A new ICP-MS technique for measuring trace elements in vapour from high pressure experiments

G.T. Nichols, T.H. Green and N.J. Pearson, GEMOC, Macquarie

Introduction

Many geologic models require the involvement of vapour to explain the observed geochemistry of rocks. Unfortunately, the trace element characteristics of vapour (or supercritical fluids) at high pressures and temperatures are presently not well constrained. We have therefore conducted a series of experiments utilizing piston-cylinder apparatus that reproduces, under controlled conditions, the high-pressures and temperatures of lower crustal or upper mantle rocks. In the experiments discussed here, we sealed approximately 0.02 to 0.04 grams of rock-powder (basanite) and water in Ag50Pd50 or Ag70Pd30 capsules and ran the experiments between 24 and 140 hours depending on temperature. The run conditions varied between 15-30 kbar and 825-1220°C. The Mt Leura (Queensland) basanite was enriched in La, Sm, Lu, Y, Hf, Nb, Ta, Th, U, Cs and Rb totalling 1.045 wt%, to enhance analytical precision. This composition and experimental conditions, were selected to approximate a vapour-saturated upper mantle rock.

Analytical Techniques

After experiments were quenched, we removed the AgPd capsules holding the vapour + rock components and enclosed them in 25 mm diameter epoxy-resin mounts. The first stage in the analytical process involved the release and immediate analysis of the vapour phase using the Macquarie University Laser Ablation Microprobe (LAM) attached to a Perkin-Elmer 5100 Inductively Coupled Plasma - Mass Spectrometer (ICP-MS). This instrument is described in detail by Norman et al. (1996). The laser is a Q-switched, frequency quadrupled Nd:YAG laser, operating at 266 nm (UV). All analyses were performed using a repetition rate of 4 Hz and energy of ~1 mJ/pulse. These conditions produced a sampling area ~30 µm in diameter. The sample-mounts, and standards, are housed in a sealed cell and the ablated material is transported by a stream of high purity Ar to the ICP-MS. The sample cell is mounted on a petrographic microscope fitted with CCD-TV, enabling viewing of the ablation process. Data acquisition was monitored on a real-time graphics display. The laser was used to drill through the AgPd capsule to release the vapour ± liquid, at which time time the laser was turned off. The vapour signals were transient, typically ² 40 seconds duration. The following trace elements were analysed Be, Ti, Ni, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, 147Sm, 152Sm, Ho, Yb, Lu, Hf, Ta, Pb, Th, U with dwell times of 20 ms, using peak hopping mode and with one sweep across the mass-range per reading.

After vapour analysis the metal capsules were sectioned and the experimental products exposed by polishing. Major element analyses of primary minerals, spherules and glass-matrix, were determined using a Cameca SX50 electron microprobe.

The trace element compositions of sufficiently large experimental grains that include minerals, spherules, and glass matrix were analysed by LAM ICP-MS under the same conditions described for the vapour analysis, except that a longer dwell time of 50 ms was employed. In these determinations the signals were essentially steady-state, displaying nearly constant counts per second for analysis times between 120 to 185 seconds.

Data Reduction

Signals are displayed during the data reduction process; for vapour analyses the transient peak, corresponding to the moment of capsule piercing, was selected for integration and comparison with background counts. Raw counts were then quantified using the NIST610 glass as an external calibration standard and 44Ca as an internal standard using the electron microprobe data for CaO. To successfully compare signals for the vapour + H2O signal it was necessary to select a normalisation factor, as we were not able to provide an absolute concentration for any particular element. In this case we selected a somewhat arbitrary value of Sr=100 ppm, which was based on the concentration of Sr in spherules analysed using the CSIRO proton-microprobe. Selecting other normalisation values, or other normalising elements, does not change the relative shape of the trace element patterns, but translates the signal either up or down such that the "absolute" concentrations vary.

The accuracy and precision of the quantitative analysis by LAM ICP-MS is discussed in Norman et al. (1996). As an example in this application, we have propagated uncertainties on an individual analysis of a spherule (analysis sph29, run 1627), and as a relative percentage errors range from ~14 to ~20% (e.g., Lu, 1150 ± 170 ppm; Pb, 2.54 ± 0.50 ppm). These values include ICP-MS counting statistics as well as the error on the internal standard (CaO) determined by electron microprobe. In this example, the standard deviation on CaO (5.09 ± 0.71 wt%; 13.9% rsd) is the main contribution to the propagated uncertainty on the trace element concentrations. The magnitude of the standard deviation may reflect true compositional variation but may also in part be due to variable Na loss under the electron beam depending on the size and composition of individual spherules.

Discussion and Results

In addition to primary minerals such as garnet, clinopyroxene, amphibole and rutile, the quenched experiments produced a number of vapour components. The unmixing of experimental vapour, during quench, ensures that the proportions of components are poorly constrained, and this provides one of the major complexities in the complete analysis of the equilibrium vapour phase. They include spheroidal shaped solids that are usually ~20 µm in diameter (between 5 and 50 µm), which we have termed "spherules". Additionally, experiments yield water and vapour (probably a C-O-N mixture) that we have analysed directly with the LAM ICP-MS.

Because of these complexities, we modified our experiments to include vitreous carbon-spheres that provide both porosity, and absorb the trace element signal of vapour present during an experiment. The analysed carbon-spheres show trace element patterns almost identical to those of the spherules we analysed in separate runs using either identical LAM ICP-MS methods, or the proton-microprobe. Since the trace element patterns of spherules and carbon-spheres are so similar we infer that these patterns are close to the equilibrium vapour, because quenched vapour is unlikely to have penetrated the carbon-spheres during the extremely rapid experimental quench. We conclude that although some spherules must form during the quench process (based on textural evidence), others must represent quenched solute components from primary equilibrium vapour.

Our analyses of the vapour components indicate the following trace element characteristics. Quenched spherules and the carbon-spheres have trace element patterns dominated by Pb and Ni depletions, and relative enrichments in Ti, and Rb. Nb is weakly preferred over Ta, and Hf over Zr. Experimental vapour measured by capsule piercing has trace element signals displaying strong enrichment in Pb and Rb. Nb is variably enriched over Ta, and Zr over Hf. La, Sm, Ti, Y and Lu display variable depletions, and in some experiments were below analytical detection.

Reference

MD Norman, NJ Pearson, A Sharma and WL Griffin 1996. Geostandards Newsletter, 20, 247-261.

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