Markus Wälle

Performance characteristics of ultra-violet femtosecond laser ablation inductively coupled plasma mass spectrometry at ∼265 and ∼200 nm

The analytical figures of merit of ultra-violet femtosecond laser ablation inductively coupled plasma mass spectrometry (UV-fs-LA-ICP-MS) using the 3rd and 4th harmonics of Ti:Sapphire (∼265 and ∼200 nm, respectively) were explored. For this purpose, elemental ratios of aerosols produced by LA of silicate glass (SRM NIST 610) were studied under varying fluence conditions ranging from moderate values of 2 J cm−2 up to 30 J cm−2, taking into account e.g. laser-induced (66Zn/65Cu) and particle size-related (238U/232Th) phenomena. It could, for instance, be shown that signal ratios were less dependent on the wavelength or laser repetition rate chosen. Furthermore, fractionation indices defined using the temporal drift of elemental ratios over two equal parts of the acquired signal were subject to systematic changes for threshold-close fluences. As a consequence, corresponding 42Ca-normalized values were found to deviate by more than 20% from unity. In contrast, LA at higher fluences resulted in less pronounced discrepancies, falling below 5% even for the most critical elements such as 66Zn, 111Cd, and 208Pb. The complete suppression of particle size-related fractionation quantified on the basis of the 238U/232Th-system turned out to be highly consistent with the absence of μ-sized particles which were measured by optical particle counting (OPC). The relative fraction of particles >0.5 μm was determined to be less than 5%, independent on the wavelength, fluence, or laser repetition rate chosen. Moreover, our results indicate the occurrence of ICP-induced elemental fractionation during analysis due to an increased mass loading of the plasma source if medium or high fluences are applied. Using the 66Zn/65Cu-ratio as a thermometric probe, the change in plasma ionization temperature among low and high mass loading conditions was estimated to be −900 K. Nevertheless, UV-fs-LA-ICP-MS analysis of different matrices (silicate glass SRM NIST 610 and brass (Zn ∼20%)) performed within the high fluence range was found to enhance the accuracy for non-matrix-matched calibration. Evidence is given that the enhancement observed mainly depends upon the suppression of laser- and/or transport-induced fractionation.

Analysis of laser-produced aerosols by inductively coupled plasma mass spectrometry: transport phenomena and elemental fractionation.

The transport phenomena of laser-produced aerosols prior to analysis by inductively coupled plasma mass spectrometry (ICPMS) were examined. Aerosol particles were visualized over the cross section of a transport tube attached to the outlet of a conventional ablation cell by light scattering using a pulsed laser source. Experiments were carried out under laminar or turbulent in-cell flow conditions applying throughputs of up to 2.0 L/min and reveal the nature of aerosol transportation to strongly depend on both flow rate and carrier gas chosen. For instance, laser ablation (LA) using laminar in-cell flow and helium as aerosol carrier resulted in stationary but inhomogeneous dispersion patterns. In addition, aerosols appear to be separated into two coexisting phases consisting of (i) dispersed particles that accumulate at the boundary layer of several vortex channel flows randomly arranged along the tube axis and (ii) larger fragments moving inside. The occurrence of these fragments was found to affect the accuracy of Si-, Zn-, and Cd-specific ICPMS analyses of aerosols released by LA of silicate glass (SRM NIST610). Accuracy drifts of more than 10% were observed for helium flow rates of >1 L/min, most probably, due to preferential evaporation and diffusion losses of volatile constituents inside the ICP. The utilization of turbulent in-cell flow made the vortex channels collapse and resulted in an almost complete aerosol homogenization. In contrast, LA using argon as aerosol carrier generally yielded a higher degree of dispersion, which was nearly independent of the flow conditions applied. To illustrate the differences among laminar and turbulent in-cell flow, furthermore, the velocity field inside the ablation cell was simulated by computational fluid dynamics.

Fluid mixing forms basement-hosted Pb-Zn deposits: Insight from metal and halogen geochemistry of individual fluid inclusions

Fluid mixing across unconformities between crystalline basement and overlying sedimentary basins is commonly invoked as an efficient chemical mechanism for ore deposition, but the origin of basement brines and the process of ore formation have rarely been linked by direct evidence. Using laser ablation–inductively coupled plasma–mass spectrometry microanalysis of individual fluid inclusions with an improved detection approach for anion components, we determined simultaneously the ore metal concentrations and the Cl/Br ratio in texturally well constrained inclusion assemblages from a basement-hosted quartz-fluorite-barite-Pb-Zn vein system. An inverse correlation between the Pb + Zn concentrations and the Cl/Br mass ratios in the fluid inclusions provides clear evidence for mixing of a basement-derived metal-rich brine and a metal-poor formation water that acquired its salinity from halite dissolution in Triassic evaporites of the sedimentary cover. This mixing of two distinct brines with comparable salinity is recorded during the growth of individual quartz crystals containing small galena inclusions, demonstrating the transient and episodic nature of fluid mixing during mineral deposition.

The effect of quadrupole ICPMS interface and ion lens design on argide formation. Implications for LA-ICPMS analysis of PGE's in geological samples

The argide levels in laser ablation ICPMS were found to vary 30 times between commonly used quadrupole ICPMS, while other polyatomic interferences like oxides vary less than 3 times. Investigating important parameters revealed that the pressure between skimmer and extraction lenses is key to a low argide rate. The Agilent 7700x with a relatively high pressure due to the restricted gas flow and a small volume between skimmer and ion lenses has the lowest argide levels. The relatively high pressure allows more collisions and breaking apart of weakly bound argide ions. Apparent light PGE (ruthenium, rhodium and palladium) concentration from cobalt, nickel, copper and zinc argides was estimated to be up to 0.00025 times the metal concentration.

Magmatic salt melt and vapor: Extreme fluids forming porphyry gold deposits in shallow subvolcanic settings

The recently discovered Biely Vrch deposit in the Western Carpathian magmatic arc is the most extreme example of a porphyry gold deposit, being practically free of copper, molybdenum or any other sulfide minerals. Microanalytical data on fluid inclusions in quartz veinlets, including a characteristic type of banded veinlets, show that this deposit formed from nearly anhydrous Fe-K-Na-Cl salt melts containing ∼10 ppm Au, coexisting with hydrous vapor of very low density. This exceptional fluid evolution required an Fe-rich dioritic source magma that was emplaced at shallow subvolcanic depth (<3.5 km), directly exsolving a hypersaline liquid and magmatic vapor at high temperature (∼850 °C). During ascent to the level of the porphyry intrusion (0.5–1 km), fluid expansion at high temperature but low pressure led to halite precipitation and further water loss to the vapor, generating an increasingly Fe-K-rich salt melt that transported high concentrations of Au but negligible Cu into the fractured porphyry stock. The low sulfur fugacity resulting from fluid expansion suppressed precipitation of sulfide, explaining the gold-only enrichment in this globally recurring but rare type of gold ore.

Analysis of brass and silicate glass by femtosecond laser ablation inductively coupled plasma mass spectrometry using liquid standard calibration

Due to the lack of reference materials and increasing requirements on the compositional analysis of aerosols produced by nanosecond laser ablation (ns-LA) using inductively coupled plasma mass spectrometry (ICP-MS), in recent years, the utilization of liquid standards for calibration has attracted particular attention. However, no attempt has yet been made to study the capabilities of liquid standard calibration in combination with femtosecond (fs) LA-ICP-MS even though a benchmark would help answer the question under which circumstances the operation of fs laser systems is recommended to increase precision or accuracy of quantification. In the scope of this study, analyses of brass and silicate glass were carried out by fs-LA-ICP-MS applying on-line addition of desolvated as well as non-desolvated liquid standards. Depending on the material considered, accuracies varied from a few up to 50% for critical elements such as Zn and Cd. The average deviation calculated from concentration values of all elements monitored during the analysis of silicate glass amounted to 11.1% and 8.4% for desolvated and non-desolvated aerosols implying similar but slightly improved performance under “wet” plasma conditions. Furthermore, the Li-, Be-, Na-, Mg-, Al-, Sr-, Cd-, Ba-, and Pb-specific analysis of silicate glass applying “matrix-matched” calibration was found to result in accuracies comparable to those obtained by liquid standard calibration except for Zn and Cd, which improved by 9% and 25% when using a solid calibration material, respectively. In contrast, the precision of analysis decreased due to a higher uncertainty of concentration values specified for the external standard used.

Fluid inclusion measurements by laser ablation sector-field ICP-MS

A state-of-the-art, highly sensitive sector-field inductively coupled plasma mass spectrometer (SF-ICP-MS) was evaluated for analyzing fluid inclusions in combination with a 193 nm excimer laser ablation, with the aim of expanding the technique to smaller inclusions and/or lower element concentrations. A well-defined assemblage of numerous co-genetic fluid inclusions and thin layers (50 nm) of chromium and copper containing traces of gold were studied regarding reproducibility and detection limits. For comparison, aliquots were analyzed with a quadrupole (Elan 6100 DRC) and the SF-ICP-MS instrument (Element XR). While using an element menu of 21 elements covering the mass range from lithium (7 amu) to lead (208 amu), approximately ten times lower detection limits were found for the SF-ICP-MS, whereby the cycle time increased by only 20% compared with the quadrupole instrument. An absolute detection limit for e.g. gold of 1 femtogram (10−15 g) was found for fluid inclusions analysis. Detection efficiencies of 2.6 × 10−5 counts per ablated atom for copper and 3.1 × 10−5 for chromium were determined by ablating the thin layers. The detection efficiency for sodium in the fluid inclusions was 2 × 10−7, about five times lower than that expected from the sensitivity obtained on NIST SRM 610 glass.

Trace elements in fluid inclusions of sediment-hosted gold deposits indicate a magmatic-hydrothermal origin of the Carlin ore trend

The Carlin-type deposits in Nevada (western USA) constitute the world9s second-largest gold ore province. These structurally and stratigraphically controlled, sediment-hosted ore bodies are characterized by carbonate dissolution attending hydrothermal precipitation of gold-rich arsenian pyrite. The origin of the mineralizing fluids and the source of the gold remain debated. Conceptual models, favoring either sedimentary, metamorphic, or magmatic fluid sources, are based on isotopic tracers, giving ambiguous results. Here we use the trace element compositions of fluid inclusions to separate geochemical signals of the large-scale fluid source from effects of deposit-scale fluid interaction with the sedimentary host rocks. Specifically, we compare the ratios of Rb, K, B, As, Sr, and Ba between clearly magmatic-hydrothermal Cu-Au ores at Copper Canyon in the Battle Mountain–Eureka trend with the Gold Quarry and Chukar Footwall deposits on the Carlin trend that contain high-grade gold in similar sedimentary host rocks. Results indicate that both ore districts can be related to upper crustal hydrous magmatic intrusions, but are now exposed at different levels of erosion and formed at different distances from their magmatic fluid source. Fluid compositions are best explained by separation of a deep magmatic fluid into Rb-K–enriched brine and B-As-Au–enriched vapor, followed by cooling and contraction of the magmatic vapor phase to an epithermal liquid, which reacted with Sr-Ba–bearing sedimentary rocks during ascent and eventual precipitation of Au-rich arsenian pyrite.

Trace element diffusion and incorporation in quartz during heating experiments

Heating of quartz crystals in order to study melt and high-temperature fluid inclusions is a common practice to constrain major physical and chemical parameters of magmatic and hydrothermal processes. Diffusion and modification of trace element content in quartz and its hosted melt inclusions have been investigated through step-heating experiments of both matrix-free quartz crystals and quartz crystals associated with sulfides and other minerals using a Linkam TS1500 stage. Magmatic and hydrothermal quartz were successively analyzed after each heating step for Cu, Al, and Ti using electron probe micro-analyzer. After the last heating step, quartz crystals and their hosted melt inclusions were analyzed by laser ablation inductively coupled plasma mass spectrometry and compared to unheated samples. Heated samples reveal modification of Cu, Li, Na, and B contents in quartz and modification of Cu, Li, Ag, and K concentrations in melt inclusions. Our results show that different mechanisms of Cu, Li, and Na incorporation occur in magmatic and hydrothermal quartz. Heated magmatic quartz records only small, up to a few ppm, enrichment in Cu and Na, mostly substituting for Li. By contrast, heated hydrothermal quartz shows enrichment up to several hundreds of ppm in Cu, Li, and Na, which substitute for originally present H. This study reveals that the composition of both quartz and its hosted melt inclusions may be significantly modified upon heating experiments, leading to erroneous quantification of elemental concentrations. In addition, each quartz crystal also becomes significantly enriched in Cu in the sub-surface layer during heating. We propose that sub-surface Cu enrichment is a direct indication of Cu diffusion in quartz externally sourced from both the surrounding sulfides as well as the copper pins belonging to the heating device. Our study shows that the chemical compositions of both heated quartz and its hosted inclusions must be interpreted with great caution to avoid misleading geological interpretations.

Hydrothermal fluids in epithermal and porphyry Au deposits in the Central Slovakia Volcanic Field

Abstract The Neogene Central Slovakia Volcanic Field in the Carpathian arc contains various Au deposits, hosted by central zones of large andesite stratovolcanoes. Fluids involved in mineralization have been studied at three different types of deposit, mostly by fluid inclusion and stable isotope techniques. The Rozália mine in the Štiavnica stratovolcano hosts intermediate sulphidation-style Au–Ag epithermal mineralization in subhorizontal veins related to hydrothermal activity during an early stage of caldera collapse. Associated fluids of low salinity underwent extensive boiling at 280–330 °C on transition from suprahydrostatic towards hydrodynamic conditions at shallow depths (c. 550 m) from fluids of mixed magmatic and meteoric origin. The Kremnica ore field hosts a large system of low sulphidation-style Au–Ag veins contemporaneous with rhyolite magmatism and situated on resurgent horst faults. Fluids were of low salinity, predominantly of meteoric origin, and showed gradual decrease in temperature along the system (c. 270–140 °C) related to a decrease in erosion level from c. 500 to c. 50 m. The Biely Vrch Au-porphyry deposit in the Javorie stratovolcano is associated with quartz stockwork in diorite porphyry intrusion. The major type of ore-bearing fluid was high temperature magmatic vapour (720–<380 °C) accompanied by Fe-rich salt melt. Gold precipitated in a high-temperature but low-pressure subvolcanic environment. Supplementary material: Stable isotope data and fluid inclusion microthermometric data are available at http://www.geolsoc.org.uk/SUP18752.