The development and geological application of a whole rock-LA-ICP-MS technique for the determination of high field strength elements

Whole rock, laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) was carried out on synthetic and natural glasses in an attempt to improve the precision and accuracy of the determination of high field strength element (HFSE) abundances. Compared to more conventional methods of analysis, such as X-ray fluorescence and ICP-optical emission spectrometry, ICP-MS offers lower limits of detection. Neutron activation analysis typically has worse precision than ICP-MS and is more labour intensive. The determination of HFSE by solution nebulisation ICP-MS is compromised by problems associated with solution instability and the possibility of incomplete digestion of mineral phases in acid. To avoid the problems inherent in solution nebulisation ICP-MS, a solid-sampling technique was developed by preparing synthetic glasses that were fused using an iridium strip heater and analysed with LA-ICP-MS. The results for fused geostandard reference materials (G-2, NBS688, MRG-1, SY-2, SY-3, and AGV-1) have an accuracy of within 11% of literature values for REE determinations and within 7% for HFSE determinations. The average precision (lσ) of analysis is within 5% for REE determinations and within 4% for HFSE determinations. -- An investigation was carried out on the application of isotope dilution (ID) to whole rock LA-ICP-MS through spiking the sample powder with an isotopically enriched solution (⁹¹Zr, ¹⁷⁹Hf, ¹⁴⁵Nd, ¹⁵¹Eu, ¹⁶¹Dy and ¹⁷¹Yb) before fusion. The results for geostandards BHVO-1, MRG-1, AGV-1 and BCR-2 analysed with this ID-LA-ICP-MS technique show an accuracy of within 4% of literature values for Zr and Hf. The LD-Zr values determined by isotope dilution were used as an internal standard, with external calibration on NIST SRM 612 glass, for the determination of Nb, Ta, La, Ce, Pr, Sm, Gd, Ho, Tm and Lu. The accuracy achieved with this methodology is within 3% for Nb, 6% for Ta and 8% for REE determinations. -- Over 100 geological samples from a variety of tectonic settings have been analysed by whole rock LA-ICP-MS in order to determine accurate and precise HFSE values. The main emphasis of this study was to investigate the variation of Nb/Ta and Zr/Hf ratios and assess the potential for these element pairs to fractionate within geological systems. The Nb/Ta ratios for virtually all of the mid-ocean ridge and ocean island basalt samples, and a majority of the island arc samples, overlapped within the chondritic range (17.5 ± 2). Because Nb/Ta does not appear to fractionate among very diverse tectonic settings (such as MORB, OIB and most arcs), the non-chondritic values found for a few of the arc samples (Nb/Ta = 11 to 26), and all of the boninitic (Nb/Ta = 9 to 12) and continental crust samples (Nb/Ta = 4 to 9), suggest that very unusual mantle processes characterise the genesis of these rocks. These processes might include fractionation related to metasomatism or the existence of an accessory phase such as rutile or amphibole. In comparison to Nb/Ta, Zr/Hf ratios showed a greater tendency to fractionate below the chondritic margin of error (36.3 ± 4) with overall values from 19 to 45 and a mean value of 31. This may reflect the greater tendency of Zr to fractionate from Hf related to the greater difference in the bulk partition coefficients of these elements