Quantitative mapping of trace elements in agate using LA-ICP-MS
© Park et al. 2015
Received: 15 November 2015
Accepted: 6 December 2015
Published: 17 December 2015
Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is an optimized technique for the quantitative mapping of trace elements on geological materials. Agate is an aggregate of microcrystalline silica showing a variety of colorful bands due to the species and concentrations of transition metals and is thus an appropriate sample to test the availability, efficiency, and detection limit of LA-ICP-MS trace element mapping.
Two fortification agate samples selected for elemental mapping are characterized by the colorful and bluish banding layers, respectively. The colorful agate sample consists mainly of two growth zones varying in color from pale orange-red brown to gray tones, whereas the blue agate sample simply shows alternating blue and white bands. Two different types of mineral inclusions, Fe oxide/hydroxide and hollandite (Ba(Mn4+ 6Mn3+ 2)O16), were identified in the inner and outer halves of the former sample, respectively. The distribution patterns of eight trace elements in these samples, including Al, Co, Cr, Cu, Fe, Mn, Ni, and Ti, were investigated, using both electron microprobe and LA-ICP-MS.
Aluminum and Fe are the most abundant trace elements in both agate samples up to 5000 ppm although their concentrations in the colorful agate sample are an order of magnitude higher than those in the blue agate sample. The concentrations of other elements in the agate samples are several parts per million levels. The quantitative maps of the colorful agate sample have shown that Fe was mainly distributed in the inner half, but Al, Cu, and Mn in the outer half. In particular, Fe and Mn were primarily concentrated as small spots. On the other hand, the oscillatory zoning patterns are apparent in the quantitative maps of the blue agate sample, showing a correlative relationship between Al and Ti but an antithetic relationship between Al and Fe. The Al and Fe distribution patterns of the colorful agate sample are reproduced in the Al Kα and Fe Kα X-ray maps, but not clear in blue agate.
Quantitative elemental maps using LA-ICP-MS are privileged to visualize trace elements distributions.
Inductively coupled plasma mass spectrometry (ICP-MS) has become the most widely accepted analytical technique for simultaneously measuring a range of trace elements at variable concentration levels (Durrant 1999; Günther and Heinrich 1999; Günther et al. 2000; Sylvester 2001a; Heinrich et al. 2003). Laser ablation (LA) system is powerful and versatile for in situ microsampling of solid materials including geological samples particularly when it is attached to ICP-MS (e.g., Jackson et al. 1992; Norman et al. 1996, Günther et al. 1997; Jeffries et al. 1998; Niemax 2001; Heinrich et al. 2003; Günther and Hattendorf 2005; Large et al. 2009; Koenig et al. 2009; Ulrich et al. 2009). The development and application of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) have grown with a dramatic rate, and in situ analyses for elemental abundances and isotopic compositions are now performed routinely in numerous laboratories worldwide.
In addition to LA-ICP-MS, both ion and electron microprobes are commonly used for in situ analysis of trace elements (Müller et al. 2003). An electron microprobe has advantages of the smallest sampling volume and well-established data reduction procedures as well as quantitative calibration methods, but extreme care is required for elements at very low abundances. In many cases, even long counting times coupled with a detailed background characterization may not achieve the required sensitivity of a trace element (Müller et al. 2003). Ion microprobe has the main strength on both high sensitivity and small sampling volume. However, the severe matrix effect in this technique commonly prevents us from quantitative analyses in the absence of appropriate standard materials (Hinton 1990). LA-ICP-MS analyses combine the strength of the above techniques: relatively straightforward calibration procedures with a low detection limit. A glass reference sample is applicable to the quantitative analyses of trace elements for different types of materials with acceptable uncertainties. However, LA-ICP-MS has relatively poorer spatial resolution and requires relatively larger sampling volumes than both electron and ion microprobes (Woodhead et al. 2007).
The quantitative mapping of trace elements in geological samples has been facilitated by LA-ICP-MS (Treble et al. 2003; Fairchild et al. 2006; Montagna et al. 2007). It is also possible with other microanalytical techniques such as ion microprobe, proton-induced X-ray emission (PIXE), and synchrotron X-ray fluorescence (Woodhead et al. 2007; Ulrich et al. 2009). In contrast to the above methods, LA-ICP-MS trace element mapping is the only technique performing at low limits of detection for a wide range of elements in a relatively short acquisition time. The application of quantitative element distribution maps using LA-ICP-MS has been accelerated with the development of analytical protocol and easy-to-use software such as Iolite (Woodhead et al. 2007; Ulrich et al. 2009).
Agate is an aggregate of various forms of microcrystalline silica, mainly chalcedony, and is generally formed by chemical precipitation in rock cavities (Götze et al. 2001). Chalcedony occurs commonly in a semi-transparent or translucent state, with waxy luster, and varies widely in color, in contrast to colorless, transparent microcrystalline quartz (Polk 2012). Transition metals incorporated during the precipitation color agates (Götze et al. 2001; Möckel et al. 2009). The species and concentration of transition metals in agate are commonly reflected by its rhythmic color banding (Heaney and Davis 1995). The shade of color is further modified when these elements work in combination. Thus, agate is an appropriate sample to test the availability, efficiency, and detection limit of LA-ICP-MS trace element mapping. In this study, the distribution patterns of trace elements in two agates were investigated, using LA-ICP-MS, for demonstrating (1) the advantages of LA-ICP-MS trace element mapping through a correlation between the colors and concentrations of trace elements in the agate samples and (2) the analytical uncertainties and detection limits on the analyses of trace elements best optimized in the study.
The correlation between the chemical composition and color of agate bands
Red, brown, black, green
Pink, violet, black
Green, yellow, red
Red, blue, green
LA-ICP-MS instrument settings
RF power (W)
Ar nebulizer flow (L/min)
Ar auxiliary gas (L/min)
Ar cooling gas (L/min)
Laser wavelength (nm)
He carrier gas (L/min)
Repetition rate (Hz)
Energy density (J/cm2)
Dwell time (ms)
Scan speed (μm)
Beam size (μm)
NIST612 SRM glass was used as the external standard and 29Si as the internal standard assuming stoichiometric quartz. The oxidation rate was monitored using the ThO+/Th+ ratios of the external standard and was maintained below 1 % during the analyses. The data reduction and image processing of chemical data were conducted with Iolite software (Hellstrom et al. 2008; Ulrich et al. 2009; Paton et al. 2011). The main advantage of this software is its capability to render line scans into quantitative element distribution images. Outlier rejection was applied to each line scan based on a 2σ threshold. The dwell times for each trace element were 10 ms per analyte.
Aluminum was the most abundant trace element in both agate samples and also showed the widest variation in concentration ranging from several thousands of parts per million to hundreds of parts per million, with sharp boundaries between the compositionally distinct bands (Figs. 5b and 6b). The compositional variation and rhythmic banding of Al in the samples are typical for silica precipitation at low temperatures below 300 °C (Rusk et al. 2011). Agates commonly contain considerable amounts of mineral and fluid inclusions possibly responsible for their different coloration (Götze et al. 2001). The quantitative Fe and Mn elemental maps and microscopic images of the colorful agate sample clearly showed spot-shaped numeral mineral inclusions definitely changing from Fe oxide/hydroxide in the inner half to Mn-bearing hollandite in the outer half (Fig. 5c, e). The distribution of Fe ions was highlighted in either Fe oxide/hydroxide inclusions or chalcedony structure as a substitute for Si4+ particularly in the inner half (Fig. 5c), whereas that of Mn ions was mainly restricted to hollandite inclusions in the outer half (Fig. 5e). The distribution patterns indicate that the valence state of Mn remained tri- and tetra-valent during the silica precipitation of outer half and further suggest that redox conditions and/or fluid compositions might change significantly between two distinct growth zones (Barnaby and Rimstidt 1989; Götze et al. 2012). The change is also reflected by the different coloration of two growth zones in the colorful agate sample from pale orange-red brown to gray in combination with Cu and Co (Table 1; Fig. 5). On the other hand, the blue agate sample does not show conspicuous mineral inclusions. Aluminum, Fe, and Ti ions were distributed in the sample as a replacement for Si4+. The oscillatory zoning patterns of Al, Fe, and Ti were apparent in the sample (Fig. 6b–d). Such fluctuations of trace element concentrations might be associated with abundance of microcrystalline quartz (Götze et al. 2001). The partitioning of trace elements into chalcedony stronger than that into microcrystalline quartz might result in the concentration of transition metals at bluish bands.
The quantitative maps of trace elements in two agate samples show several lines of advantages on the LA-ICP-MS mapping technique. The quantitative Mn and Ti maps of colorful and blue agate samples, respectively, suggest that the compositional zonation of trace elements can be observed with their concentrations down to several parts per million levels (Figs. 5e and 6d). Analytical time spent for the quantitative mapping was ca. 18 h. Moggi-Cecchi et al. (2002) suggested that long counting times and special operating conditions are required to obtain X-ray distribution maps for trace elements below 1000 ppm. The maps can further be semi-quantified by electron microprobe spot analyses. Rusk et al. (2008) lowered the detection limits of semi-quantitative maps of trace elements down to several tens of parts per million levels, but the amount of time spent for the mapping was ca. 90 h on the 500 × 500 pixel map size. The qualitative Al Kα and Fe Kα X-ray maps of the colorful agate sample have reproduced the distribution patterns of trace elements similar to their quantitative maps (Fig. 5b, c, f, g), but the reproduction failed in other qualitative maps. Based upon the qualitative Al Kα X-ray map of the blue agate, the detection limits of qualitative mapping in the study were around 100–200 ppm (Fig. 6b, f), and it took 15 h for a common electron microprobe equipped with four WDS to map the distribution of eight trace elements in the blue agate sample with 500 × 500 pixel resolution. The potential limitations of laser ablation imaging that have commonly been mentioned lie in (1) its destructive nature with respect to other microsampling methods (Woodhead et al. 2007) and (2) its relatively large beam size resulting in relatively low spatial resolution (Ulrich et al. 2009). The ablated depth of agates has been minimized to ca. 5 μm in the study by a series of parallel line scans. It is also noted that the presence and distribution of ~10-μm-long mineral inclusions were well identified in the quantitative maps (Fig. 5c, e) although their size is much smaller than the beam size of laser ablation.
The quantitative distribution maps of trace elements in two agate samples were acquired by a series of equally long line scan and a subsequent merging of line scans with Iolite software. These maps have particularly shown the fluctuations and concentrations of trace elements in the agate samples which cannot be visualized by conventional spot analysis. The colorful agate sample showed a variation of trace elements from Fe-rich inner half to Mn-rich outer half. The quantitative maps of the blue agate sample showed a correlative distribution relationship between Al and Ti, but an antithetic distribution relationship between Al and Fe. The distribution patterns resulted mainly from alternating microcrystalline quartz-rich and chalcedony bands. The detection limits of quantitative maps in the study were estimated as several parts per million levels with the RSD values of ca. 10 %, whereas those of X-ray qualitative maps as 100–200 ppm. These results clearly demonstrate that LA-ICP-MS can provide valuable data for interpretations of compositional variations in association with microstructures, chemical/physical phase boundaries, and geological and environmental heterogeneous samples.
This work was supported by KBSI grant (T33612) to C.-S. Park. We thank anonymous reviewers for their constructive comments and suggestions.
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