Direct Quantitative Determination of Trace Elements in Fine‐Grained Whole Rocks by Laser Ablation‐Inductively Coupled Plasma‐Mass Spectrometry

Abstract

Previous laser ablation‐ICP‐MS bulk analyses have been confined to volcanic glasses and glass disks or powder pellets similar to those used for XRF analysis. This study proposes a method to determine twenty trace elements (fourteen rare earth elements, Sc, Y, Zr, Nb, Hf and Ta) by LA‐ICP‐MS directly from polished thick sections and rock slabs of six fine‐grained crystalline and aphanitic rocks (five volcanic rocks and one pelitic tillite). Laser scanning of eight to ten 20 mm long linear tracks using a spot size of 160 μm, with a total ablated area of 26–32 mm2, was performed. Quantification was carried out by (a) internal standardisation using Si and (b) without applying internal standardisation. In the latter method, external determination of one element in conventional LA‐ICP‐MS quantification is no longer needed. Although the fine‐grained rocks studied contained variable amounts of volatiles (up to 4%), this method gave results that agree within 10% relative with those obtained by internal standardisation using Si. Two USGS basalt glass reference materials (BCR‐2G and BHVO‐2G) were used for external calibration. The results and the associated trace element patterns and ratios of elemental pairs obtained from both methods of quantification showed good agreement with the results from solution nebulisation ICP‐MS within 20% (mostly within 10%) relative. Fine‐grained rocks are common and include volcanic, sedimentary and low‐grade metamorphic rocks (e.g., basalt, andesite, rhyolite, shale, mudstone, tillite, loess, pelite and slate) and their trace element contents and associated ratios are important geochemical tracers in studies focusing on the composition and evolution of the crust and mantle. Our method provides a simple and quantitative way to determine trace elements in fine‐grained rocks even with those displaying complex textures.

Calibration and correction of LA-ICP-MS and LA-MC-ICP-MS analyses for element contents and isotopic ratios

“…The 100% normalization calibration is summed up as Equation (5) according to Liu et al (2008). This method has been used to analyze the major and trace elements of silicate glasses (Guillong et al, 2005;Halicz and Günther, 2004;Liu et al, 2008), silicate minerals (He et al, 2013;Hu et al, 2008a;Liu et al, 2008;Liu et al, 2010b), carbonate minerals , silicate whole-rocks (Li et al, 2011a;Zhu et al, 2013) and metal oxides Zong et al, 2015).…”

Applications of LA-ICP-MS in the elemental analyses of geological samples

“…The direct analysis of a rock slice using LA-ICP-MS can only be performed on cryptocrystalline volcanic rock, volcanic glass or fine-grained pelite because of the selective ablation of different minerals [24]. The pressing of a powder pellet without binders simplified the sample preparation and avoided any undesirable dilution of the sample; however, the particle size of the samples needs to be sufficiently reduced (e.g.…”

“…As an improvement to the summed signal normalization technique, Halicz and Günther [43] calculated the concentration of the internal standard based on normalization of the sum of all metal oxides to 100 wt.% to determine the trace-element compositions of anhydrous silicates. This method has been used to analyze the major and trace elements of silicate glasses [40,43,44], silicate minerals [40,45,46], carbonate minerals [47], silicate whole-rocks [23,24] and metal oxides [48]. The generic formula for this calibration method is demonstrated as follows: ) of the calibration curve can be calculated based on the concentration-weighted average, which leads to the greatest reduction in uncertainty introduced through the relatively high uncertainties of those elements with low concentrations in the reference materials [40] because, for a given element, high concentrations can be determined more precisely than low concentrations, and the reference values of high concentrations are typically more accurate than those of low concentrations.…”

“…Based on the sample preparation technique, LA-ICP-MS methods have been applied to the bulk analysis of various samples, including (1) the direct analysis of a rock slice [24,126], (2) the analysis of pressed-powder pellets both with binders [1, 26,127] and without binders [128], and (3) the analysis of fused glasses fused both with a flux agent [6, 129,130] and without a flux agent [22,23,25,61,[131][132][133][134].…”

“…As a consequence, LA-ICP-MS has been used in the geosciences not only as a single-spot analysis method, especially where trace element or isotope concentrations are required at high spatial resolution (e.g. the analysis of a single fluid/melt inclusion, a single zircon grain, or the zonation of a mineral) [2,4,5,9-16] or distribution analyses of elements and isotopes of rocks and minerals are required [17][18][19][20][21], but also for bulk analyses of rocks and soils [22][23][24][25][26][27][28][29].…”