Christoph A. Heinrich

Quantitative multi-element analysis of minerals, fluid and melt inclusions by laser-ablation inductively-coupled-plasma mass-spectrometry

Laser-ablation ICPMS has become widely accessible as a powerful and efficient multi-element microanalytical technique. One of its key strengths is the ability to analyse a wide concentration range from major (tens of wt.%) to trace (ng/g) levels in minerals and their microscopic inclusions. An ArF excimer laser system (λ = 193 nm) with imaging optics for controlled UV ablation and simultaneous petrographic viewing was designed specifically for representative sampling and quantitative multi-element analysis of microscopic fluid, melt and mineral inclusions beneath the sample surface. After a review of the requirements and recent technical developments, results are presented which together document the reliability and reproducibility of quantitative microanalysis of complex samples such as zoned crystals or fluid and melt inclusions in various host minerals. Analytical errors due to elemental fractionation are reduced to the typical precision achieved by quadrupole LA-ICPMS in multi-element mode (2–5% RSD). This progress is largely due to the small size of aerosol particles generated by the optimized UV optical system. Depth profiling yields representative and accurate concentration results at a resolution of ∼0.1 μm perpendicular to the ablation surface. Ablation is largely matrix-insensitive for different elements, such that silicate and borate glasses, silicates and oxide minerals, or direct liquid ablation can be used interchangeably for external standardization of any homogeneous or heterogeneous material. The absolute ablation rate is material dependent, however, so that quantitative LA-ICPMS analysis requires an internal standard (i.e., an independent constraint such as the absolute concentration of one element). Our approach to quantifying fluid and melt inclusion compositions is described in detail. Experiments with synthetic fluid inclusions show that accurate results are obtained by combining the LA-ICPMS analysis of element concentration ratios with a microthermometric measurement of the NaCl equivalent concentration and an empirical description of the effect of major cations on the final melting temperatures of ice, hydrohalite or halite. Expected calibration errors for NaCl-H2O-dominated fluids are smaller than the typical analytical scatter within an assemblage of simultaneously trapped fluid inclusions. Analytical precision is limited by representative ablation of all phases in heterogeneous inclusions and the integration of transient ICPMS signals, to typically ±10 to 20% RSD. Element concentrations in devitrified and even coarsely crystallized silicate melt inclusions can be reconstituted from LA-ICPMS signals. Deconvolution of inclusion and host signals with internal standardization automatically corrects for sidewall crystallization after melt entrapment at high temperature. A test using melt inclusions in a midocean ridge basalt, a summary of published geochemical studies and a new application to REE analysis of coexisting fluids and mineral phases in carbonatite-related veins illustrate the versatility and some of the strengths and limitations of LA-ICPMS, in comparison with other microanalytical techniques.

Capabilities of an Argon Fluoride 193 nm Excimer Laser for LaserAblation Inductively Coupled Plasma Mass Spectometry Microanalysis ofGeological Materials

Recent developments in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) have demonstrated its potential for in situ microanalysis for major, minor and trace elements in solids, such as minerals. With the low backgrounds and high sensitivity of new ICP-MS instruments, limits of detection of 1–10 ng g -1 in a 40 µm ablation pit for many elements can be reached. Fractionation effects due to different ablation rates of various elements have prevented quantification without matrix-matched standards with 1064 nm Nd:YAG lasers. These effects have been reduced but not eliminated using shorter UV wavelengths ( e.g . a quadrupled Nd:YAG 266 nm). Excimer lasers with wavelengths below 200 nm are expected to reduce fractionation effects further, but they present a serious challenge to the design of optical systems, especially if high-resolution UV ablation needs to be combined with high quality visual observation, which is essential for the study of complex materials, such as geological samples. An LA system was developed using an homogenized UV laser beam (193 nm, Argon Fluoride excimer) with a common UV–visual objective on a modified petrographic microscope with reflected and transmitted light illumination, in combination with a Perkin-Elmer Elan 6000 ICP-MS instrument. The optical system allows imaging of both visible and UV laser light onto the sample surface at the same time. Laser operating parameters and their influence on the ablation process were investigated using NIST SRM 612/610. Fractionation effects due to differential ablation of various elements as a function of time can be reduced to interelement correlation coefficients of r =0.9 or better and have become insignificant within the precision of quadrupole ICP-MS using this new optical design. Energy densities and repetition rates need to be kept within limited ranges for accurate and reproducible determinations of trace elements such as Zn, U and Pb, which have previously presented strong fractionation problems. LA-ICP-MS determinations on natural hornblende, augite, and garnet, calibrated against NIST SRM 612 using any major element as an internal standard, agree well with independent literature data. These experiments with the Argon Fluoride 193 nm excimer system demonstrate a greatly reduced matrix dependence of the ablation process, which facilitates in situ analysis of unknown samples.

Metal fractionation between magmatic brine and vapor, determined by microanalysis of fluid inclusions

The major and trace element compositions of individual fluid inclusions from a range of magmatic-hydrothermal ore deposits were analyzed by laser-ablation inductively coupled plasma-mass spectrometry, to explore the behavior of ore-forming components during fluid phase separation (“boiling”) in high-temperature saline fluid systems. Data from 13 samples showing unambiguous evidence for coeval trapping of a liquid brine and a coexisting vapor phase identify two groups of elements with drastically different geochemical behavior. Na, K, Fe, Mn, Zn, Rb, Cs, Ag, Sn, Pb, and Tl preferentially partition into the brine (probably as Cl complexes), whereas Cu, As, Au (probably as HS complexes), and B selectively partition into the vapor. Fluid phase separation is probably a major, previously underestimated process in the chemical differentiation that contributes to the extreme range of selective element enrichments in magmatic-hydrothermal systems, from deep plutons through porphyry-style and greisen deposits to epithermal mineralization and volcanic fumaroles.

Gold concentrations of magmatic brines and the metal budget of porphyry copper deposits

Porphyry copper–molybdenum–gold deposits are the most important metal resources formed by hydrothermal processes associated with magmatism. It remains controversial, however, whether the metal content of porphyry-style and other magmatic–hydrothermal deposits is dominantly controlled by metal partitioning between magma and an exsolving magmatic fluid phase, or by scavenging of metals from solid upper-crustal rocks by surface-derived fluids. It also remains unknown to what degree the metal content in such deposits is affected by selective mineral precipitation from the ore fluid. Extremely saline fluids, precipitating quartz and ore minerals in veins have been inferred to have a significant magma-derived component, on the basis of geological, isotopic, and experimental evidence,. Here we report gold and copper concentrations of single fluid inclusions in quartz, determined by laser-ablation inductively coupled plasma mass spectrometry. The results show that the Au/Cu ratio of primary high-temperature brines is identical to the bulk Au/Cu ratio in two of the worlds largest copper–gold ore bodies. This indicates that the bulk metal budget of such deposits is primarily controlled by the composition of the incoming fluid, which is, in turn, likely to be controlled by the crystallization process in an underlying magma chamber.

Enhanced sensitivity in laser ablation-ICP mass spectrometry using helium-argon mixtures as aerosol carrier

Laser ablation-ICP-MS is a sensitive and accurate technique for major to trace multi-element analysis at high spatial resolution on the scale of 10 µm. A wide variety of samples can be studied quantitatively, including minerals and their solid, liquid or melt inclusions as required for geochemical studies. As the desired spatial resolution increases, however, detection limits become severely constrained by the total amount of sample material reaching the ICP. Detection limits are therefore determined by the ablation rate and by the efficiency of removal of ablated aerosol particles from the ablation spot and their transport into the plasma. Properties of the carrier gas are known to affect the ablation process and the efficiency of particle transportation. This study explores the effects of different ablation-cell configurations and the use of helium, dry argon and argon moistened with water for the transport of aerosols into an ICP-MS, using a prototype 193 nm ArF excimer laser. Deposition of visible particles deposited around the ablation pit is significantly reduced when helium is used instead of argon. A moderate flux of helium through the chamber, mixed with moistened argon immediately downstream from the ablation chamber, leads to at least a 2-3-fold increase in the signal intensities across the entire mass range when compared with argon gas only. Background intensities above mass 85 are significantly reduced, but polyatomic interferences in the low mass region increase by an order of magnitude, owing to oxide formation caused by the water load. A high flux of helium, mixed just behind the ablation cell with dry argon, yields a 2-3-fold sensitivity enhancement, in addition to greatly reduced background intensity across the entire mass range. This results in one order of magnitude improvement in detection limits for most elements. These modifications permit the routine determination of minor concentrations of chlorine in microscopic fluid inclusions or the analysis of minerals, such as trace element concentrations in quartz (e.g., Na and Li down to 500 ng g –1 , using a 40 µ ablation pit). Furthermore, this improved sensitivity has recently yielded the first quantitative determination of gold concentrations (∼0.1 µg g –1 ) and full rare-earth element patterns in single 25 µm fluid inclusions.

Quantitative analysis of major, minor and trace elements in fluid inclusions using laser ablation–inductively coupled plasmamass spectrometry

Microscopic fluid inclusions in minerals are the main source of information about the chemical composition of fluids associated with large-scale material transport in the Earths interior. Hydrothermal transport processes are responsible for the natural enrichment of metal resources in many ore deposits. For the multi-element analysis of the microscopic fluid inclusions (typically 5–50 µm in diameter), LA-ICP-MS has become one of the most promising techniques owing to the recent progress in laser optics design and the development of high-sensitivity ICP mass spectrometers. The quantitative analyses of 19 major, minor and trace elements covering a concentration range of five orders of magnitude were carried out on 39 single natural fluid inclusions, together with a number of experiments to optimise controlled ablation and to test the calibration procedure. A modified commercial ICP-MS instrument was used together with a prototype ablation system based on a 193 nm excimer laser. In a stepwise opening procedure for complex polyphase inclusions, a small hole (4 µm pit) was first drilled for the partial release of liquid and vapor, followed by complete drilling out using a pit covering the entire inclusion. Controlled ablation improves the reproducibility of element ratios to less than 20% for most major, minor and trace elements measured in an assemblage of cogenetic inclusions (including elements that are initially present as solid precipitates within the inclusion), provided that the entire transient ICP-MS signal is integrated. Element ratios were calculated from integrated intensity ratios using an external standard, either a NIST SRM glass or an aqueous standard solution ablated directly through a plastic film. Absolute concentrations were calculated from the element ratiosviaan internal standard element, whose concentration was determined prior to ablation. Microthermometric measurements of phase transitions were used to determine total salinity from known phase diagrams, by measuring either the depression of ice-melting temperature, or the temperature of dissolution of NaCl crystals. Salinity can be related to the concentration of Na (or in some cases Cl), which serves as the internal standard element for the quantification of trace element concentrations. Calculated limits of detection are in the ng g–1to µg g–1region, depending on the volume of the inclusions. The accuracy of the overall analysis, including internal and external calibration, is typically between 5 and 20%, as demonstrated on alkali elements in synthetic fluid inclusions of known composition.

Kyanite-eclogite to amphibolite fades evolution of hydrous mafic and pelitic rocks, Adula nappe, Central Alps

In the southern Adula nappe (Central Alps), two stages of regional metamorphism have affected mafic and pelitic rocks. Earlier eclogite facies with a regional zonation from glaucophane eclogites to kyanite-hornblende eclogites was followed by a Tertiary overprint which varied from greenschist to high-grade amphibolite facies. Despite a common metamorphic history, contrasting equilibration conditions are often recorded by high-pressure mafic eclogite and adjacent predominantly lower-pressure pelite assemblages. This pressure contrast may be explained by different overprinting rates of the two bulk compositions during unloading. The rates are controlled by a mechanism in which dehydrating metapelites provide the H2O required for simultaneous overprinting of enclosed mafic eclogites by hydration.Quantitative mass balance modelling based on corona textures is used to show that overprinting of metapelites during unloading involved dehydration reactions. The relatively rapid rate of dehydration reactions led to nearly complete reequilibration of metapelites to amphibolite facies assemblages.After the formation during high-pressure metamorphism of mafic eclogites, later lower-pressure reequilibration by hydration to amphibolites was slow, and therefore incomplete, because it depended on large scale transport of H2O from adjacent, dehydrating metapelites.The facies contrast observed between rocks of different bulk composition is thus a consequence of the general tendency of metamorphic rocks to retain the most dehydrated assemblage as the final recorded state.

Copper deposition by fluid cooling in intrusion-centered systems: New insights from the Bingham porphyry ore deposit, Utah

Quartz veins in porphyry copper deposits record the physiochemical evolution of fluids in subvolcanic magmatic-hydrothermal systems. We have combined cathodoluminescence (CL) petrography with fluid-inclusion microthermometry to unravel the growth history of individual quartz veins and to link this history to copper ore formation at Bingham, Utah. Early barren quartz veins with K-feldspar + biotite (potassic) alteration selvages occur throughout the 2 km vertical exposure of quartz monzonite porphyry stock. At depths of 500 m to at least 1350 m below the orebody, fluid inclusions in these barren veins trapped a single-phase CO 2 -bearing fluid containing ∼2-12 wt% NaCl e q u i v . Within and to depths of 500 m below the orebody, early quartz veins contain abundant hypersaline liquid (38-50 wt% NaCl e q u i v ) and vapor-rich inclusions trapped together at temperatures of 560-350 °C and pressures of 550-140 bar, consistent with fluctuations between lithostatic and hydrostatic pressure at paleodepths of 1.4 to 2.1 km. CL petrography shows that bornite and chalcopyrite were deposited together with a later generation of quartz and K-feldspar in microscopic fractures and dissolution vugs in early barren quartz veins and wall rock. This late quartz contains hypersaline liquid (36-46 wt% NaCl e q u i v ) and vapor-rich inclusions trapped at 380-330 °C and at 160-120 bar hydrostatic pressure. We conclude that a single-phase magmatic-hydrothermal fluid underwent phase separation to hypersaline liquid (or brine) and vapor ∼500 m below the base of the orebody at a paleodepth of ∼2.5 km. Brine and vapor continued to ascend and formed multiple generations of barren quartz veins with potassic selvages. Thermal decline to temperatures below 400 °C was the main driving force for copper-iron sulfide deposition, given the lack of evidence of mixing of brines with low-salinity waters, the lack of correspondence of the ore zone with the initiation of phase separation, and no change in wall-rock alteration style.

Magmatic vapor contraction and the transport of gold from the porphyry environment to epithermal ore deposits

Fluid-phase stability relations combined with thermodynamic modeling using fluid-inclusion analyses and new gold-solubility experiments lead to an integrated geological interpretation linking epithermal gold mineralization and porphyry-style ore formation to the cooling of hydrous magma chambers. The essential chemical requirement for gold transport to low temperatures is an initial excess of sulfide over Fe in the magmatic fluid, which is best achieved by condensing out Fe-rich brine from a buoyant, low- to medium-salinity vapor enriched in volatile S. This vapor can contract directly to an aqueous liquid, by cooling at elevated pressure above the critical curve of the salt-water fluid system. Physical and chemical conditions are matched when magmatic fluid is released through a gradually downward-retracting interface of crystallizing magma beneath a porphyry stock, predicting the consistent zoning and overprinting relations of alteration and mineralization observed in magmatic hydrothermal systems.

Comparison of the ablation behaviour of 266 nm Nd:YAG and 193 nm ArF excimer lasers for LA-ICP-MS analysis

The ablation characteristics of a 266 nm Nd:YAG laser and a 193 nm excimer laser were compared by successive experiments by inductively coupled plasma mass spectrometry (ICP-MS), using the same ablation cell without changing the carrier-gas flows within comparative experiments. Both laser-optical systems have a fairly flat-topped lateral energy distribution yielding pan-shaped ablation pits on the sample. Comparative experiments with the two optical systems were carried out with argon and with helium+argon as carrier gases. For both lasers and both gas set-ups, signals of 40 s duration were recorded with a pulse rate of 10 Hz, with similar fluence adjusted to give comparable rates of material ablation. ICP-MS signal intensities were normalised to the total ablated volume, to compare the effects of lasers and gases on transport efficiency, ionisation efficiency and response and time-dependent element fractionation. The accuracy of trace element results was tested using two materials that allow stable ablation with both lasers but have significantly different matrix compositions (SRM 612 silicate glass from NIST and AGV-1 geological reference material in lithium tetraborate fusion). The time-averaged rate of material ablation is similar for both lasers and independent of the carrier gas in the sample chamber, but decays more rapidly with the 266 nm system. In argon, the signal unit response per volume of ablated material is similar with both lasers. In helium, the signal intensity with the 266 nm laser is enhanced slightly (maximum two-fold) compared with argon, but with the 193 nm system a consistent 2-3-fold signal enhancement is achieved. The use of helium reduces the amount of visible (>1 µm size) particle deposition in the ablation cell, irrespective of laser wavelength, and tests with successive ablations of chemically contrasting samples indicate memory effects with the 266 nm system that are absent in the same experiment with the 193 nm system. The limits of detection for both lasers were further improved by the use of He owing to the decrease in background intensities. Time-dependent element fractionation during a 40 s single-spot ablation is almost eliminated with the 193 nm system (<10%), but with the 266 nm laser inter-element intensity ratios comparing the first and the second halves of the ablation period varied by up to 60% for some elements. Results for the cross-calibration between silicate (SRM 612) and borate glasses (AGV-1) obtained with both lasers indicate that the fractionation with the 266 nm system is similar for these two matrices, but this is not generally true for silicate minerals. The 193 nm system gives slightly better reproducibility between multiple analyses of one sample compared with the 266 nm system, yielding 2-5% RSDs for major and minor elements and 7-15% for concentrations below 10 ppm.