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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 mm. 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 m ablation pit). Furthermore, this improved sensitivity has recently yielded the first quantitative determination of gold concentrations (~0.1 mg g−1) and full rare-earth element patterns in single 25 mm fluid inclusions. net signal intensity for 238U+ in ICP-MS. Other authors have

Recent developments in laser ablation inductively coupledfor trace element analysis of geological samples and other environmental materials, owing to the increased sensitivity of plasma mass spectrometry (LA-ICP-MS) have demonstrated its potential for in situ microanalysis for major, minor and ICP-MS and the more eÃcient interaction of UV lasers with solid samples. The use of UV laser beams has led to a more trace elements in solids, such as minerals. With the low backgrounds and high sensitivity of new ICP-MS instruments, controlled ablation process.1 With new generation ICP-MS instruments, modifications of the sample cone geometry and limits of detection of 1-10 ng g−1 in a 40 mm ablation pit for many elements can be reached. Fractionation eÂects due to changes in the torch box configuration, limits of detection for ten selected rare earth elements (REE) of less than 10 ng g−1 diÂerent ablation rates of various elements have prevented quantification without matrix-matched standards with 1064 nm in a 35-40 mm pit can now be reached.2,3 Earlier work by Hirata and Nesbitt,4 Fryer et. al.5 and JeÂries et al.6 demon-Nd5YAG lasers. These eÂects have been reduced but not eliminated using shorter UV wavelengths (e.g. a quadrupled strated the limitations of LA-ICP-MS analysis because of significant fractionation eÂects observed during the ablation Nd5YAG 266 nm). Excimer lasers with wavelengths below 200 nm are expected to reduce fractionation eÂects further, but for some elements, especially Zn, Pb and U, which are of major interest in geological samples. Stix et al.7 reported matrix they present a serious challenge to the design of optical systems, especially if high-resolution UV ablation needs to be eÂects for mineral analysis using synthetic glass standards.Various strategies applied to LA-ICP-MS to minimise these combined with high quality visual observation, which is essential for the study of complex materials, such as geological eÂects have been reported, including moving the stage during ablation,4 spraying water onto the ablation site or the use of samples. An LA system was developed using an homogenized UV laser beam (193 nm, Argon Fluoride excimer) with a several internal standards for specified groups of elements of geochemical interest.8 However, none of these approaches can common UV-visual objective on a modified petrographic microscope with reflected and transmitted light illumination, in be used for routine LA analysis or match the improved ablation characteristics oÂered by the 193 nm excimer system described. 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. EXPERIMENTAL Fractionation eÂects due to diÂerential ablation of various ICP-MS Instrumentation elements as a function of time can be reduced to interelement correlation coeÃcients of r=0.9 or better and have becom...

In most published hydrothermal ore deposit models, the main agent of metal transport is an aqueous liquid. However, there is increasing evidence from volcanic vapors, geothermal systems (continental and submarine), vapor-rich fluid inclusions, and experimental studies that the vapor phase may be an important and even dominant ore fluid in some hydrothermal systems. This paper reviews the evidence for the transport of metals by vapor (which we define as an aqueous fluid of any composition with a density lower than its critical density), clarifies some of the thermodynamic controls that may make such transport possible, and suggests a model for the formation of porphyry and epithermal deposits that involves precipitation of the ores from vapor or a vaporderived fluid.Analyses of vapor (generally >90% water) released from volcanic fumaroles at temperatures from 500°to over 900°C and near-atmospheric pressure typically yield concentrations of ore metals in the parts per billion to parts per million range. These vapors also commonly deposit appreciable quantities of ore minerals as sublimates. Much higher metal concentrations (from ppm to wt %) are observed in vapor inclusions trapped at pressures of 200 to 1,000 bars in deeper veins at lower temperatures (400°-650°C). Moreover, concentrations of some metals, notably Cu and Au, are commonly higher in vapor inclusions than they are in inclusions of coexisting hypersaline liquid (brine). Experiments designed to determine the concentration of Cu, Sn, Ag, and Au in HCl-bearing water vapor at variable although relatively low pressures (up to 180 bars) partly explain this difference. These experiments show that metal solubility is orders of magnitude higher than predicted by volatility data for water-free systems, and furthermore that it increases sharply with increasing water fugacity and correlates positively with the fugacity of HCl. Thermodynamic analysis shows that metal solubility is greatly enhanced by reaction of the metal with HCl and by hydration, which results in the formation of species such as MeClm.nH2O. Nonetheless, the concentrations measured by these experiments are considerably lower than those measured in experiments involving aqueous liquids or determined for vapor fluid inclusions. A possible explanation for this and for the apparent preference of metals such as Cu and Au for the vapor over the coexisting brine in some natural settings is suggested by limited experimental studies of metal partitioning between vapor and brine. These studies show that, whereas Cu, Fe, and Zn all partition strongly into the liquid in chloride-bearing sulfur-free systems, Cu partitions preferentially into the vapor in the presence of significant concentrations of sulfur. We therefore infer that high concentrations of Cu and Au in vapor inclusions reflect the strong preference of sulfur for the vapor phase and the formation of sulfur-bearing metallic gas species.Phase stability relationships in the system NaCl-H2O indicate how vapor transport of metals may occur in nature, b...

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Porphyry-type ore deposits are major resources of copper and gold, precipitated from fluids expelled by crustal magma chambers. The metals are typically concentrated in confined ore shells within vertically extensive vein networks, formed through hydraulic fracturing of rock by ascending fluids. Numerical modeling shows that dynamic permeability responses to magmatic fluid expulsion can stabilize a front of metal precipitation at the boundary between lithostatically pressured up-flow of hot magmatic fluids and hydrostatically pressured convection of cooler meteoric fluids. The balance between focused heat advection and lateral cooling controls the most important economic characteristics, including size, shape, and ore grade. This self-sustaining process may extend to epithermal gold deposits, venting at active volcanoes, and regions with the potential for geothermal energy production.

Hawaii 23 indicate that equilibrium-line altitudes were ϳ900 m lower than at present (ϳ780 m reduction relative to lower sea level). But limiting dates for these advances range widely (20-15 kyr ago for Huascarán, 30-10 kyr for Kilimanjaro, Ͼ15 kyr for New Guinea and 22-9 kyr for Hawaii), so the synchronicity of lowered equilibrium-line altitudes across the tropics remains uncertain. For Kilimanjaro and Huascarán, the OSU simulation is 3-5 ЊC cooler than the control, yielding approximate temperature-related depressions of equilibrium-line altitudes of 550-900 m (based on a nominal tropical lapse rate of 5.5 ЊC km −1 ). In both areas net moisture at the LGM is greater than that of the control, which would further enhance glacier growth. The OSU simulation is thus consistent with regional glacier advances in the Andes and east Africa. We did not modify CLIMAP SSTs near New Guinea and Hawaii, and as a result temperatures in the OSU simulation are essentially the same as those of the CLIMAP simulation (3 ЊC cooler and 1 ЊC warmer than the control, respectively) and perhaps inconsistent with local glaciation, indicating the need for further study of these regions 24 .The OSU simulation helps to resolve some, but not all, disagreements between land and ocean data. We did not consider climate feedbacks associated with LGM vegetation, and these may yield further modelled cooling over land 25 . Although the ice-age tropics in the OSU simulation are generally cooler and drier than the control, some regions such as the Andean highlands are cooler and wetter, suggesting substantial variation within the tropics both regionally and with altitude, consistent with recent data compilations 26 . The LGM cooling in the OSU simulation agrees well with recent ocean and atmosphere-ocean model simulations over the eastern Pacific, the equatorial Atlantic and regions of Africa and Asia 9,27-29 . To the extent that our AGCM results are modelindependent, this agreement suggests convergence of data and models in these regions. In the western Pacific, the OSU reconstruction agrees with one ocean simulation 29 and one coupled atmosphereocean simulation 28 , but disagrees with a coupled atmosphere-ocean simulation that yields a cooling of 4-6 ЊC (ref. 27). Nevertheless, our results highlight the potentially widespread influences of regional SST changes in circulation patterns and moisture fluxes associated with tropical-extratropical temperature gradients, and understore the need to reconstruct both the amplitude and the geographical distribution of LGM climate changes in much greater detail. Ⅺ