Development of analytical methods and standard reference materials for analysis of trace elements and isotopic ratios in sulphides

Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) is a micro-sampling technique for the analysis of isotopic ratios and elemental concentrations in solid materials. It has become a widely used analytical technique for geological research; however its application has been limited in the field of ore deposit research by the small range of mineral specific reference materials available and an incomplete understanding of the fundamental processes occurring during the laser-sample interaction for sulphide minerals. In this study the influence of laser parameters such as fluence, spot size and repetition rate on the ablation of sulphide minerals were investigated for a range of laser ablation systems: a solid state 213 nm wavelength laser, ~4 ns pulse width (New Wave UP213); a solid state 193 nm wavelength laser, ~4 ns pulse width (New Wave UP193ss); and an ArF excimer 193 nm wavelength laser, ~20 ns pulse width (Resonetics Resolution S155). These laser systems were coupled with Agilent quadrupole ICP-MS instruments for analysis. The ablation characteristics of a range of sulphide minerals were investigated: for bornite, chalcopyrite, pentlandite, pyrite, pyrrhotite, sphalerite and tetrahedrite. The ablation craters in each mineral were imaged using a Field Emission Scanning Electron Microscope (FE-SEM) and significant melting was found to occur around the rim and on the crater base for some minerals. The amount of melting was partially dependent on the type of laser with the 213 nm laser producing slightly more melting than the 193 nm excimer laser. However, the relative order of minerals for the amount of melting observed was the same for both lasers, with tetrahedrite > bornite, chalcopyrite > pentlandite, pyrrhotite > pyrite, sphalerite. There is also a strong correlation between the amount of melting and the bond strength as estimated from the Gibbs free energy of formation. This suggests that the extent of melting occurring during ablation is primarily dependent on the physical properties of the mineral, rather than the type of laser used. Three types of particles produced during ablation were identified: large droplets of solidified melt (0.1-1.0 ˜í¬¿m), rounded condensate particles (20-100 nm) and loose agglomerations of very fine condensates. For some minerals the proportions and size of each particle type were different between laser systems, indicating that the particle formation processes are influenced by the laser wavelength and pulse width. This study has demonstrated that there can be significant elemental and isotopic fractionation for S compared to other major elements in sulphide minerals (eg. Fe, Cu, Ni and Co). When using pyrite as the reference material (RM) for mass bias correction of 34S/32S isotopic ratios there can be significant fractionation: of 1.5 ‚ÄövÑ‚àû for pyrrhotite and 2.6 ‚ÄövÑ‚àû for bornite with a 213 nm laser. Similarly using pyrite as the RM for calibration of S concentrations using Fe as the internal standard can cause an over estimation of S by up to 50% for tetrahedrite and 30-40% for bornite and chalcopyrite. These differences can be reduced however, to <1 ‚ÄövÑ‚àû for 34S/32S ratios and 10-15% for chalcopyrite by using high fluence (4.6 J cm-2) with a 193 nm excimer laser. Despite this improvement in mineral dependent fractionation at high fluence, the amount of down-hole fractionation is more significant for spot ablations under these conditions and therefore is not recommended for routine analyses. There is a positive correlation between the elemental fractionation between S and Fe and the amount of melting around the ablation crater. This is attributed to preferential volatilisation of S from molten material due to its lower boiling point compared to Fe. The need for matrix match reference materials for accurate LA-ICP-MS has been demonstrated for the ns pulse width lasers used, however there are few widely available RM's for sulphide minerals. Two new RM's have been developed in this study: NiS-3 for PGE and Au trace element analysis in the range 20-24 ˜í¬¿g g-1 and PPP-1 pyrite for S isotopic analysis with ˜í¬•34SV-CDT 5.3 ¬¨¬± 0.2 ‚ÄövÑ‚àû. Two additional secondary isotope RMs were also developed: Po-10 pyrrhotite (˜í¬•34SV-CDT 6.0 ¬¨¬± 0.3 ‚ÄövÑ‚àû) and N-11 bornite (˜í¬•34SV-CDT -4.4 ¬¨¬± 0.6 ‚ÄövÑ‚àû). The NiS-3 RM was used to quantify the PGE and Au concentrations in four RMs used and developed in other LA-ICP-MS facilities: 8b, PGE-A, Po727-T1, Po724-T and the Lombard meteorite. These concentrations were compared against their published values and the consistency for calibration was investigated between these RMs. Po727-T1 and 8b were consistent for all elements to within 5%, however all other RMs showed some inconsistencies. Po724-T, Po727-T1 and the Lombard meteorite were found to be homogeneous for all elements with the analytical uncertainties, whereas the other RMs were heterogeneous for some elements especially Os in 8b and Pd in NiS-3. The interface tubing through which the ablated material is transported from the laser ablation sample chamber to the ICP-MS can have an influence on both the precision and the length of time for the signal to fall to background levels after analysis (washout time). This study investigated the effects of interface tubing configuration on the precision and washout of LA-ICP-MS analyses of sulphides. The washout time for the S can be longer than the signals for other elements: when using a single straight interface tube the S signal from ablating pyrite takes >120 s to washout, compared to 1-2 s for Fe. In contrast, the washout time for S after pyrrhotite ablation is ~3 s indicating differing transport mechanisms for S between these two Fe-sulphide minerals. Additional components were added to the interface: a 'squid' mixing device (Laurin Technic) and a tight coil of small inner diameter Tygon tubing. These components were found to improve both the precision (from 0.66 to 0.28 ‚ÄövÑ‚àû uncertainty on a single 34S/32S analysis) and the washout time for S after pyrite ablation (to ~20 s). By gaining a better understanding of the ablation processes in sulphide minerals, improvements in the washout time for S, and the characterisation of new mineral specific RMs, new applications for quadrupole ICP-MS have been developed: 1) Characterising the 34S/32S isotopic ratios in pyrite and pyrrhotite to within 1 ‚ÄövÑ‚àû precision and accuracy; and 2) being able to accurately measure S in a range of sulphide minerals to enable it to be used as the internal standard element or for quantification of analyses by normalising to 100 % element totals.