GEMOC ARC National Key CentreSharma, A.L., Alard. O., Elhlou, S. and Pearson, N.J., GEMOC Macquarie
In this study we present accurate determinations of LILE (Rb, Sr, Ba,
Sc,U, Th, Pb) , HFSE (Hf, Zr, Nb, Ta) and 14 REE in ultramafic (Peridotites
JP-1, PCC-1) and basalt( BCR-2, BIR-1) rock standards. Due to their low
abundances, quantitative determinations of these element are difficult.
It has been recently suggested that HNO3/HF attack and
evaporation gives lower recovery especially for REE in ICP-MS (Yokoyama
et al., 1999). Yokoyama and coworkers have proposed an effective
HF/HClO4 digestion technique in order to avoid fluoride
precipitation of REE.
This study assessed the benefits of perchloric over nitric digestion
techniques for ultramafic and mafic rock standards (Peridotites : JP1,
PCC-1, basalts BCR-2, BIR-1, BHVO-2).
Three different digestions were tested. The first one involved double 4ml HF:HNO3 (1:1) attack and evaporation, followed by 10 ml HNO3 (6N) digestion and evaporation to incipient dryness. The second technique involved 4 ml HF/ HNO3 (1:1) attack, followed by a 2ml HF/HClO4 (1:1) attacks. Samples were then refluxed 2 times with decreasing amounts of HClO4 at high temperature (up to 190oC). After evaporation, 0.5 ml of 6N HCl was added and evaporated. This technique is a slight modification to that of Yokoyama et al. (1999). The third technique was developed by Ionov et al. (1992) and involves larger volumes of HClO4. It consists of a primary attack using HF/HClO4 (2:2.5) and a second HF/HClO4 (1:1.5) digestion, followed by three consecutive attacks of 0.5, 0.25, 0.1 ml HClO4 and evaporation at increasing temperature (150o, 180o, 190oC respectively). All trace elements have been determined by ICPMS using external calibration. Samples were taken up in 100 ml of 2% HNO3, achieving a dilution factor ca. 1000. We compared data obtained by various analytical techniques and data obtained by Laser Ablation ICP-MS on quenched basaltic glasses BIR-1 and BCR-2.
The detection limits for most elements are in the range of 1 to 10 ppb. Our values for PCC-1, BCR-2, BIR-1 and BHVO-1 are comparable to those obtained by ID-SSMS (Jochum et al., 1989) and other ICPMS work (Govindaraju, 1989). The solution values obtained for BCR-2 and BIR-1 are in good agreement with LAM-ICPMS values (this work and Norman et al., 1996). The LAM-ICPMS values are free of dissolution and interference problems, thus suggesting that our solution values are accurate. The average of 22 analyses of JP-1 is within error of those published by Ionov et al. (1992). However systematic discrepancies with the Japanese Geological Survey's working values are observed. It is noted that HFSE/REE ratios (eg. Hf/Sm, Nb/Ce) and HFSE ratios (eg. Nb/Ta, Zr/Hf) vary widely for the ultramafic rocks depending on the digestion. This variability is also observed in literature thereby making it difficult to assess the true fractionation between HFSE and REE due to the various digestions.
Higher recovery of REE is obtained with HClO4 digestion for ultramafic rocks compared with that for HNO3 digestion. The benefits of the HClO4 attack for basaltic rocks are less obvious with a recovery of only ~10-15 % better than that for HNO3 /HF digestion. On the other hand the use of HClO4 significantly raises the acid blank due to the difficulty of safely and efficiently distilling HClO4 . The detection limits for most elements is at least 2 times worse with HF/HClO4 than with HF/HNO3. The small net gain in sensitivity from the use of HClO4 must be weighed against the disadvantages (especially safety issues) involved in its use. Further work is continuing to further assess the advantages and drawbacks of the two types of acid digestions and further to characterize the ratio of HFSE relative to REE in these rock standards
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Norman, M.D., Pearson, N.J., Sharma, A.L., Griffin, W.L. (1996), Quantitative analysis of trace elements in geological materials by laser ablation ICP-MS: instrumental operating conditions and calibration values of NIST glasses, Geostandards Newsletter, 2 : 247-261.
Yokoyama,T., Makishima, A., Nakamura, E (1999), Evaluation of the coprecipitation of incompatible elements with fluoride during silicate rock dissolution by acid digestion, Chemical Geology, 157 : 175-187.