range, from 0.17 to 0.93. The presence of sulphate minerals indicates that the sulphur existed as SO;- (and associated species) in the fluids. Chlorinities based on microthermometric data range from 2.5–11.4 m, which indicates SO;- molalities of 1–22.7. Charge imbalances indicate that other anions, probably HCO3− and CO;-, are important constituents of the fluids. Fenites around the Oka complex are characterized by the replacement of quartz-alkali feldspar gneisses by aegirine, nepheline, and K-feldspar. The dominance of Na and lack of Si in the fluid inclusions is consistent with the precipitation of predominantly nepheline and aegirine in the fenites. Phase equilibria in the K2ONa2OAl2O3SiO2 system indicate that the development of K-feldspar in the fenites requires lower NaClKCl ratios and higher silica activities in the fluids that can only be explained by fluid-rock interaction at low fluid/rock ratios. The nature of the fluid-rock reactions that result in fenitization may play a more important role than has previously been recognized in controlling the development of potassic versus sodic fenites and on the spatial and temporal evolution of fenites.

Occurrence of Fracture-Hosted Impsonite and Petroleum Fluid Inclusions, Quebec City Region, Canada (1)

Impsonite--a high-rank solid petroleum residue--occurs as fracture fillings in Cambrian-Ordovician deep-water sedimentary rocks of the Taconian Allochthon in the region surrounding Quebec City. A material of similar appearance occurs as solid, opaque, black inclusions in small euhedral quartz crystals that formed contemporaneously with the impsonite. Raman microprobe spectroscopy reveals that this material is comprised, in part, of disordered graphitic carbon. Sheaths of nonfluorescent fluid (gas or supercritical vapor) surrounding some of these solid inclusions may represent volatile products generated during postentrapment thermal maturation. Raman spectra reveal the presence of CH[4] and possibly CO[2] and C[2]H[6]. Petroleum fluid inclusions, entrapped in the quartz contemporaneously with the impsonite precursor, are comprised of a heterogeneous and variable mixture of organic phases, including a viscous fluorescent liquid and a nonfluorescent supercritical fluid. These fluid inclusions occasionally contain a reddish brown translucent solid (bitumen?), a colorless birefringent solid (paraffin?), and may also include an aqueous phase. Two distinct populations of fluid inclusions can be distinguished based upon their fluorescence spectra. The mode of occurrence and the composition of these materials suggest that the fracture-fill impsonite is a solid residuum of the petroleum in the inclusions. Reflectance of the fracture-fill impsonite increases from north to south (1.3-1.9%) and crosscuts nappe boundaries, whereas reflectance of dispersed organic matter from the same strata increases from south to north (1.7-2.2%) and changes abruptly at the nappe boundaries. Dispersed organic matter reflectances indicate pre-Taconian maturation beyond the petroleum window, whereas the emplacement and maturation of fracture-fill material is post-Taconian. Strata underlying the nappes contain probable source rocks that are still within the oil window. Petroleum may have migrated into overlying strata through fractures, whereupon it was destroyed by subsequent catagenetic processes. The interpreted paragenetic sequence of pretectonic maturation, nappe emplacement, maturation, and oil generat on from the underlying autochthon, and subsequent degradation of the petroleum may serve as a model for oil- and gas-producing fold-and-thrust belts elsewhere.

Source of fluids forming distal Zn-Pb-Ag skarns: Evidence from laser ablation–inductively coupled plasma–mass spectrometry analysis of fluid inclusions from El Mochito, Honduras

The compositions of flinclusions trapped in garnet, pyroxene, and sphalerite, measured with laser ablation‐inductively coupled plasma‐mass spectrometry, from the distal, dominantly limestone-hosted, El Mochito Zn-Pb-Ag skarn, Honduras, are very similar to those of proximal magmatic fl uids in granitoid-related mineral deposits. However, the El Mochito mineralizing fl uids are chemically distinct from basinal brines, demonstrating that in distal skarns, magmatic fl uids dominate, and that most element concentrations and ratios are unaffected by water-rock interaction. High fl uid temperatures (~325‐425 °C) throughout the paragenesis are also consistent with this hypothesis. A variety of elements and element ratios (e.g., Rb, Mn, K/Na, Mn/Na, Rb/Na, Zn/Na) clearly discriminates between magmatic fl uids and basinal brines. Elevated Ca most likely resulted from carbonate dissolution along the fl ow path. Zinc and Pb contents indicate that the ore metals were precipitated in the deposit in proportions that approached their ratios in the fl uid.

785 nm femtosecond laser ablation for improved precision and reduction of interferences in Sr isotope analyses using MC-ICP-MS

The femtosecond (fs) laser ablation (LA) MC-ICP-MS Sr isotope analysis of calcium-rich materials is reported. Independent Sr isotope ratio determination using solution nebulization (SN) MC-ICP-MS after chromatographic separation of Sr was also conducted. Material studied includes a marine carbonate saggital otolith certified (for trace metals) reference material FEBS-1, and igneous fluorite. Analyses of the FEBS-1 using LA analyses at repetition rates of 5 to 200 Hz with energies from 0.1 to 0.6 mJ pulse−1 and SN analyses demonstrate that fs LA Sr isotope analysis produced similar within-run precision to the SN Sr isotope analysis when the same total number of ions was collected in an analysis. The results demonstrate that the precision for fs LA Sr isotope analyses is more controlled by Poisson counting statistics than for nanosecond (ns) LA analyses, and improvements (≥ 2 fold) in external precision, within-run precision, and accuracy can be obtained in comparison with reported ns LA-based results. Good external precision (1SD, n = 17) ± 0.000007, good within-run precision (1SE) ± 0.000010, as well as good accuracy (0.000006) determinations of Sr isotope ratios in carbonate materials can be obtained in situ with a spatial resolution of 60 to 90 μm when using a Faraday detector signal intensity at mass 88 of greater than 10 V and a total integration time of 151 s. In this contribution, it is further shown that an accurate 84Sr/86Sr ratio of 0.056560 ± 0.000015 to 0.056525 ± 0.000007 can be measured in fs LA Sr isotope analysis of FEBS-1. Previously reported Ca argide and Ca dimer polyatomic ion interferences were not observed to be significant in this study. In addition, this study reports the criteria necessary for effective interference corrections from doubly-charged ions of Er and Yb in LA Sr isotope analysis of fluorite samples, which have Sr concentrations > 650 ppm, and a rare-earth element (REE) Er/Sr or Yb/Sr concentration ratio of less than 0.01.

Solute transport across basement/cover interfaces by buoyancy-driven thermohaline convection: Implications for the formation of unconformity-related uranium deposits

Previous studies have suggested that buoyancy-driven convection could only have developed in thick sandstone aquifers within Proterozoic intracratonic sedimentary basins that host unconformity-related uranium deposits. On the other hand, oxygen and hydrogen isotopic and fluid inclusion studies of quartz and carbonate veins have shown that basinal brines have interacted with basement rocks and basement-derived fluids pervasively. Finite element modeling was conducted to investigate the potential mechanisms driving fluid interaction across a basement/cover unconformity. We firstly constructed a simplified conceptual model by integrating the known features shared by the Athabasca, Thelon and Kombolgie basins and their Phanerozoic counterparts. Based on this conceptual model, various numerical scenarios were designed to examine buoyancy-driven (heat and solute transport are coupled) fluid flow patterns and the corresponding solute transport. The results show that thermohaline convection may have penetrated into the basement for up to 1 to 2 km below the unconformity, when typical hydrological parameters for these Proterozoic hydrogeological units were used. Fluid flow velocities in the sandstone sequence were several orders of magnitude larger than those in the basement. If a uranium source (a pore fluid with 500 mg/l uranium) was assumed to be located in the center of the basin below the unconformity, uranium was able to gradually spread into the sandstone aquifer through thermohaline convection without considering any contribution from fluid-rock interaction. The uranium concentration of basinal fluids above the uranium source approached 15 and 24 mg/l after 1 and 5 m.y. of modeling time, respectively. If the uranium source was initially located at the center of the aquifer, a uranium plume developed and percolated down to 2 km below the unconformity at 5 m.y. The location of the uranium source also affects the solute transport efficiency. A uranium source located around the sloping basal unconformity, either in the basin fill or basement, close to the basin margin, led to a wider uranium plume than if it was located near the center of the basin. Given appropriate hydrological conditions, thermohaline convection could have caused widespread interaction of basinal brines with basement rocks or basement-derived fluids in Proterozoic basins where unconformity-related uranium deposits have developed, and that enough uranium could have been leached from the uranium-rich basement to form large, high-grade unconformity-related uranium deposits.

C-O-H-N-salt fluids associated with contact metamorphism, McGerrigle Mountains, Quebec: A Raman spectroscopic study

Abstract Contact metamorphism and metasomatism of a sequence of Cambro-Ordovician pelitic, calcareous, and psammitic metasediments in the thermal aureole of the Devonian McGerrigle pluton has resulted in the generation of a wide variety of fluid compositions which can be described by the system C-O-H-N-salt. Microthermometric and Raman spectroscopic data on fluid inclusions trapped in quartzsulfide veins adjacent to the intrusive contact indicate that three fluid types existed in the aureole: (1) H 2 O (2) H 2 O + CO 2 , and (3) CH 2 ± N 2 ± CO 2 . The aqueous fluids have salinities which range from 0 to 50 eq. wt% NaCl + CaCl 2 . This range in fluid compositions resulted from the progressive devolatilization of a variety of lithologies and the release of waters from the intrusion. The range in salinity of the aqueous fluids is attributed to variable contributions of high salinity waters from the intrusions and lower salinity dehydration and formation waters. Some of the CO 2 may be orthomagmatic; however, most of it probably came from decarbonation of calcareous, and possibly carbonaceous, lithologies. CH 4 and N 2 were generated in pelitic units during thermal metamorphism, CH 4 from reactions involving bitumen and graphite, and N 2 from NH 2 + in silicate minerals and possibly from bitumen. The restriction of high mole fractions of CH 4 and N 2 to water-free fluid inclusions can be explained by fluid immiscibility within the fracture system. The partitioning of CO 2 between water-poor and water-rich fluids was less extreme, and, in some cases, CO 2 and H 2 O may have been completely miscible.